Quantum Sciences

  • Warm-starting quantum optimization
    Quantum 5, 479 (2021). https://doi.org/10.22331/q-2021-06-17-479 There is an increasing interest in quantum algorithms for problems of integer programming and combinatorial optimization. Classical solvers for such problems employ relaxations, which replace binary variables with continuous ones, for instance in the form of higher-dimensional matrix-valued problems (semidefinite programming). Under the Unique Games Conjecture, these relaxations often provide…
  • Experimental localisation of quantum entanglement through monitored classical mediator
    Quantum 5, 478 (2021). https://doi.org/10.22331/q-2021-06-17-478 Quantum entanglement is a form of correlation between quantum particles that cannot be increased via local operations and classical communication. It has therefore been proposed that an increment of quantum entanglement between probes that are interacting solely via a mediator implies non-classicality of the mediator. Indeed, under certain assumptions regarding…
  • Quantum marginal problem and incompatibility
    Quantum 5, 476 (2021). https://doi.org/10.22331/q-2021-06-15-476 One of the basic distinctions between classical and quantum mechanics is the existence of fundamentally incompatible quantities. Such quantities are present on all levels of quantum objects: states, measurements, quantum channels, and even higher order dynamics. In this manuscript, we show that two seemingly different aspects of quantum incompatibility: the…
  • Visualizing the emission of a single photon with frequency and time resolved spectroscopy
    Quantum 5, 474 (2021). https://doi.org/10.22331/q-2021-06-10-474 At the dawn of Quantum Physics, Wigner and Weisskopf obtained a full analytical description (a $textit{photon portrait}$) of the emission of a single photon by a two-level system, using the basis of frequency modes (Weisskopf and Wigner, "Zeitschrift für Physik", 63, 1930). A direct experimental reconstruction of this portrait demands…
  • Certifying dimension of quantum systems by sequential projective measurements
    Quantum 5, 472 (2021). https://doi.org/10.22331/q-2021-06-10-472 This work analyzes correlations arising from quantum systems subject to sequential projective measurements to certify that the system in question has a quantum dimension greater than some $d$. We refine previous known methods and show that dimension greater than two can be certified in scenarios which are considerably simpler than…
  • Witnessing Wigner Negativity
    Quantum 5, 471 (2021). https://doi.org/10.22331/q-2021-06-08-471 Negativity of the Wigner function is arguably one of the most striking non-classical features of quantum states. Beyond its fundamental relevance, it is also a necessary resource for quantum speedup with continuous variables. As quantum technologies emerge, the need to identify and characterize the resources which provide an advantage over…
  • Efficient Bayesian phase estimation using mixed priors
    Quantum 5, 469 (2021). https://doi.org/10.22331/q-2021-06-07-469 We describe an efficient implementation of Bayesian quantum phase estimation in the presence of noise and multiple eigenstates. The main contribution of this work is the dynamic switching between different representations of the phase distributions, namely truncated Fourier series and normal distributions. The Fourier-series representation has the advantage of being…
  • Efficient qubit phase estimation using adaptive measurements
    Quantum 5, 467 (2021). https://doi.org/10.22331/q-2021-06-04-467 Estimating correctly the quantum phase of a physical system is a central problem in quantum parameter estimation theory due to its wide range of applications from quantum metrology to cryptography. Ideally, the optimal quantum estimator is given by the so-called quantum Cramér-Rao bound, so any measurement strategy aims to obtain…
  • Analyzing the barren plateau phenomenon in training quantum neural networks with the ZX-calculus
    Quantum 5, 466 (2021). https://doi.org/10.22331/q-2021-06-04-466 In this paper, we propose a general scheme to analyze the gradient vanishing phenomenon, also known as the barren plateau phenomenon, in training quantum neural networks with the ZX-calculus. More precisely, we extend the barren plateaus theorem from unitary 2-design circuits to any parameterized quantum circuits under certain reasonable assumptions.…
  • Quantum advantage from energy measurements of many-body quantum systems
    Quantum 5, 465 (2021). https://doi.org/10.22331/q-2021-06-02-465 The problem of sampling outputs of quantum circuits has been proposed as a candidate for demonstrating a quantum computational advantage (sometimes referred to as quantum "supremacy"). In this work, we investigate whether quantum advantage demonstrations can be achieved for more physically-motivated sampling problems, related to measurements of physical observables. We…
  • Modeling and mitigation of cross-talk effects in readout noise with applications to the Quantum Approximate Optimization Algorithm
    Quantum 5, 464 (2021). https://doi.org/10.22331/q-2021-06-01-464 Measurement noise is one of the main sources of errors in currently available quantum devices based on superconducting qubits. At the same time, the complexity of its characterization and mitigation often exhibits exponential scaling with the system size. In this work, we introduce a correlated measurement noise model that can…
  • Time-optimal quantum transformations with bounded bandwidth
    Quantum 5, 462 (2021). https://doi.org/10.22331/q-2021-05-27-462 In this paper, we derive sharp lower bounds, also known as quantum speed limits, for the time it takes to transform a quantum system into a state such that an observable assumes its lowest average value. We assume that the system is initially in an incoherent state relative to the…
  • Parallel entangling gate operations and two-way quantum communication in spin chains
    Quantum 5, 460 (2021). https://doi.org/10.22331/q-2021-05-26-460 The power of a quantum circuit is determined through the number of two-qubit entangling gates that can be performed within the coherence time of the system. In the absence of parallel quantum gate operations, this would make the quantum simulators limited to shallow circuits. Here, we propose a protocol to…
  • Versatile Super-Sensitive Metrology Using Induced Coherence
    Quantum 5, 458 (2021). https://doi.org/10.22331/q-2021-05-26-458 We theoretically analyze the phase sensitivity of the Induced-Coherence (Mandel-Type) Interferometer, including the case where the sensitivity is "boosted" into the bright input regime with coherent-light seeding. We find scaling which reaches below the shot noise limit, even when seeding the spatial mode which does not interact with the sample…
  • How dynamics constrains probabilities in general probabilistic theories
    Quantum 5, 457 (2021). https://doi.org/10.22331/q-2021-05-21-457 We introduce a general framework for analysing general probabilistic theories, which emphasises the distinction between the dynamical and probabilistic structures of a system. The dynamical structure is the set of pure states together with the action of the reversible dynamics, whilst the probabilistic structure determines the measurements and the outcome…
  • Contextual Subspace Variational Quantum Eigensolver
    Quantum 5, 456 (2021). https://doi.org/10.22331/q-2021-05-14-456 We describe the $textit{contextual subspace variational quantum eigensolver}$ (CS-VQE), a hybrid quantum-classical algorithm for approximating the ground state energy of a Hamiltonian. The approximation to the ground state energy is obtained as the sum of two contributions. The first contribution comes from a noncontextual approximation to the Hamiltonian, and is…
  • Multi-time correlations in the positive-P, Q, and doubled phase-space representations
    Quantum 5, 455 (2021). https://doi.org/10.22331/q-2021-05-10-455 A number of physically intuitive results for the calculation of multi-time correlations in phase-space representations of quantum mechanics are obtained. They relate time-dependent stochastic samples to multi-time observables, and rely on the presence of derivative-free operator identities. In particular, expressions for time-ordered normal-ordered observables in the positive-P distribution are derived…
  • Qubit-efficient encoding schemes for binary optimisation problems
    Quantum 5, 454 (2021). https://doi.org/10.22331/q-2021-05-04-454 We propose and analyze a set of variational quantum algorithms for solving quadratic unconstrained binary optimization problems where a problem consisting of $n_c$ classical variables can be implemented on $mathcal O(log n_c)$ number of qubits. The underlying encoding scheme allows for a systematic increase in correlations among the classical variables…
  • Quantum Chaos is Quantum
    Quantum 5, 453 (2021). https://doi.org/10.22331/q-2021-05-04-453 It is well known that a quantum circuit on $N$ qubits composed of Clifford gates with the addition of $k$ non Clifford gates can be simulated on a classical computer by an algorithm scaling as $text{poly}(N)exp(k)$[1]. We show that, for a quantum circuit to simulate quantum chaotic behavior, it is…
  • Error mitigation on a near-term quantum photonic device
    Quantum 5, 452 (2021). https://doi.org/10.22331/q-2021-05-04-452 Photon loss is destructive to the performance of quantum photonic devices and therefore suppressing the effects of photon loss is paramount to photonic quantum technologies. We present two schemes to mitigate the effects of photon loss for a Gaussian Boson Sampling device, in particular, to improve the estimation of the…
  • Local master equations bypass the secular approximation
    Quantum 5, 451 (2021). https://doi.org/10.22331/q-2021-05-01-451 Master equations are a vital tool to model heat flow through nanoscale thermodynamic systems. Most practical devices are made up of interacting sub-system, and are often modelled using either $textit{local}$ master equations (LMEs) or $textit{global}$ master equations (GMEs). While the limiting cases in which either the LME or the GME…
  • Designing locally maximally entangled quantum states with arbitrary local symmetries
    Quantum 5, 450 (2021). https://doi.org/10.22331/q-2021-05-01-450 One of the key ingredients of many LOCC protocols in quantum information is a multiparticle (locally) maximally entangled quantum state, aka a critical state, that possesses local symmetries. We show how to design critical states with arbitrarily large local unitary symmetry. We explain that such states can be realised in…
  • Shortcuts to Squeezed Thermal States
    Quantum 5, 449 (2021). https://doi.org/10.22331/q-2021-05-01-449 Squeezed state in harmonic systems can be generated through a variety of techniques, including varying the oscillator frequency or using nonlinear two-photon Raman interaction. We focus on these two techniques to drive an initial thermal state into a final squeezed thermal state with controlled squeezing parameters – amplitude and phase…
  • Certifying optimality for convex quantum channel optimization problems
    Quantum 5, 448 (2021). https://doi.org/10.22331/q-2021-05-01-448 We identify necessary and sufficient conditions for a quantum channel to be optimal for any convex optimization problem in which the optimization is taken over the set of all quantum channels of a fixed size. Optimality conditions for convex optimization problems over the set of all quantum measurements of a…
  • Photonic quantum data locking
    Quantum 5, 447 (2021). https://doi.org/10.22331/q-2021-04-28-447 Quantum data locking is a quantum phenomenon that allows us to encrypt a long message with a small secret key with information-theoretic security. This is in sharp contrast with classical information theory where, according to Shannon, the secret key needs to be at least as long as the message. Here…
Translate »