# Publications

## Theoretical description and experimental simulation of quantum entanglement near open time-like curves via pseudo-density operators.

Nature communications **10** (2019) 182-

Closed timelike curves are striking predictions of general relativity allowing for time-travel. They are afflicted by notorious causality issues (e.g. grandfather's paradox). Quantum models where a qubit travels back in time solve these problems, at the cost of violating quantum theory's linearity-leading e.g. to universal quantum cloning. Interestingly, linearity is violated even by open timelike curves (OTCs), where the qubit does not interact with its past copy, but is initially entangled with another qubit. Non-linear dynamics is needed to avoid violating entanglement monogamy. Here we propose an alternative approach to OTCs, allowing for monogamy violations. Specifically, we describe the qubit in the OTC via a pseudo-density operator-a unified descriptor of both temporal and spatial correlations. We also simulate the monogamy violation with polarization-entangled photons, providing a pseudo-density operator quantum tomography. Remarkably, our proposal applies to any space-time correlations violating entanglement monogamy, such as those arising in black holes.

## Introductory quantum physics and relativity (Second edition)

, 2018

© 2018 by World Scientific Publishing Co. Pte. Ltd. All right reserved. This book is a revised and updated version of Introductory Quantum Physics and Relativity. Based on lectures given as part of the undergraduate degree programme at the University of Leeds, it has been extended in line with recent developments in the field. The book contains all the material required for quantum physics and relativity in the first three years of a traditional physics degree, in addition to more interesting and up-to-date extensions and applications which include quantum field theory, entanglement, and quantum information science. The second edition is unique as an undergraduate textbook as it combines quantum physics and relativity at an introductory level. It expounds the foundations of these two subjects in detail, but also illustrates how they can be combined. It discusses recent applications, but also exposes undergraduates to cutting-edge research topics, such as laser cooling, Bose-Einstein condensation, tunneling microscopes, lasers, nonlocality, and quantum teleportation.

## Geometry of quantum correlations in space-time

PHYSICAL REVIEW A **98** (2018) ARTN 052312

## Operational effects of the UNOT gate on classical and quantum correlations

SCIENCE BULLETIN **63** (2018) 765-770

## Maximum one-shot dissipated work from Rényi divergences.

Physical review. E **97** (2018) 052135-

Thermodynamics describes large-scale, slowly evolving systems. Two modern approaches generalize thermodynamics: fluctuation theorems, which concern finite-time nonequilibrium processes, and one-shot statistical mechanics, which concerns small scales and finite numbers of trials. Combining these approaches, we calculate a one-shot analog of the average dissipated work defined in fluctuation contexts: the cost of performing a protocol in finite time instead of quasistatically. The average dissipated work has been shown to be proportional to a relative entropy between phase-space densities, to a relative entropy between quantum states, and to a relative entropy between probability distributions over possible values of work. We derive one-shot analogs of all three equations, demonstrating that the order-infinity Rényi divergence is proportional to the maximum possible dissipated work in each case. These one-shot analogs of fluctuation-theorem results contribute to the unification of these two toolkits for small-scale, nonequilibrium statistical physics.

## Squeezing Enhances Quantum Synchronization.

Physical review letters **120** (2018) 163601-

It is desirable to observe synchronization of quantum systems in the quantum regime, defined by the low number of excitations and a highly nonclassical steady state of the self-sustained oscillator. Several existing proposals of observing synchronization in the quantum regime suffer from the fact that the noise statistics overwhelm synchronization in this regime. Here, we resolve this issue by driving a self-sustained oscillator with a squeezing Hamiltonian instead of a harmonic drive and analyze this system in the classical and quantum regime. We demonstrate that strong entrainment is possible for small values of squeezing, and in this regime, the states are nonclassical. Furthermore, we show that the quality of synchronization measured by the FWHM of the power spectrum is enhanced with squeezing.

## Law and Disorder

New Scientist **237** (2018) 32-35

© 2018 Reed Business Information Ltd, England We are building machines to undermine nature's most rigid rule, says physicist Vlatko Vedral

## Causal Asymmetry in a Quantum World

PHYSICAL REVIEW X **8** (2018) ARTN 031013

## Proton tunnelling in hydrogen bonds and its implications in an induced-fit model of enzyme catalysis

PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES **474** (2018) ARTN 20180037

## Measuring quantumness: from theory to observability in interferometric setups

EUROPEAN PHYSICAL JOURNAL D **72** (2018) ARTN 219

## When can gravity path-entangle two spatially superposed masses?

PHYSICAL REVIEW D **98** (2018) ARTN 046001

## From micro to macro: Adventures of a wandering physicist

, 2018

© 2018 by World Scientific Publishing Co. Pte. Ltd. All rights reserved. This is a popular science book exploring the limits of scientific explanation. In particular, it debates if all sciences will ultimately be reducible to physics. The journey starts with physics itself, where there is a gap between the micro (quantum) and the macro (classical) and moves into chemistry, biology and the social sciences. Written by a practising scientist, this volume offers a personal perspective on various topics and incorporates the latest research.

## Quantum plug n' play: modular computation in the quantum regime

NEW JOURNAL OF PHYSICS **20** (2018) ARTN 013004

## Experimental Verification of a Jarzynski-Related Information-Theoretic Equality by a Single Trapped Ion.

Physical review letters **120** (2018) 010601-010601

Most nonequilibrium processes in thermodynamics are quantified only by inequalities; however, the Jarzynski relation presents a remarkably simple and general equality relating nonequilibrium quantities with the equilibrium free energy, and this equality holds in both the classical and quantum regimes. We report a single-spin test and confirmation of the Jarzynski relation in the quantum regime using a single ultracold ^{40}Ca^{+} ion trapped in a harmonic potential, based on a general information-theoretic equality for a temporal evolution of the system sandwiched between two projective measurements. By considering both initially pure and mixed states, respectively, we verify, in an exact and fundamental fashion, the nonequilibrium quantum thermodynamics relevant to the mutual information and Jarzynski equality.

## Quantum-gravity effects could in principle be witnessed in neutrino-like oscillations

NEW JOURNAL OF PHYSICS **20** (2018) ARTN 083011

## Device-Independent Tests of Quantum Measurements.

Physical review letters **118** (2017) 250501-

We consider the problem of characterizing the set of input-output correlations that can be generated by an arbitrarily given quantum measurement. Our main result is to provide a closed-form, full characterization of such a set for any qubit measurement, and to discuss its geometrical interpretation. As applications, we further specify our results to the cases of real and complex symmetric, informationally complete measurements and mutually unbiased bases of a qubit, in the presence of isotropic noise. Our results provide the optimal device-independent tests of quantum measurements.

## Witnessing the quantumness of a system by observing only its classical features

NPJ QUANTUM INFORMATION **3** (2017) ARTN 41

## Entropic equality for worst-case work at any protocol speed

NEW JOURNAL OF PHYSICS **19** (2017) ARTN 043013