# Publications

## Operational advantage of basis-independent quantum coherence

EPL **125** (2019) ARTN 50005

## Emergence of correlated proton tunnelling in water ice.

Proceedings. Mathematical, Physical, and Engineering Sciences **475** (2019) 20180867-20180867

Several experimental and theoretical studies report instances of concerted or correlated multiple proton tunnelling in solid phases of water. Here, we construct a pseudo-spin model for the quantum motion of protons in a hexameric H2O ring and extend it to open system dynamics that takes environmental effects into account in the form of O-H stretch vibrations. We approach the problem of correlations in tunnelling using quantum information theory in a departure from previous studies. Our formalism enables us to quantify the coherent proton mobility around the hexagonal ring by one of the principal measures of coherence, the l 1 norm of coherence. The nature of the pairwise pseudo-spin correlations underlying the overall mobility is further investigated within this formalism. We show that the classical correlations of the individual quantum tunnelling events in long-time limit is sufficient to capture the behaviour of coherent proton mobility observed in low-temperature experiments. We conclude that long-range intra-ring interactions do not appear to be a necessary condition for correlated proton tunnelling in water ice.

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

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

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.

## Engineering statistical transmutation of identical quantum particles

PHYSICAL REVIEW B **99** (2019) ARTN 045430

## Uncertainty equality with quantum memory and its experimental verification

NPJ QUANTUM INFORMATION **5** (2019) ARTN 39

## 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) 052135-052135

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) 163601-163601

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

## Experimental test of the relation between coherence and path information

Communications Physics **1** (2018)

© 2018, The Author(s). Quantum coherence stemming from the superposition behaviour of a particle beyond the classical realm, serves as one of the most fundamental features in quantum mechanics. The wave-particle duality phenomenon, which shares the same origin, has a strong relationship with quantum coherence. Recently, an elegant relation between quantum coherence and path information has been theoretically derived. Here, we experimentally test such new duality by l1-norm measure and the minimum-error state discrimination. We prepare three classes of two-photon states encoded in polarisation degree of freedom, with one photon serving as the target and the other photon as the detector. We observe that wave-particle-like complementarity and Bagan’s equality, defined by the duality relation between coherence and path information, is well satisfied. Our results may shed new light on the original nature of wave-particle duality and on the applications of quantum coherence as a fundamental resource in quantum technologies.

## Probing quantum features of photosynthetic organisms

NPJ QUANTUM INFORMATION **4** (2018) ARTN 60

## 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

## Interaction of Phase-Modulated Femtosecond Pulses with an Optically Dense Quasi-Resonant Medium of Rubidium Vapors

OPTICS AND SPECTROSCOPY **125** (2018) 667-672

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

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