Physics

Incompatibility measures in multiparameter quantum estimation under hierarchical quantum measurements

PRA: Quantum information - Fri, 2022-06-24 12:00

Author(s): Hongzhen Chen, Yu Chen, and Haidong Yuan

The incompatibility of the measurements constrains the achievable precisions in multiparameter quantum estimation. Understanding the tradeoff induced by such incompatibility is a central topic in quantum metrology. Here we provide an approach to study the incompatibility under general p-local measur…

[Phys. Rev. A 105, 062442] Published Fri Jun 24, 2022

Categories: Journals, Physics

Robust optimization for quantum reinforcement learning control using partial observations

PRA: Quantum information - Fri, 2022-06-24 12:00

Author(s): Chen Jiang, Yu Pan, Zheng-Guang Wu, Qing Gao, and Daoyi Dong

The current quantum reinforcement learning control models often assume that the quantum states are known a priori for control optimization. However, full observation of the quantum state is experimentally infeasible due to the exponential scaling of the number of required quantum measurements on the…

[Phys. Rev. A 105, 062443] Published Fri Jun 24, 2022

Categories: Journals, Physics

Quantum signal processing for simulating cold plasma waves

PRA: Quantum information - Fri, 2022-06-24 12:00

Author(s): I. Novikau, E. A. Startsev, and I. Y. Dodin

Numerical modeling of radio-frequency waves in plasma with sufficiently high spatial and temporal resolution remains challenging even with modern computers. However, such simulations can be sped up using quantum computers in the future. Here, we propose how to do such modeling for cold plasma waves,…

[Phys. Rev. A 105, 062444] Published Fri Jun 24, 2022

Categories: Journals, Physics

Quantum state transfer between a frequency-encoded photonic qubit and a quantum-dot spin in a nanophotonic waveguide

PRA: Quantum information - Fri, 2022-06-24 12:00

Author(s): Ming Lai Chan, Ziv Aqua, Alexey Tiranov, Barak Dayan, Peter Lodahl, and Anders S. Sørensen

We propose a deterministic yet fully passive scheme to transfer the quantum state from a frequency-encoded photon to the spin of a quantum dot mediated by a nanophotonic waveguide. We assess the quality of the state transfer by studying the effects of all relevant experimental imperfections on the s…

[Phys. Rev. A 105, 062445] Published Fri Jun 24, 2022

Categories: Journals, Physics

Temperature-Induced Disorder-Free Localization. (arXiv:2206.11273v1 [cond-mat.dis-nn])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

Disorder-free localization is a paradigm of strong ergodicity breaking that has been shown to occur in global quenches of lattice gauge theories when the system is initialized in a superposition over an extensive number of gauge sectors. Here, we show that preparing the system in a thermal Gibbs ensemble without any coherences between different gauge sectors also gives rise to disorder-free localization, with temperature acting as a disorder strength. We demonstrate our findings by calculating the quench dynamics of the imbalance of thermal ensembles in both $\mathrm{U}(1)$ and $\mathbb{Z}_2$ lattice gauge theories through exact diagonalization, showing greater localization with increasing ensemble temperature. Furthermore, we show how adding terms linear in local pseudogenerators can enhance temperature-induced disorder-free localization due to the dynamical emergence of an enriched local symmetry. Our work expands the realm of disorder-free localization into finite-temperature physics, and shows counterintuitively that certain quantum nonergodic phenomena can become more prominent at high temperature. We discuss the accessibility of our conclusions in current quantum simulation and computing platforms.

Categories: Journals, Physics

Topological classification of Higher-order topological phases with nested band inversion surfaces. (arXiv:2206.11296v1 [cond-mat.mes-hall])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

Higher-order topological phases (HOTPs) hold gapped bulk bands and topological boundary states localized in boundaries with codimension higher than one. In this work, we provide a unified construction and topological characterization of HOTPs for the full Altland-Zirnbauer tenfold symmetry classes, based on a method known as the nested band inversion surfaces (BISs). Specifically, HOTPs built on this method are decomposed into a series of subsystems, and higher-order topological boundary states emerges from the interplay of their 1st-order topology. Our analysis begins with a general discussion of HOTPs in continuous Hamiltonians for each symmetry class, then moves on to several lattice examples illustrating the topological characterization based on the nested-BIS method. Despite that the example minimal models possess several spatial symmetries, our method does not rely on any spatial symmetry, and can be easily extended into arbitrary orders of topology in dimensions. Furthermore, we extend our discussion to systems with asymmetric boundary states induced by two different mechanisms, namely crossed BISs that break a $\mathcal{C}_4$ rotation symmetry, and non-Clifford operators that break certain chiral-mirror symmetries of the minimal models.

Categories: Journals, Physics

Perturbation theory under the truncated Wigner approximation reveals how system-environment entanglement formation drives quantum decoherence. (arXiv:2206.11306v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

Quantum decoherence is the disappearance of simple phase relations within a discrete quantum system as a result of interactions with an environment. For many applications, the question is not necessarily how to avoid (inevitable) system-environment interactions, but rather how to design environments that optimally preserve a system's phase relations in spite of such interactions. The formation of system-environment entanglement is a major driving mechanism for decoherence, and a detailed understanding of this process could inform strategies for conserving coherence optimally. This requires scalable, flexible, and systematically improvable quantum dynamical methods that retain detailed information about the entanglement properties of the environment, yet very few current methods offer this combination of features. Here, we address this need by introducing a theoretical framework wherein we combine the truncated Wigner approximation with standard time-dependent perturbation theory allowing for computing expectation values of operators in the combined system-environment Hilbert space. We demonstrate the utility of this framework by applying it to the spin-boson model, representative of qubits and simple donor-acceptor systems. For this model, our framework provides an analytical description of perturbative contributions to expectation values. We monitor how quantum decoherence at zero temperature is accompanied by entanglement formation with individual environmental degrees of freedom. Based on this entanglement behavior, we find that the selective suppression of low-frequency environmental modes is particularly effective for mitigating quantum decoherence.

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Modular architectures to deterministically generate graph states. (arXiv:2206.11307v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

Graph states are a family of stabilizer states which can be tailored towards various applications in photonic quantum computing and quantum communication. In this paper, we present a modular design based on quantum dot emitters coupled to a waveguide and optical fiber delay lines to deterministically generate N-dimensional cluster states and other useful graph states such as tree states and repeater states. Unlike previous proposals, our design requires no two-qubit gates on quantum dots and at most one optical switch, thereby, minimizing challenges usually posed by these requirements. Furthermore, we discuss the error model for our design and demonstrate a fault-tolerant quantum memory with an error threshold of 0.53% in the case of a 3d graph state on a Raussendorf-Harrington-Goyal (RHG) lattice. We also provide a fundamental upper bound on the correctable loss in the fault-tolerant RHG state based on the percolation theory, which is 1.24 dB or 0.24 dB depending on whether the state is directly generated or obtained from a simple cubic cluster state, respectively.

Categories: Journals, Physics

Number-conserving solution for dynamical quantum backreaction in a Bose-Einstein condensate. (arXiv:2206.11317v1 [cond-mat.quant-gas])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

We provide a number-conserving approach to the backreaction problem of small quantum fluctuations onto a classical background for the exactly soluble dynamical evolution of a Bose-Einstein condensate, experimentally realizable in the ultracold gas laboratory. A force density exerted on the gas particles which is of quantum origin is uniquely identified as the deviation from the classical Eulerian force density. The backreaction equations are then explored for the specific example of a finite size uniform density condensate initially at rest. By assuming that the condensate starts from a non-interacting regime, and in its ground state, we fix a well-defined initial vacuum condition, which is driven out-of-equilibrium by instantaneously turning on the interactions. The assumption of this initial vacuum accounts for the ambiguity in choosing a vacuum state for interacting condensates, which is due to phase diffusion and the ensuing condensate collapse. As a major finding, we reveal that the time evolution of the condensate cloud leads to condensate density corrections that cannot in general be disentangled from the quantum depletion in measurements probing the power spectrum of the total density. Furthermore, while the condensate is initially at rest, quantum fluctuations give rise to a nontrivial condensate flux, from which we demonstrate that the quantum force density attenuates the classical Eulerian force. Finally, the knowledge of the particle density as a function of time for a condensate at rest determines, to order $N^0$, where $N$ is the total number of particles, the quantum force density, thus offering a viable route for obtaining experimentally accessible quantum backreaction effects.

Categories: Journals, Physics

Informationally complete measures of quantum entanglement. (arXiv:2206.11336v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

Although quantum entanglement has already been verified experimentally and applied in quantum computing, quantum sensing and quantum networks, most of the existing measures cannot characterize the entanglement faithfully. In this work, by exploiting the Schmidt decomposition of a bipartite state $|\psi\rangle_{AB}$, we first establish a one-to-one correspondence relation between the characteristic polynomial of the reduced state $\rho_A$ and the polynomials its trace. Then we introduce a family of entanglement measures which are given by the complete eigenvalues of the reduced density matrices of the system. Specific measures called informationally complete entanglement measures (ICEMs) are presented to illustrate the advantages. It is demonstrated that such ICEMs can characterize finer and distinguish better the entanglement than existing well-known entanglement measures. They also give rise to criteria of state transformations under local operation and classical communication. Moreover, we show that the ICEMs can be efficiently estimated on a quantum computer. The fully separability, entanglement and genuine multipartite entanglement can detected faithfully on quantum devices.

Categories: Journals, Physics

Quantifying NV-center Spectral Diffusion by Symmetry. (arXiv:2206.11362v1 [cond-mat.mes-hall])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

The spectrally narrow, spin-dependent optical transitions of nitrogen vacancy (NV) center defects in diamond can be harnessed for quantum networking applications. Key to such networking schemes is the generation of indistinguishable photons. Two challenges limit scalability in such systems: defect-to-defect variations of the optical transition frequencies caused by local strain variation, and spectral diffusion of the optical frequencies on repeated measurement caused by photoexcitation of nearby charge traps. In this experimental study we undertake a group theoretic approach to quantifying spectral diffusion and strain, decomposing each into components corresponding to Jahn-Teller symmetries of the NV center. We investigate correlations between the components of strain, spectral diffusion, and depth from surface, finding that strain and spectral diffusion are each dominated by longitudinal perturbations. We also find a weak negative correlation between transverse static strain and total spectral diffusion suggesting that transverse strain provides some degree of protection from spectral diffusion. Additionally, we find that spectral diffusion becomes more pronounced with increasing depth in the diamond bulk. Our symmetry-decomposed technique for quantifying spectral diffusion can be valuable for understanding how a given nanoscale charge trap environment influences spectral diffusion and for developing strategies of mitigation.

Categories: Journals, Physics

Quantum metrology timing limits of the Hong-Ou-Mandel interferometer and of general two-photon measurements. (arXiv:2206.11387v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

We examine the precision limits of Hong-Ou-Mandel (HOM) timing measurements, as well as precision limits applying to generalized two-photon measurements. As a special case, we consider the use of two-photon measurements using photons with variable bandwidths and frequency correlations. When the photon bandwidths are not equal, maximizing the measurement precision involves a trade-off between high interference visibility and strong frequency anticorrelations, with the optimal precision occuring when the photons share non-maximal frequency anticorrelations. We show that a generalized measurement has precision limits that are qualitatively similar to those of the HOM measurement whenever the generalized measurement is insensitive to the net delay of both photons. By examining the performance of states with more general frequency distributions, our analysis allows for engineering of the joint spectral amplitude for use in realistic situations, in which both photons may not have ideal spectral properties.

Categories: Journals, Physics

Optimal linear cyclic quantum heat engines cannot benefit from strong coupling. (arXiv:2206.11453v1 [cond-mat.stat-mech])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

Uncovering whether strong system-bath coupling can be an advantageous operation resource for energy conversion can facilitate the development of efficient quantum heat engines (QHEs). Yet, a consensus on this ongoing debate is still lacking owing to challenges arising from treating strong couplings. Here we conclude the debate for optimal linear cyclic QHEs operated under a small temperature difference by revealing the detrimental role of strong system-bath coupling in their optimal operations. We analytically demonstrate that both the efficiency at maximum power and maximum efficiency of strong-coupling linear cyclic QHEs are upper bounded by their weak-coupling counterparts and, particularly, experience a quadratic suppression relative to the Carnot limit under strong time-reversal symmetry breaking.

Categories: Journals, Physics

Delocalized and Dynamical Catalytic Randomness and Information Flow. (arXiv:2206.11469v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

We generalize the theory of catalytic quantum randomness to delocalized and dynamical settings. First, we expand the resource theory of randomness (RTR) by calculating the amount of entropy catalytically extractable from a correlated or dynamical randomness source. In doing so, we show that no entropy can be catalytically extracted when one cannot implement local projective measurement on randomness source without altering its state. The RTR, as an archetype of the `concave' resource theory, is complementary to the convex resource theories in which the amount of randomness required to erase the resource is a resource measure. As an application, we prove that quantum operation cannot be hidden in correlation between two parties without using randomness, which is the dynamical generalization of the no-hiding theorem. Second, we study the physical properties of information flow. Popularized quotes like "information is physical" or "it from bit" suggest the matter-like picture of information that can travel with the definite direction while leaving detectable traces on its region of departure. To examine the validity of this picture, we focus on that catalysis of randomness models directional flow of information with the distinguished source and recipient. We show that classical information can always spread from its source without altering its source or its surrounding context, like an immaterial entity, while quantum information cannot. We suggest an approach to formal definition of semantic quantum information and claim that utilizing semantic information is equivalent to using a partially depleted information source. By doing so, we unify the utilization of semantic and non-semantic quantum information and conclude that one can always extract more information from an incompletely depleted classical randomness source, but it is not possible for quantum randomness sources.

Categories: Journals, Physics

Quantum Approximation of Normalized Schatten Norms and Applications to Learning. (arXiv:2206.11506v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

Efficient measures to determine similarity of quantum states, such as the fidelity metric, have been widely studied. In this paper, we address the problem of defining a similarity measure for quantum operations that can be \textit{efficiently estimated}. Given two quantum operations, $U_1$ and $U_2$, represented in their circuit forms, we first develop a quantum sampling circuit to estimate the normalized Schatten 2-norm of their difference ($\| U_1-U_2 \|_{S_2}$) with precision $\epsilon$, using only one clean qubit and one classical random variable. We prove a Poly$(\frac{1}{\epsilon})$ upper bound on the sample complexity, which is independent of the size of the quantum system. We then show that such a similarity metric is directly related to a functional definition of similarity of unitary operations using the conventional fidelity metric of quantum states ($F$): If $\| U_1-U_2 \|_{S_2}$ is sufficiently small (e.g. $\leq \frac{\epsilon}{1+\sqrt{2(1/\delta - 1)}}$) then the fidelity of states obtained by processing the same randomly and uniformly picked pure state, $|\psi \rangle$, is as high as needed ($F({U}_1 |\psi \rangle, {U}_2 |\psi \rangle)\geq 1-\epsilon$) with probability exceeding $1-\delta$. We provide example applications of this efficient similarity metric estimation framework to quantum circuit learning tasks, such as finding the square root of a given unitary operation.

Categories: Journals, Physics

On Classifying Images using Quantum Image Representation. (arXiv:2206.11509v1 [quant-ph])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

In this paper, we consider different Quantum Image Representation Methods to encode images into quantum states and then use a Quantum Machine Learning pipeline to classify the images. We provide encouraging results on classifying benchmark datasets of grayscale and colour images using two different classifiers. We also test multi-class classification performance.

Categories: Journals, Physics

Quantum Many-Body Scars: A Quasiparticle Perspective. (arXiv:2206.11528v1 [cond-mat.str-el])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

Weakly interacting quasiparticles play a central role in the low-energy description of many phases of quantum matter. At higher energies, however, quasiparticles cease to be well-defined in generic many-body systems due to a proliferation of decay channels. In this review, we discuss the phenomenon of quantum many-body scars, which can give rise to certain species of stable quasiparticles throughout the energy spectrum. This goes along with a set of unusual non-equilibrium phenomena including many-body revivals and non-thermal stationary states. We provide a pedagogical exposition of this physics via a simple yet comprehensive example, that of a spin-1 XY model. We place our discussion in the broader context of symmetry-based constructions of many-body scar states, projector embeddings, and Hilbert space fragmentation. We conclude with a summary of experimental progress and theoretical puzzles.

Categories: Journals, Physics

Exact bounds on the energy gap of transverse-field Ising chains by mapping to random walks. (arXiv:2206.11575v1 [cond-mat.stat-mech])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

Based on a relationship with continuous-time random walks discovered by Igl\'oi, Turban, and Rieger, we derive exact lower and upper bounds on the lowest energy gap of open transverse-field Ising chains, which are explicit in the parameters and are generally valid for arbitrary sets of possibly random couplings and fields. In the homogeneous chain and in the random chain with uncorrelated parameters, both the lower and upper bounds are found to show the same finite-size scaling in the ferromagnetic phase and at the critical point, demonstrating the ability of these bounds to infer the correct finite-size scaling of the critical gap. Applying the bounds to random transverse-field Ising chains with coupling-field correlations, a model which is relevant for adiabatic quantum computing, the finite-size scaling of the gap is shown to be related to that of sums of independent random variables. We determine the critical dynamical exponent of the model and reveal the existence of logarithmic corrections at special points.

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Metamorphic InAs/InGaAs QWs with electron mobilities exceeding $7\times10^5cm^2/Vs$. (arXiv:2206.11590v1 [cond-mat.mtrl-sci])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

We present a study on the influence of strain-relieving InAlAs buffer layers on metamorphic InAs/InGaAs quantum wells grown by molecular beam epitaxy on GaAs. Residual strain in the buffer layer, the InGaAs barrier and the InAs wells were assessed by X-ray diffraction and high-resolution transmission electron microscopy. By carefully choosing the composition profile and thicknesses of the buffer layer, virtually unstrained InGaAs barriers embedding an InAs quantum well with thickness up to 7nm can be grown. This allows reaching low-temperature electron mobilities much higher than previously reported for samples obtained by metamorphic growth on GaAs, and comparable to the values achieved for samples grown on InP substrates.

Categories: Journals, Physics

Wetting critical behavior within the Lindblad dissipative dynamics. (arXiv:2206.11662v1 [cond-mat.stat-mech])

arXiv.org: Quantum Physics - Fri, 2022-06-24 06:45

We investigate the critical behavior, both in space and time, of the wetting interface within the coexistence region around the first-order phase transition of a fully-connected quantum Ising model in a slab geometry. For that, we employ the Lindblad master equation in which temperature is inherited by the coupling to a dissipative bath rather than being a functional parameter as in the conventional Cahn's free energy. Lindblad's approach gives not only access to the dissipative dynamics and steady-state configuration of the wetting interface throughout the whole phase diagram but also shows that the wetting critical behavior can be successfully exploited to characterize the phase diagram as an alternative to the direct evaluation of the free energies of the competing phases.

Categories: Journals, Physics