Quantum Sciences

  • Amplification of quadratic Hamiltonians
    Quantum 4, 271 (2020). https://doi.org/10.22331/q-2020-05-25-271 Speeding up the dynamics of a quantum system is of paramount importance for quantum technologies. However, in finite dimensions and without full knowledge of the details of the system, it is easily shown to be $impossible$. In contrast we show that continuous variable systems described by a certain class of … Read MoreRead more
  • Time crystallinity in open quantum systems
    Quantum 4, 270 (2020). https://doi.org/10.22331/q-2020-05-25-270 Time crystals are genuinely non-equilibrium quantum phases of matter that break time-translational symmetry. While in non-equilibrium closed systems time crystals have been experimentally realized, it remains an open question whether or not such a phase survives when systems are coupled to an environment. Although dissipation caused by the coupling to … Read MoreRead more
  • Quantum Natural Gradient
    Quantum 4, 269 (2020). https://doi.org/10.22331/q-2020-05-25-269 A quantum generalization of Natural Gradient Descent is presented as part of a general-purpose optimization framework for variational quantum circuits. The optimization dynamics is interpreted as moving in the steepest descent direction with respect to the Quantum Information Geometry, corresponding to the real part of the Quantum Geometric Tensor (QGT), … Read MoreRead more
  • Ramsey interferometry with generalized one-axis twisting echoes
    Quantum 4, 268 (2020). https://doi.org/10.22331/q-2020-05-15-268 We consider a large class of Ramsey interferometry protocols which are enhanced by squeezing and un-squeezing operations before and after a phase signal is imprinted on the collective spin of $N$ particles. We report an analytical optimization for any given particle number and strengths of (un-)squeezing. These results can be … Read MoreRead more
  • Classical simulation of linear optics subject to nonuniform losses
    Quantum 4, 267 (2020). https://doi.org/10.22331/q-2020-05-14-267 We present a comprehensive study of the impact of non-uniform, i.e. path-dependent, photonic losses on the computational complexity of linear-optical processes. Our main result states that, if each beam splitter in a network induces some loss probability, non-uniform network designs cannot circumvent the efficient classical simulations based on losses. To … Read MoreRead more
  • Classical zero-knowledge arguments for quantum computations
    Quantum 4, 266 (2020). https://doi.org/10.22331/q-2020-05-14-266 We show that every language in QMA admits a classical-verifier, quantum-prover zero-knowledge argument system which is sound against quantum polynomial-time provers and zero-knowledge for classical (and quantum) polynomial-time verifiers. The protocol builds upon two recent results: a computational zero-knowledge proof system for languages in QMA, with a quantum verifier, introduced … Read MoreRead more
  • Conditions for superdecoherence
    Quantum 4, 265 (2020). https://doi.org/10.22331/q-2020-05-14-265 Decoherence is the main obstacle to quantum computation. The decoherence rate per qubit is typically assumed to be constant. It is known, however, that quantum registers coupling to a single reservoir can show a decoherence rate per qubit that increases linearly with the number of qubits. This effect has been … Read MoreRead more
  • How many qubits are needed for quantum computational supremacy?
    Quantum 4, 264 (2020). https://doi.org/10.22331/q-2020-05-11-264 Quantum computational supremacy arguments, which describe a way for a quantum computer to perform a task that cannot also be done by a classical computer, typically require some sort of computational assumption related to the limitations of classical computation. One common assumption is that the polynomial hierarchy ($mathsf{PH}$) does not … Read MoreRead more
  • An Adaptive Optimizer for Measurement-Frugal Variational Algorithms
    Quantum 4, 263 (2020). https://doi.org/10.22331/q-2020-05-11-263 Variational hybrid quantum-classical algorithms (VHQCAs) have the potential to be useful in the era of near-term quantum computing. However, recently there has been concern regarding the number of measurements needed for convergence of VHQCAs. Here, we address this concern by investigating the classical optimizer in VHQCAs. We introduce a novel … Read MoreRead more
  • The type-independent resource theory of local operations and shared randomness
    Quantum 4, 262 (2020). https://doi.org/10.22331/q-2020-04-30-262 In space-like separated experiments and other scenarios where multiple parties share a classical common cause but no cause-effect relations, quantum theory allows a variety of nonsignaling resources which are useful for distributed quantum information processing. These include quantum states, nonlocal boxes, steering assemblages, teleportages, channel steering assemblages, and so on. … Read MoreRead more
  • The type-independent resource theory of local operations and shared randomness
    Quantum 4, 262 (2020). https://doi.org/10.22331/q-2020-04-30-262 In space-like separated experiments and other scenarios where multiple parties share a classical common cause but no cause-effect relations, quantum theory allows a variety of nonsignaling resources which are useful for distributed quantum information processing. These include quantum states, nonlocal boxes, steering assemblages, teleportages, channel steering assemblages, and so on. … Read MoreRead more
  • Symmetries and monotones in Markovian quantum dynamics
    Quantum 4, 261 (2020). https://doi.org/10.22331/q-2020-04-30-261 What can one infer about the dynamical evolution of quantum systems just by symmetry considerations? For Markovian dynamics in finite dimensions, we present a simple construction that assigns to each symmetry of the generator a family of scalar functions over quantum states that are monotonic under the time evolution. The … Read MoreRead more
  • Symmetries and monotones in Markovian quantum dynamics
    Quantum 4, 261 (2020). https://doi.org/10.22331/q-2020-04-30-261 What can one infer about the dynamical evolution of quantum systems just by symmetry considerations? For Markovian dynamics in finite dimensions, we present a simple construction that assigns to each symmetry of the generator a family of scalar functions over quantum states that are monotonic under the time evolution. The … Read MoreRead more
  • Device-independent quantum key distribution with single-photon sources
    Quantum 4, 260 (2020). https://doi.org/10.22331/q-2020-04-30-260 $textit{Device-independent quantum key distribution}$ protocols allow two honest users to establish a secret key with minimal levels of trust on the provider, as security is proven without any assumption on the inner working of the devices used for the distribution. Unfortunately, the implementation of these protocols is challenging, as it … Read MoreRead more
  • Device-independent quantum key distribution with single-photon sources
    Quantum 4, 260 (2020). https://doi.org/10.22331/q-2020-04-30-260 $textit{Device-independent quantum key distribution}$ protocols allow two honest users to establish a secret key with minimal levels of trust on the provider, as security is proven without any assumption on the inner working of the devices used for the distribution. Unfortunately, the implementation of these protocols is challenging, as it … Read MoreRead more
  • The first law of general quantum resource theories
    Quantum 4, 259 (2020). https://doi.org/10.22331/q-2020-04-30-259 We extend the tools of quantum resource theories to scenarios in which multiple quantities (or resources) are present, and their interplay governs the evolution of physical systems. We derive conditions for the interconversion of these resources, which generalise the first law of thermodynamics. We study reversibility conditions for multi-resource theories, … Read MoreRead more
  • The first law of general quantum resource theories
    Quantum 4, 259 (2020). https://doi.org/10.22331/q-2020-04-30-259 We extend the tools of quantum resource theories to scenarios in which multiple quantities (or resources) are present, and their interplay governs the evolution of physical systems. We derive conditions for the interconversion of these resources, which generalise the first law of thermodynamics. We study reversibility conditions for multi-resource theories, … Read MoreRead more
  • Contextual advantage for state-dependent cloning
    Quantum 4, 258 (2020). https://doi.org/10.22331/q-2020-04-27-258 A number of noncontextual models exist which reproduce different subsets of quantum theory and admit a no-cloning theorem. Therefore, if one chooses noncontextuality as one's notion of classicality, no-cloning cannot be regarded as a nonclassical phenomenon. In this work, however, we show that there are aspects of the phenomenology of … Read MoreRead more
  • Contextual advantage for state-dependent cloning
    Quantum 4, 258 (2020). https://doi.org/10.22331/q-2020-04-27-258 A number of noncontextual models exist which reproduce different subsets of quantum theory and admit a no-cloning theorem. Therefore, if one chooses noncontextuality as one's notion of classicality, no-cloning cannot be regarded as a nonclassical phenomenon. In this work, however, we show that there are aspects of the phenomenology of … Read MoreRead more
  • Call for editors 2020
    Today Quantum celebrates 3 years since the first publications! We count now with over 250 published articles. This growth was made possible by our incredible team of editors, countless expert referees, and the support of authors and the quantum community. Thank you! With a steadily growing submission rate, and several of the founding editors phasing … Read MoreRead more
  • Call for editors 2020
    Today Quantum celebrates 3 years since the first publications! We count now with over 250 published articles. This growth was made possible by our incredible team of editors, countless expert referees, and the support of authors and the quantum community. Thank you! With a steadily growing submission rate, and several of the founding editors phasing … Read MoreRead more
  • Mitigation of readout noise in near-term quantum devices by classical post-processing based on detector tomography
    Quantum 4, 257 (2020). https://doi.org/10.22331/q-2020-04-24-257 We propose a simple scheme to reduce readout errors in experiments on quantum systems with finite number of measurement outcomes. Our method relies on performing classical post-processing which is preceded by Quantum Detector Tomography, i.e., the reconstruction of a Positive-Operator Valued Measure (POVM) describing the given quantum measurement device. If … Read MoreRead more
  • Mitigation of readout noise in near-term quantum devices by classical post-processing based on detector tomography
    Quantum 4, 257 (2020). https://doi.org/10.22331/q-2020-04-24-257 We propose a simple scheme to reduce readout errors in experiments on quantum systems with finite number of measurement outcomes. Our method relies on performing classical post-processing which is preceded by Quantum Detector Tomography, i.e., the reconstruction of a Positive-Operator Valued Measure (POVM) describing the given quantum measurement device. If … Read MoreRead more
  • Improving Variational Quantum Optimization using CVaR
    Quantum 4, 256 (2020). https://doi.org/10.22331/q-2020-04-20-256 Hybrid quantum/classical variational algorithms can be implemented on noisy intermediate-scale quantum computers and can be used to find solutions for combinatorial optimization problems. Approaches discussed in the literature minimize the expectation of the problem Hamiltonian for a parameterized trial quantum state. The expectation is estimated as the sample mean of … Read MoreRead more
  • Improving Variational Quantum Optimization using CVaR
    Quantum 4, 256 (2020). https://doi.org/10.22331/q-2020-04-20-256 Hybrid quantum/classical variational algorithms can be implemented on noisy intermediate-scale quantum computers and can be used to find solutions for combinatorial optimization problems. Approaches discussed in the literature minimize the expectation of the problem Hamiltonian for a parameterized trial quantum state. The expectation is estimated as the sample mean of … Read MoreRead more