1,356 research outputs found
Quantum Thermodynamics
Quantum thermodynamics is an emerging research field aiming to extend
standard thermodynamics and non-equilibrium statistical physics to ensembles of
sizes well below the thermodynamic limit, in non-equilibrium situations, and
with the full inclusion of quantum effects. Fuelled by experimental advances
and the potential of future nanoscale applications this research effort is
pursued by scientists with different backgrounds, including statistical
physics, many-body theory, mesoscopic physics and quantum information theory,
who bring various tools and methods to the field. A multitude of theoretical
questions are being addressed ranging from issues of thermalisation of quantum
systems and various definitions of "work", to the efficiency and power of
quantum engines. This overview provides a perspective on a selection of these
current trends accessible to postgraduate students and researchers alike.Comment: 48 pages, improved and expanded several sections. Comments welcom
Coherence and measurement in quantum thermodynamics
Thermodynamics is a highly successful macroscopic theory widely used across
the natural sciences and for the construction of everyday devices, from car
engines and fridges to power plants and solar cells. With thermodynamics
predating quantum theory, research now aims to uncover the thermodynamic laws
that govern finite size systems which may in addition host quantum effects.
Here we identify information processing tasks, the so-called "projections",
that can only be formulated within the framework of quantum mechanics. We show
that the physical realisation of such projections can come with a non-trivial
thermodynamic work only for quantum states with coherences. This contrasts with
information erasure, first investigated by Landauer, for which a thermodynamic
work cost applies for classical and quantum erasure alike. Implications are
far-reaching, adding a thermodynamic dimension to measurements performed in
quantum thermodynamics experiments, and providing key input for the
construction of a future quantum thermodynamic framework. Repercussions are
discussed for quantum work fluctuation relations and thermodynamic single-shot
approaches.Comment: 6 pages + appendix, 4 figures, v2: changed presentation, critically
discuss interpretation as measurement, added new conclusions; previous title:
"Quantum measurement and its role in thermodynamics
Entanglement and separability of quantum harmonic oscillator systems at finite temperature
In the present paper we study the entanglement properties of thermal (a.k.a.
Gibbs) states of quantum harmonic oscillator systems as functions of the
Hamiltonian and the temperature. We prove the physical intuition that at
sufficiently high temperatures the thermal state becomes fully separable and we
deduce bounds on the critical temperature at which this happens. We show that
the bound becomes tight for a wide class of Hamiltonians with sufficient
translation symmetry. We find, that at the crossover the thermal energy is of
the order of the energy of the strongest normal mode of the system and quantify
the degree of entanglement below the critical temperature. Finally, we discuss
the example of a ring topology in detail and compare our results with previous
work in an entanglement-phase diagram.Comment: 10 pages, 5 figure
Thermodynamics of discrete quantum processes
We define thermodynamic configurations and identify two primitives of
discrete quantum processes between configurations for which heat and work can
be defined in a natural way. This allows us to uncover a general second law for
any discrete trajectory that consists of a sequence of these primitives,
linking both equilibrium and non-equilibrium configurations. Moreover, in the
limit of a discrete trajectory that passes through an infinite number of
configurations, i.e. in the reversible limit, we recover the saturation of the
second law. Finally, we show that for a discrete Carnot cycle operating between
four configurations one recovers Carnot's thermal efficiency.Comment: 14pages, 9 figure
Leggett-Garg inequalities for quantum fluctuating work
The Leggett-Garg inequalities serve to test whether or not quantum
correlations in time can be explained within a classical macrorealistic
framework. We apply this test to thermodynamics and derive a set of Leggett-
Garg inequalities for the statistics of fluctuating work done on a quantum
system unitarily driven in time. It is shown that these inequalities can be
violated in a driven two-level system, thereby demonstrating that there exists
no general macrorealistic description of quantum work. These violations are
shown to emerge within the standard Two-Projective-Measurement scheme as well
as for alternative definitions of fluctuating work that are based on weak
measurement. Our results elucidate the influences of temporal correlations on
work extraction in the quantum regime and highlight a key difference between
quantum and classical thermodynamics.Comment: v2, 1 figure, accepted version to appear in Entropy (Special Issue on
"Quantum Thermodynamics II"
Entropy production and time-asymmetry in the presence of strong interactions
It is known that the equilibrium properties of open classical systems that
are strongly coupled to a heat bath are described by a set of thermodynamic
potentials related to the system's Hamiltonian of mean force. By adapting this
framework to a more general class of non-equilibrium states, we show that the
equilibrium properties of the bath can be well-defined, even when the system is
arbitrarily far from equilibrium and correlated with the bath. These states,
which retain a notion of temperature, take the form of conditional equilibrium
distributions. For out-of-equilibrium processes we show that the average
entropy production quantifies the extent to which the system-bath state is
driven away from the conditional equilibrium distribution. In addition, we show
that the stochastic entropy production satisfies a generalised Crooks relation
and can be used to quantify time-asymmetry of correlated non-equilibrium
processes. These results naturally extend the familiar properties of entropy
production in weakly-coupled systems to the strong coupling regime.
Experimental measurements of the entropy production at strong coupling could be
pursued using optomechanics or trapped ion systems, which allow strong coupling
to be engineered.Comment: 8 pages, 1 figure, comments welcom
Quantum correlation of light scattered by disordered media
We study theoretically how multiple scattering of light in a disordered
medium can spontaneously generate quantum correlations. In particular we focus
on the case where the input state is Gaussian and characterize the correlations
between two arbitrary output modes. As there is not a single all-inclusive
measure of correlation, we characterise the output correlations with three
measures: intensity fluctuations, entanglement, and quantum discord. We found
that, while a single mode coherent state input can not produce quantum
correlations, any other Gaussian input will produce them in one form or
another. This includes input states that are usually regarded as more classical
than coherent ones, such as thermal states, which will produce a non zero
quantum discord
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