[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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.
[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.
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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