Quantum Sciences

  • Bell nonlocality with a single shot
    Quantum 4, 353 (2020). https://doi.org/10.22331/q-2020-10-28-353 In order to reject the local hidden variables hypothesis, the usefulness of a Bell inequality can be quantified by how small a $p$-value it will give for a physical experiment. Here we show that to obtain a small expected $p$-value it is sufficient to have a large gap between the … Read More
  • Optimization of the surface code design for Majorana-based qubits
    Quantum 4, 352 (2020). https://doi.org/10.22331/q-2020-10-28-352 The surface code is a prominent topological error-correcting code exhibiting high fault-tolerance accuracy thresholds. Conventional schemes for error correction with the surface code place qubits on a planar grid and assume native CNOT gates between the data qubits with nearest-neighbor ancilla qubits. Here, we present surface code error-correction schemes using … Read More
  • Classical Simulations of Quantum Field Theory in Curved Spacetime I: Fermionic Hawking-Hartle Vacua from a Staggered Lattice Scheme
    Quantum 4, 351 (2020). https://doi.org/10.22331/q-2020-10-28-351 We numerically compute renormalized expectation values of quadratic operators in a quantum field theory (QFT) of free Dirac fermions in curved two-dimensional (Lorentzian) spacetime. First, we use a staggered-fermion discretization to generate a sequence of lattice theories yielding the desired QFT in the continuum limit. Numerically-computed lattice correlators are then … Read More
  • Emergence of the Born rule in quantum optics
    Quantum 4, 350 (2020). https://doi.org/10.22331/q-2020-10-26-350 The Born rule provides a fundamental connection between theory and observation in quantum mechanics, yet its origin remains a mystery. We consider this problem within the context of quantum optics using only classical physics and the assumption of a quantum electrodynamic vacuum that is real rather than virtual. The connection … Read More
  • A three-player coherent state embezzlement game
    Quantum 4, 349 (2020). https://doi.org/10.22331/q-2020-10-26-349 We introduce a three-player nonlocal game, with a finite number of classical questions and answers, such that the optimal success probability of $1$ in the game can only be achieved in the limit of strategies using arbitrarily high-dimensional entangled states. Precisely, there exists a constant $0 <cleq 1$ such that … Read More
  • Transforming graph states to Bell-pairs is NP-Complete
    Quantum 4, 348 (2020). https://doi.org/10.22331/q-2020-10-22-348 Critical to the construction of large scale quantum networks, i.e. a quantum internet, is the development of fast algorithms for managing entanglement present in the network. One fundamental building block for a quantum internet is the distribution of Bell pairs between distant nodes in the network. Here we focus on … Read More
  • Environmentally Induced Entanglement – Anomalous Behavior in the Adiabatic Regime
    Quantum 4, 347 (2020). https://doi.org/10.22331/q-2020-10-22-347 Considering two non-interacting qubits in the context of open quantum systems, it is well known that their common environment may act as an entangling agent. In a perturbative regime the influence of the environment on the system dynamics can effectively be described by a unitary and a dissipative contribution. For … Read More
  • A generalization of CHSH and the algebraic structure of optimal strategies
    Quantum 4, 346 (2020). https://doi.org/10.22331/q-2020-10-21-346 $textit{Self-testing}$ has been a rich area of study in quantum information theory. It allows an experimenter to interact classically with a black box quantum system and to test that a specific entangled state was present and a specific set of measurements were performed. Recently, self-testing has been central to high-profile … Read More
  • Quantum prescriptions are more ontologically distinct than they are operationally distinguishable
    Quantum 4, 345 (2020). https://doi.org/10.22331/q-2020-10-21-345 Based on an intuitive generalization of the Leibniz principle of `the identity of indiscernibles', we introduce a novel ontological notion of classicality, called bounded ontological distinctness. Formulated as a principle, bounded ontological distinctness equates the distinguishability of a set of operational physical entities to the distinctness of their ontological counterparts. … Read More
  • Bounding sets of sequential quantum correlations and device-independent randomness certification
    Quantum 4, 344 (2020). https://doi.org/10.22331/q-2020-10-19-344 An important problem in quantum information theory is that of bounding sets of correlations that arise from making local measurements on entangled states of arbitrary dimension. Currently, the best-known method to tackle this problem is the NPA hierarchy; an infinite sequence of semidefinite programs that provides increasingly tighter outer approximations … Read More
  • Probing nonclassicality with matrices of phase-space distributions
    Quantum 4, 343 (2020). https://doi.org/10.22331/q-2020-10-15-343 We devise a method to certify nonclassical features via correlations of phase-space distributions by unifying the notions of quasiprobabilities and matrices of correlation functions. Our approach complements and extends recent results that were based on Chebyshev's integral inequality [65]. The method developed here correlates arbitrary phase-space functions at arbitrary points … Read More
  • A quantum extension of SVM-perf for training nonlinear SVMs in almost linear time
    Quantum 4, 342 (2020). https://doi.org/10.22331/q-2020-10-15-342 We propose a quantum algorithm for training nonlinear support vector machines (SVM) for feature space learning where classical input data is encoded in the amplitudes of quantum states. Based on the classical SVM-perf algorithm of Joachims [1], our algorithm has a running time which scales linearly in the number of … Read More
  • Yao.jl: Extensible, Efficient Framework for Quantum Algorithm Design
    Quantum 4, 341 (2020). https://doi.org/10.22331/q-2020-10-11-341 We introduce $texttt{Yao}$, an extensible, efficient open-source framework for quantum algorithm design. $texttt{Yao}$ features generic and differentiable programming of quantum circuits. It achieves state-of-the-art performance in simulating small to intermediate-sized quantum circuits that are relevant to near-term applications. We introduce the design principles and critical techniques behind $texttt{Yao}$. These include … Read More
  • Transfer learning in hybrid classical-quantum neural networks
    Quantum 4, 340 (2020). https://doi.org/10.22331/q-2020-10-09-340 We extend the concept of transfer learning, widely applied in modern machine learning algorithms, to the emerging context of hybrid neural networks composed of classical and quantum elements. We propose different implementations of hybrid transfer learning, but we focus mainly on the paradigm in which a pre-trained classical network is … Read More
  • Weak-ergodicity-breaking via lattice supersymmetry
    Quantum 4, 339 (2020). https://doi.org/10.22331/q-2020-10-07-339 We study the spectral properties of $D$-dimensional $N=2$ supersymmetric lattice models. We find systematic departures from the eigenstate thermalization hypothesis (ETH) in the form of a degenerate set of ETH-violating supersymmetric (SUSY) doublets, also referred to as many-body scars, that we construct analytically. These states are stable against arbitrary SUSY-preserving … Read More
  • Morphophoric POVMs, generalised qplexes, and 2-designs
    Quantum 4, 338 (2020). https://doi.org/10.22331/q-2020-09-30-338 We study the class of quantum measurements with the property that the image of the set of quantum states under the measurement map transforming states into probability distributions is similar to this set and call such measurements morphophoric. This leads to the generalisation of the notion of a qplex, where … Read More
  • Self-testing of quantum systems: a review
    Quantum 4, 337 (2020). https://doi.org/10.22331/q-2020-09-30-337 Self-testing is a method to infer the underlying physics of a quantum experiment in a black box scenario. As such it represents the strongest form of certification for quantum systems. In recent years a considerable self-testing literature has developed, leading to progress in related device-independent quantum information protocols and deepening … Read More
  • Shortcuts to Adiabaticity in Driven Open Quantum Systems: Balanced Gain and Loss and Non-Markovian Evolution
    Quantum 4, 336 (2020). https://doi.org/10.22331/q-2020-09-28-336 A universal scheme is introduced to speed up the dynamics of a driven open quantum system along a prescribed trajectory of interest. This framework generalizes counterdiabatic driving to open quantum processes. Shortcuts to adiabaticity designed in this fashion can be implemented in two alternative physical scenarios: one characterized by the … Read More
  • Thermodynamics of ultrastrongly coupled light-matter systems
    Quantum 4, 335 (2020). https://doi.org/10.22331/q-2020-09-28-335 We study the thermodynamic properties of a system of two-level dipoles that are coupled ultrastrongly to a single cavity mode. By using exact numerical and approximate analytical methods, we evaluate the free energy of this system at arbitrary interaction strengths and discuss strong-coupling modifications of derivative quantities such as the … Read More
  • De-Signing Hamiltonians for Quantum Adiabatic Optimization
    Quantum 4, 334 (2020). https://doi.org/10.22331/q-2020-09-24-334 Quantum fluctuations driven by non-stoquastic Hamiltonians have been conjectured to be an important and perhaps essential missing ingredient for achieving a quantum advantage with adiabatic optimization. We introduce a transformation that maps every non-stoquastic adiabatic path ending in a classical Hamiltonian to a corresponding stoquastic adiabatic path by appropriately adjusting … Read More
  • Communication through coherent control of quantum channels
    Quantum 4, 333 (2020). https://doi.org/10.22331/q-2020-09-24-333 A completely depolarising quantum channel always outputs a fully mixed state and thus cannot transmit any information. In a recent Letter[3], it was however shown that if a quantum state passes through two such channels in a quantum superposition of different orders—a setup known as the “quantum switch''—then information can … Read More
  • Informationally restricted quantum correlations
    Quantum 4, 332 (2020). https://doi.org/10.22331/q-2020-09-24-332 Quantum communication leads to strong correlations, that can outperform classical ones. Complementary to previous works in this area, we investigate correlations in prepare-and-measure scenarios assuming a bound on the information content of the quantum communication, rather than on its Hilbert-space dimension. Specifically, we explore the extent of classical and quantum … Read More
  • Kitaev’s quantum double model as an error correcting code
    Quantum 4, 331 (2020). https://doi.org/10.22331/q-2020-09-24-331 Kitaev's quantum double models in 2D provide some of the most commonly studied examples of topological quantum order. In particular, the ground space is thought to yield a quantum error-correcting code. We offer an explicit proof that this is the case for arbitrary finite groups. Actually a stronger claim is … Read More
  • Distillation of maximally correlated bosonic matter from many-body quantum coherence
    Quantum 4, 330 (2020). https://doi.org/10.22331/q-2020-09-24-330 We construct quantum coherence resource theories in symmetrized Fock space (QCRTF), thereby providing an information-theoretic framework that connects analyses of quantum coherence in discrete-variable (DV) and continuous variable (CV) bosonic systems. Unlike traditional quantum coherence resource theories, QCRTF can be made independent of the single-particle basis and allow to quantify … Read More
  • Additive-error fine-grained quantum supremacy
    Quantum 4, 329 (2020). https://doi.org/10.22331/q-2020-09-24-329 It is known that several sub-universal quantum computing models, such as the IQP model, the Boson sampling model, the one-clean qubit model, and the random circuit model, cannot be classically simulated in polynomial time under certain conjectures in classical complexity theory. Recently, these results have been improved to “fine-grained" versions … Read More