Quantum Physics
- [1] arXiv:2405.09613 [pdf, ps, html, other]
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Title: Computable entanglement costComments: 7+24 pages, no figuresSubjects: Quantum Physics (quant-ph); Statistical Mechanics (cond-mat.stat-mech); Mathematical Physics (math-ph)
Quantum information theory is plagued by the problem of regularisations, which require the evaluation of formidable asymptotic quantities. This makes it computationally intractable to gain a precise quantitative understanding of the ultimate efficiency of key operational tasks such as entanglement manipulation. Here we consider the problem of computing the asymptotic entanglement cost of preparing noisy quantum states under quantum operations with positive partial transpose (PPT). A previously claimed solution to this problem is shown to be incorrect. We construct instead an alternative solution in the form of two hierarchies of semi-definite programs that converge to the true asymptotic value of the entanglement cost from above and from below. Our main result establishes that this convergence happens exponentially fast, thus yielding an efficient algorithm that approximates the cost up to an additive error $\varepsilon$ in time $\mathrm{poly}\big(D,\,\log(1/\varepsilon)\big)$, where $D$ is the underlying Hilbert space dimension. To our knowledge, this is the first time that an asymptotic entanglement measure is shown to be efficiently computable despite no closed-form formula being available.
- [2] arXiv:2405.09615 [pdf, ps, other]
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Title: Characterizing MPS and PEPS Preparable via Measurement and FeedbackSubjects: Quantum Physics (quant-ph); Strongly Correlated Electrons (cond-mat.str-el)
Preparing long-range entangled states poses significant challenges for near-term quantum devices. It is known that measurement and feedback (MF) can aid this task by allowing the preparation of certain paradigmatic long-range entangled states with only constant circuit depth. Here we systematically explore the structure of states that can be prepared using constant-depth local circuits and a single MF round. Using the framework of tensor networks, the preparability under MF translates to tensor symmetries. We detail the structure of matrix-product states (MPS) and projected entangled-pair states (PEPS) that can be prepared using MF, revealing the coexistence of Clifford-like properties and magic. Furthermore, we provide analytic solutions to states exhibiting MF symmetries akin to the symmetry-protected topological order in one dimension and the topological order in two dimensions, and we discuss their characteristics. Finally, we discuss the analogous implementation of operators via MF, providing a structural theorem that connects to the well-known Clifford teleportation.
- [3] arXiv:2405.09622 [pdf, ps, html, other]
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Title: Holevo Cram\'er-Rao bound: How close can we get without entangling measurements?Comments: 22 pages, 9 figures, 10 appendices; presented at AIP Summer Meeting 2023Subjects: Quantum Physics (quant-ph); Mathematical Physics (math-ph); Data Analysis, Statistics and Probability (physics.data-an)
In multi-parameter quantum metrology, the resource of entanglement can lead to an increase in efficiency of the estimation process. Entanglement can be used in the state preparation stage, or the measurement stage, or both, to harness this advantage; here we focus on the role of entangling measurements. Specifically, entangling or collective measurements over multiple identical copies of a probe state are known to be superior to measuring each probe individually, but the extent of this improvement is an open problem. It is also known that such entangling measurements, though resource-intensive, are required to attain the ultimate limits in multi-parameter quantum metrology and quantum information processing tasks. In this work we investigate the maximum precision improvement that collective quantum measurements can offer over individual measurements for estimating parameters of qudit states, calling this the 'collective quantum enhancement'. We show that, whereas the maximum enhancement can, in principle, be a factor of $n$ for estimating $n$ parameters, this bound is not tight for large $n$. Instead, our results prove an enhancement linear in dimension of the qudit is possible using collective measurements and lead us to conjecture that this is the maximum collective quantum enhancement in any local estimation scenario.
- [4] arXiv:2405.09626 [pdf, ps, html, other]
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Title: Permutation tests for quantum state identityComments: 15 pages, 1 figureSubjects: Quantum Physics (quant-ph)
The quantum analogue of the equality function, known as the quantum state identity problem, is the task of deciding whether $n$ unknown quantum states are equal or unequal, given the promise that all states are either pairwise orthogonal or identical. Under the one-sided error requirement, it is known that the permutation test is optimal for this task, and for two input states this coincides with the well-known Swap test. Until now, the optimal measurement in the general two-sided error regime was unknown. Under more specific promises, the problem can be solved approximately or even optimally with simpler tests, such as the circle test.
This work attempts to capture the underlying structure of (fine-grained formulations of) the quantum state identity problem. Using tools from semi-definite programming and representation theory, we (i) give an optimal test for any input distribution without the one-sided error requirement by writing the problem as an SDP, giving the exact solutions to the primal and dual programs and showing that the two values coincide; (ii) propose a general $G$-test which uses an arbitrary subgroup $G$ of $\text{S}_n$, giving an analytic expression of the performance of the specific test, and (iii) give an approximation of the permutation test using only a classical permutation and $n-1$ Swap tests. - [5] arXiv:2405.09628 [pdf, ps, other]
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Title: Quantum Dynamics in Krylov Space: Methods and ApplicationsPratik Nandy, Apollonas S. Matsoukas-Roubeas, Pablo Martínez-Azcona, Anatoly Dymarsky, Adolfo del CampoComments: 64 pages, 27 figuresSubjects: Quantum Physics (quant-ph); Statistical Mechanics (cond-mat.stat-mech); Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th); Chaotic Dynamics (nlin.CD)
The dynamics of quantum systems unfolds within a subspace of the state space or operator space, known as the Krylov space. This review presents the use of Krylov subspace methods to provide a compact and computationally efficient description of quantum evolution, with emphasis on nonequilibrium phenomena of many-body systems with a large Hilbert space. It provides a comprehensive update of recent developments, focused on the quantum evolution of operators in the Heisenberg picture as well as pure and mixed states. It further explores the notion of Krylov complexity and associated metrics as tools for quantifying operator growth, their bounds by generalized quantum speed limits, the universal operator growth hypothesis, and its relation to quantum chaos, scrambling, and generalized coherent states. A comparison of several generalizations of the Krylov construction for open quantum systems is presented. A closing discussion addresses the application of Krylov subspace methods in quantum field theory, holography, integrability, quantum control, and quantum computing, as well as current open problems.
- [6] arXiv:2405.09631 [pdf, ps, html, other]
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Title: Quantum switch instabilities with an open controlOtavio A. D. Molitor, André H. A. Malavazi, Roberto Dobal Baldijão, Alexandre C. Orthey Jr., Ismael L. Paiva, Pedro R. DieguezComments: 7+8 pages, 3+7 figuresSubjects: Quantum Physics (quant-ph)
The superposition of causal order shows promise in various quantum technologies. However, the fragility of quantum systems arising from environmental interactions, leading to dissipative behavior and irreversibility, demands a deeper understanding of the possible instabilities in the coherent control of causal orders. In this work, we employ a collisional model to investigate the impact of an open control system on the generation of interference between two causal orders. We present the environmental instabilities for the switch of two arbitrary quantum operations and examine the influence of environmental temperature on each potential outcome of control post-selection. Additionally, we explore how environmental instabilities affect protocol performance, including switching between mutually unbiased measurement observables and refrigeration powered by causal order superposition, providing insights into broader implications.
- [7] arXiv:2405.09641 [pdf, ps, html, other]
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Title: Entanglement dynamics of two modes coupled through a dissipative movable mirror in an optomechanical systemComments: 8 pages, 5 figuresSubjects: Quantum Physics (quant-ph); Mathematical Physics (math-ph)
Nonclassical states are an important class of states in quantum mechanics, especially for their applications in quantum information theory. Optomechanical systems serve as an invaluable platform for exploring and harnessing these states test bed for search and application of such states. In this study, we focused on the studied the mirror-in-the-middle optomechanical system. We observed that in the absence of losses, a coherent state evolves into a entangled one. Furthermore, we demonstrate that the generation of a Schrödinger-cat state depends on the optomechanical coupling. We exactly solved the Gorini-Kossalokowinki-Sudarshan-Lindblad master equation, highlighting the direct influence of the reservoir on the dynamics when mechanical losses are considered. We later discussed vacuum one-photon superposition states to obtain exact entanglement dynamics using concurrence as a quantifier. Our results show that the overall entanglement of the system is attenuated by mechanical losses in the mirror.
- [8] arXiv:2405.09644 [pdf, ps, html, other]
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Title: Optimizing Tensor Contraction Paths: A Greedy Algorithm Approach With Improved Cost FunctionsSubjects: Quantum Physics (quant-ph); Discrete Mathematics (cs.DM); Mathematical Software (cs.MS)
Finding efficient tensor contraction paths is essential for a wide range of problems, including model counting, quantum circuits, graph problems, and language models. There exist several approaches to find efficient paths, such as the greedy and random greedy algorithm by Optimized Einsum (opt_einsum), and the greedy algorithm and hypergraph partitioning approach employed in cotengra. However, these algorithms require a lot of computational time and resources to find efficient contraction paths. In this paper, we introduce a novel approach based on the greedy algorithm by opt_einsum that computes efficient contraction paths in less time. Moreover, with our approach, we are even able to compute paths for large problems where modern algorithms fail.
- [9] arXiv:2405.09706 [pdf, ps, html, other]
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Title: Canonical transformations applied to the non-free Landau electronSubjects: Quantum Physics (quant-ph)
The method previously used to solve Schrödinger equation by a unitary transformation for a electron under the influence of a constant magnetic field is used to obtain a non-free Landau electron wave function. The physical meaning of this wave function is discussed based on the conserved properties of the transformed Hamiltonian.
- [10] arXiv:2405.09724 [pdf, ps, html, other]
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Title: Parametrized Energy-Efficient Quantum Kernels for Network Service Fault DiagnosisComments: Submitted to the IEEE International Conference on Quantum Computing and Engineering 2024Subjects: Quantum Physics (quant-ph)
In quantum kernel learning, the primary method involves using a quantum computer to calculate the inner product between feature vectors, thereby obtaining a Gram matrix used as a kernel in machine learning models such as support vector machines (SVMs). However, a method for consistently achieving high performance has not been established. In this study, we investigate the diagnostic accuracy using a commercial dataset of a network service fault diagnosis system used by telecommunications carriers, focusing on quantum kernel learning, and propose a method to stably achieve high performance.We show significant performance improvements and an efficient achievement of high performance over conventional methods can be attained by applying quantum entanglement in the portion of the general quantum circuit used to create the quantum kernel, through input data parameter mapping and parameter tuning related to relative phase angles. Furthermore, experimental validation of the quantum kernel was conducted using IBM' s superconducting quantum computer IBM-Kawasaki, and its practicality was verified while applying the error suppression feature of Q-CTRL' s Fire Opal.
- [11] arXiv:2405.09767 [pdf, ps, html, other]
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Title: Compact quantum algorithms that can potentially maintain quantum advantage for solving time-dependent differential equationsComments: 29 pages, 10 figures, 1 tableSubjects: Quantum Physics (quant-ph); Applied Physics (physics.app-ph); Computational Physics (physics.comp-ph); Fluid Dynamics (physics.flu-dyn)
Many claims of computational advantages have been made for quantum computing over classical, but they have not been demonstrated for practical problems. Here, we present algorithms for solving time-dependent PDEs governing fluid flow problems. We build on an idea based on linear combination of unitaries to simulate non-unitary, non-Hermitian quantum systems, and generate hybrid quantum-classical algorithms that efficiently perform iterative matrix-vector multiplication and matrix inversion operations. These algorithms lead to low-depth quantum circuits that protect quantum advantage, with the best-case asymptotic complexities that are near-optimal. We demonstrate the performance of the algorithms by conducting: (a) ideal state-vector simulations using an in-house, high performance, quantum simulator called $\textit{QFlowS}$; (b) experiments on a real quantum device (IBM Cairo); and (c) noisy simulations using Qiskit Aer. We also provide device specifications such as error-rates (noise) and state sampling (measurement) to accurately perform convergent flow simulations on noisy devices.
- [12] arXiv:2405.09785 [pdf, ps, html, other]
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Title: Interference between distinguishable photonsComments: 7 pages, 5 figures. Comments are welcome. arXiv admin note: substantial text overlap with arXiv:2404.05158Subjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall)
Two-photon interference (TPI) lies at the heart of photonic quantum technologies. TPI is generally regarded as quantum interference stemming from the indistinguishability of identical photons, hence a common intuition prevails that TPI would disappear if photons are distinguishable. Here we disprove this perspective and uncover the essence of TPI. We report the first demonstration of TPI between distinguishable photons with their frequency separation up to $10^4$ times larger than their linewidths. We perform time-resolved TPI between an independent laser and single photons with ultralong coherence time ($>10\ \mu$s). We observe a maximum TPI visibility of $72\%\pm 2\%$ well above the $50\%$ classical limit indicating the quantum feature, and simultaneously a broad visibility background and a classical beat visibility of less than $50\%$ reflecting the classical feature. These visibilities are independent of the photon frequency separation and show no difference between distinguishable and indistinguishable photons. Based on a general wave superposition model, we derive the cross-correlation functions which fully reproduce and explain the experiments. Our results reveal that TPI as the fourth-order interference arises from the second-order interference of two photons within the mutual coherence time and TPI is not linked to the photon indistinguishability. This work provides new insights into the nature of TPI with great implications in both quantum optics and photonic quantum technologies.
- [13] arXiv:2405.09803 [pdf, ps, html, other]
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Title: Quantum annealing for finding maximum-weight independent set of unit-disk graphsComments: 12 pages, 8 figuresSubjects: Quantum Physics (quant-ph)
Recent progress in quantum computing and quantum simulation of many-body systems with arrays of neutral atoms using Rydberg excitation brought unforeseen opportunities towards computational advantage in solving various optimization problems. The problem of maximum-weight independent set (MWIS) of unit-disk graphs is an example of NP-hard optimization problems. It involves finding the largest set of vertices with the maximum sum of their weights for a graph which has edges connecting all pairs of vertices within a unit distance. This problem can be solved using quantum annealing with an array of interacting Rydberg atoms. For a particular graph, a spatial arrangement of atoms represents vertices of the graph, while the detuning from the resonance at Rydberg excitation defines weights of these vertices. The edges of the graph can be drawn according to the unit disk criterion. MWIS can be obtained by applying a variational quantum adiabatic algorithm (VQAA). We consider driving the quantum system of interacting atoms to the many-body ground state using a non-linear quasi-adiabatic profile for sweeping the Rydberg detuning. We also propose using a quantum wire which is a set of auxiliary atoms of a different chemical element to mediate strong coupling between the remote vertices of the graph. We investigate this effect for different lengths of the quantum wire. We also investigate the quantum phases of matter realizing commensurate and incommensurate phases in 1D and 2D spatial arrangement of the atomic array.
- [14] arXiv:2405.09808 [pdf, ps, html, other]
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Title: Phase Retrieval from the Hong-Ou-Mandel Dip to Characterize the Phase Spectrum of Independent Pulses at the Single-Photon LevelSubjects: Quantum Physics (quant-ph); Optics (physics.optics)
Measuring the phase spectrum at the single-photon level is essential for the full characterization of the temporal-spectral mode of quantum sources. We present a phase retrieval algorithm-based method to recover the phase spectrum difference between two independent pulses from their Hong-Ou-Mandel interference pattern and intensity spectra. Our confirmatory experiment with coherent state pulses confirms the accuracy of the recovered phase spectrum difference to within plus or minus 0.1 rad. The method we employ is readily generalizable to the measurement of single-photon wave packets and even correlated photon pairs.
- [15] arXiv:2405.09829 [pdf, ps, html, other]
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Title: Quantum Systems from Random Probabilistic AutomataComments: 26 pages, 9 figuresSubjects: Quantum Physics (quant-ph); Cellular Automata and Lattice Gases (nlin.CG)
Probabilistic cellular automata with deterministic updating are quantum systems. We employ the quantum formalism for an investigation of random probabilistic cellular automata, which start with a probability distribution over initial configurations. The properties of the deterministic updating are randomly distributed over space and time. We are interested in a possible continuum limit for a very large number of cells. As an example we consider bits with two colors, moving to the left or right on a linear chain. At randomly distributed scattering points, they change direction and color. A numerical simulation reveals the typical features of quantum systems. We find particular initial probability distributions which reemerge periodically after a certain number of time steps, as produced by the periodic evolution of energy eigenstates in quantum mechanics. Using a description in terms of wave functions allows to introduce statistical observables for momentum and energy. They characterize the probabilistic information without taking definite values for a given bit configuration, with a conceptual status similar to temperature in classical statistical thermal equilibrium. Conservation of energy and momentum are essential ingredients for the understanding of the evolution of our stochastic probabilistic automata. This evolution resembles in some aspects a single Dirac fermion in two dimensions with a random potential.
- [16] arXiv:2405.09845 [pdf, ps, html, other]
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Title: Nanomechanically induced transparency in $\mathcal{PT}$-symmetric optical cavitiesComments: 7 pages, 6 figuresSubjects: Quantum Physics (quant-ph); Optics (physics.optics)
In this paper, we analytically present the phenomena of nanomechanically induced transparency (NMIT) and transmission rate in a parity-time-symmetric ($\mathcal{PT}$-symmetric) opto-nanomechanical system (ONMS) where a levitated dielectric nanospheres is trapped near the antinodes closest to right mirror of passive cavity which further coupled to an active cavity via hoping factor. We find that the phenomenon of NMIT may be generated from the output probe field in the presence of an effective opto-nanomechanical coupling between the cavity field and the nanosphere, whose steady-state position is influenced by the Coulomb interaction between the cavity mirror and the nanosphere. In addition, the width and height of the transparency window can be controlled through the effective optomechanical coupling, which is readily adjusted by altering changing the nanosphere's radius and the Coulomb interaction. One of the most interesting result is the transition NMIT behavior in $\mathcal{PT}$-symmetric and broken $\mathcal{PT}$-symmetric regime. We show that the presence of nanosphere in the passive cavity enhances the width and transmission rate of NMIT window in passive-passive regime and in passive-active regime, a notable decrease of sideband amplification has been observed. These results show that our scheme may find some potential applications for optical signal processing an and quantum information processing.
- [17] arXiv:2405.09860 [pdf, ps, html, other]
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Title: Optimal Switching Networks for Paired-Egress Bell State Analyzer PoolsMarii Koyama, Claire Yun, Amin Taherkhani, Naphan Benchasattabuse, Bernard Ousmane Sane, Michal Hajdušek, Shota Nagayama, Rodney Van MeterComments: 11 pages, 8 figures, 1 tableSubjects: Quantum Physics (quant-ph); Networking and Internet Architecture (cs.NI)
To scale quantum computers to useful levels, we must build networks of quantum computational nodes that can share entanglement for use in distributed forms of quantum algorithms. In one proposed architecture, node-to-node entanglement is created when nodes emit photons entangled with stationary memories, with the photons routed through a switched interconnect to a shared pool of Bell state analyzers (BSAs). Designs that optimize switching circuits will reduce loss and crosstalk, raising entanglement rates and fidelity. We present optimal designs for switched interconnects constrained to planar layouts, appropriate for silicon waveguides and Mach-Zehnder interferometer (MZI) $2 \times 2$ switch points. The architectures for the optimal designs are scalable and algorithmically structured to pair any arbitrary inputs in a rearrangeable, non-blocking way. For pairing $N$ inputs, $N(N - 2)/4$ switches are required, which is less than half of number of switches required for full permutation switching networks. An efficient routing algorithm is also presented for each architecture. These designs can also be employed in reverse for entanglement generation using a shared pool of entangled paired photon sources.
- [18] arXiv:2405.09861 [pdf, ps, html, other]
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Title: An Implementation and Analysis of a Practical Quantum Link Architecture Utilizing Entangled Photon SourcesKento Samuel Soon, Michal Hajdušek, Shota Nagayama, Naphan Benchasattabuse, Kentaro Teramoto, Ryosuke Satoh, Rodney Van MeterComments: 8 pages, 8 figuresSubjects: Quantum Physics (quant-ph); Networking and Internet Architecture (cs.NI)
Quantum repeater networks play a crucial role in distributing entanglement. Various link architectures have been proposed to facilitate the creation of Bell pairs between distant nodes, with entangled photon sources emerging as a primary technology for building quantum networks. Our work advances the Memory-Source-Memory (MSM) link architecture, addressing the absence of practical implementation details. We conduct numerical simulations using the Quantum Internet Simulation Package (QuISP) to analyze the performance of the MSM link and contrast it with other link architectures. We observe a saturation effect in the MSM link, where additional quantum resources do not affect the Bell pair generation rate of the link. By introducing a theoretical model, we explain the origin of this effect and characterize the parameter region where it occurs. Our work bridges theoretical insights with practical implementation, which is crucial for robust and scalable quantum networks.
- [19] arXiv:2405.09862 [pdf, ps, html, other]
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Title: Performance of Quantum Networks Using Heterogeneous Link ArchitecturesKento Samuel Soon, Naphan Benchasattabuse, Michal Hajdušek, Kentaro Teramoto, Shota Nagayama, Rodney Van MeterComments: 10 pages, 10 figuresSubjects: Quantum Physics (quant-ph); Networking and Internet Architecture (cs.NI)
The heterogeneity of quantum link architectures is an essential theme in designing quantum networks for technological interoperability and possibly performance optimization. However, the performance of heterogeneously connected quantum links has not yet been addressed. Here, we investigate the integration of two inherently different technologies, with one link where the photons flow from the nodes toward a device in the middle of the link, and a different link where pairs of photons flow from a device in the middle towards the nodes. We utilize the quantum internet simulator QuISP to conduct simulations. We first optimize the existing photon pair protocol for a single link by taking the pulse rate into account. Here, we find that increasing the pulse rate can actually decrease the overall performance. Using our optimized links, we demonstrate that heterogeneous networks actually work. Their performance is highly dependent on link configuration, but we observe no significant decrease in generation rate compared to homogeneous networks. This work provides insights into the phenomena we likely will observe when introducing technological heterogeneity into quantum networks, which is crucial for creating a scalable and robust quantum internetwork.
- [20] arXiv:2405.09876 [pdf, ps, html, other]
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Title: Engineering Challenges in All-photonic Quantum RepeatersComments: 9 pages, 4 figures, 2 tables; comments welcome!Subjects: Quantum Physics (quant-ph); Networking and Internet Architecture (cs.NI)
Quantum networking, heralded as the next frontier in communication networks, envisions a realm where quantum computers and devices collaborate to unlock capabilities beyond what is possible with the Internet. A critical component for realizing a long-distance quantum network, and ultimately, the Quantum Internet, is the quantum repeater. As with the race to build a scalable quantum computer with different technologies, various schemes exist for building quantum repeaters. This article offers a gentle introduction to the two-way ``all-photonic quantum repeaters,'' a recent addition to quantum repeater technologies. In contrast to conventional approaches, these repeaters eliminate the need for quantum memories, offering the dual benefits of higher repetition rates and intrinsic tolerance to both quantum operational errors and photon losses. Using visualization and simple rules for manipulating graph states, we describe how all-photonic quantum repeaters work. We discuss the problem of the increased volume of classical communication required by this scheme, which places a huge processing requirement on the end nodes. We address this problem by presenting a solution that decreases the amount of classical communication by three orders of magnitude. We conclude by highlighting other key open challenges in translating the theoretical all-photonic framework into real-world implementation, providing insights into the practical considerations and future research directions of all-photonic quantum repeater technology.
- [21] arXiv:2405.09881 [pdf, ps, html, other]
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Title: Scalable Timing Coordination of Bell State Analyzers in Quantum NetworksYoshihiro Mori, Toshihiko Sasaki, Rikizo Ikuta, Kentaro Teramoto, Hiroyuki Ohno, Michal Hajdušek, Rodney Van Meter, Shota NagayamaComments: 7 pages, 9 figures. Submitted to the IEEE Quantum Week 2024Subjects: Quantum Physics (quant-ph); Networking and Internet Architecture (cs.NI)
The optical Bell State Analyzer (BSA) plays a key role in the optical generation of entanglement in quantum networks. The optical BSA is effective in controlling the timing of arriving photons to achieve interference. It is unclear whether timing synchronization is possible even in multi-hop and complex large-scale networks, and if so, how efficient it is. We investigate the scalability of BSA synchronization mechanisms over multiple hops for quantum networks both with and without memory in each node. We first focus on the exchange of entanglement between two network nodes via a BSA, especially effective methods of optical path coordination in achieving the simultaneous arrival of photons at the BSA. In optical memoryless quantum networks, including repeater graph state networks, we see that the quantum optical path coordination works well, though some possible timing coordination mechanisms have effects that cascade to adjacent links and beyond, some of which was not going to work well of timing coordination. We also discuss the effect of quantum memory, given that end-to-end extension of entangled states through multi-node entanglement exchange is essential for the practical application of quantum networks. Finally, cycles of all-optical links in the network topology are shown to may not be to synchronize, this property should be taken into account when considering synchronization in large networks.
- [22] arXiv:2405.09885 [pdf, ps, html, other]
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Title: Symmetry breaking and non-ergodicity in a driven-dissipative ensemble of multi-level atoms in a cavityEnrique Hernandez, Elmer Suarez, Igor Lesanovsky, Beatriz Olmos, Philippe W. Courteille, Sebastian SlamaSubjects: Quantum Physics (quant-ph); Atomic Physics (physics.atom-ph)
Dissipative light-matter systems can display emergent collective behavior. Here, we report a $\mathbb{Z}_2$-symmetry-breaking phase transition in a system of multi-level $^{87}$Rb atoms strongly coupled to a weakly driven two-mode optical cavity. In the symmetry-broken phase, non-ergodic dynamics manifests in the emergence of multiple stationary states with disjoint basins of attraction. This feature enables the amplification of a small atomic population imbalance into a characteristic macroscopic cavity transmission signal. Our experiment does not only showcase strongly dissipative atom-cavity systems as platforms for probing non-trivial collective many-body phenomena, but also highlights their potential for hosting technological applications in the context of sensing, density classification, and pattern retrieval dynamics within associative memories.
- [23] arXiv:2405.09899 [pdf, ps, html, other]
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Title: Quantum Metrology with Higher-order Exceptional Points in Atom-cavity MagnonicsSubjects: Quantum Physics (quant-ph)
Exceptional points (EPs), early arising from non-Hermitian physics, significantly amplify the system's response to minor perturbations, and act as a useful concept to enhance measurement in metrology. In particular, such a metrological enhancement grows dramatically with the EP's order. However, the Langevin noises intrinsically existing in the non-Hermitian systems diminish this enhancement. In this study, we propose a protocol for quantum metrology with the construction of higher-order EPs (HOEPs) in atom-cavity system through Hermitian magnon-photon interaction. The construction of HOEPs utilizes the atom-cavity non-Hermitian-like dynamical behavior but avoids the external Langevin noises via the Hermitian interaction. A general analysis is exhibited for the construction of arbitrary $n$-th order EP (EPn). As a demonstration of the superiority of these HOEPs in quantum metrology, we work out an EP3/4-based atomic sensor with sensitivity being orders of magnitude higher than that achievable in an EP2-based one. We further unveil the mechanism behind the sensitivity enhancement from HOEPs. The experimental establishment for this proposal is suggested with potential candidates. This EP-based atomic sensor, taking advantage of the atom-light interface, offers new insight into quantum metrology with HOEPs.
- [24] arXiv:2405.09977 [pdf, ps, other]
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Title: Generalized Conditional DisplacementShiran Even-Haim, Asaf A. Diringer, Ron Ruimy, Gefen Baranes, Alexey Gorlach, Shay Hacohen-Gourgy, Ido KaminerSubjects: Quantum Physics (quant-ph)
Conditional displacement with a qubit ancilla is a critical component in continuous-variable error correction protocols. We present the generalized conditional displacement operator, conditioned on a qudit ancilla, showing how it enhances error-correction with Gottesman-Kitaev-Preskill (GKP) codes and exploring potential implementations.
- [25] arXiv:2405.09988 [pdf, ps, html, other]
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Title: Blueprint for all-to-all connected superconducting spin qubitsComments: 14 pages, 8 figuresSubjects: Quantum Physics (quant-ph); Superconductivity (cond-mat.supr-con)
Andreev (or superconducting) spin qubits (ASQs) have recently emerged as a promising qubit platform that combines superconducting circuits with semiconductor spin degrees of freedom. While recent experiments have successfully coupled two ASQs, how to realize a scalable architecture for extending this coupling to multiple distant qubits remains an open question. In this work, we resolve this challenge by introducing an architecture that achieves all-to-all connectivity between multiple remote ASQs. Our approach enables selective connectivity between any qubit pair while maintaining all other qubit pairs uncoupled. Furthermore, we demonstrate the feasibility of efficient readout using circuit quantum electrodynamics techniques and compare different readout configurations. Our architecture shows promise both for gate-based quantum computing and for analog quantum simulation applications by offering higher qubit connectivity than alternative solid-state platforms.
- [26] arXiv:2405.10033 [pdf, ps, html, other]
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Title: Tight scaling of key rate for differential-phase-shift quantum key distributionComments: 11 pages, 1 figureSubjects: Quantum Physics (quant-ph)
The performance of quantum key distribution (QKD) protocols is evaluated based on the ease of implementation and key generation rate. Among major protocols, the differential-phase-shift (DPS) protocol has the advantage of simple implementation using a train of coherent pulses and a passive detection unit. Unfortunately, however, its key rate is known to be at least proportional to $\eta^2$ with respect to channel transmission $\eta\to0$. If one can only prove the rate proportional to $\eta^2$ and cannot improve the analysis beyond that, then the DPS protocol will be deemed inferior to other major protocols, such as the decoy BB84 protocol. In this paper, we consider a type of DPS protocol in which the phase of each emitted block comprising $n$ pulses is randomized and significantly improve the analysis of its key rate. Specifically, we reveal that the key rate is proportional to $\eta^{1+\frac{1}{n-2}}$ and this rate is tight. This implies that the DPS protocol can achieve a key rate proportional to $\eta$ for a large number of $n$, which is the same scaling as the decoy BB84 protocol. Our result suggests that the DPS protocol can achieve a combination of both advantages of ease of implementation and a high key generation rate.
- [27] arXiv:2405.10052 [pdf, ps, html, other]
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Title: Deep Neural Network-assisted improvement of quantum compressed sensing tomographyComments: 11 pages, 8 figures, github hyperlink includedSubjects: Quantum Physics (quant-ph)
Quantum compressed sensing is the fundamental tool for low-rank density matrix tomographic reconstruction in the informationally incomplete case. We examine situations where the acquired information is not enough to allow one to obtain a precise compressed sensing reconstruction. In this scenario, we propose a Deep Neural Network-based post-processing to improve the initial reconstruction provided by compressed sensing. The idea is to treat the estimated state as a noisy input for the network and perform a deep-supervised denoising task. After the network is applied, a projection onto the space of feasible density matrices is performed to obtain an improved final state estimation. We demonstrate through numerical experiments the improvement obtained by the denoising process and exploit the possibility of looping the inference scheme to obtain further advantages. Finally, we test the resilience of the approach to out-of-distribution data.
- [28] arXiv:2405.10055 [pdf, ps, html, other]
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Title: What are kets?Comments: Bulletin of the EATCS 141 October 2023Subjects: Quantum Physics (quant-ph); Logic in Computer Science (cs.LO)
According to Dirac's bra-ket notation, in an inner-product space, the inner product $\langle x\,|\,y\rangle$ of vectors $x,y$ can be viewed as an application of the bra $\langle x|$ to the ket $|y\rangle$. Here $\langle x|$ is the linear functional $|y\rangle \mapsto \langle x\,|\,y\rangle$ and $|y\rangle$ is the vector $y$. But often -- though not always -- there are advantages in seeing $|y\rangle$ as the function $a \mapsto a\cdot y$ where $a$ ranges over the scalars. For example, the outer product $|y\rangle\langle x|$ becomes simply the composition $|y\rangle \circ \langle x|$. It would be most convenient to view kets sometimes as vectors and sometimes as functions, depending on the context. This turns out to be possible. While the bra-ket notation arose in quantum mechanics, this note presupposes no familiarity with quantum mechanics.
- [29] arXiv:2405.10107 [pdf, ps, html, other]
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Title: Interferometric Purcell suppression of spontaneous emission in a superconducting qubitAlec Yen, Yufeng Ye, Kaidong Peng, Jennifer Wang, Gregory Cunningham, Michael Gingras, Bethany M. Niedzielski, Hannah Stickler, Kyle Serniak, Mollie E. Schwartz, Kevin P. O'BrienComments: 10 pages, 10 figuresSubjects: Quantum Physics (quant-ph)
In superconducting qubits, suppression of spontaneous emission is essential to achieve fast dispersive measurement and reset without sacrificing qubit lifetime. We show that resonator-mediated decay of the qubit mode to the feedline can be suppressed using destructive interference, where the readout resonator is coupled to the feedline at two points. This "interferometric Purcell filter" does not require dedicated filter components or impedance mismatch in the feedline, making it suitable for applications such as all-pass readout. We design and fabricate a device with the proposed scheme and demonstrate suppression of resonator-mediated decay that exceeds 2 orders of magnitude over a bandwidth of 400 MHz.
- [30] arXiv:2405.10113 [pdf, ps, html, other]
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Title: Master thesis: High-rate multipartite quantum secret sharing with continuous variablesComments: Master thesisSubjects: Quantum Physics (quant-ph)
Quantum cryptography has undergone substantial growth and development within the multi-disciplinary field of quantum information in recent years. The field is constantly advancing with new protocols being developed, security measures being improved, and the first practical applications of these technologies being deployed in optical fibers and free space optical beams. In this paper, we present a comprehensive review of a cutting-edge metropolitan-scale protocol for continuous-variable quantum cryptography. The protocol allows an arbitrary number of users to send modulated coherent states to a relay, where a generalised Bell detection creates secure multipartite correlations. These correlations are then distilled into a shared secret key, providing a secure method for quantum secret-sharing. This novel approach to quantum cryptography has the potential to offer high-rate secure multipartite communication using readily available optical components, making it a promising advancement in the field.
- [31] arXiv:2405.10119 [pdf, ps, html, other]
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Title: Applications of Quantum Machine Learning for Quantitative FinanceComments: comments are welcomeSubjects: Quantum Physics (quant-ph)
Machine learning and quantum machine learning (QML) have gained significant importance, as they offer powerful tools for tackling complex computational problems across various domains. This work gives an extensive overview of QML uses in quantitative finance, an important discipline in the financial industry. We examine the connection between quantum computing and machine learning in financial applications, spanning a range of use cases including fraud detection, underwriting, Value at Risk, stock market prediction, portfolio optimization, and option pricing by overviewing the corpus of literature concerning various financial subdomains.
- [32] arXiv:2405.10125 [pdf, ps, html, other]
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Title: A Recursive Lower Bound on the Energy Improvement of the Quantum Approximate Optimization AlgorithmComments: 18 pages, 13 figuresSubjects: Quantum Physics (quant-ph)
The quantum approximate optimization algorithm (QAOA) uses a quantum computer to implement a variational method with $2p$ layers of alternating unitary operators, optimized by a classical computer to minimize a cost function. While rigorous performance guarantees exist for the QAOA at small depths $p$, the behavior at large depths remains less clear, though simulations suggest exponentially fast convergence for certain problems. In this work, we gain insights into the deep QAOA using an analytic expansion of the cost function around transition states. Transition states are constructed in a recursive manner: from the local minima of the QAOA with $p$ layers we obtain transition states of the QAOA with $p+1$ layers, which are stationary points characterized by a unique direction of negative curvature. We construct an analytic estimate of the negative curvature and the corresponding direction in parameter space at each transition state. The expansion of the QAOA cost function along the negative direction to the quartic order gives a lower bound of the QAOA cost function improvement. We provide physical intuition behind the analytic expressions for the local curvature and quartic expansion coefficient. Our numerical study confirms the accuracy of our approximations and reveals that the obtained bound and the true value of the QAOA cost function gain have a characteristic exponential decrease with the number of layers $p$, with the bound decreasing more rapidly. Our study establishes an analytical method for recursively studying the QAOA that is applicable in the regime of high circuit depth.
- [33] arXiv:2405.10154 [pdf, ps, html, other]
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Title: Quantum CZ Gate based on Single Gradient MetasurfaceSubjects: Quantum Physics (quant-ph); Optics (physics.optics)
We propose a scheme to realize quantum controlled-Z (CZ) gates through single gradient metasurface. Using its unique parallel beam-splitting feature, i.e., a series of connected beam splitters with the same splitting ratio, one metasurface can support a CZ gate, several independent CZ gates, or a cascaded CZ gates. Taking advantage of the input polarization determined output path-locking feature, both polarization-encoded and path-encoded CZ gates can be demonstrated on the same metasurface, which further improves the integration level of quantum devices. Our research paves the way for integrating quantum logical function through the metasurface.
- [34] arXiv:2405.10176 [pdf, ps, html, other]
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Title: Topological, multi-mode amplification induced by non-reciprocal, long-range dissipative couplingsComments: 17 pages, 8 figuresSubjects: Quantum Physics (quant-ph)
Non-reciprocal couplings or drivings are known to induce steady-state, directional, amplification in driven-dissipative bosonic lattices. This amplification phenomena has been recently linked to the existence of a non-zero topological invariant defined with the system's dynamical matrix, and thus, it depends critically on the couplings' structure. In this work, we demonstrate the emergence of unconventional, non-reciprocal, long-range dissipative couplings induced by the interaction of the bosonic chain with a chiral, multi-mode channel, and then study their impact on topological amplification phenomena. We show that these couplings can lead to topological invariant values greater than one which induce topological, multi-mode amplification and metastability behaviour not predicted in other setups. Besides, we also show how these couplings can also stabilize topological amplifying phases in the presence of local parametric drivings. Finally, we conclude by showing how such phenomena can be naturally obtained in two-dimensional topological insulators hosting multiple edge modes.
- [35] arXiv:2405.10226 [pdf, ps, html, other]
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Title: Geometric phase amplification in a clock interferometer for enhanced metrologySubjects: Quantum Physics (quant-ph)
High-precision measurements are crucial for testing the fundamental laws of nature and for advancing the technological frontier. Clock interferometry, where particles with an internal clock are coherently split and recombined along two spatial paths, has sparked significant interest due to its fundamental implications, especially at the intersection of quantum mechanics and general relativity. Here, we demonstrate that a clock interferometer provides metrological improvement with respect to its technical-noise-limited counterpart employing a single internal quantum state. This enhancement around a critical working point can be interpreted as a geometric-phase-induced signal-to-noise ratio gain. In our experimental setup, we infer a precision enhancement of 8.8 decibels when measuring a small difference between external fields. We estimate that tens of decibels of precision enhancement could be attained for measurements with a higher atom flux. This opens the door to the development of a superior probe for fundamental physics as well as a high-performance sensor for various technological applications.
- [36] arXiv:2405.10242 [pdf, ps, html, other]
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Title: Quantum State Learning Implies Circuit Lower BoundsComments: 53 pagesSubjects: Quantum Physics (quant-ph)
We establish connections between state tomography, pseudorandomness, quantum state synthesis, and circuit lower bounds. In particular, let $\mathfrak{C}$ be a family of non-uniform quantum circuits of polynomial size and suppose that there exists an algorithm that, given copies of $|\psi \rangle$, distinguishes whether $|\psi \rangle$ is produced by $\mathfrak{C}$ or is Haar random, promised one of these is the case. For arbitrary fixed constant $c$, we show that if the algorithm uses at most $O(2^{n^c})$ time and $2^{n^{0.99}}$ samples then $\mathsf{stateBQE} \not\subset \mathsf{state}\mathfrak{C}$. Here $\mathsf{stateBQE} := \mathsf{stateBQTIME}[2^{O(n)}]$ and $\mathsf{state}\mathfrak{C}$ are state synthesis complexity classes as introduced by Rosenthal and Yuen (ITCS 2022), which capture problems with classical inputs but quantum output. Note that efficient tomography implies a similarly efficient distinguishing algorithm against Haar random states, even for nearly exponential-time algorithms. Because every state produced by a polynomial-size circuit can be learned with $2^{O(n)}$ samples and time, or $O(n^{\omega(1)})$ samples and $2^{O(n^{\omega(1)})}$ time, we show that even slightly non-trivial quantum state tomography algorithms would lead to new statements about quantum state synthesis. Finally, a slight modification of our proof shows that distinguishing algorithms for quantum states can imply circuit lower bounds for decision problems as well. This help sheds light on why time-efficient tomography algorithms for non-uniform quantum circuit classes has only had limited and partial progress. Our work parallels results by Arunachalam et al. (FOCS 2021) that revealed a similar connection between quantum learning of Boolean functions and circuit lower bounds for classical circuit classes, but modified for the purposes of state tomography and state synthesis.
- [37] arXiv:2405.10245 [pdf, ps, html, other]
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Title: A Graph-Theoretical Framework to Analyse Zero Discord Quantum StatesSubjects: Quantum Physics (quant-ph)
This article comprehensively explores matrices and their prerequisites for achieving positive semidefiniteness. The study delves into a series of theorems concerning pure quantum states in the context of weighted graphs. The main objective of this study is to establish a graph-theoretic framework for the study of quantum discord and to identify the necessary and sufficient conditions for zero quantum discord states using unitary operators. This research aims to advance the understanding of quantum discord and its implications for quantum information theory with a graph-theoretic framework.
- [38] arXiv:2405.10259 [pdf, ps, other]
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Title: Energy-limited quantum dynamicsComments: Comments welcomeSubjects: Quantum Physics (quant-ph); Mathematical Physics (math-ph)
We consider quantum systems with energy constraints. In general, quantum channels and continuous-time dynamics need not satisfy energy conservation. Physically meaningful channels, however, can only introduce a finite amount of energy to the system, and continuous-time dynamics may only increase the energy gradually over time. We systematically study such "energy-limited" channels and dynamics. For Markovian dynamics, energy-limitedness is equivalent to a single operator inequality in the Heisenberg picture. By tracking the output energy, we observe that the energy-constrained operator and diamond norms of Shirokov and Winter satisfy submultiplicativity estimates with respect to energy-limited channels. This makes for a powerful toolkit for quantitative analyses of dynamical problems in finite and infinite-dimensional systems. As an application, we derive state-dependent bounds for quantum speed limits and related problems that outperform the usual operator/diamond norm estimates, which have to account for fluctuations in high-energy states.
- [39] arXiv:2405.10264 [pdf, ps, html, other]
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Title: Architectures and random properties of symplectic quantum circuitsComments: 13+7 pages, 8 figuresSubjects: Quantum Physics (quant-ph); Machine Learning (cs.LG)
Parametrized and random unitary (or orthogonal) $n$-qubit circuits play a central role in quantum information. As such, one could naturally assume that circuits implementing symplectic transformation would attract similar attention. However, this is not the case, as $\mathbb{SP}(d/2)$ -- the group of $d\times d$ unitary symplectic matrices -- has thus far been overlooked. In this work, we aim at starting to right this wrong. We begin by presenting a universal set of generators $\mathcal{G}$ for the symplectic algebra $i\mathfrak{sp}(d/2)$, consisting of one- and two-qubit Pauli operators acting on neighboring sites in a one-dimensional lattice. Here, we uncover two critical differences between such set, and equivalent ones for unitary and orthogonal circuits. Namely, we find that the operators in $\mathcal{G}$ cannot generate arbitrary local symplectic unitaries and that they are not translationally invariant. We then review the Schur-Weyl duality between the symplectic group and the Brauer algebra, and use tools from Weingarten calculus to prove that Pauli measurements at the output of Haar random symplectic circuits can converge to Gaussian processes. As a by-product, such analysis provides us with concentration bounds for Pauli measurements in circuits that form $t$-designs over $\mathbb{SP}(d/2)$. To finish, we present tensor-network tools to analyze shallow random symplectic circuits, and we use these to numerically show that computational-basis measurements anti-concentrate at logarithmic depth.
- [40] arXiv:2405.10274 [pdf, ps, html, other]
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Title: Simultaneous Haar Indistinguishability with Applications to Unclonable CryptographySubjects: Quantum Physics (quant-ph); Cryptography and Security (cs.CR)
Unclonable cryptography is concerned with leveraging the no-cloning principle to build cryptographic primitives that are otherwise impossible to achieve classically. Understanding the feasibility of unclonable encryption, one of the key unclonable primitives, satisfying indistinguishability security in the plain model has been a major open question in the area. So far, the existing constructions of unclonable encryption are either in the quantum random oracle model or are based on new conjectures.
We present a new approach to unclonable encryption via a reduction to a novel question about nonlocal quantum state discrimination: how well can non-communicating -- but entangled -- players distinguish between different distributions over quantum states? We call this task simultaneous state indistinguishability. Our main technical result is showing that the players cannot distinguish between each player receiving independently-chosen Haar random states versus all players receiving the same Haar random state.
We leverage this result to present the first construction of unclonable encryption satisfying indistinguishability security, with quantum decryption keys, in the plain model. We also show other implications to single-decryptor encryption and leakage-resilient secret sharing. - [41] arXiv:2405.10279 [pdf, ps, html, other]
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Title: Photon emission from macroscopic currentsSubjects: Quantum Physics (quant-ph); Optics (physics.optics)
Coherent states are a well-established tool of quantum optics to describe electromagnetic waves in terms of photons. However, they do not describe the near-field regime of radiation sources. Instead, we generically use classical solutions of Maxwell's equations to describe radiation in the near-field regime. The classical solutions provide linear relations between currents and emitted electromagnetic fields, whereas evolution of states at the quantum level proceeds through unitary time evolution operators involving photon operators. This begs questions how the classical radiation equations relate to unitary quantum evolution, and how we can describe macroscopic fields from antennas or magnetic coils in terms of elementary photons. The present paper answers both questions through the construction of generalized Glauber states for radiation emitters.
- [42] arXiv:2405.10282 [pdf, ps, html, other]
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Title: GKLS Vector Field Dynamics for Gaussian StatesComments: 33 pages, 6 figuresSubjects: Quantum Physics (quant-ph); Mathematical Physics (math-ph)
We construct the vector field associated with the GKLS generator for systems described by Gaussian states. This vector field is defined on the dual space of the algebra of operators, restricted to operators quadratic in position and momentum. It is shown that the GKLS dynamics accepts a decomposition principle, that is, this vector field can be decomposed in three parts, a conservative Hamiltonian component, a gradient-like and a Choi-Krauss vector field. The last two terms are considered a "perturbation" associated with dissipation. Examples are presented for a harmonic oscillator with different dissipation terms.
- [43] arXiv:2405.10284 [pdf, ps, html, other]
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Title: Quantum Vision Transformers for Quark-Gluon ClassificationMarçal Comajoan Cara, Gopal Ramesh Dahale, Zhongtian Dong, Roy T. Forestano, Sergei Gleyzer, Daniel Justice, Kyoungchul Kong, Tom Magorsch, Konstantin T. Matchev, Katia Matcheva, Eyup B. UnluComments: 14 pages, 8 figures. Published in MDPI Axioms 2024, 13(5), 323Journal-ref: Axioms 2024, 13(5), 323Subjects: Quantum Physics (quant-ph); Machine Learning (cs.LG); High Energy Physics - Phenomenology (hep-ph)
We introduce a hybrid quantum-classical vision transformer architecture, notable for its integration of variational quantum circuits within both the attention mechanism and the multi-layer perceptrons. The research addresses the critical challenge of computational efficiency and resource constraints in analyzing data from the upcoming High Luminosity Large Hadron Collider, presenting the architecture as a potential solution. In particular, we evaluate our method by applying the model to multi-detector jet images from CMS Open Data. The goal is to distinguish quark-initiated from gluon-initiated jets. We successfully train the quantum model and evaluate it via numerical simulations. Using this approach, we achieve classification performance almost on par with the one obtained with the completely classical architecture, considering a similar number of parameters.
- [44] arXiv:2405.10294 [pdf, ps, html, other]
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Title: Corrections to adiabatic behavior for long pathsComments: 10 pages, 0 figuresSubjects: Quantum Physics (quant-ph); High Energy Physics - Lattice (hep-lat); Nuclear Theory (nucl-th)
The cost and the error of the adiabatic theorem for preparing the final eigenstate are discussed in terms of path length. Previous studies in terms of the norm of the Hamiltonian and its derivatives with the spectral gap are limited to describe the cost of adiabatic state preparation for large systems. We argue that total time is not a good measure for determining the computational difficulty of adiabatic quantum computation by developing a no-go theorem. From the result of time-periodic Hamiltonian cases, we suggest that there are proxies for computational cost which typically grow as path length increases when the error is kept fixed and small and consider possible conjectures on how general the behavior is.
- [45] arXiv:2405.10306 [pdf, ps, html, other]
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Title: Fault Tolerance Embedded in a Quantum-Gap-Estimation Algorithm with Trial-State OptimizationComments: 5 pages, 3 figures, 1 tableSubjects: Quantum Physics (quant-ph); Strongly Correlated Electrons (cond-mat.str-el)
We construct a hybrid quantum algorithm to estimate gaps in many-body energy spectra and prove that it is inherently fault-tolerant to global multi-qubit depolarizing noise. Using trial-state optimization without active error correction, we show that the spectral peak of an exact target gap can be amplified beyond the noise threshold, thereby reducing gap-estimate error. We numerically verify fault tolerance using the Qiskit Aer simulator with a model of common mid-circuit noise channels. Our results reveal the potential for accurate quantum simulations on near-term noisy quantum computers.
New submissions for Friday, 17 May 2024 (showing 45 of 45 entries )
- [46] arXiv:2405.09571 (cross-list from eess.SP) [pdf, ps, html, other]
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Title: The Best Radar Ranging Pulse to Resolve Two ReflectorsComments: 8 pages, 8 figuresSubjects: Signal Processing (eess.SP); Data Analysis, Statistics and Probability (physics.data-an); Optics (physics.optics); Quantum Physics (quant-ph)
Previous work established fundamental bounds on subwavelength resolution for the radar range resolution problem, called superradar [Phys. Rev. Appl. 20, 064046 (2023)]. In this work, we identify the optimal waveforms for distinguishing the range resolution between two reflectors of identical strength. We discuss both the unnormalized optimal waveform as well as the best square-integrable pulse, and their variants. Using orthogonal function theory, we give an explicit algorithm to optimize the wave pulse in finite time to have the best performance. We also explore range resolution estimation with unnormalized waveforms with multi-parameter methods to also independently estimate loss and time of arrival. These results are consistent with the earlier single parameter approach of range resolution only and give deeper insight into the ranging estimation problem. Experimental results are presented using radio pulse reflections inside coaxial cables, showing robust range resolution smaller than a tenth of the inverse bandedge, with uncertainties close to the derived Cramér-Rao bound.
- [47] arXiv:2405.09669 (cross-list from cond-mat.stat-mech) [pdf, ps, html, other]
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Title: Bounds on Fluctuations of First Passage Times for Counting Observables in Classical and Quantum Markov ProcessesSubjects: Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)
We study the statistics of first passage times (FPTs) of trajectory observables in both classical and quantum Markov processes. We consider specifically the FPTs of counting observables, that is, the times to reach a certain threshold of a trajectory quantity which takes values in the positive integers and is non-decreasing in time. For classical continuous-time Markov chains we rigorously prove: (i) a large deviation principle (LDP) for FPTs, whose corollary is a strong law of large numbers; (ii) a concentration inequality for the FPT of the dynamical activity, which provides an upper bound to the probability of its fluctuations to all orders; and (iii) an upper bound to the probability of the tails for the FPT of an arbitrary counting observable. For quantum Markov processes we rigorously prove: (iv) the quantum version of the LDP, and subsequent strong law of large numbers, for the FPTs of generic counts of quantum jumps; (v) a concentration bound for the the FPT of total number of quantum jumps, which provides an upper bound to the probability of its fluctuations to all orders, together with a similar bound for the sub-class of quantum reset processes which requires less strict irreducibility conditions; and (vi) a tail bound for the FPT of arbitrary counts. Our results allow to extend to FPTs the so-called "inverse thermodynamic uncertainty relations" that upper bound the size of fluctuations in time-integrated quantities. We illustrate our results with simple examples.
- [48] arXiv:2405.09683 (cross-list from physics.optics) [pdf, ps, other]
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Title: Plasmonic Nanocavity to Boost Single Photon Emission from Defects in Thin Hexagonal Boron NitrideMohammadjavad Dowran, Ufuk Kilic, Suvechhya Lamichhane, Adam Erickson, Joshua Barker, Mathias Schubert, Sy-Hwang Liou, Christos Argyropoulos, Abdelghani LaraouiSubjects: Optics (physics.optics); Quantum Physics (quant-ph)
Efficient and compact single photon emission platforms operating at room temperature with ultrafast speed and high brightness will be fundamental components of the emerging quantum communications and computing fields. However, so far, it has been very challenging to design practical deterministic single photon emitters based on nanoscale solid state materials that meet the fast emission rate and strong brightness demands. Here we provide a solution to this longstanding problem by using metallic nanocavities integrated with hexagonal boron nitride (hBN) flakes with defects acting as nanoscale single photon emitters (SPEs) at room temperature. The presented hybrid nanophotonic structure creates a rapid speedup and large enhancement in single photon emission at room temperature. Hence, the nonclassical light emission performance is substantially improved compared to plain hBN flakes and hBN on gold layered structures without nanocavity. Extensive theoretical calculations are also performed to accurately model the new hybrid nanophotonic system and prove that the incorporation of plasmonic nanocavity is key to the efficient SPE performance. The proposed quantum nanocavity single photon source is expected to be an element of paramount importance to the envisioned room temperature integrated quantum photonic networks.
- [49] arXiv:2405.09728 (cross-list from cond-mat.stat-mech) [pdf, ps, html, other]
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Title: Hidden zero modes and topology of multiband non-Hermitian systemsSubjects: Statistical Mechanics (cond-mat.stat-mech); Strongly Correlated Electrons (cond-mat.str-el); Mathematical Physics (math-ph); Quantum Physics (quant-ph)
In a finite non-Hermitian system, the number of zero modes does not necessarily reflect the topology of the system. This is known as the breakdown of the bulk-boundary correspondence and has lead to misconceptions about the topological protection of edge modes in such systems. Here we show why this breakdown does occur and that it typically results in hidden zero modes, extremely long-lived zero energy excitations, which are only revealed when considering the singular value instead of the eigenvalue spectrum. We point out, furthermore, that in a finite multiband non-Hermitian system with Hamiltonian $H$, one needs to consider also the reflected Hamiltonian $\tilde H$, which is in general distinct from the adjoint $H^\dagger$, to properly relate the number of protected zeroes to the winding number of $H$.
- [50] arXiv:2405.09745 (cross-list from hep-th) [pdf, ps, html, other]
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Title: Pseudoentropy sum rule by analytical continuation of the superposition parameterComments: 33 pagesSubjects: High Energy Physics - Theory (hep-th); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)
In this paper, we establish a sum rule that connects the pseudoentropy and entanglement entropy of a superposition state. Through analytical continuation of the superposition parameter, we demonstrate that the transition matrix and density matrix of the superposition state can be treated in a unified manner. Within this framework, we naturally derive sum rules for the (reduced) transition matrix, pseudo Rényi entropy, and pseudoentropy. Furthermore, we demonstrate the close relationship between the sum rule for pseudoentropy and the singularity structure of the entropy function for the superposition state after analytical continuation. We also explore potential applications of the sum rule, including its relevance to understanding the gravity dual of non-Hermitian transition matrices and establishing upper bounds for the absolute value of pseudoentropy.
- [51] arXiv:2405.09812 (cross-list from cond-mat.other) [pdf, ps, other]
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Title: Mean-field and cumulant approaches to modelling organic polariton physicsComments: PhD ThesisSubjects: Other Condensed Matter (cond-mat.other); Quantum Physics (quant-ph)
In this thesis we develop methods for many-body open quantum systems and apply them to systems of organic polaritons. The methods employ a mean-field approach to reduce the dimensionality of large-scale problems. Initially assuming the absence of correlations in the many-body state, this approach is built upon in two ways.
First, we show how the mean-field approximation can be combined with matrix product operator methods to efficiently simulate the non-Markovian dynamics of a many-body system with strong coupling to multiple environments. We apply this method to calculate the threshold and photoluminescence for a realistic model of an organic laser.
Second, we extend the mean-field description by systematically including higher-order correlations via cumulant expansions of the Heisenberg equations of motion. We investigate the validity and convergence properties of these expansions, both with respect to expansion order and system size, for many-body systems with many-to-one network structures. We then show how the cumulant expansions may be used to calculate spatially resolved dynamics of organic polaritons. This enables a study of organic polariton transport in which we observe reversible conversion to dark exciton states and sub-group-velocity propagation.
The methods established in this work offer versatile tools for analysing large, many-body open quantum systems and investigating finite-size effects. Their application reveals the intricate dynamics of organic polaritons resulting from the interplay of strong light-matter coupling and vibrational effects. - [52] arXiv:2405.10009 (cross-list from math.SP) [pdf, ps, html, other]
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Title: Dirac operators on the half-line: stability of spectrum and non-relativistic limitComments: 17 pages, 1 figureSubjects: Spectral Theory (math.SP); Mathematical Physics (math-ph); Analysis of PDEs (math.AP); Quantum Physics (quant-ph)
We consider Dirac operators on the half-line, subject to generalised infinite-mass boundary conditions. We derive sufficient conditions which guarantee the stability of the spectrum against possibly non-self-adjoint potential perturbations and study the optimality of the obtained results. Finally, we establish a non-relativistic limit which makes a relationship of the present model to the Robin Laplacian on the half-line.
- [53] arXiv:2405.10015 (cross-list from cond-mat.mes-hall) [pdf, ps, html, other]
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Title: Non-Hermitian Topology in Hermitian Topological MatterComments: 8+10 pages, 5+1 figuresSubjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
Non-Hermiticity leads to distinctive topological phenomena absent in Hermitian systems. However, connection between such intrinsic non-Hermitian topology and Hermitian topology has remained largely elusive. Here, considering the bulk and boundary as an environment and system, we demonstrate that anomalous boundary states in Hermitian topological insulators exhibit non-Hermitian topology. We study the self-energy capturing the particle exchange between the bulk and boundary, and demonstrate that it detects Hermitian topology in the bulk and induces non-Hermitian topology at the boundary. As an illustrative example, we show the non-Hermitian topology and concomitant skin effect inherently embedded within chiral edge states of Chern insulators. We also find the emergence of hinge states within effective non-Hermitian Hamiltonians at surfaces of three-dimensional topological insulators. Furthermore, we comprehensively classify our correspondence across all the tenfold symmetry classes of topological insulators and superconductors. Our work uncovers a hidden connection between Hermitian and non-Hermitian topology, and provides an approach to identifying non-Hermitian topology in quantum matter.
- [54] arXiv:2405.10032 (cross-list from physics.chem-ph) [pdf, ps, html, other]
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Title: On the use of complex GTOs for the evaluation of radial integrals involving oscillating functionsComments: 19 pages, 4 figuresJournal-ref: Advances in Quantum Chemistry 88, 133-149 (2023)Subjects: Chemical Physics (physics.chem-ph); Quantum Physics (quant-ph)
We study two classes of radial integrals involving a product of bound and continuum one-electron states. Using a representation of the continuum part with an expansion on complex Gaussian Type Orbitals, such integrals can be performed analytically. We investigate the reliability of this scheme for low-energy physical parameters. This study serves as a premise in view of potential applications in molecular scattering processes.
- [55] arXiv:2405.10062 (cross-list from cond-mat.dis-nn) [pdf, ps, html, other]
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Title: Phenomenology of many-body localization in bond-disordered spin chainsComments: comments most welcome!!!Subjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Quantum Gases (cond-mat.quant-gas); Statistical Mechanics (cond-mat.stat-mech); Quantum Physics (quant-ph)
Many-body localization (MBL) hinders the thermalization of quantum many-body systems in the presence of strong disorder. In this work, we study the MBL regime in bond-disordered spin-1/2 XXZ spin chain, finding the multimodal distribution of entanglement entropy in eigenstates, sub-Poissonian level statistics, and revealing a relation between operators and initial states required for examining the breakdown of thermalization in the time evolution of the system. We employ a real space renormalization group scheme to identify these phenomenological features of the MBL regime that extend beyond the standard picture of local integrals of motion relevant for systems with disorder coupled to on-site operators. Our results pave the way for experimental probing of MBL in bond-disordered spin chains.
- [56] arXiv:2405.10079 (cross-list from cond-mat.mes-hall) [pdf, ps, html, other]
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Title: Emergence of moir\'e superlattice potential in graphene by twisted-hBN layersTianyu Zhang (1), Chengxin Xiao (2), Hongxia Xue (1), Wang Yao (2), Dong-Keun Ki (1) ((1) Department of Physics and HK Institute of Quantum Science \& Technology, The University of Hong Kong, Pokfulam Road, Hong Kong, China, (2) New Cornerstone Science Laboratory, Department of Physics, The University of Hong Kong, Hong Kong, China)Comments: 10 pages, 5 figures, 1 tableSubjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)
Moiré superlattices formed in stacks of two or more 2D crystals with similar lattice structures have recently become excellent platforms to reveal new physics in low-dimensional systems. They are, however, highly sensitive to the angle and lattice constant differences between the associated crystals, limiting the range of the material choice and the possible moiré patterns for a given 2D crystal. Here, we present a novel approach to realize an atomically flat substrate with a periodic moiré pattern that can induce the moiré potential on the material on top by van der Waals (vdW) interactions, without suffering from the lattice and angle mismatch. By constructing a twisted hBN (thBN) moiré substrate at an angle of about 1$^\circ$, we show that the graphene on top, aligned around 15$^\circ$ with the neighboring hBN layers, exhibits typical transport properties under a hexagonal moiré potential, including multiple satellite Dirac points (DPs), Hofstadter butterfly effect, and Brown-Zak oscillations. All features point to the existence of the moiré potential in graphene formed by thBN with $\sim$1$^\circ$ twist angle. Further statistical study shows that the twist from a parallel interface between the hBN layers is critical to induce the moiré potential. Our study demonstrates that the thBN moiré substrate can be used to investigate moiré physics in arbitrary materials without being constrained by their lattice constants.
- [57] arXiv:2405.10108 (cross-list from hep-th) [pdf, ps, html, other]
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Title: Quantum Field Theory in Curved Spacetime Approach to the Backreaction of Dynamical Casimir EffectComments: Undergraduate thesisSubjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc); Quantum Physics (quant-ph)
In this thesis, we investigate the dynamical Casimir effect, the creation of particles from vacuum by dynamical boundary conditions or dynamical background, and its backreaction to the motion of the boundary. The backreaction of particle creation to the boundary motion is studied using quantum field theory in curved spacetime technique, in 1+1 dimension and 3+1 dimension. The relevant quantities in these quantum field processes are carefully analyzed, including regularization of the UV and IR divergent of vacuum energy, and estimation of classical backreaction effects like radiation pressure. We recovered the qualitative result of backreaction in 1+1 dimensions. In the 3+1 dimension, we find that the backreaction tends to slow down the system to suppress the further particle creation, similar to the case of cosmological particle creation.
- [58] arXiv:2405.10151 (cross-list from physics.atom-ph) [pdf, ps, html, other]
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Title: Relativistic EELS scattering cross-sections for microanalysis based on Dirac solutionsZezhong Zhang, Ivan Lobato, Hamish Brown, Dirk Lamoen, Daen Jannis, Johan Verbeeck, Sandra Van Aert, Peter D. NellistComments: 52 pages, 8 figuresSubjects: Atomic Physics (physics.atom-ph); Materials Science (cond-mat.mtrl-sci); Applied Physics (physics.app-ph); Quantum Physics (quant-ph)
The rich information of electron energy-loss spectroscopy (EELS) comes from the complex inelastic scattering process whereby fast electrons transfer energy and momentum to atoms, exciting bound electrons from their ground states to higher unoccupied states. To quantify EELS, the common practice is to compare the cross-sections integrated within an energy window or fit the observed spectrum with theoretical differential cross-sections calculated from a generalized oscillator strength (GOS) database with experimental parameters. The previous Hartree-Fock-based and DFT-based GOS are calculated from Schrödinger's solution of atomic orbitals, which does not include the full relativistic effects. Here, we attempt to go beyond the limitations of the Schrödinger solution in the GOS tabulation by including the full relativistic effects using the Dirac equation within the local density approximation, which is particularly important for core-shell electrons of heavy elements with strong spin-orbit coupling. This has been done for all elements in the periodic table (up to Z = 118) for all possible excitation edges using modern computing capabilities and parallelization algorithms. The relativistic effects of fast incoming electrons were included to calculate cross-sections that are specific to the acceleration voltage. We make these tabulated GOS available under an open-source license to the benefit of both academic users as well as allowing integration into commercial solutions.
- [59] arXiv:2405.10199 (cross-list from cond-mat.dis-nn) [pdf, ps, html, other]
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Title: Quantum criticality and Kibble-Zurek scaling in the Aubry-Andr\'{e}-Stark modelComments: 10 pages, 11 figuresSubjects: Disordered Systems and Neural Networks (cond-mat.dis-nn); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)
We explore quantum criticality and Kibble-Zurek scaling (KZS) in the Aubry-Andre-Stark (AAS) model, where the Stark field of strength $\varepsilon$ is added onto the one-dimensional quasiperiodic lattice. We perform scaling analysis and numerical calculations of the localization length, inverse participation ratio (IPR), and energy gap between the ground and first excited states to characterize critical properties of the delocalization-localization transition. Remarkably, our scaling analysis shows that, near the critical point, the localization length $\xi$ scales with $\varepsilon$ as $\xi\propto\varepsilon^{-\nu}$ with $\nu\approx0.3$ a new critical exponent for the AAS model, which is different from the counterparts for both the pure Aubry-Andre (AA) model and Stark model. The IPR $\mathcal{I}$ scales as $\mathcal{I}\propto\varepsilon^{s}$ with the critical exponent $s\approx0.098$, which is also different from both two pure models. The energy gap $\Delta E$ scales as $\Delta E\propto \varepsilon^{\nu z}$ with the same critical exponent $z\approx2.374$ as that for the pure AA model. We further reveal hybrid scaling functions in the overlap between the critical regions of the Anderson and Stark localizations. Moreover, we investigate the driven dynamics of the localization transitions in the AAS model. By linearly changing the Stark (quasiperiodic) potential, we calculate the evolution of the localization length and the IPR, and study their dependence on the driving rate. We find that the driven dynamics from the ground state is well described by the KZS with the critical exponents obtained from the static scaling analysis. When both the Stark and quasiperiodic potentials are relevant, the KZS form includes the two scaling variables. This work extends our understanding of critical phenomena on localization transitions and generalizes the application of the KZS to hybrid models.
- [60] arXiv:2405.10263 (cross-list from cs.LG) [pdf, ps, other]
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Title: On Partially Unitary LearningComments: A working algorithm implementing Partially Unitary Learning arXiv:2212.14810 is developed and generalizedSubjects: Machine Learning (cs.LG); Numerical Analysis (math.NA); Quantum Physics (quant-ph); Machine Learning (stat.ML)
The problem of an optimal mapping between Hilbert spaces $IN$ of $\left|\psi\right\rangle$ and $OUT$ of $\left|\phi\right\rangle$ based on a set of wavefunction measurements (within a phase) $\psi_l \to \phi_l$, $l=1\dots M$, is formulated as an optimization problem maximizing the total fidelity $\sum_{l=1}^{M} \omega^{(l)} \left|\langle\phi_l|\mathcal{U}|\psi_l\rangle\right|^2$ subject to probability preservation constraints on $\mathcal{U}$ (partial unitarity). Constructed operator $\mathcal{U}$ can be considered as a $IN$ to $OUT$ quantum channel; it is a partially unitary rectangular matrix of the dimension $\dim(OUT) \times \dim(IN)$ transforming operators as $A^{OUT}=\mathcal{U} A^{IN} \mathcal{U}^{\dagger}$. An iteration algorithm finding the global maximum of this optimization problem is developed and it's application to a number of problems is demonstrated. A software product implementing the algorithm is available from the authors.
- [61] arXiv:2405.10268 (cross-list from nucl-th) [pdf, ps, html, other]
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Title: The Magic in Nuclear and Hypernuclear ForcesComments: 14 pages, 6 figuresSubjects: Nuclear Theory (nucl-th); High Energy Physics - Phenomenology (hep-ph); Quantum Physics (quant-ph)
Toward an improved understanding of the role of quantum information in nuclei and exotic matter, we examine the magic (non-stabilizerness) in low-energy strong interaction processes. As stabilizer states can be prepared efficiently using classical computers, and include classes of entangled states, it is magic and fluctuations in magic, along with entanglement, that determine resource requirements for quantum simulations. As a measure of fluctuations in magic induced by scattering, the "magic power" of the S-matrix is introduced. Using experimentally-determined scattering phase shifts and mixing parameters, the magic power in nucleon-nucleon and hyperon-nucleon scattering, along with the magic in the deuteron, are found to exhibit interesting features. The $\Sigma^-$-baryon is identified as a potential candidate catalyst for enhanced spreading of magic and entanglement in dense matter, depending on in-medium decoherence.
- [62] arXiv:2405.10285 (cross-list from hep-lat) [pdf, ps, html, other]
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Title: Interacting chiral fermions on the lattice with matrix product operator normsJutho Haegeman, Laurens Lootens, Quinten Mortier, Alexander Stottmeister, Atsushi Ueda, Frank VerstraeteSubjects: High Energy Physics - Lattice (hep-lat); Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
We develop a formalism for simulating one-dimensional interacting chiral fermions on the lattice without breaking any local symmetries by defining a Fock space endowed with a semi-definite norm defined in terms of matrix product operators. This formalism can be understood as a second-quantized form of Stacey fermions, hence providing a possible solution for the fermion doubling problem and circumventing the Nielsen-Ninomiya theorem. We prove that the emerging theory is hermitian by virtue of the fact that it gives rise to a hermitian generalized eigenvalue problem and that it has local features as it can be simulated using tensor network methods similar to the ones used for simulating local quantum Hamiltonians. We also show that the scaling limit of the free model recovers the chiral fermion field. As a proof of principle, we consider a single Weyl fermion on a periodic ring with Hubbard-type nearest-neighbor interactions and construct a variational generalized DMRG code demonstrating that the ground states of the system for large system sizes can be determined efficiently.
Cross submissions for Friday, 17 May 2024 (showing 17 of 17 entries )
- [63] arXiv:2008.10617 (replaced) [pdf, ps, html, other]
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Title: Do qubits dream of entangled sheep? Quantum measurement without classical outputNoah Lupu-Gladstein, Aharon Brodutch, Hugo Ferretti, Weng-Kian Tham, Arthur Ou Teen Pang, Kent Bonsma-Fisher, Aephraim M. SteinbergSubjects: Quantum Physics (quant-ph)
Quantum mechanics is usually formulated with an implicit assumption that agents who can observe and interact with the world are external to it and have a classical memory. However, there is no accepted way to define the quantum-classical cut and no a priori reason to rule out fully quantum agents with coherent quantum memories. In this work, we introduce an entirely quantum notion of measurement, called a sensation, to account for quantum agents that experience the world through quantum sensors. Sensations eschew probabilities and instead describe a deterministic flow of quantum information. We quantify the information gain and disturbance of a sensation using concepts from quantum information theory and find that sensations always disturb at least as much as they inform. Viewing measurements as sensations could lead to a new understanding of quantum theory in general and to new results in the context of quantum networks.
- [64] arXiv:2112.14998 (replaced) [pdf, ps, html, other]
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Title: Optimal control of a quantum sensor: A fast algorithm based on an analytic solutionComments: Main text: 21 pages, 7 figuresSubjects: Quantum Physics (quant-ph)
Quantum sensors can show unprecedented sensitivities, provided they are controlled in a very specific, optimal way. Here, we consider a spin sensor of time-varying fields in the presence of dephasing noise, and we show that the problem of finding the pulsed control field that optimizes the sensitivity (i.e., the smallest detectable signal) can be mapped to the determination of the ground state of a spin chain. We find an approximate but analytic solution of this problem, which provides a \emph{lower bound} for the sensitivity and a pulsed control very close to optimal, which we further use as initial guess for realizing a fast simulated annealing algorithm. We experimentally demonstrate the sensitivity improvement for a spin-qubit magnetometer based on a nitrogen-vacancy center in diamond.
- [65] arXiv:2202.03523 (replaced) [pdf, ps, html, other]
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Title: Resource Marginal ProblemsComments: 14+16 pages and 3 figures. New results added and largely rewritten. Accepted for publication in QuantumSubjects: Quantum Physics (quant-ph); Mathematical Physics (math-ph)
We introduce the resource marginal problems, which concern the possibility of having a resource-free target subsystem compatible with a given collection of marginal density matrices. By identifying an appropriate choice of resource R and target subsystem T, our problems reduce, respectively, to the well-known marginal problems for quantum states and the problem of determining if a given quantum system is a resource. More generally, we say that a set of marginal states is resource-free incompatible with a target subsystem T if all global states compatible with this set must result in a resourceful state in T of type R. We show that this incompatibility induces a resource theory that can be quantified by a monotone and obtain necessary and sufficient conditions for this monotone to be computable as a conic program with finite optimum. We further show, via the corresponding witnesses, that (1) resource-free incompatibility is equivalent to an operational advantage in some channel-discrimination tasks, and (2) some specific cases of such tasks fully characterize the convertibility between marginal density matrices exhibiting resource-free incompatibility. Through our framework, one sees a clear connection between any marginal problem -- which implicitly involves some notion of incompatibility -- for quantum states and a resource theory for quantum states. We also establish a close connection between the physical relevance of resource marginal problems and the ground state properties of certain many-body Hamiltonians. In terms of application, the universality of our framework leads, for example, to a further quantitative understanding of the incompatibility associated with the recently-proposed entanglement marginal problems and entanglement transitivity problems.
- [66] arXiv:2209.06191 (replaced) [pdf, ps, html, other]
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Title: Universal measurement-based quantum computation in a one-dimensional architecture enabled by dual-unitary circuitsComments: V2: Published version. Contains improved main theorem and other minor modificationsSubjects: Quantum Physics (quant-ph); Strongly Correlated Electrons (cond-mat.str-el)
A powerful tool emerging from the study of many-body quantum dynamics is that of dual-unitary circuits, which are unitary even when read `sideways', i.e., along the spatial direction. Here, we show that this provides the ideal framework to understand and expand on the notion of measurement-based quantum computation (MBQC). In particular, applying a dual-unitary circuit to a many-body state followed by appropriate measurements effectively implements quantum computation in the spatial direction. We show how the dual-unitary dynamics generated by the dynamics of the paradigmatic one-dimensional kicked Ising chain with certain parameter choices generate resource states for universal deterministic MBQC. Specifically, after $k$ time-steps, equivalent to a depth-$k$ quantum circuit, we obtain a resource state for universal MBQC on $\sim 3k/4$ encoded qubits. Our protocol allows generic quantum circuits to be `rotated' in space-time and gives new ways to exchange between resources like qubit number and coherence time in quantum computers. Beyond the practical advantages, we also interpret the dual-unitary evolution as generating an infinite sequence of new symmetry-protected topological phases with spatially modulated symmetries, which gives a vast generalization of the well-studied one-dimensional cluster state and shows that our protocol is robust to symmetry-respecting deformations.
- [67] arXiv:2211.05813 (replaced) [pdf, ps, html, other]
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Title: Decoherence of a 2-Path System by Infrared PhotonsSubjects: Quantum Physics (quant-ph); High Energy Physics - Theory (hep-th)
We calculate the decoherence caused by photon emission for a charged particle travelling through an interferometer; the decoherence rate gives a quantitative measure of how much "which-path" quantum information is gained by the electromagnetic field. We isolate the quantum information content of both leading and sub-leading soft photons, and show that it can be extracted entirely from information about the endpoints of the particle's paths. When infrared dressing is used to cure the infrared divergences in the theory, the leading order soft photons then give no contribution to decoherence, and carry no quantum information. The sub-leading soft photons in contrast may carry finite which-path information, and the sub-leading contribution to decoherence takes an extremely simple, time-independent form depending only on the size of the interferometer. An interesting open question is whether or not dressing should also be applied at sub-leading order; we discuss the possibility of answering this question experimentally.
- [68] arXiv:2211.06167 (replaced) [pdf, ps, html, other]
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Title: Tuning for Quantum Speedup in Directed Lackadaisical Quantum WalksSubjects: Quantum Physics (quant-ph)
Quantum walks constitute an important tool for designing quantum algorithms and information processing tasks. In a lackadaisical walk, in addition to the possibility of moving out of a node, the walker can remain on the same node with some probability. This is achieved by introducing self-loops, parameterized by self-loop strength $l$, attached to the nodes such that large $l$ implies a higher likelihood for the walker to be trapped at the node. In this work, {\it directed}, lackadaisical quantum walks is studied. Depending on $l$, two regimes are shown to exist -- one in which classical walker dominates and the other dominated by the quantum walker. In the latter case, we also demonstrate the existence of two distinct scaling regimes with $l$ for quantum walker on a line and on a binary tree. Surprisingly, a significant quantum-induced speedup is realized for large $l$. By tuning the initial state, the extent of this speedup can be manipulated.
- [69] arXiv:2302.02459 (replaced) [pdf, ps, other]
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Title: A High Performance Compiler for Very Large Scale Surface Code ComputationsComments: accepted in QuantumSubjects: Quantum Physics (quant-ph)
We present the first high performance compiler for very large scale quantum error correction: it translates an arbitrary quantum circuit to surface code operations based on lattice surgery. Our compiler offers an end to end error correction workflow implemented by a pluggable architecture centered around an intermediate representation of lattice surgery instructions. Moreover, the compiler supports customizable circuit layouts, can be used for quantum benchmarking and includes a quantum resource estimator. The compiler can process millions of gates using a streaming pipeline at a speed geared towards real-time operation of a physical device. We compiled within seconds 80 million logical surface code instructions, corresponding to a high precision Clifford+T implementation of the 128-qubit Quantum Fourier Transform (QFT). Our code is open-sourced at \url{this https URL}.
- [70] arXiv:2303.08395 (replaced) [pdf, ps, html, other]
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Title: Stationary Two-State System in Optics using Layered MaterialsComments: 6 pages, 2 figuresSubjects: Quantum Physics (quant-ph); Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Materials Science (cond-mat.mtrl-sci)
In scenarios where electrons are confined to a flat surface, such as graphene, quantizing electrodynamics reveals intriguing insights. We find that one of Maxwell's equations manifests as part of the Hamiltonian, leading to novel constraints on physical states due to residual gauge invariance. We identify two quantum states with zero energy expectation values: one replicates the scattering and absorption of light, a phenomenon familiar in classical optics, while the other is more fundamentally associated with photon creation. These states form an inseparable two-state system, giving a new formula for reflection and transmission coefficients with photon emission effects. Notably, there exists a special thickness of the surface where these states decouple, offering intriguing possibilities for exploring physics through symmetry-based perturbations involving concepts of parity, axial gauge fields, and surface deformation.
- [71] arXiv:2303.09430 (replaced) [pdf, ps, html, other]
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Title: Global optimization of MPS in quantum-inspired numerical analysisSubjects: Quantum Physics (quant-ph)
This work discusses the solution of partial differential equations (PDEs) using matrix product states (MPS). The study focuses on the search for the lowest eigenstates of a Hamiltonian equation, for which five algorithms are introduced: imaginary-time evolution, steepest gradient descent, an improved gradient descent, an implicitly restarted Arnoldi method, and density matrix renormalization group (DMRG) optimization. The first four methods are engineered using a framework of limited-precision linear algebra, where operations between MPS and matrix product operators (MPOs) are implemented with finite resources. All methods are benchmarked using the PDE for a quantum harmonic oscillator in up to two dimensions, over a regular grid with up to $2^{28}$ points. Our study reveals that all MPS-based techniques outperform exact diagonalization techniques based on vectors, with respect to memory usage. Imaginary-time algorithms are shown to underperform any type of gradient descent, both in terms of calibration needs and costs. Finally, Arnoldi like methods and DMRG asymptotically outperform all other methods, including exact diagonalization, as problem size increases, with an exponential advantage in memory and time usage.
- [72] arXiv:2306.17083 (replaced) [pdf, ps, other]
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Title: Optimal mixers restricted to subspaces and the stabilizer formalismSubjects: Quantum Physics (quant-ph)
We present a novel formalism to both understand and construct mixers that preserve a given subspace. The method connects and utilizes the stabilizer formalism that is used in error correcting codes. This can be useful in the setting when the quantum approximate optimization algorithm (QAOA), a popular meta-heuristic for solving combinatorial optimization problems, is applied in the setting where the constraints of the problem lead to a feasible subspace that is large but easy to specify. The proposed method gives a systematic way to construct mixers that are resource efficient in the number of controlled not gates and can be understood as a generalization of the well-known X and XY mixers and a relaxation of the Grover mixer: Given a basis of any subspace, a resource efficient mixer can be constructed that preserves the subspace. The numerical examples provided show a dramatic reduction of CX gates when compared to previous results. We call our approach logical X-Mixer or logical X QAOA ($\textbf{LX-QAOA}$), since it can be understood as dividing the subspace into code spaces of stabilizers S and consecutively applying logical rotational X gates associated with these code spaces. Overall, we hope that this new perspective can lead to further insight into the development of quantum algorithms.
- [73] arXiv:2307.06833 (replaced) [pdf, ps, html, other]
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Title: Impact of unreliable devices on stability of quantum computationsSubjects: Quantum Physics (quant-ph); Emerging Technologies (cs.ET)
Noisy intermediate-scale quantum (NISQ) devices are valuable platforms for testing the tenets of quantum computing, but these devices are susceptible to errors arising from de-coherence, leakage, cross-talk and other sources of noise. This raises concerns regarding the stability of results when using NISQ devices since strategies for mitigating errors generally require well-characterized and stationary error models. Here, we quantify the reliability of NISQ devices by assessing the necessary conditions for generating stable results within a given tolerance. We use similarity metrics derived from device characterization data to derive and validate bounds on the stability of a 5-qubit implementation of the Bernstein-Vazirani algorithm. Simulation experiments conducted with noise data from IBM Washington, spanning January 2022 to April 2023, revealed that the reliability metric fluctuated between 41% and 92%. This variation significantly surpasses the maximum allowable threshold of 2.2% needed for stable outcomes. Consequently, the device proved unreliable for consistently reproducing the statistical mean in the context of the Bernstein-Vazirani circuit.
- [74] arXiv:2307.16339 (replaced) [pdf, ps, html, other]
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Title: Non-Kochen-Specker ContextualityComments: 19 pages, 5 figures, 1 table; 8-dim part of Fig. 3 has been corrected ((h) and (i) are not isomorphic); Nothing else in the rest of the paper is affected by this correction; The correction is published in Entropy 2024, 26(2), 100; this https URLJournal-ref: Entropy 2023, 25(8), 1117Subjects: Quantum Physics (quant-ph); Mathematical Physics (math-ph)
Quantum contextuality supports quantum computation and communication. One of its main vehicles is hypergraphs. The most elaborated are the Kochen-Specker ones, but there is also another class of contextual sets that are not of this kind. Their representation has been mostly operator-based and limited to special constructs in three- to six-dim spaces, a notable example of which is the Yu-Oh set. Previously, we showed that hypergraphs underlie all of them, and in this paper, we give general methods - whose complexity does not scale up with the dimension - for generating such non-Kochen-Specker hypergraphs in any dimension and give examples in up to 16-dim spaces. Our automated generation is probabilistic and random, but the statistics of accumulated data enable one to filter out sets with the required size and structure.
- [75] arXiv:2308.04098 (replaced) [pdf, ps, html, other]
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Title: Molecular docking via quantum approximate optimization algorithmComments: 23 pages, 17 figures, All comments are welcomeJournal-ref: Phys. Rev. Applied 21, 034036 (2024)Subjects: Quantum Physics (quant-ph); Chemical Physics (physics.chem-ph); Biomolecules (q-bio.BM)
Molecular docking plays a pivotal role in drug discovery and precision medicine, enabling us to understand protein functions and advance novel therapeutics. Here, we introduce a potential alternative solution to this problem, the digitized-counterdiabatic quantum approximate optimization algorithm (DC-QAOA), which utilizes counterdiabatic driving and QAOA on a quantum computer. Our method was applied to analyze diverse biological systems, including the SARS-CoV-2 Mpro complex with PM-2-020B, the DPP-4 complex with piperidine fused imidazopyridine 34, and the HIV-1 gp120 complex with JP-III-048. The DC-QAOA exhibits superior performance, providing more accurate and biologically relevant docking results, especially for larger molecular docking problems. Moreover, QAOA-based algorithms demonstrate enhanced hardware compatibility in the noisy intermediate-scale quantum era, indicating their potential for efficient implementation under practical docking scenarios. Our findings underscore quantum computing's potential in drug discovery and offer valuable insights for optimizing protein-ligand docking processes.
- [76] arXiv:2308.05871 (replaced) [pdf, ps, html, other]
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Title: Globally optimal interferometry with lossy twin Fock probesComments: 13 pages, 2 figuresJournal-ref: Front. Phys. (2024) 12:1369786Subjects: Quantum Physics (quant-ph); Quantum Gases (cond-mat.quant-gas); Mathematical Physics (math-ph)
Parity or quadratic spin (e.g., $J_{z}^{2}$) readouts of a Mach-Zehnder (MZ) interferometer probed with a twin Fock input state allow to saturate the optimal sensitivity attainable among all mode-separable states with a fixed total number of particles, but only when the interferometer phase $\theta$ is near zero. When more general Dicke state probes are used, the parity readout saturates the quantum Fisher information (QFI) at $\theta=0$, whereas better-than-standard quantum limit performance of the $J_{z}^{2}$ readout is restricted to an $o(\sqrt{N})$ occupation imbalance. We show that a method of moments readout of two quadratic spin observables $J_{z}^{2}$ and $J_{+}^{2}+J_{-}^{2}$ is globally optimal for Dicke state probes, i.e., the error saturates the QFI for all $\theta$. In the lossy setting, we derive the time-inhomogeneous Markov process describing the effect of particle loss on twin Fock states, showing that method of moments readout of four at-most-quadratic spin observables is sufficient for globally optimal estimation of $\theta$ when two or more particles are lost. The analysis culminates in a numerical calculation of the QFI matrix for distributed MZ interferometry on the four mode state $\vert {N\over 4},{N\over 4},{N\over 4},{N\over 4}\rangle$ and its lossy counterparts, showing that an advantage for estimation of any linear function of the local MZ phases $\theta_{1}$, $\theta_{2}$ (compared to independent probing of the MZ phases by two copies of $\vert {N\over 4},{N\over 4}\rangle$) appears when more than one particle is lost.
- [77] arXiv:2309.08155 (replaced) [pdf, ps, other]
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Title: Designs from Local Random Quantum Circuits with SU(d) SymmetryComments: 12+38 pagesSubjects: Quantum Physics (quant-ph); Mathematical Physics (math-ph)
The generation of $k$-designs (pseudorandom distributions that emulate the Haar measure up to $k$ moments) with local quantum circuit ensembles is a problem of fundamental importance in quantum information and physics. Despite the extensive understanding of this problem for ordinary random circuits, the crucial situations where symmetries or conservation laws are in play are known to pose fundamental challenges and remain little understood. We construct, for the first time, explicit local unitary ensembles that can achieve high-order unitary $k$-designs under transversal continuous symmetry, in the particularly important SU$(d)$ case. Specifically, we define the Convolutional Quantum Alternating group (CQA) generated by 4-local SU$(d)$-symmetric Hamiltonians as well as associated 4-local SU$(d)$-symmetric random unitary circuit ensembles, and prove that they form and converge to SU$(d)$-symmetric $k$-designs, respectively, for all $k < n(n-3)/2$ with $n$ being the number of qudits. A key technique that we employ to obtain the results is the Okounkov--Vershik approach to $S_n$ representation theory. To study the convergence time of the CQA ensemble, we develop a numerical method using the Young orthogonal form and $S_n$ branching rule. We provide strong evidence for a subconstant spectral gap and certain convergence time scales of various important circuit architectures, which contrast with the symmetry-free case. We also provide comprehensive explanations of the difficulties and limitations in rigorously analyzing the convergence time using methods that have been effective for cases without symmetries, including Knabe's local gap threshold and Nachtergaele's martingale methods. This suggests that a novel approach is likely necessary for understanding the convergence time of SU$(d)$-symmetric local random circuits.
- [78] arXiv:2309.10192 (replaced) [pdf, ps, html, other]
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Title: Geometric Ramsey Interferometry with a Tripod SchemeComments: 4 figures, 6 pages including referencesSubjects: Quantum Physics (quant-ph); Atomic Physics (physics.atom-ph)
Ramsey interferometry is a key technique for precision spectroscopy and to probe the coherence of quantum systems. Typically, an interferometer is constructed using two quantum states and involves a time-dependent interaction with two short resonant electromagnetic pulses. Here, we explore a different type of Ramsey interferometer where we perform quantum state manipulations by geometrical means, eliminating the temporal dependence of the interaction. We use a resonant tripod scheme in ultracold strontium atoms where the interferometric operation is restricted to a two-dimensional dark-state subspace in the dressed-state picture. The observed interferometric phase accumulation is due to an effective geometric scalar term in the dark-state subspace, which remarkably does not vanish during the free evolution time when the light-matter interaction is turned off. This study opens the door for more robust interferometers operating on multiple input-output ports.
- [79] arXiv:2310.01316 (replaced) [pdf, ps, html, other]
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Title: Entanglement of Nanophotonic Quantum Memory Nodes in a Telecom NetworkCan M. Knaut, Aziza Suleymanzade, Yan-Cheng Wei, Daniel R. Assumpcao, Pieter-Jan Stas, Yan Qi Huan, Bartholomeus Machielse, Erik N. Knall, Madison Sutula, Gefen Baranes, Neil Sinclair, Chawina De-Eknamkul, David S. Levonian, Mihir K. Bhaskar, Hongkun Park, Marko Lončar, Mikhail D. LukinComments: 23 pages, 16 figuresJournal-ref: Nature 629, 573 (2024)Subjects: Quantum Physics (quant-ph)
A key challenge in realizing practical quantum networks for long-distance quantum communication involves robust entanglement between quantum memory nodes connected via fiber optical infrastructure. Here, we demonstrate a two-node quantum network composed of multi-qubit registers based on silicon-vacancy (SiV) centers in nanophotonic diamond cavities integrated with a telecommunication (telecom) fiber network. Remote entanglement is generated via the cavity-enhanced interactions between the SiV's electron spin qubits and optical photons. Serial, heralded spin-photon entangling gate operations with time-bin qubits are used for robust entanglement of separated nodes. Long-lived nuclear spin qubits are used to provide second-long entanglement storage and integrated error detection. By integrating efficient bi-directional quantum frequency conversion of photonic communication qubits to telecom frequencies (1350 nm), we demonstrate entanglement of two nuclear spin memories through 40 km spools of low-loss fiber and a 35 km long fiber loop deployed in the Boston area urban environment, representing an enabling step towards practical quantum repeaters and large-scale quantum networks.
- [80] arXiv:2310.08813 (replaced) [pdf, ps, html, other]
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Title: A Unifying Quantum Speed Limit For Time-Independent Hamiltonian EvolutionComments: revised, 22 pages, 3 figures, to appear in J.Phys.ASubjects: Quantum Physics (quant-ph)
Quantum speed limit (QSL) is the study of fundamental limits on the evolution time of quantum systems. For instance, under the action of a time-independent Hamiltonian, the evolution time between an initial and a final quantum state obeys various mutually complementary lower bounds. They include the Mandelstam-Tamm, Margolus-Levitin, Luo-Zhang, dual ML and Lee-Chau bounds. Here we show that the Mandelstam-Tamm bound can be obtained by optimizing the Lee-Chau bound over a certain parameter. More importantly, we report a QSL that includes all the above bounds as special cases before optimizing over the physically meaningless reference energy level of a quantum system. This unifying bound depends on a certain parameter $p$. For any fixed $p$, we find all pairs of time-independent Hamiltonian and initial pure quantum state that saturate this unifying bound. More importantly, these pairs allow us to compute this bound accurately and efficiently using an oracle that returns certain $p$th moments related to the absolute value of energy of the quantum state. Moreover, this oracle can be simulated by a computationally efficient and accurate algorithm for finite-dimensional quantum systems as well as for certain infinite-dimensional quantum states with bounded and continuous energy spectra. This makes our computational method feasible in a lot of practical situations. We compare the performance of this bound for the case of a fixed $p$ as well as the case of optimizing over $p$ with existing QSLs. We find that if the dimension of the underlying Hilbert space is $\lesssim 2000$, our unifying bound optimized over $p$ can be computed accurately in a few minutes using Mathematica code with just-in-time compilation in a typical desktop. Besides, this optimized unifying QSL is at least as good as all the existing ones combined and can occasionally be a few percent to a few times better.
- [81] arXiv:2310.11653 (replaced) [pdf, ps, html, other]
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Title: Quantum work: Reconciling quantum mechanics and thermodynamicsComments: In this new version, we improved our proofs and made small changes to the main textJournal-ref: Physical Review Research 6, L022036, 2024Subjects: Quantum Physics (quant-ph)
It has been recently claimed that no protocol for measuring quantum work can satisfy standard required physical principles, casting doubts on the compatibility between quantum mechanics, thermodynamics, and the classical limit. In this Letter, we present a solution for this incompatibility. We demonstrate that the standard formulation of these principles fails to address the classical limit properly. By proposing changes in this direction, we prove that all the essential principles can be satisfied when work is defined as a quantum observable, reconciling quantum work statistics and thermodynamics.
- [82] arXiv:2310.15092 (replaced) [pdf, ps, html, other]
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Title: Dihedral Quantum CodesSubjects: Quantum Physics (quant-ph); Cryptography and Security (cs.CR); Information Theory (cs.IT)
We establish dihedral quantum codes of short block length, a class of CSS codes obtained by the lifted product construction. We present the code construction and give a formula for the code dimension, depending on the two classical codes that the CSS code is based on. We also give a lower bound on the code distance and construct an example of short dihedral quantum codes.
- [83] arXiv:2310.16709 (replaced) [pdf, ps, html, other]
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Title: Sampling reduced density matrix to extract fine levels of entanglement spectrumSubjects: Quantum Physics (quant-ph); Statistical Mechanics (cond-mat.stat-mech); Strongly Correlated Electrons (cond-mat.str-el); High Energy Physics - Theory (hep-th)
Low-lying entanglement spectrum provides the quintessential fingerprint to identify the highly entangled quantum matter with topological and conformal field-theoretical properties. However, when the entangling region acquires long boundary with the environment, such as that between long coupled chains or in two or higher dimensions, there unfortunately exists no universal yet practical method to compute the entanglement spectra with affordable computational cost. Here we propose a new scheme to overcome such difficulty and successfully extract the low-lying fine entanglement spectrum (ES). We trace out the environment via quantum Monte Carlo simulation and diagonalize the reduced density matrix to gain the ES. We demonstrate the strength and reliability of our method through long coupled spin chains and answer its long-standing controversy. Our simulation results, with unprecedentedly large system sizes, establish the practical computation scheme of the entanglement spectrum with a huge freedom degree of environment.
- [84] arXiv:2311.05592 (replaced) [pdf, ps, html, other]
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Title: Fixed-point Grover Adaptive Search for Binary Optimization ProblemsComments: 15 pages; Results substantially improved. Submitted. Comments are welcome!Subjects: Quantum Physics (quant-ph)
We study a Grover-type method for Quadratic Binary Optimization problems. In the unconstrained (QUBO) case, for an $n$-dimensional problem with $m$ nonzero terms, we construct a marker oracle for such problems with a tuneable parameter, $\Lambda \in \left[ 1, m \right] \cap \mathbb{Z}$. At $d \in \mathbb{Z}_+$ precision, the oracle uses $O \left( n + \Lambda d \right)$ qubits, has total depth $O \left( \tfrac{m}{\Lambda} \log_2 \left( n \right) + \log_2 \left( d \right) \right)$, and non-Clifford depth of $O \left( \tfrac{m}{\Lambda} \right)$. Moreover, each qubit required to be connected to at most $O \left( \log_2 \left( \Lambda + d \right) \right)$ other qubits. In the case of a maximal graph cuts, as $d = 2 \log_2 \left( n \right)$ always suffices, the depth of the marker oracle can be made as shallow as $O \left( \log_2 \left( n \right) \right)$. For all values of $\Lambda$, the non-Clifford gate count of these oracles is strictly lower (by a factor of $\sim 2$) than previous constructions.
We then introduce a novel \emph{Fixed-point Grover Adaptive Search for QUBO Problems}, using our oracle design and a hybrid Fixed-point Grover Search of Li et al. This method has better performance guarantees than previous Grover Adaptive Search methods. Finally, we give a heuristic argument that, with high probability and in $O \left( \tfrac{\log_2 \left( n \right)}{\sqrt{\epsilon}} \right)$ time, this adaptive method finds a configuration that is among the best $\epsilon 2^n$ ones. - [85] arXiv:2311.12324 (replaced) [pdf, ps, html, other]
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Title: {\AE} codesComments: 5+5 pages, 3 figures. Added appendix about non-locality of spontaneous decaySubjects: Quantum Physics (quant-ph); Atomic Physics (physics.atom-ph)
Diatomic molecular codes [arXiv:1911.00099] are designed to encode quantum information in the orientation of a diatomic molecule, allowing error correction from small torques and changes in angular momentum. Here, we directly study noise native to atomic and molecular platforms -- spontaneous emission, stray electromagnetic fields, and Raman scattering -- and show that diatomic molecular codes fail against this noise. We derive simple necessary and sufficient conditions for codes to protect against such noise. We also identify existing and develop new absorption-emission (Æ) codes that are more practical than molecular codes, require lower average momentum, can directly protect against photonic processes up to arbitrary order, and are applicable to a broader set of atomic and molecular systems.
- [86] arXiv:2312.13504 (replaced) [pdf, ps, html, other]
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Title: Annealing reduces Si$_3$N$_4$ microwave-frequency dielectric loss in superconducting resonatorsSarang Mittal, Kazemi Adachi, Nicholas E. Frattini, Maxwell D. Urmey, Sheng-Xiang Lin, Alec L. Emser, Cyril Metzger, Luca Talamo, Sarah Dickson, David Carlson, Scott B. Papp, Cindy A. Regal, Konrad W. LehnertComments: 11 pages, 7 figuresSubjects: Quantum Physics (quant-ph)
The dielectric loss of silicon nitride (Si$_3$N$_4$) limits the performance of microwave-frequency devices that rely on this material for sensing, signal processing, and quantum communication. Using superconducting resonant circuits, we measure the cryogenic loss tangent of either as-deposited or high-temperature annealed stoichiometric Si$_3$N$_4$ as a function of drive strength and temperature. The internal loss behavior of the electrical resonators is largely consistent with the standard tunneling model of two-level systems (TLS), including damping caused by resonant energy exchange with TLS and by the relaxation of non-resonant TLS. We further supplement the TLS model with a self-heating effect to explain an increase in the loss observed in as-deposited films at large drive powers. Critically, we demonstrate that annealing remedies this anomalous power-induced loss, reduces the relaxation-type damping by more than two orders of magnitude, and reduces the resonant-type damping by a factor of three. Employing infrared absorption spectroscopy, we find that annealing reduces the concentration of hydrogen in the Si$_3$N$_4$, suggesting that hydrogen impurities cause substantial dissipation.
- [87] arXiv:2312.14313 (replaced) [pdf, ps, html, other]
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Title: Lifetime Reduction of Single Germanium-Vacancy Centers in Diamond via a Tunable Open MicrocavityComments: 14 pages, 6 figures, journal accepted version after initial reviewSubjects: Quantum Physics (quant-ph)
Coupling between a single quantum emitter and an optical cavity presents a key capability for future quantum networking applications. Here, we explore interactions between individual germanium-vacancy (GeV) defects in diamond and an open microcavity at cryogenic temperatures. Exploiting the tunability of our microcavity system to characterize and select emitters, we observe a Purcell-effect-induced lifetime reduction of up to $4.5\pm0.3$, and extract coherent coupling rates up to $360\pm20$ MHz. Our results indicate that the GeV defect has favorable optical properties for cavity coupling, with a quantum efficiency of at least $0.34\pm0.05$ and likely much higher.
- [88] arXiv:2401.00725 (replaced) [pdf, ps, html, other]
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Title: Decoherence in Exchange-Coupled Quantum Spin Qubit Systems: Impact of Multiqubit Interactions and Geometric ConnectivityComments: 10 pages, 14 figuresSubjects: Quantum Physics (quant-ph)
We investigate the impact of different connectivities on the decoherence time in quantum systems under quasi-static Heisenberg noise. We considered three types of elementary units, including node, stick and triangle and connect them into ring, chain, and tree configurations. We find that rings exhibit greater stability compared to chains, contrary to the expectation that higher average connectivity leads to decreased stability. Additionally, the stick configuration is more stable than the triangle configuration. We also observe similar trends in entanglement entropy and return probability, indicating their potential use in characterizing decoherence time. Our findings provide insights into the interplay between connectivity and stability in quantum systems, with implications for the design of robust quantum technologies and quantum error correction strategies.
- [89] arXiv:2401.01592 (replaced) [pdf, ps, html, other]
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Title: Single-photon scattering in giant-atom waveguide systems with chiral couplingComments: 15 pages, 8 figuresSubjects: Quantum Physics (quant-ph)
We study single-photon scattering spectra of a giant atom chirally coupled to a one-dimensional waveguide at multiple connection points, and examine chirality induced effects in the scattering spectra. We show that the transmission spectra typically possess an anti-Lorentzian lineshape with a nonzero minimum, but by engineering the chirality of the multi-point coupling, the transmission spectrum of an incident photon can undergo a transition from complete transmission to total reflection at multiple frequency ``windows'', where the width of the anti-Lorentzian lineshape for each of the window can be flexibly tuned at a fixed frequency detuning. Moreover, we show that a perfect nonreciprocal photon scattering can be achieved due to the interplay between internal atomic spontaneous emission and the chirally external decay to the waveguide, in contrast to that induced by the non-Markovian retardation effect. We also consider the non-Markovian retardation effect on the scattering spectra, which allows for a photonic band gap even with only two chiral coupling points. The giant-atom-waveguide system with chiral coupling is a promising candidate for realizing single-photon routers with multiple channels.
- [90] arXiv:2401.16619 (replaced) [pdf, ps, html, other]
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Title: Polynomial-depth quantum algorithm for computing matrix determinantComments: 8 pages, 3 figures, minor change in presentationSubjects: Quantum Physics (quant-ph)
We propose an algorithm for calculating the determinant of a square matrix, and construct the quantum circuit realizing it, using multiqubit control gates (representable in terms of Toffoli gates, CNOTs and SWAPs), Hadamard transformations and $Z$-operators. Each row of the matrix is encoded as a pure state of some quantum system. The admitted matrix is therefore arbitrary up to the normalization of quantum states of those systems. The depth of the proposed algorithm is $O(N^3\log \, N)$ for the $N\times N$ matrix.
- [91] arXiv:2401.17980 (replaced) [pdf, ps, html, other]
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Title: No epistemic model can explain anti-distinguishability of quantum mixed preparationsComments: Comments are welcomeSubjects: Quantum Physics (quant-ph)
We address the fundamental question of whether epistemic models can reproduce the empirical predictions of general quantum preparations. This involves comparing the common quantum overlap determined by the anti-distinguishability of a set of mixed preparations with the common epistemic overlap of the probability distribution over the ontic states describing these preparations. A set of quantum mixed preparations is deemed to be non-epistemic when the epistemic overlap must be zero while the corresponding quantum overlap remains non-zero. In its strongest manifestation, a set of mixed quantum preparations is fully non-epistemic if the epistemic overlap vanishes while the quantum overlap reaches its maximum value of one. Remarkably, we show that there exist sets of non-epistemic mixed preparations even in dimension 2, when the overlap between three mixed preparations is concerned. Moreover, we present quantum mixed preparations in dimensions 3 and 4 that are fully non-epistemic concerning the overlap between four and three preparations, respectively. We also establish a generic upper bound on the average ratio between the epistemic and quantum overlap for two mixed preparations. Consequently, the ratio for certain pairs of quantum mixed preparations is shown to be arbitrarily small in two different instances, signifying they are non-epistemic in one case and fully non-epistemic in the other. Finally, we delve into some of the remarkable implications stemming from our findings.
- [92] arXiv:2403.12222 (replaced) [pdf, ps, html, other]
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Title: Multiplexed quantum state transfer in waveguidesSubjects: Quantum Physics (quant-ph)
In this article, we consider a realistic waveguide implementation of a quantum network that serves as a testbed to show how to maximize the storage and manipulation of quantum information in QED setups. We analyze two approaches using wavepacket engineering and quantum state transfer protocols. First, we propose and design a family of orthogonal photons in the time domain. These photons allow for a selective interaction with distinct targeted qubits. Yet, mode multiplexing employing resonant nodes is largely spoiled by cross-talk effects. This motivates the second approach, namely, frequency multiplexing. Here we explore the limits of frequency multiplexing through the waveguide, analyzing its capabilities to host and faithfully transmit photons of different frequencies within a given bandwidth. We perform detailed one- and two-photon simulations and provide theoretical bounds for the fidelity of coherent quantum state transfer protocols under realistic conditions. Our results show that state-of-the-art experiments can employ dozens of multiplexed photons with global fidelities fulfilling the requirements imposed by fault-tolerant quantum computing. This is with the caveat that the conditions for single-photon fidelity are met.
- [93] arXiv:2403.17868 (replaced) [pdf, ps, html, other]
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Title: An invitation to the sample complexity of quantum hypothesis testingComments: v3: 58 pages, 1 figure, correction to Corollary 10; see independent and concurrent work of Pensia, Jog, Loh at arXiv:2403.16981Subjects: Quantum Physics (quant-ph); Information Theory (cs.IT); Machine Learning (cs.LG); Statistics Theory (math.ST)
Quantum hypothesis testing (QHT) has been traditionally studied from the information-theoretic perspective, wherein one is interested in the optimal decay rate of error probabilities as a function of the number of samples of an unknown state. In this paper, we study the sample complexity of QHT, wherein the goal is to determine the minimum number of samples needed to reach a desired error probability. By making use of the wealth of knowledge that already exists in the literature on QHT, we characterize the sample complexity of binary QHT in the symmetric and asymmetric settings, and we provide bounds on the sample complexity of multiple QHT. In more detail, we prove that the sample complexity of symmetric binary QHT depends logarithmically on the inverse error probability and inversely on the negative logarithm of the fidelity. As a counterpart of the quantum Stein's lemma, we also find that the sample complexity of asymmetric binary QHT depends logarithmically on the inverse type II error probability and inversely on the quantum relative entropy, provided that the type II error probability is sufficiently small. We then provide lower and upper bounds on the sample complexity of multiple QHT, with it remaining an intriguing open question to improve these bounds. The final part of our paper outlines and reviews how sample complexity of QHT is relevant to a broad swathe of research areas and can enhance understanding of many fundamental concepts, including quantum algorithms for simulation and search, quantum learning and classification, and foundations of quantum mechanics. As such, we view our paper as an invitation to researchers coming from different communities to study and contribute to the problem of sample complexity of QHT, and we outline a number of open directions for future research.
- [94] arXiv:2404.02079 (replaced) [pdf, ps, html, other]
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Title: Coherent Control of an Optical Quantum Dot Using Phonons and PhotonsRyan A DeCrescent, Zixuan Wang, Joseph T Bush, Poolad Imany, Alex Kwiatkowski, Dileep V Reddy, Sae Woo Nam, Richard P Mirin, Kevin L SilvermanComments: 19 pages, 4 main figures, 7 supplementary figuresSubjects: Quantum Physics (quant-ph)
Genuine quantum-mechanical effects are readily observable in modern optomechanical systems comprising bosonic ("classical") optical resonators. Here we describe unique features and advantages of optical two-level systems, or qubits, for optomechanics. The qubit state can be coherently controlled using both phonons and resonant or detuned photons. We experimentally demonstrate this using charge-controlled InAs quantum dots (QDs) in surface-acoustic-wave resonators. Time-correlated single-photon counting measurements reveal the control of QD population dynamics using engineered optical pulses and mechanical motion. As a first example, at moderate acoustic drive strengths, we demonstrate the potential of this technique to maximize fidelity in quantum microwave-to-optical transduction. Specifically, we tailor the scheme so that mechanically assisted photon scattering is enhanced over the direct detuned photon scattering from the QD. Spectral analysis reveals distinct scattering channels related to Rayleigh scattering and luminescence in our pulsed excitation measurements which lead to time-dependent scattering spectra. Quantum-mechanical calculations show good agreement with our experimental results, together providing a comprehensive description of excitation, scattering and emission in a coupled QD-phonon optomechanical system.
- [95] arXiv:2404.07197 (replaced) [pdf, ps, other]
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Title: An indeterminacy-based ontology for quantum theoryComments: Several improvements to the previous versionSubjects: Quantum Physics (quant-ph); History and Philosophy of Physics (physics.hist-ph)
I present and defend a new ontology for quantum theories (or "interpretations" of quantum theory) called Generative Quantum Theory (GQT). GQT postulates different sets of features, and the combination of these different features can help generate different quantum theories. Furthermore, this ontology makes quantum indeterminacy and determinacy play an important explanatory role in accounting for when quantum systems whose values of their properties are indeterminate become determinate. The process via which determinate values arise varies between the different quantum theories. Moreover, quantum states represent quantum properties and structures that give rise to determinacy, and each quantum theory specifies a structure with certain features. I will focus on the following quantum theories: GRW, the Many-Worlds Interpretation, single-world relationalist theories such as Relational Quantum Mechanics, Bohmian Mechanics, hybrid classical-quantum theories, and Environmental Determinacy-based (EnD) Quantum Theory. I will argue that GQT should be taken seriously because it provides a series of important benefits that current widely discussed ontologies lack, namely, wavefunction realism and primitive ontology, without some of their costs. For instance, it helps generate quantum theories that are clearly compatible with relativistic causality, such as EnD Quantum Theory. Also, GQT has the benefit of providing new ways to compare and evaluate quantum theories, which may lead to philosophical and scientific progress on these issues.
- [96] arXiv:2404.08147 (replaced) [pdf, ps, other]
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Title: LinguaQuanta: Towards a Quantum Transpiler Between OpenQASM and Quipper (Extended)Comments: Added a new circuit decomposition reference (see fig. 22)Subjects: Quantum Physics (quant-ph); Software Engineering (cs.SE)
As quantum computing evolves, many important questions emerge, such as how best to represent quantum programs, and how to promote interoperability between quantum program analysis tools. These questions arise naturally in the design of quantum transpilers, which translate between quantum programming languages. In this paper, we take a step towards answering these questions by identifying challenges and best practices in quantum transpiler design. We base these recommendations on our experience designing LinguaQuanta, a quantum transpiler between Quipper and OpenQASM. First, we provide categorical specifications for quantum transpilers, which aim to encapsulate the core principles of the UNIX philosophy. We then identify quantum circuit decompositions which we expect to be useful in quantum transpilation. With these foundations in place, we then discuss challenges faced during the implementation of LinguaQuanta, such as ancilla management and stability under round translation. To show that LinguaQuanta works in practice, a short tutorial is given for the example of quantum phase estimation. We conclude with recommendations for the future of LinguaQuanta, and for quantum software development tools more broadly.
- [97] arXiv:2404.10738 (replaced) [pdf, ps, html, other]
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Title: Quantum Teleportation Coexisting with Conventional Classical Communications in Optical FiberJordan M. Thomas, Fei I. Yeh, Jim Hao Chen, Joe J. Mambretti, Scott J. Kohlert, Gregory S. Kanter, Prem KumarSubjects: Quantum Physics (quant-ph)
The ability for quantum and classical networks to operate in the same optical fibers would aid the deployment of quantum network technology. However, quantum performance can be susceptible to noise photons generated by spontaneous Raman scattering of high-power coexisting classical light. Quantum teleportation is a fundamental operation in quantum networking, but has yet to be demonstrated in fibers populated with high data rate conventional optical signals. In this paper, we demonstrate a three-node quantum state teleportation system coexisting with 400-Gbps C-band classical communications in 30.2 km of fiber. To protect quantum fidelity, Raman noise rates are suppressed using optimized O-band quantum channels and filtering in multiple degrees of freedom. Fidelity is shown to be well maintained with elevated classical powers as high as 18.7 dBm, which could support multiple classical channels with many terabits/s aggregate data rates. These results show the feasibility of advanced quantum and classical network applications operating within a unified fiber infrastructure.
- [98] arXiv:2404.16156 (replaced) [pdf, ps, html, other]
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Title: Guardians of the Quantum GANComments: 11 pages, 10 figuresSubjects: Quantum Physics (quant-ph); Hardware Architecture (cs.AR); Cryptography and Security (cs.CR); Machine Learning (cs.LG)
Quantum Generative Adversarial Networks (qGANs) are at the forefront of image-generating quantum machine learning models. To accommodate the growing demand for Noisy Intermediate-Scale Quantum (NISQ) devices to train and infer quantum machine learning models, the number of third-party vendors offering quantum hardware as a service is expected to rise. This expansion introduces the risk of untrusted vendors potentially stealing proprietary information from the quantum machine learning models. To address this concern we propose a novel watermarking technique that exploits the noise signature embedded during the training phase of qGANs as a non-invasive watermark. The watermark is identifiable in the images generated by the qGAN allowing us to trace the specific quantum hardware used during training hence providing strong proof of ownership. To further enhance the security robustness, we propose the training of qGANs on a sequence of multiple quantum hardware, embedding a complex watermark comprising the noise signatures of all the training hardware that is difficult for adversaries to replicate. We also develop a machine learning classifier to extract this watermark robustly, thereby identifying the training hardware (or the suite of hardware) from the images generated by the qGAN validating the authenticity of the model. We note that the watermark signature is robust against inferencing on hardware different than the hardware that was used for training. We obtain watermark extraction accuracy of 100% and ~90% for training the qGAN on individual and multiple quantum hardware setups (and inferencing on different hardware), respectively. Since parameter evolution during training is strongly modulated by quantum noise, the proposed watermark can be extended to other quantum machine learning models as well.
- [99] arXiv:2404.16245 (replaced) [pdf, ps, other]
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Title: Computationally Efficient Molecular Integrals of Solid Harmonic Gaussian Orbitals Using Quantum Entanglement of Angular MomentumSubjects: Quantum Physics (quant-ph); Chemical Physics (physics.chem-ph)
Evaluating multi-center molecular integrals with Cartesian Gaussian-type basis sets has been a long-standing bottleneck in electronic structure theory calculation for solids and molecules. We have developed a vector-coupling and vector-uncoupling scheme to solve molecular Coulomb integrals with solid harmonics basis functions(SHGO). Solid harmonics are eigenstates of angular momentum, making it possible to factorize molecular integrals. By combining solid harmonic addition, differential and product rules, the computationally costly multi-center four-center integrals can be factored into an angular part and a radial component dependent on the atomic positions. The potential speed-up ratio in evaluating molecular nuclear Coulomb integrals in our method can reach up to four orders of magnitude for atomic orbitals with high angular momentum quantum numbers. The foundation underpinning the mathematical efficiency is the quantum angular momentum theory, where both vector-coupling and vector-uncoupling schemes correspond to unitary Clebsch-Gordan transformations that act on quantum angular momentum states, influencing their degree of entanglement. By incorporating quantum angular momentum through these transformations, the entanglement of the states can be reduced, and the less entanglement there is for a quantum system, the easier it is to simulate. The highly efficient method unveiled here opens new avenues for accelerated material and molecule design and discovery.
- [100] arXiv:2404.18847 (replaced) [pdf, ps, html, other]
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Title: Cyclic measurements and simplified quantum state tomographyComments: 22 pages, 2 figuresSubjects: Quantum Physics (quant-ph); Mathematical Physics (math-ph)
Tomographic reconstruction of quantum states plays a fundamental role in benchmarking quantum systems and retrieving information from quantum computers. Among the informationally complete sets of quantum measurements the tight ones provide a linear reconstruction formula and minimize the propagation of statistical errors. However, implementing tight measurements in the lab is challenging due to the high number of required measurement projections, involving a series of experimental setup preparations. In this work, we introduce the notion of cyclic tight measurements, that allow us to perform full quantum state tomography while considering only repeated application of a single unitary-based quantum device during the measurement stage process. This type of measurements significantly simplifies the complexity of the experimental setup required to retrieve the quantum state of a physical system. Additionally, we design feasible setup preparation procedure that produce well-approximated cyclic tight measurements, in every finite dimension.
- [101] arXiv:2405.03306 (replaced) [pdf, ps, html, other]
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Title: Quantum advantage in batteries for Sachdev-Ye-Kitaev interactionsComments: 5 pages, no figures; some formulas revised and references added in v4Subjects: Quantum Physics (quant-ph)
A quantum advantage can be achieved in the unitary charging of quantum batteries if their cells are interacting. Here, we try to clarify with some analytical calculations whether and how this quantum advantage is achieved for sparse Sachdev-Ye-Kitaev (SYK) interactions. By performing a simple modelization, we find that for $q$-point rescaled sparse SYK interactions the quantum advantage goes as $\Gamma\sim N^{\frac{\alpha-q}{2}+\frac{1}{2}}$, where $\alpha$ is related to the connectivity and $N$ is the number of cells.
- [102] arXiv:2405.07246 (replaced) [pdf, ps, other]
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Title: ZX Graphical Calculus for Continuous-Variable Quantum ProcessesHironari Nagayoshi, Warit Asavanant, Ryuhoh Ide, Kosuke Fukui, Atsushi Sakaguchi, Jun-ichi Yoshikawa, Nicolas C. Menicucci, Akira FurusawaComments: 34 pages, 3 figuresSubjects: Quantum Physics (quant-ph)
Continuous-variable (CV) quantum information processing is a promising candidate for large-scale fault-tolerant quantum computation. However, analysis of CV quantum process relies mostly on direct computation of the evolution of operators in the Heisenberg picture, and the features of CV space has yet to be thoroughly investigated in an intuitive manner. One key ingredient for further exploration of CV quantum computing is the construction of a computational model that brings visual intuition and new tools for analysis. In this paper, we delve into a graphical computational model, inspired by a similar model for qubit-based systems called the ZX calculus, that enables the representation of arbitrary CV quantum process as a simple directed graph. We demonstrate the utility of our model as a graphical tool to comprehend CV processes intuitively by showing how equivalences between two distinct quantum processes can be proven as a sequence of diagrammatic transformations in certain cases. We also examine possible applications of our model, such as measurement-based quantum computing, characterization of Gaussian and non-Gaussian processes, and circuit optimization.
- [103] arXiv:2405.07833 (replaced) [pdf, ps, html, other]
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Title: Subradiance and Superradiant Long Range Excitation Transport among Quantum Emitter Ensembles in a WaveguideComments: 9 pages, 7 figuresSubjects: Quantum Physics (quant-ph)
In contrast to free space, in waveguides the dispersive and dissipative dipole-dipole interactions among quantum emitters exhibit a periodic behavior over remarkably long distances. We propose a novel setup exploiting this long-range periodicity in order to create highly excited subradiant states and facilitate fast controlled collective energy transport amongst far-apart ensembles coupled to a waveguide. For sufficiently large ensembles collective superradiant emission into the fiber modes dominates over its free space counterpart. We show that for a large number of emitters a fast transverse coherent pulse can create almost perfect subradiant states with up to $50\%$ excitation. On the other hand, for a coherent excitation of one sub-ensemble above an overall excitation fraction of $50\%$ we find a nearly lossless and fast energy transfer to the ground state sub-ensemble. This transport can be enhanced or suppressed by controlling the positions of the ensembles relative to each other, while it can also be realized with a random position distribution. In the optimally enhanced case this fast transfer appears as superradiant emission with subsequent superabsorption, yet, without a superradiant decay after the absorption. The highly excited subradiant states as well as the superradiant excitation transfer appear as suitable building blocks in applications like active atomic clocks, quantum batteries, quantum information protocols and quantum metrology procedures such as fiber-based Ramsey schemes.
- [104] arXiv:2405.07880 (replaced) [pdf, ps, html, other]
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Title: On Hagedorn wavepackets associated with different GaussiansComments: 26 pages, 2 figuresSubjects: Quantum Physics (quant-ph); Mathematical Physics (math-ph); Chemical Physics (physics.chem-ph)
Hagedorn functions are carefully constructed generalizations of Hermite functions to the setting of many-dimensional squeezed and coupled harmonic systems. Wavepackets formed by superpositions of Hagedorn functions have been successfully used to solve the time-dependent Schrödinger equation exactly in harmonic systems and variationally in anharmonic systems. For evaluating typical observables, such as position or kinetic energy, it is sufficient to consider orthonormal Hagedorn functions with a single Gaussian center. Here, we instead derive various relations between Hagedorn bases associated with different Gaussians, including their overlaps, which are necessary for evaluating quantities nonlocal in time, such as time correlation functions needed for computing spectra. First, we use the Bogoliubov transformation to obtain commutation relations between the ladder operators associated with different Gaussians. Then, instead of using numerical quadrature, we employ these commutation relations to derive exact recurrence relations for the overlap integrals between Hagedorn functions with different Gaussian centers. Finally, we present numerical experiments that demonstrate the accuracy and efficiency of our algebraic method as well as its suitability to treat problems in spectroscopy and chemical dynamics.
- [105] arXiv:2405.07970 (replaced) [pdf, ps, html, other]
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Title: How much entanglement is needed for emergent anyons and fermions?Subjects: Quantum Physics (quant-ph); Strongly Correlated Electrons (cond-mat.str-el)
It is known that particles with exotic properties can emerge in systems made of simple constituents such as qubits, due to long-range quantum entanglement. In this paper, we provide quantitative characterizations of entanglement necessary for emergent anyons and fermions by using the geometric entanglement measure (GEM) which quantifies the maximal overlap between a given state and any short-range entangled states. For systems with emergent anyons, based on the braiding statistics, we show that the GEM scales linearly in the system size regardless of microscopic details. The phenomenon of emergent anyons can also be understood within the framework of quantum error correction (QEC). Specifically, we show that the GEM of any 2D stabilizer codes must be at least quadratic in the code distance. Our proof is based on a generic prescription for constructing string operators, establishing a rigorous and direct connection between emergent anyons and QEC. For systems with emergent fermions, despite that the ground state subspaces could be exponentially huge and their coding properties could be rather poor, we show that the GEM also scales linearly in the system size. Our results also establish an intriguing link between quantum anomaly and entanglement: a quantum state respecting anomalous $1$-form symmetries, be it pure or mixed, must be long-range entangled and have large GEM, offering a non-trivial class of intrinsically mixed state phases.
- [106] arXiv:2405.08157 (replaced) [pdf, ps, html, other]
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Title: Towards a fiber-optic temporally multiplexed single photon sourceAgustina G. Magnoni, Laura T. Knoll, Lina Wölcken, Julián Defant, Julián Morales, Miguel A. LarotondaComments: 6 pages, 6 figuresSubjects: Quantum Physics (quant-ph); Optics (physics.optics)
We demonstrate the feasibility of implementing a photon source with sub-Poissonian emission statistics through temporal multiplexing of a continuous wave heralded photon source in the optical communications wavelength range. We use the time arrival information of a heralding photon to actively modify the delay of the heralded photon in an all-fiber assembly, in order to synchronize the output with with respect to an external clock. Within this synchronized operating regime we show that the addition of a single temporal correcting stage can improve the figure of merit for single photon emission of a heralded photon source. We obtain a brightness improvement factor of approximately 1.8 and an enhancement of the signal-to-noise ratio, quantified by the coincidence-to-accidental counts ratio. These results, clear the way for integrated optics non-classical photon sources in the optical communication band.
- [107] arXiv:2310.06698 (replaced) [pdf, ps, html, other]
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Title: Simulating the Transverse Field Ising Model on the Kagome Lattice using a Programmable Quantum AnnealerComments: 12 + 6 pages, 7 + 8 figuresSubjects: Statistical Mechanics (cond-mat.stat-mech); Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)
The presence of competing interactions due to geometry leads to frustration in quantum spin models. As a consequence, the ground state of such systems often displays a large degeneracy that can be lifted due to thermal or quantum effects. One such example is the antiferromagnetic Ising model on the Kagome lattice. It was shown that while the same model on the triangular lattice is ordered at zero temperature for small transverse field due to an order by disorder mechanism, the Kagome lattice resists any such effects and exhibits only short range spin correlations and a trivial paramagnetic phase. We embed this model on the latest architecture of D-Wave's quantum annealer, the Advantage2 prototype, which uses the highly connected Zephyr graph. Using advanced embedding and calibration techniques, we are able to embed a Kagome lattice with mixed open and periodic boundary conditions of 231 sites on the full graph of the currently available prototype. Through forward annealing experiments, we show that under a finite longitudinal field the system exhibits a one-third magnetization plateau, consistent with a classical spin liquid state of reduced entropy. An anneal-pause-quench protocol is then used to extract an experimental ensemble of states resulting from the equilibration of the model at finite transverse and longitudinal field. This allows us to construct a partial phase diagram and confirm that the system exits the constrained Hilbert space of the classical spin liquid when subjected to a transverse field. We connect our results to previous theoretical results and quantum Monte Carlo simulation, which helps us confirm the validity of the quantum simulation realized here, thereby extracting insight into the performance of the D-Wave quantum annealer to simulate non-trivial quantum systems in equilibrium.
- [108] arXiv:2310.17950 (replaced) [pdf, ps, html, other]
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Title: Resilient Intraparticle Entanglement and its Manifestation in Spin Dynamics of Disordered Dirac MaterialsSubjects: Mesoscale and Nanoscale Physics (cond-mat.mes-hall); Quantum Physics (quant-ph)
Topological quantum matter exhibits novel transport phenomena driven by entanglement between internal degrees of freedom, as for instance generated by spin-orbit coupling effects. Here we report on a direct connection between the mechanism driving spin relaxation and the intertwined dynamics between spin and sublattice degrees of freedom in disordered graphene. Beyond having a direct observable consequence, such intraparticle entanglement is shown to be resilient to disorder, pointing towards a novel resource for quantum information processing.
- [109] arXiv:2402.06343 (replaced) [pdf, ps, html, other]
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Title: Blackbody heat capacity at constant pressureComments: 8 pages, no figures. Version accepted for publication in JSTATSubjects: Statistical Mechanics (cond-mat.stat-mech); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
At first glance, the title of this work seems to be improper. And the reason is well known. Since blackbody pressure depends only on temperature, one cannot take the derivative of the thermodynamic quantities with respect to one of them, keeping the other constant. That is, the heat capacity at constant pressure, $C_{P}$, as well as, the coefficient of thermal expansion, $\alpha$, and the isothermal compressibility, $\kappa_{T}$, are ill-defined quantities. This work will show that when the perfect conducting nature of the walls of a blackbody cavity is taken into account, $C_{P}$, $\alpha$ and $\kappa_{T}$ are in fact well defined, and they are related by the usual thermodynamic relations, as expected. Two geometries will be considered, namely, a spherical shell and a cubic box. It will be shown that $C_{P}$, $\alpha$ and $\kappa_{T}$ depend very much on the geometry of the cavity. Issues regarding thermodynamic stability will be addressed, revealing that they also depend on the cavity's geometry. It is argued that these findings may be amenable to experimental verification.
- [110] arXiv:2402.16067 (replaced) [pdf, ps, html, other]
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Title: Log-majorization and matrix norm inequalities with application to quantum informationComments: 21 pages, Remark 2.4 is revised and references are addedSubjects: Functional Analysis (math.FA); Operator Algebras (math.OA); Quantum Physics (quant-ph)
We are concerned with log-majorization for matrices in connection with the multivariate Golden--Thompson trace inequality and the Karcher mean (i.e., a multivariate extension of the weighted geometric mean). We show an extension of Araki's log-majorization and apply it to the $\alpha$-$z$-Rényi divergence in quantum information. We discuss the equality cases in the multivariate trace inequality of Golden--Thompson type and in the norm inequality for the Karcher mean. The paper includes an appendix to correct the proof of the author's old result on the equality case in the norm inequality for the weighted geometric mean.
- [111] arXiv:2402.17627 (replaced) [pdf, ps, html, other]
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Title: Many-body perturbation theory for strongly correlated effective Hamiltonians using effective field theory methodsComments: 16 pages, 5 figures, 5 tablesSubjects: Quantum Gases (cond-mat.quant-gas); Strongly Correlated Electrons (cond-mat.str-el); Nuclear Theory (nucl-th); Quantum Physics (quant-ph)
Introducing low-energy effective Hamiltonians is usual to grasp most correlations in quantum many-body problems. For instance, such effective Hamiltonians can be treated at the mean-field level to reproduce some physical properties of interest. Employing effective Hamiltonians that contain many-body correlations renders the use of perturbative many-body techniques difficult because of the overcounting of correlations. In this work, we develop a strategy to apply an extension of the many-body perturbation theory starting from an effective interaction that contains correlations beyond the mean field level. The goal is to re-organize the many-body calculation to avoid the overcounting of correlations originating from the introduction of correlated effective Hamiltonians in the description. For this purpose, we generalize the formulation of the Rayleigh-Schrödinger perturbation theory by including free parameters adjusted to reproduce the appropriate limits. In particular, the expansion in the bare weak-coupling regime and the strong-coupling limit serves as a valuable input to fix the value of the free parameters appearing in the resulting expression. This method avoids double counting of correlations using beyond-mean-field strategies for the description of many-body systems. The ground state energy of various systems relevant for ultracold atomic, nuclear, and condensed matter physics is reproduced qualitatively beyond the domain of validity of the standard many-body perturbation theory. Finally, our method suggests interpreting the formal results obtained as an effective field theory using the proposed reorganization of the many-body calculation. The results, like ground state energies, are improved systematically by considering higher orders in the extended many-body perturbation theory while maintaining a straightforward polynomial expansion.
- [112] arXiv:2403.07056 (replaced) [pdf, ps, other]
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Title: Gravitational back-reaction is magicalComments: 62 pages, 20 figures; title changed, Theorem 1 and 2 refined, references addedSubjects: High Energy Physics - Theory (hep-th); General Relativity and Quantum Cosmology (gr-qc); Quantum Physics (quant-ph)
We study the interplay between magic and entanglement in quantum many-body systems. We show that non-local magic, which is supported by the quantum correlations is lower bounded by the non-flatness of entanglement spectrum and upper bounded by the amount of entanglement in the system. We then argue that a smoothed version of non-local magic bounds the hardness of classical simulations for incompressible states. In conformal field theories, we conjecture that the non-local magic should scale linearly with entanglement entropy but sublinearly when an approximation of the state is allowed. We support the conjectures using both analytical arguments based on unitary distillation and numerical data from an Ising CFT. If the CFT has a holographic dual, then we prove that the non-local magic vanishes if and only if there is no gravitational back-reaction. Furthermore, we show that non-local magic is approximately equal to the rate of change of the minimal surface area in response to the change of cosmic brane tension in the bulk.
- [113] arXiv:2403.07084 (replaced) [pdf, ps, html, other]
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Title: Is the effective potential, effective for dynamics?Comments: 51 pages, 5 figures, more references and discussionsJournal-ref: Phys. Rev. D 109, 105021 (2024)Subjects: High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)
We critically examine the applicability of the effective potential within dynamical situations and find, in short, that the answer is negative. An important caveat of the use of an effective potential in dynamical equations of motion is an explicit violation of energy conservation.
An \emph{adiabatic} effective potential is introduced in a consistent quasi-static approximation, and its narrow regime of validity is discussed. Two ubiquitous instances in which even the adiabatic effective potential is not valid in dynamics are studied in detail: parametric amplification in the case of oscillating mean fields, and spinodal instabilities associated with spontaneous symmetry breaking. In both cases profuse particle production is directly linked to the failure of the effective potential to describe the dynamics. We introduce a consistent, renormalized, energy conserving dynamical framework that is amenable to numerical implementation. Energy conservation leads to the emergence of asymptotic highly excited, entangled stationary states from the dynamical evolution. As a corollary, decoherence via dephasing of the density matrix in the adiabatic basis is argued to lead to an emergent entropy, formally equivalent to the entanglement entropy. The results suggest novel characterization of asymptotic equilibrium states in terms of order parameter vs. energy density. - [114] arXiv:2403.17069 (replaced) [pdf, ps, other]
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Title: Tensor network formulation of symmetry protected topological phases in mixed statesComments: Appendix D is fixedSubjects: Strongly Correlated Electrons (cond-mat.str-el); Quantum Physics (quant-ph)
We define and classify symmetry-protected topological (SPT) phases in mixed states based on the tensor network formulation of the density matrix. In one dimension, we introduce strong injective matrix product density operators (MPDO), which describe a broad class of short-range correlated mixed states, including the locally decohered SPT states. We map strong injective MPDO to a pure state in the doubled Hilbert space and define the SPT phases according to the cohomology class of the symmetry group in the doubled state. Although the doubled state exhibits an enlarged symmetry, the possible SPT phases are also constrained by the Hermiticity and the semi-positivity of the density matrix. We here obtain a complete classification of SPT phases with a direct product of strong $G$ and weak $K$ unitary symmetry given by the cohomology group $H^2(G, \text{U}(1))\oplus H^1(K, H^1(G, \text{U}(1)))$. The SPT phases in our definition are preserved under symmetric local circuits consisting of non-degenerate channels. This motivates an alternative definition of SPT phases according to the equivalence class of mixed states under a ``one-way" connection using symmetric non-degenerate channels. In locally purifiable MPDO with strong symmetry, we prove that this alternative definition reproduces the cohomology classification. We further extend our results to two-dimensional mixed states described by strong semi-injective tensor network density operators and classify the possible SPT phases.