Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session P27: General Quantum Information and Quantum ComputationFocus
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Sponsoring Units: DQI Chair: Todd Brun, University of Southern California Room: BCEC 160C |
Wednesday, March 6, 2019 2:30PM - 2:42PM |
P27.00001: Fundamental limits to quantum channel discrimination Stefano Pirandola, Riccardo Laurenza, Cosmo Lupo What is the ultimate performance for discriminating two arbitrary quantum channels acting on a finite-dimensional Hilbert space? Here we address this basic question by deriving a general and fundamental lower bound. More precisely, we investigate the symmetric discrimination of two arbitrary qudit channels by means of the most general protocols based on adaptive (feedback-assisted) quantum operations. In this general scenario, we first show how port-based teleportation can be used to completely simplify these adaptive protocols into a much simpler non-adaptive form, designing a new form of teleportation stretching. Then, we prove that the minimum error probability affecting the channel discrimination cannot beat a bound determined by the Choi matrices of the channels, establishing an ultimate and elegant formula for quantum hypothesis testing. As a consequence of this bound, we derive the ultimate limits for adaptive quantum illumination and single-photon quantum optical resolution. Finally, we show how the methodology can also be applied to other tasks, such as quantum metrology, quantum communication and secret key generation. |
Wednesday, March 6, 2019 2:42PM - 2:54PM |
P27.00002: Quantum Dimension Witness and Assisted Quantum State Discrimination Uman Khalid, Youngmin Jeong, Hyundong Shin The relevance between quantum information processing tasks such as witnessing the dimension of an unknown quantum system and quantum state discrimination is indebted to the statistics obtained from the quantum measurement. A quantum dimension witness (QDW) provides a lower bound on the minimum dimensions that are necessary to describe the quantum correlation originated from the measurements on an unknown quantum system. Such measurement-based quantum correlations (MbQCs) are not only observer dependent but also depend on how strongly an observer perturbs the unobserved system. Based on the aforementioned measurement scenario, we present a quantitative measure that is both the QDW and MbQC measure for arbitrary dimensional bipartite quantum systems. The quantitative measure serves well for bipartite mixed quantum systems and vanishes only for a non-quantum system. We also show that MbQCs are more general than traditional quantum correlations in an optimal assisted quantum state discrimination task. |
Wednesday, March 6, 2019 2:54PM - 3:06PM |
P27.00003: Fidelity estimation via local measurements in the presence of arbitrarily correlated noise Liangzhong Ruan, Stephanie Wehner Fidelity estimation for entangled states shared by remote nodes is an essential building block for the quality control in quantum networks. In the literature, the issue of minimizing the efficiency loss in fidelity estimation due to the limitation of local operations and the issue of characterizing the accuracy of fidelity estimation protocols in the presence of arbitrary noise remain interesting challenges. This work addresses these two challenges, designs a protocol for estimating the average fidelity of qubit pairs shared by remote agents, whose efficiency reaches the fundamental upper bound achievable with local operations, and characterizes the performance of the proposed protocol in the presence of arbitrary noise. Analysis has been performed to characterize which factors affect the estimation accuracy and how. Numerical tests have been performed to check the preciseness and robustness of the proposed protocol, as well as to characterize the proper parameter setting. The analysis and numerical tests have identified the distinctive factors that affect the estimation accuracy in the presence of arbitrary noise, and have given clear instructions on how to properly address these factors. |
Wednesday, March 6, 2019 3:06PM - 3:18PM |
P27.00004: Effective Hamiltonian theory of the geometric evolution of quantum systems Vladyslav Shkolnykov, Guido Burkard We present an effective Hamiltonian description of the quantum dynamics of a generalized Lambda system undergoing adiabatic evolution [1]. We assume the system to be initialized in the dark subspace and show that its holonomic evolution can be viewed as a conventional Hamiltonian dynamics in an appropriately chosen extended Hilbert space. In contrast to the existing approaches, our method does not require the calculation of the non-Abelian Berry connection and can be applied without any parametrization of the dark subspace, which becomes a challenging problem with increasing system size. |
Wednesday, March 6, 2019 3:18PM - 3:30PM |
P27.00005: Dense Measurements for Gate-Model Quantum Computers Laszlo Gyongyosi, Sandor Imre The measurement procedure is a fundamental cornerstone of gate-model quantum computers. The main issues in current quantum measurement strategies are the high number of measurement rounds to determine a global optimal measurement output and the low success probability of finding a global optimal measurement output. Each measurement round requires preparing the quantum system and applying quantum operations and measurements with high-precision control in the physical layer. These issues result in high-cost measurements with a low probability of success. Here, we define a novel measurement for gate-model quantum computers called dense quantum measurement. The dense measurement strategy aims at fixing the main drawbacks of standard quantum measurements by achieving a significant reduction in the number of necessary measurement rounds and by radically improving the success probabilities of finding global optimal outputs. We provide application scenarios for quantum circuits with arbitrary unitary sequences. The dense measurement also provides an experimentally implementable solution for practical quantum computations. |
Wednesday, March 6, 2019 3:30PM - 3:42PM |
P27.00006: Why the Quantum William Stuckey To answer Wheeler's question ``Why the quantum?'' via quantum information theory per Bub, one must explain why the world is quantum rather than classical and why the world is quantum rather than superquantum, i.e., ``Why the Tsirelson bound?'' We show that the quantum correlations resulting from two Bell basis states, which uniquely produce the Tsirelson bound for the Clauser-Horne-Shimony-Holt quantity, can be derived from the conservation of angular momentum (on average) for the quantum exchange of momentum. This explanation of the Tsirelson bound does not require hidden variables or `causal influences'. Since superquantum correlations exceed quantum correlations, we know that they would violate conservation of angular momentum and we show how this happens using the Popescu-Rohrlich correlations. Thus, quantum correlations responsible for the Tsirelson bound satisfy conservation of angular momentum for the quantum exchange of momentum while both classical and superquantum correlations can fail to satisfy this constraint. We generalize the result to conservation per any measurement associated with a Bell basis state. While this constraint is not surprising per se, the details on how it obtains evidence a deeper principle at work in Nature, i.e., no preferred reference frame. |
Wednesday, March 6, 2019 3:42PM - 3:54PM |
P27.00007: Wavefunction; The Guided Energy of Wave Desmond Agbolade Ademola This paper expound the true nature of wavefunction and answered questions that arise from quantum foundation such as what is nature of wavefunction? Is mathematical formalism of wavefunction correct? Does wavefunction collapse during measurement? Do quantum physical entanglement and many world interpretations really exist? In addition, is there uncertainty in the physical reality of our nature as being concluded in the quantum foundation? The fundamental analysis presented in this work show that mathematical formalism of wavefunction was wrongly formulated. Because, we discovered that, wavefunction is the guided energy of wave by the reason that the universe and everything in it from large particle to the smallest tiny particle, physical and imaginary, visible and invisible, existing and non-existing has guided energy which is wavefunction.We further show that wavefunction is not collapse, only disengaged and engaged gradually. The gradual process of wavefunction disengagement display how the particle-like behavior return to wave-like behavior and the gradual process of wavefunction engagement also display how wave-like behavior return to the particle-like behavior. Convergence and divergence of wavefunction exhibit precise position of a particle and wave amplitude respectively. |
Wednesday, March 6, 2019 3:54PM - 4:06PM |
P27.00008: Quaternion Series Spin Douglas Sweetser
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Wednesday, March 6, 2019 4:06PM - 4:18PM |
P27.00009: ABSTRACT WITHDRAWN
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Wednesday, March 6, 2019 4:18PM - 4:30PM |
P27.00010: Entropic Energy-Time Uncertainty Relation Patrick J Coles, Vishal Katariya, Seth Lloyd, Iman Marvian, Mark M Wilde Energy-time uncertainty plays an important role in quantum foundations and technologies, and it was even discussed by the founders of quantum mechanics. However, standard approaches (e.g., Robertson's uncertainty relation) do not apply to energy-time uncertainty because, in general, there is no Hermitian operator associated with time. Following previous approaches, we quantify time uncertainty by how well one can read off the time from a quantum clock. We then use entropy to quantify the information-theoretic distinguishability of the various time states of the clock. Our main result is an entropic energy-time uncertainty relation for general time-independent Hamiltonians, stated for both the discrete-time and continuous-time cases. Our uncertainty relation is strong, in the sense that it allows for a quantum memory to help reduce the uncertainty, and this formulation leads us to reinterpret it as a bound on the relative entropy of asymmetry. Due to the operational relevance of entropy, we anticipate that our uncertainty relation will have information-processing applications. |
Wednesday, March 6, 2019 4:30PM - 4:42PM |
P27.00011: Duality and free energy analyticity bounds for few-body Ising models with extensive homology rank Yi Jiang, Ilya Dumer, Alexey Kovalev, Leonid Pryadko We consider pairs of few-body Ising models such that each model can be obtained from the dual of the other after freezing k spins on large-degree sites. Such a pair of Ising models can be interpreted as a two-chain complex with k being the rank of the first homology group. Our focus is on the case where k is extensive. In the presence of bond disorder, we prove the existence of a low-T weak-disorder region where additional summation over the defects has no effect on the free energy density f(T) in the thermodynamical limit, and of a high-T region where in the ferromagnetic case an extensive homological defect does not affect f(T). We also discuss the convergence of the high- and low-temperature series for the free energy density, prove the analyticity of limiting f(T) at high and low temperatures, and construct inequalities for the critical point(s) where analyticity is lost. As an application, we prove multiplicity of the conventionally defined critical points for Ising models on all {f, d} tilings of the infinite hyperbolic plane. For these infinite graphs, we show that critical temperatures with free and wired boundary conditions differ, T(f)c < T(w)c . |
Wednesday, March 6, 2019 4:42PM - 4:54PM |
P27.00012: Complexity phase transition in interacting and long-range bosonic Hamiltonians Nishad Maskara, Abhinav Deshpande, Cong Minh Tran, Bill Fefferman, Michael Foss-Feig, Alexey V Gorshkov We investigate the complexity of sampling from time-evolved states due to bosonic Hamiltonians. We obtain timescales for which approximate sampling is easy and hard, generalising the results in Ref. [1] to systems with interacting bosons and to systems with long-range couplings. The easiness results rely on recent developments in the simulation of spatially local Hamiltonians [2,3]. For free bosons with long-range hops, where the strength of the hopping term decays with distance as a power law, we observe the hardness setting in earlier than the nearest-neighbor case. We also obtain hardness results for interacting bosons. Along the way, we develop methods and tools that are of independent interest. Our work maps out the timescale between easiness and hardness of sampling on a "complexity phase diagram", giving a testbed for exploring physical manifestations of the computational complexity of simulating interacting quantum systems. |
Wednesday, March 6, 2019 4:54PM - 5:06PM |
P27.00013: Unitary designs for continuous variable systems Thomas Schuster, Quntao Zhuang, Beni Yoshida, Norman Yao The study of information scrambling in many-body systems has sharpened our understanding of quantum chaos. In discrete variable (DV) systems (finite-dimensional, e.g. spins), the scrambling ‘strength’ of a unitary is often measured by its closeness to a Haar random unitary. This leads to a hierarchy of increasingly fine-grained measures of scrambling known as ‘unitary k-designs’. Here, we extend the notion of unitary designs to continuous variable (CV) systems (infinite-dimensional, e.g. photons). Although there is no generalization of Haar randomness to CV systems, we show that averages of physical quantities over Haar random unitaries remain well-defined in the CV limit, and use this to define CV unitary designs. Surprisingly, Gaussian unitaries, despite being non-interacting, form a CV 2-design and can therefore `quasi-scramble' information. Extending further, we show that unitary 4-designs maximize the phase space volume of generic time-evolved operators. |
Wednesday, March 6, 2019 5:06PM - 5:18PM |
P27.00014: Scrambling and complexity in phase space Quntao Zhuang, Thomas Schuster, Beni Yoshida, Norman Yao In this talk, we will describe extensions of the study of scrambling and complexity to infinite-dimensional continuous variable (CV) systems. Unlike their discrete variable (DV) cousins, continuous variable systems exhibit two complementary domains of information scrambling: 1) scrambling in the phase space of a single mode and 2) scrambling across multiple modes. Moreover, for each of these domains, we identify two distinct "types" of scrambling; strict scrambling, where an initial operator localized in phase space spreads out and quasi-scrambling, where a local ensemble of operators distorts but the overall phase space volume remains fixed. To characterize these behaviors, we introduce a CV out-of-time-order correlator (OTOC) based upon displacement operators, which can be experimentally measured. By studying operator spreading and entanglement formation in a random local Gaussian circuit ensemble, we infer the dynamics of generic, chaotic, locally-interacting systems.Our work opens the door to experimentally probing phase space scrambling in CV systems, including cavity QED and quantum optics architectures. |
Wednesday, March 6, 2019 5:18PM - 5:30PM |
P27.00015: Characterizing the performance of continuous-variable Gaussian quantum gates Kunal Sharma, Mark M Wilde The required set of operations for universal continuous-variable quantum computation can be primarily be divided into two categories: Gaussian and non-Gaussian operations. Furthermore, any Gaussian operation can be decomposed as a sequence of phase-space displacements and symplectic transformations. Although Gaussian operations are ubiquitous in quantum optics, their experimental realizations are generally approximations of the ideal Gaussian unitaries. In this work, we study different performance metrics to analyze how well these experimental approximations simulate the ideal Gaussian operations. In particular, we find that none of these experimental approximations converge uniformly to the ideal Gaussian processes. However, convergence occurs in the strong sense, or if the discrimination strategy is energy bounded, then the convergence is uniform. We indicate how these energy-constrained bounds could be used for experimental implementations of these Gaussian operations in order to achieve any desired accuracy. |
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