# Publications

## Quantum information: Are we nearly there yet?

New Scientist **219** (2013) 8-

Living systems might be employing quantum information processing. If we can learn their secrets, personal quantum computers no longer seem so fanciful an idea. © 2013 Reed Business Information Ltd, England.

## Requirement of dissonance in assisted optimal state discrimination.

Sci Rep **3** (2013) 2134-

A fundamental problem in quantum information is to explore what kind of quantum correlations is responsible for successful completion of a quantum information procedure. Here we study the roles of entanglement, discord, and dissonance needed for optimal quantum state discrimination when the latter is assisted with an auxiliary system. In such process, we present a more general joint unitary transformation than the existing results. The quantum entanglement between a principal qubit and an ancilla is found to be completely unnecessary, as it can be set to zero in the arbitrary case by adjusting the parameters in the general unitary without affecting the success probability. This result also shows that it is quantum dissonance that plays as a key role in assisted optimal state discrimination and not quantum entanglement. A necessary criterion for the necessity of quantum dissonance based on the linear entropy is also presented. PACS numbers: 03.65.Ta, 03.67.Mn, 42.50.Dv.

## Operational Significance of Discord Consumption

2013 CONFERENCE ON LASERS AND ELECTRO-OPTICS EUROPE AND INTERNATIONAL QUANTUM ELECTRONICS CONFERENCE (CLEO EUROPE/IQEC) (2013)

## Topological features of good resources for measurement-based quantum computation

MATHEMATICAL STRUCTURES IN COMPUTER SCIENCE **23** (2013) 441-453

## Quantum Non-Demolition Detection of Polar Molecule Complexes: Dimers, Trimers, Tetramers

Laser Physics: international journal **23** (2013) 015501-

The optical nondestructive method for in situ detection of the bound states of ultracold polar molecules is developed. It promises a minimally destructive measurement scheme up to a physically exciting quantum non-demolition (QND) level. The detection of molecular complexes beyond simple pairs of quantum particles (dimers, known, e.g., from the BEC-BCS theory) is suggested, including three-body (trimers) and four-body (tertramers) complexes trapped by one-dimensional tubes. The intensity of scattered light is sensitive to the molecule number fluctuations beyond the mean-density approximation. Such fluctuations are very different for various complexes, which leads to radically different light scattering. This type of research extends "quantum optics of quantum gases" to the field of ultracold molecules. Merging the quantum optical and ultracold gas problems will advance the experimental efforts towards the study of the light-matter interaction at its ultimate quantum level, where the quantizations of both light and matter are equally important.

## Correlations in quantum physics

International Journal of Modern Physics B **27** (2013)

We provide a historical perspective of how the notion of correlations has evolved within quantum physics. We begin by reviewing Shannon's information theory and its first application in quantum physics, due to Everett, in explaining the information conveyed during a quantum measurement. This naturally leads us to Lindblad's information theoretic analysis of quantum measurements and his emphasis of the difference between the classical and quantum mutual information. After briefly summarizing the quantification of entanglement using these ideas, we arrive at the concept of quantum discord, which naturally captures the boundary between entanglement and classical correlations. Finally we discuss possible links between discord, which the generation of correlations in thermodynamic transformations of coupled harmonic oscillators. © 2013 World Scientific Publishing Company.

## Nonclassicality of optomechanical devices in experimentally realistic operating regimes

Physical Review A - Atomic, Molecular, and Optical Physics **88** (2013)

Enforcing a nonclassical behavior in mesoscopic systems is important for the study of the boundaries between the quantum and the classical world. Recent experiments have shown that optomechanical devices are promising candidates to pursue such investigations. Here we consider two different setups where the indirect coupling between a three-level atom and the movable mirrors of a cavity is achieved. The resulting dynamics is able to conditionally prepare a nonclassical state of the mirrors by means of projective measurements operated over a pure state of the atomic system. The nonclassical features are persistent against incoherent thermal preparation of the mechanical systems and their dissipative dynamics. © 2013 American Physical Society.

## Wigner rotations and an apparent paradox in relativistic quantum information

PHYSICAL REVIEW A **87** (2013) ARTN 042102

## Majorana fermions in s-wave noncentrosymmetric superconductor with Dresselhaus (110) spin-orbit coupling

Physical Review B - Condensed Matter and Materials Physics **87** (2013)

The asymmetric spin-orbit interactions play a crucial role in realizing topological phases in a noncentrosymmetric superconductor (NCS). We investigate the edge states and the vortex core states in the s-wave NCS with Dresselhaus (110) spin-orbit coupling by both numerical and analytical methods. In particular, we demonstrate that there exists a semimetal phase characterized by the flat Andreev bound states in the phase diagram of the s-wave Dresselhaus NCS which supports the emergence of Majorana fermions. The flat dispersion implies a peak in the density of states which has a clear experimental signature in the tunneling conductance measurements and the Majorana fermions proposed here should be experimentally detectable. © 2013 American Physical Society.

## Extracting quantum work statistics and fluctuation theorems by single-qubit interferometry

Physical Review Letters **110** (2013)

We propose an experimental scheme to verify the quantum nonequilibrium fluctuation relations using current technology. Specifically, we show that the characteristic function of the work distribution for a nonequilibrium quench of a general quantum system can be extracted by Ramsey interferometry of a single probe qubit. Our scheme paves the way for the full characterization of nonequilibrium processes in a variety of quantum systems, ranging from single particles to many-body atomic systems and spin chains. We demonstrate our idea using a time-dependent quench of the motional state of a trapped ion, where the internal pseudospin provides a convenient probe qubit. © 2013 American Physical Society.

## Witnessing the quantumness of a single system: From anticommutators to interference and discord

Physical Review A - Atomic, Molecular, and Optical Physics **87** (2013)

We introduce a method to witness the quantumness of a system. The method relies on the fact that the anticommutator of two classical states is always positive. By contrast, we show that there is always a nonpositive anticommutator due to any two quantum states. We notice that interference depends on the trace of the anticommutator of two states, and it is therefore operationally more suitable to detect quantumness by looking at anticommutators of states rather than their commutators. © 2013 American Physical Society.

## The classical-quantum boundary for correlations: Discord and related measures

Reviews of Modern Physics **84** (2012)

One of the best signatures of nonclassicality in a quantum system is the existence of correlations that have no classical counterpart. Different methods for quantifying the quantum and classical parts of correlations are among the more actively studied topics of quantum-information theory over the past decade. Entanglement is the most prominent of these correlations, but in many cases unentangled states exhibit nonclassical behavior too. Thus distinguishing quantum correlations other than entanglement provides a better division between the quantum and classical worlds, especially when considering mixed states. Here different notions of classical and quantum correlations quantified by quantum discord and other related measures are reviewed. In the first half, the mathematical properties of the measures of quantum correlations are reviewed, related to each other, and the classical-quantum division that is common among them is discussed. In the second half, it is shown that the measures identify and quantify the deviation from classicality in various quantum-information- processing tasks, quantum thermodynamics, open-system dynamics, and many-body physics. It is shown that in many cases quantum correlations indicate an advantage of quantum methods over classical ones. © 2012 American Physical Society.

## Unifying Typical Entanglement and Coin Tossing: On Randomization in Probabilistic Theories

Communications in Mathematical Physics **316** (2012) 441-487

It is well-known that pure quantum states are typically almost maximally entangled, and thus have close to maximally mixed subsystems. We consider whether this is true for probabilistic theories more generally, and not just for quantum theory. We derive a formula for the expected purity of a subsystem in any probabilistic theory for which this quantity is well-defined. It applies to typical entanglement in pure quantum states, coin tossing in classical probability theory, and randomization in post-quantum theories; a simple generalization yields the typical entanglement in (anti)symmetric quantum subspaces. The formula is exact and simple, only containing the number of degrees of freedom and the information capacity of the respective systems. It allows us to generalize statistical physics arguments in a way which depends only on coarse properties of the underlying theory. The proof of the formula generalizes several randomization notions to general probabilistic theories. This includes a generalization of purity, contributing to the recent effort of finding appropriate generalized entropy measures. © 2012 Springer-Verlag Berlin Heidelberg.

## Classical to quantum in large-number limit

Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences **370** (2012) 4810-4820

We construct a quantumness witness following the work of Alicki & van Ryn (AvR). We reformulate the AvR test by defining it for quantum states rather than for observables. This allows us to identify the necessary quantities and resources to detect quantumness for any given system. The first quantity turns out to be the purity of the system. When applying the witness to a system with even moderate mixedness, the protocol is unable to reveal any quantumness. We then show that having many copies of the system leads the witness to reveal quantumness. This seems contrary to the Bohr correspondence, which asserts that, in the large-number limit, quantum systems become classical, whereas the witness shows quantumness when several non-quantum systems, as determined by the witness, are considered together. However, the resources required to detect the quantumness increase dramatically with the number of systems. We apply the quantumness witness for systems that are highly mixed but in the large-number limit that resembles nuclear magnetic resonance (NMR) systems. We make several conclusions about detecting quantumness in NMR-like systems. © 2012 The Royal Society.

## Topological order in 1D Cluster state protected by symmetry

Quantum Information Processing **11** (2012) 1961-1968

We demonstrate how to construct the Z2 × Z2 global symmetry which protects the ground state degeneracy of cluster states for open boundary conditions. Such a degeneracy ultimately arises because the set of stabilizers do not span a complete set of integrals of motion of the cluster state Hamiltonian for open boundary conditions. By applying control phase transformations, our construction makes the stabilizers into the Pauli operators spanning the qubit Hilbert space from which the degeneracy comes. © Springer Science+Business Media, LLC 2011.

## Emergent Thermodynamics in a Quenched Quantum Many-Body System

Physical Review Letters **109** (2012)

We study the statistics of the work done, fluctuation relations, and irreversible entropy production in a quantum many-body system subject to the sudden quench of a control parameter. By treating the quench as a thermodynamic transformation we show that the emergence of irreversibility in the nonequilibrium dynamics of closed many-body quantum systems can be accurately characterized. We demonstrate our ideas by considering a transverse quantum Ising model that is taken out of equilibrium by an instantaneous change of the transverse field. © 2012 American Physical Society.

## Information-theoretic lower bound on energy cost of stochastic computation

PROCEEDINGS OF THE ROYAL SOCIETY A-MATHEMATICAL PHYSICAL AND ENGINEERING SCIENCES **468** (2012) 4058-4066

## When Casimir meets Kibble-Zurek

PHYSICA SCRIPTA **T151** (2012) ARTN 014071