Quantum

Paper

Causal orders, quantum circuits and spacetime: distinguishing between definite and superposed causal orders

Nikola Paunković and Marko Vojinović,

Quantum 4, 275 (2020).

We study the notion of causal orders for the cases of (classical and quantum) circuits and spacetime events. We show that every circuit can be immersed into a classical spacetime, preserving…

Paper

Energy upper bound for structurally stable $N$-passive states

Raffaele Salvia and Vittorio Giovannetti,

Quantum 4, 274 (2020).

Passive states are special configurations of a quantum system which exhibit no energy decrement at the end of an arbitrary cyclic driving of the model Hamiltonian. When applied to an increas…

Paper

Modelling equilibration of local many-body quantum systems by random graph ensembles

Daniel Nickelsen and Michael Kastner,

Quantum 4, 273 (2020).

We introduce structured random matrix ensembles, constructed to model many-body quantum systems with local interactions. These ensembles are employed to study equilibration of isolated many-…

Paper

Scaling of variational quantum circuit depth for condensed matter systems

Carlos Bravo-Prieto, Josep Lumbreras-Zarapico, Luca Tagliacozzo, and José I. Latorre,

Quantum 4, 272 (2020).

We benchmark the accuracy of a variational quantum eigensolver based on a finite-depth quantum circuit encoding ground state of local Hamiltonians. We show that in gapped phases, the accurac…

Paper

Amplification of quadratic Hamiltonians

Christian Arenz, Denys I. Bondar, Daniel Burgarth, Cecilia Cormick, and Herschel Rabitz,

Quantum 4, 271 (2020).

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…

Paper

Time crystallinity in open quantum systems

Andreu Riera-Campeny, Maria Moreno-Cardoner, and Anna Sanpera,

Quantum 4, 270 (2020).

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 re…

Paper

Quantum Natural Gradient

James Stokes, Josh Izaac, Nathan Killoran, and Giuseppe Carleo,

Quantum 4, 269 (2020).

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 interpret…

Paper

Ramsey interferometry with generalized one-axis twisting echoes

Marius Schulte, Victor J. Martínez-Lahuerta, Maja S. Scharnagl, and Klemens Hammerer,

Quantum 4, 268 (2020).

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 o…

Paper

Classical simulation of linear optics subject to nonuniform losses

Daniel Jost Brod and Michał Oszmaniec,

Quantum 4, 267 (2020).

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,…

Paper

Classical zero-knowledge arguments for quantum computations

Thomas Vidick and Tina Zhang,

Quantum 4, 266 (2020).

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 cl…

Paper

Conditions for superdecoherence

Joris Kattemölle and Jasper van Wezel,

Quantum 4, 265 (2020).

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…

Paper

How many qubits are needed for quantum computational supremacy?

Alexander M. Dalzell, Aram W. Harrow, Dax Enshan Koh, and Rolando L. La Placa,

Quantum 4, 264 (2020).

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 comp…

Paper

An Adaptive Optimizer for Measurement-Frugal Variational Algorithms

Jonas M. Kübler, Andrew Arrasmith, Lukasz Cincio, and Patrick J. Coles,

Quantum 4, 263 (2020).

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 num…

Paper

The type-independent resource theory of local operations and shared randomness

David Schmid, Denis Rosset, and Francesco Buscemi,

Quantum 4, 262 (2020).

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 r…

Paper

Symmetries and monotones in Markovian quantum dynamics

Georgios Styliaris and Paolo Zanardi,

Quantum 4, 261 (2020).

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 assign…

Paper

Device-independent quantum key distribution with single-photon sources

Jan Kołodyński, Alejandro Máttar, Paul Skrzypczyk, Erik Woodhead, Daniel Cavalcanti, Konrad Banaszek, and Antonio Acín,

Quantum 4, 260 (2020).

$\textit{Device-independent quantum key distribution}$ protocols allow two honest users to establish a secret key with minimal levels of trust on the provider, as security is proven without an…

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