Vlatko Vedral

Università degli Studi di Palermo, Palermo, Sicily, Italy

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Publications (283)1223.06 Total impact

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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 out-of-equilibrium transformation. The non-equilibrium 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.
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    ABSTRACT: Fluctuation-dissipation 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 "one-shot statistical mechanics." Rooted in one-shot information theory, one-shot 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 one-shot results. We demonstrate the usefulness of this bridge between frameworks in several ways. Using Crooks' Theorem, we derive a bound on one-shot work quantities. These bounds are tighter, in certain parameter regimes, than a bound in the fluctuation literature and a bound in the one-shot literature. Our bounds withstand tests by numerical simulations of an information-theoretic Carnot engine. By analyzing data from DNA-hairpin experiments, we show that experiments used to test fluctuation theorems also test one-shot results. Additionally, we derive one-shot analogs of a known equality between a relative entropy and the average work dissipated as heat. Our unification of experimentally tested fluctuation relations with one-shot statistical mechanics is intended to bridge one-shot theory to applications.
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    ABSTRACT: The quantum uncertainty principle stipulates that when one observable is predictable there must be some other observables that are unpredictable. The principle is viewed as holding the key to many quantum phenomena and understanding it deeper is of great interest in the study of the foundations of quantum theory. Here we show that apart from being restrictive, the principle also plays a positive role as the enabler of non-classical dynamics in an interferometer. First we note that instantaneous action at a distance should not be possible. We show that for general probabilistic theories this heavily curtails the non-classical dynamics. We prove that there is a trade-off with the uncertainty principle that allows theories to evade this restriction. On one extreme, non-classical theories with maximal certainty have their non-classical dynamics absolutely restricted to only the identity operation. On the other extreme, quantum theory minimizes certainty in return for maximal non-classical dynamics.
    Nature Communications 08/2014; 5:4592. · 10.74 Impact Factor
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    Tristan Farrow, Vlatko Vedral
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    ABSTRACT: A generic and intuitive model for coherent energy transport in multiple minima systems coupled to a quantum mechanical bath is shown. Using a simple spin-boson system, we illustrate how a generic donor-acceptor system can be brought into resonance using a narrow band of vibrational modes, such that the transfer efficiency of an electron-hole pair (exciton) is made arbitrarily high. Coherent transport phenomena in nature are of renewed interest since the discovery that a photon captured by the light-harvesting complex (LHC) in photosynthetic organisms can be conveyed to a chemical reaction centre with near-perfect efficiency. Classical explanations of the transfer use stochastic diffusion to model the hopping motion of a photo-excited exciton. This accounts inadequately for the speed and efficiency of the energy transfer measured in a series of recent landmark experiments. Taking a quantum mechanical perspective can help capture the salient features of the efficient part of that transfer. To show the versatility of the model, we extend it to a multiple minima system comprising seven-sites, reminiscent of the widely studied Fenna-Matthews-Olson (FMO) light-harvesting complex. We show that an idealised transport model for multiple minima coupled to a narrow-band phonon can transport energy with arbitrarily high efficiency.
    Scientific reports. 06/2014; 4.
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    ABSTRACT: We introduce a simple and efficient technique to verify quantum discord in unknown Gaussian states and certain class of non-Gaussian states. We also demonstrate that discord between bipartite systems can be consumed to encode information that can only be accessed by coherent quantum interaction.
    CLEO: QELS_Fundamental Science; 06/2014
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    Tristan Farrow, Vlatko Vedral
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    ABSTRACT: We review canonical experiments on systems that have pushed the boundary between the quantum and classical worlds towards much larger scales, and discuss their unique features that enable quantum coherence to survive. Because the types of systems differ so widely, we use a case by case approach to identifying the different parameters and criteria that capture their behaviour in a quantum mechanical framework. We find it helpful to categorise systems into three broad classes defined by mass, spatio-temporal coherence, and number of particles. The classes are not mutually exclusive and in fact the properties of some systems fit into several classes. We discuss experiments by turn, starting with interference of massive objects like macromolecules and micro-mechanical resonators, followed by self-interference of single particles in complex molecules, before examining the striking advances made with superconducting qubits. Finally, we propose a theoretical basis for quantifying the macroscopic features of a system to lay the ground for a more systematic comparison of the quantum properties in disparate systems.
    Optics Communications 06/2014; · 1.44 Impact Factor
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    ABSTRACT: We study the physics of quantum phase transitions from the perspective of nonequilibrium thermodynamics. For first-order quantum phase transitions, we find that the average work done per quench in crossing the critical point is discontinuous. This leads us to introduce the quantum latent work in analogy with the classical latent heat of first order classical phase transitions. For second order quantum phase transitions the irreversible work is closely related to the fidelity susceptibility for weak sudden quenches of the system Hamiltonian. We demonstrate our ideas with numerical simulations of first, second, and infinite order phase transitions in various spin chain models.
    Physical review. E, Statistical, nonlinear, and soft matter physics. 06/2014; 89(6-1):062103.
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    ABSTRACT: We systematically study the dc/ac current response in Majorana nanowire with or without shortrange Coulomb interaction and disorder. For dc voltage, there is a zero-bias conductance peak which signi?es the existence of Majorana fermion and is in accordance with previous experiment on InSb nanowire. We also consider the ac current response mediated by Majorana fermion and ?nd that the current is enhanced in step with the increase of level broadening and the decrease of temperature, and ?nally saturates at high voltage. To discuss the in uences of short-range interaction and disorder on Majorana nanowire, we implement the bosonization and renormalization group methods to obtain the phase diagram of the Hamiltonian and ?nd that there is a topological phase transition in the interplay of superconductivity and disorder.
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    ABSTRACT: We extend the concept of superadiabatic dynamics, or transitionless quantum driving, to quantum open systems whose evolution is governed by a master equation in the Lindblad form. We provide the general framework needed to determine the control strategy required to achieve superadiabaticity. We apply our formalism to two examples consisting of a two-level system coupled to environments with time-dependent bath operators.
    New Journal of Physics 05/2014; 16(5):053017. · 4.06 Impact Factor
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    ABSTRACT: While we have intuitive notions of structure and complexity, the formalization of this intuition is non-trivial. The statistical complexity is a popular candidate. It is based on the idea that the complexity of a process can be quantified by the complexity of its simplest mathematical model - the model that requires the least past information for optimal future prediction. Here we review how such models, known as $\epsilon$-machines can be further simplified through quantum logic, and explore the resulting consequences for understanding complexity. In particular, we propose a new measure of complexity based on quantum $\epsilon$-machines. We apply this to a simple system undergoing constant thermalization. The resulting quantum measure of complexity aligns more closely with our intuition of how complexity should behave.
    European Physical Journal Plus 04/2014; 129(9). · 1.30 Impact Factor
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    ABSTRACT: The thermodynamic implications for the out-of-equilibrium dynamics of quantum systems are to date largely unexplored, especially for quantum many-body systems. In this paper we investigate the paradigmatic case of an array of nearest-neighbor coupled quantum harmonic oscillators interacting with a thermal bath and subjected to a quench of the inter-oscillator coupling strength. We study the work done on the system and its irreversible counterpart, and characterize analytically the fluctuation relations of the ensuing out-of-equilibrium dynamics. Finally, we showcase an interesting functional link between the dissipated work produced across a two-element chain and their degree of general quantum correlations. Our results suggest that, for the specific model at hand, the non-classical features of a harmonic system can influence significantly its thermodynamics.
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    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. 01/2014; 4:6995.
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    ABSTRACT: Quantum illumination employs entanglement to detect reflecting objects in environments so noisy that all entanglement is destroyed. This appears paradoxical: the benefit of entanglement outlasts entanglement itself. Here we demonstrate that the resilience of quantum illumination is due to quantum discord - a more resilient form of quantum correlations. We prove a direct equality between the performance gain in quantum illumination and the amount of discord which is expended to resolve the target. Discord outlasts entanglement; and in harnessing this discord, quantum illumination outperforms all conventional techniques. This simultaneously explains why quantum illumination thrives in entanglement breaking noise, and confirms discord is a resource of immediate value.
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    ABSTRACT: Landauer's principle states that it costs at least kTln2 of work to reset one bit in the presence of a heat bath at temperature T. The bound of kTln2 is achieved in the unphysical infinite-time limit. Here we ask what is possible if one is restricted to finite-time protocols. We prove analytically that it is possible to reset a bit with a work cost close to kTln2 in a finite time. We construct an explicit protocol that achieves this, which involves changing the system's Hamiltonian avoiding quantum coherences, and thermalising. Using concepts and techniques pertaining to single-shot statistical mechanics, we further develop the limit on the work cost, proving that the heat dissipated is close to the minimal possible not just on average, but guaranteed with high confidence in every run. Moreover we exploit the protocol to design a quantum heat engine that works near the Carnot efficiency in finite time.
    Physical Review Letters 11/2013; 113(10). · 7.73 Impact Factor
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    ABSTRACT: A Comment on the Letter by S. W. Kim et al., Phys. Rev. Lett. 106, 070401 (2011). The authors of the Letter offer a Reply.
    Physical Review Letters 11/2013; 111(18):188901. · 7.73 Impact Factor
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    ABSTRACT: A Comment on the Letter by S. W. Kim , Phys. Rev. Lett. 106, 070401 (2011).PRLTAO0031-900710.1103/PhysRevLett.106.070401 The authors of the Letter offer a Reply.
    Physical Review Letters 11/2013; · 7.73 Impact Factor
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    Jayne Thompson, Mile Gu, Kavan Modi, Vlatko Vedral
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    ABSTRACT: In classical computation each subroutine can be treated as a black box -- when we use preprogrammed operations we need not know their exact physical implementation. This modularity is highly desirable, as a complex problem can decompose into smaller problems with known solutions. Here we identify a general condition where applying an unknown quantum process as a subroutine is impossible, which immediately forbids applying a black-box unitary conditioned on a quantum mechanical degree of freedom. This prevents several quantum protocols, including deterministic quantum computation with one qubit, from operating on truly unknown inputs. We present a method to avoid this situation for certain computational problems. We apply this method to construct a modular version of Shor's factoring algorithm, reducing its complexity, and the extent to which a quantum circuit needs to be tailored to factor specific numbers.
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    ABSTRACT: In a recently published letter [S. W. Kim, T. Sagawa, S. DeLiberato, and M. Ueda, PRL 106, 070401 (2011)] the influence of particle statistics on extractable work in the Szilard engine was discussed. We point out that the expressions given there suggest no work extraction is possible in the low temperature limit if more than two particles are used and thus are not optimal. We argue that the optimal extractable work is in general higher and in particular non-decreasing in the number of particles.
    Physical Review Letters. 09/2013; 111(18).
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    ABSTRACT: We consider one-dimensional Hamiltonian systems whose ground states display symmetry-protected topological order. We show that ground states within the topological phase cannot be connected with each other through local operations and classical communication between a bipartition of the system. Our claim is demonstrated by analyzing the entanglement spectrum and Rényi entropies of different physical systems that provide examples for symmetry-protected topological phases. Specifically, we consider the spin-1/2 cluster-Ising model and a class of spin-1 models undergoing quantum phase transitions to the Haldane phase. Our results provide a probe for symmetry-protected topological order. Since the picture holds even at the system's local scale, our analysis can serve as a local experimental test for topological order.
    Physical review. B, Condensed matter 09/2013; 88(12). · 3.77 Impact Factor
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    ABSTRACT: Enforcing a non-classical behavior in mesoscopic systems is important for the study of the boundaries between quantum and 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 non-classical state of the mirrors by means of projective measurements operated over a pure state of the atomic system. The non-classical features are persistent against incoherent thermal preparation of the mechanical systems and their dissipative dynamics.
    Physical Review A 09/2013; 88(1). · 3.04 Impact Factor

Publication Stats

11k Citations
1,223.06 Total Impact Points


  • 2014
    • Università degli Studi di Palermo
      • Dipartimento di Fisica e Chimica
      Palermo, Sicily, Italy
  • 1995–2014
    • University of Oxford
      • • Department of Physics
      • • Department of Materials
      Oxford, England, United Kingdom
  • 2013
    • Tianjin University
      • Department of Physics
      Tianjin, Tianjin Shi, China
    • Masaryk University
      Brünn, South Moravian, Czech Republic
  • 2012
    • Northeast Institute of Geography and Agroecology
      • Institute of Physics
      Beijing, Beijing Shi, China
  • 2006–2012
    • National University of Singapore
      • • Centre for Quantum Technologies
      • • Department of Physics
      Singapore, Singapore
    • University of Porto
      Oporto, Porto, Portugal
    • Federal University of Minas Gerais
      • Departamento de Matemática
      Belo Horizonte, Estado de Minas Gerais, Brazil
    • University of Cambridge
      Cambridge, England, United Kingdom
  • 1996–2012
    • Imperial College London
      • • Department of Physics
      • • Section of Statistics
      Londinium, England, United Kingdom
  • 2011
    • Università degli Studi di Bari Aldo Moro
      • Dipartimento di Matematica
      Bari, Apulia, Italy
    • ETH Zurich
      • Institute for Theoretical Physics
      Zürich, ZH, Switzerland
    • Federal University of Pernambuco
      • Department of Physics
      Arrecife, Pernambuco, Brazil
    • Macquarie University
      Sydney, New South Wales, Australia
  • 2004–2011
    • University of Leeds
      • School of Physics and Astronomy
      Leeds, ENG, United Kingdom
  • 2004–2010
    • University of Vienna
      • • Faculty of Physics
      • • Basic Experimental Physics Training and Didactics Group
      Vienna, Vienna, Austria
  • 2007
    • Indian Institute of Technology Madras
      • Department of Electrical Engineering
      Chennai, State of Tamil Nadu, India
  • 2000
    • University of Catania
      Catania, Sicily, Italy
    • Uppsala University
      Uppsala, Uppsala, Sweden