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…

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.…

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…

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…

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…

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…

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…

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…

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…

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…

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…

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…

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