Publications (308)1254.96 Total impact

Article: The power of one qumode
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ABSTRACT: Although quantum computers are capable of solving problems like factoring exponentially faster than the bestknown classical algorithms, determining the resources responsible for their computational power remains unclear. An important class of problems where quantum computers possess an advantage is phase estimation, which includes applications like factoring. We introduce a new computational model based on a single squeezed state resource that can perform phase estimation, which we call the power of one qumode. This model is inspired by an interesting computational model known as deterministic quantum computing with one quantum bit (DQC1). Using the power of one qumode, we identify that the amount of squeezing is sufficient to quantify the resource requirements of different computational problems based on phase estimation. In particular, it establishes a quantitative relationship between the resources required for factoring and DQC1. For example, we find the squeezing required to factor has an exponential scaling whereas no squeezing (i.e., a coherent state) is already sufficient to solve the hardest problem in DQC1.  [Show abstract] [Hide abstract]
ABSTRACT: Can quantum information fundamentally change the way we perceive what is complex? Here, we study statistical complexity, a popular quantifier of complexity that captures the minimal memory required to model a process. We construct a quantum variant of this measure, and evaluate it analytically for a classical Ising spin chain. The resulting complexity measure  quantum statistical complexity  exhibits drastically different qualitative behaviour. Whereas the classical complexity of the spin chain grows monotonically with temperature, its quantum complexity rises to a maximum at some finite temperature then tends back towards zero for higher temperatures. This demonstrates that our notion of what is complex depends fundamentally on which information theory we employ.  [Show abstract] [Hide abstract]
ABSTRACT: Recent results in quantum information theory characterize quantum coherence in the context of resource theories. Here we study the relation between quantum coherence and quantum discord, a kind of quantum correlation which appears even in nonentangled states. We prove that the creation of quantum discord with multipartite incoherent operations is bounded by the amount of quantum coherence consumed in its subsystems during the process. We show how the interplay between quantum coherence consumption and creation of quantum discord works in the preparation of multipartite quantum correlated states and in the model of deterministic quantum computation with one qubit (DQC1). 
Article: Photonic Maxwell's demon
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ABSTRACT: We report an experimental realisation of Maxwell's demon in a photonic setup. We show that a measurement at the singlephoton level followed by a feedforward operation allows the extraction of work from intense thermal light into an electric circuit. The interpretation of the experiment stimulates the derivation of a new equality relating work extraction to information acquired by measurement. We derive a bound using this relation and show that it is in agreement with the experimental results. Our work puts forward photonic systems as a platform for experiments related to information in thermodynamics.  [Show abstract] [Hide abstract]
ABSTRACT: Living organisms capitalize on their ability to predict their environment to maximize their available free energy, and invest this energy in turn to create new complex structures. Is there a preferred method by which this manipulation of structure should be done? Our intuition is "simpler is better," but this is only a guiding principal. Here, we substantiate this claim through thermodynamic reasoning. We present a new framework for the manipulation of patterns (structured sequences of data) by predictive devices. We identify the dissipative costs and how they can be minimized by the choice of memory in these predictive devices. For pattern generation, we see that simpler is indeed better. However, contrary to intuition, when it comes to extracting work from a pattern, any device capable of making statistically accurate predictions can recover all available energy.  [Show abstract] [Hide abstract]
ABSTRACT: Two approaches to smallscale and quantum thermodynamics are fluctuation relations and oneshot statistical mechanics. Fluctuation relations (such as Crooks’ theorem and Jarzynski's equality) relate nonequilibrium behaviors to equilibrium quantities such as free energy. Oneshot statistical mechanics involves statements about every run of an experiment, not just about averages over trials. We investigate the relation between the two approaches. We show that both approaches feature the same notions of work and the same notions of probability distributions over possible work values. The two approaches are alternative toolkits with which to analyze these distributions. To combine the toolkits, we show how oneshot work quantities can be defined and bounded in contexts governed by Crooks’ theorem. These bounds provide a new bridge from oneshot theory to experiments originally designed for testing fluctuation theorems.New Journal of Physics 09/2015; 17(9). DOI:10.1088/13672630/17/9/095003 · 3.56 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The use of the von Neumann entropy in formulating the laws of thermodynamics has recently been challenged. It is associated with the average work whereas the work guaranteed to be extracted in any single run of an experiment is the more interesting quantity in general. We show that an expression that quantifies majorization determines the optimal guaranteed work. We argue it should therefore be the central quantity of statistical mechanics, rather than the von Neumann entropy. In the limit of many identical and independent subsystems (asymptotic i.i.d) the von Neumann entropy expressions are recovered but in the nonequilbrium regime the optimal guaranteed work can be radically different to the optimal average. Moreover our measure of majorization governs which evolutions can be realized via thermal interactions, whereas the nondecrease of the von Neumann entropy is not sufficiently restrictive. Our results are inspired by singleshot information theory.New Journal of Physics 07/2015; 17(7). DOI:10.1088/13672630/17/7/073001 · 3.56 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Thermodynamics describes largescale, slowly evolving systems. Two modern approaches generalize thermodynamics: fluctuation theorems, which concern finitetime nonequilibrium processes, and oneshot statistical mechanics, which concerns small scales and finite numbers of trials. Combining these approaches, we calculate a oneshot 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 phasespace densities, one between quantum states, and one between probability distributions over possible values of work. We derive oneshot analogs of all three equations, demonstrating that the orderinfinity R\'enyi divergence is proportional to the maximum dissipated work in each case. These oneshot analogs of fluctuationtheorem results contribute to the unification of these two toolkits for smallscale, nonequilibrium statistical physics. 
Article: Majorana transport in superconducting nanowire with Rashba and Dresselhaus spin–orbit couplings
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ABSTRACT: The tunneling experiment is a key technique for detecting Majorana fermion (MF) in solid state systems. We use Keldysh nonequilibrium Green function method to study twolead tunneling in superconducting nanowire with Rashba and Dresselhaus spinorbit couplings. A zerobias dc conductance peak appears in our setup which signifies the existence of MF and is in accordance with previous experimental results on InSb nanowire. Interestingly, due to the exotic property of MF, there exists a hole transmission channel which makes the currents asymmetric at the left and right leads. The ac current response mediated by MF is also studied here. To discuss the impacts of Coulomb interaction and disorder on the transport property of Majorana nanowire, we use the renormalization group method to study the phase diagram of the wire. It is found that there is a topological phase transition under the interplay of superconductivity and disorder. We find that the Majorana transport is preserved in the superconductingdominated topological phase and destroyed in the disorderdominated nontopological insulator phase.Journal of Physics Condensed Matter 05/2015; 27(22):225302. DOI:10.1088/09538984/27/22/225302 · 2.35 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We suggest that quantum macroscopicity should be quantified in terms of coherence, and propose a set of conditions that should be satisfied by any measure of macroscopic coherence. We show that this enables a rigorous justification of a previously proposed measure of macroscopicity based on the quantum Fisher information, while another measure does not satisfy important monotonicity criteria.  [Show abstract] [Hide abstract]
ABSTRACT: We derive an equality for nonequilibrium statistical mechanics. The equality concerns the worstcase work output of a timedependent Hamiltonian protocol in the presence of a Markovian heat bath. It has the form "worstcase work = penalty  optimum". The equality holds for all rates of changing the Hamiltonian and can be used to derive the optimum by setting the penalty to 0. The optimum term contains the max entropy of the initial state, rather than the von Neumann entropy, thus recovering recent results from singleshot statistical mechanics. We apply the equality to an electron box.  [Show abstract] [Hide abstract]
ABSTRACT: Entanglement can be well quantified by R\'{e}nyi $\alpha$entropy which is a generalization of the standard von Neumann entropy. Here we study the measure of entanglement R\'{e}nyi $\alpha$entropy for arbitrary twoqubit states. We show that entanglement of two states may be incomparable, contrary to other wellaccepted entanglement measures. These facts impose constraint on the convertibility of entangled states by local operations and classical communication. We find that when $\alpha $ is larger than a critical value, the entanglement measure by R\'{e}nyi $\alpha$entropy is determined solely by concurrence which is a well accepted measure of entanglement. When $\alpha $ is small, the entanglement R\'{e}nyi $\alpha$entropy of Werner state is obtained. Interestingly, we show that entanglement R\'{e}nyi $\alpha$entropy of Werner state is always less than any pure entangled state when $\alpha $ is close to zero, even this Werner state is close to a maximally entangled state and the concurrence is larger. We also conclude that the optimal decomposition of a general mixed state cannot be the same for all $\alpha $.  [Show abstract] [Hide abstract]
ABSTRACT: The lowtemperature physics of quantum manybody systems is largely governed by the structure of their ground states. Minimizing the energy of local interactions, ground states often reflect strong properties of locality such as the area law for entanglement entropy and the exponential decay of correlations between spatially separated observables. In this letter we present a novel characterization of locality in quantum states, which we call `local reversibility'. It characterizes the type of operations that are needed to reverse the action of a general disturbance on the state. We prove that unique ground states of gapped local Hamiltonian are locally reversible. This way, we identify new fundamental features of manybody ground states, which cannot be derived from the aforementioned properties. We use local reversibility to distinguish between states enjoying microscopic and macroscopic quantum phenomena. To demonstrate the potential of our approach, we prove specific properties of ground states, which are relevant both to critical and noncritical theories.  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the notion of quantumness based on the noncommutativity of the algebra of observables and introduce a measure of quantumness based on the mutual incompatibility of quantum states. Since it relies on the full algebra of observables, our measure for composed systems is partition independent and witnesses the global quantum nature of a state. We show that such quantity can be experimentally measured with an interferometric setup and that, when an arbitrary bipartition is introduced, it detects the oneway quantum correlations restricted to one of the two subsystems. We finally show that, by combining only two projective measurements and carrying out the interference procedure, our measure becomes an efficient universal witness of quantum discord and nonclassical correlations.  [Show abstract] [Hide abstract]
ABSTRACT: In general relativity, closed timelike curves can break causality with remarkable and unsettling consequences. At the classical level, they induce causal paradoxes disturbing enough to motivate conjectures that explicitly prevent their existence. At the quantum level, resolving such paradoxes induce radical benefits  from cloning unknown quantum states to solving problems intractable to quantum computers. Instinctively, one expects these benefits to vanish if causality is respected. Here we show that in harnessing entanglement, we can efficiently solve NPcomplete problems and clone arbitrary quantum states  even when all timetravelling systems are completely isolated from the past. Thus, the many defining benefits of closed timelike curves can still be harnessed, even when causality is preserved. Our results unveil the subtle interplay between entanglement and general relativity, and significantly improve the potential of probing the radical effects that may exist at the interface between relativity and quantum theory.12/2014; 1. DOI:10.1038/npjqi.2015.7  [Show abstract] [Hide abstract]
ABSTRACT: Maxwell's daemon is a popular personification of a principle connecting information gain and extractable work in thermodynamics. A Szilard Engine is a particular hypothetical realization of Maxwell's daemon, which is able to extract work from a single thermal reservoir by measuring the position of particle(s) within the system. Here we investigate the role of particle statistics in the whole process; namely, how the extractable work changes if instead of classical particles fermions or bosons are used as the working medium. We give a unifying argument for the optimal work in the different cases: the extractable work is determined solely by the information gain of the initial measurement, as measured by the mutual information, regardless of the number and type of particles which constitute the working substance.Scientific Reports 11/2014; 4:6995. DOI:10.1038/srep06995 · 5.58 Impact Factor 
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ABSTRACT: We investigate the thermodynamical properties of quantum fields in curved spacetime. Our approach is to consider quantum fields in curved spacetime as a quantum system undergoing an outofequilibrium transformation. The nonequilibrium features are studied by using a formalism which has been developed to derive fluctuation relations and emergent irreversible features beyond the linear response regime. We apply these ideas to an expanding universe scenario, therefore avoiding assumptions on the relation between entropy and quantum matter. We provide a fluctuation theorem which allows us to understand particle production due to the expansion of the universe as an entropic increase. Our results pave the way towards a different understanding of the thermodynamics of relativistic and quantum systems in our universe.  [Show abstract] [Hide abstract]
ABSTRACT: Fluctuationdissipation relations, such as Crooks' Theorem and Jarzynski's Equality, are powerful tools in quantum and classical nonequilibrium statistical mechanics. We link these relations to a newer approach known as "oneshot statistical mechanics." Rooted in oneshot information theory, oneshot statistical mechanics concerns statements true of every implementation of a protocol, not only of averages. We show that two general models for work extraction in the presence of heat baths obey fluctuation relations and oneshot results. We demonstrate the usefulness of this bridge between frameworks in several ways. Using Crooks' Theorem, we derive a bound on oneshot work quantities. These bounds are tighter, in certain parameter regimes, than a bound in the fluctuation literature and a bound in the oneshot literature. Our bounds withstand tests by numerical simulations of an informationtheoretic Carnot engine. By analyzing data from DNAhairpin experiments, we show that experiments used to test fluctuation theorems also test oneshot results. Additionally, we derive oneshot analogs of a known equality between a relative entropy and the average work dissipated as heat. Our unification of experimentally tested fluctuation relations with oneshot statistical mechanics is intended to bridge oneshot theory to applications. 
Article: Universal Optimal Quantum Correlator
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ABSTRACT: Recently, a novel operational strategy to access quantum correlation functions of the form Tr[A rho B] was provided in [F. Buscemi, M. Dall'Arno, M. Ozawa, and V. Vedral, arXiv:1312.4240]. Here we propose a realization scheme, that we call partial expectation values, implementing such strategy in terms of a unitary interaction with an ancillary system followed by the measurement of an observable on the ancilla. Our scheme is universal, being independent of rho, A, and B, and it is optimal in a statistical sense. Our scheme is suitable for implementation with present quantum optical technology, and provides a new way to test uncertainty relations.International Journal of Quantum Information 09/2014; 12(07n08). DOI:10.1142/S0219749915600023 · 0.88 Impact Factor
Publication Stats
15k  Citations  
1,254.96  Total Impact Points  
Top Journals
 Physical Review A (54)
 Physical Review Letters (32)
 New Journal of Physics (17)
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 Nature (8)
Institutions

19952015

University of Oxford
 Department of Physics
Oxford, England, United Kingdom


2014

Università degli Studi di Palermo
 Dipartimento di Fisica e Chimica
Palermo, Sicily, Italy


2013

Masaryk University
 Faculty of Informatics
Brünn, South Moravian, Czech Republic


19962012

Imperial College London
 Section of Statistics
Londinium, England, United Kingdom


2011

Federal University of Pernambuco
 Department of Physics
Arrecife, Pernambuco, Brazil


20042011

University of Leeds
 School of Physics and Astronomy
Leeds, ENG, United Kingdom


2006

University of Vienna
 Basic Experimental Physics Training and Didactics Group
Wien, Vienna, Austria 
University of Cambridge
 Department of Applied Mathematics and Theoretical Physics
Cambridge, England, United Kingdom 
Federal University of Minas Gerais
 Departamento de Física
Cidade de Minas, Minas Gerais, Brazil
