702 research outputs found
Multimode Fock states with large photon number: effective descriptions and applications in quantum metrology
We develop general tools to characterise and efficiently compute relevant
observables of multimode -photon states generated in non-linear decays in
one-dimensional waveguides. We then consider optical interferometry in a
Mach-Zender interferometer where a -mode photonic state enters in each arm
of the interferometer. We derive a simple expression for the Quantum Fisher
Information in terms of the average photon number in each mode, and show that
it can be saturated by number-resolved photon measurements that do not
distinguish between the different modes.Comment: 18 pages, 11 figures. V2: Minor change
Work and entropy production in generalised Gibbs ensembles
Recent years have seen an enormously revived interest in the study of
thermodynamic notions in the quantum regime. This applies both to the study of
notions of work extraction in thermal machines in the quantum regime, as well
as to questions of equilibration and thermalisation of interacting quantum
many-body systems as such. In this work we bring together these two lines of
research by studying work extraction in a closed system that undergoes a
sequence of quenches and equilibration steps concomitant with free evolutions.
In this way, we incorporate an important insight from the study of the dynamics
of quantum many body systems: the evolution of closed systems is expected to be
well described, for relevant observables and most times, by a suitable
equilibrium state. We will consider three kinds of equilibration, namely to (i)
the time averaged state, (ii) the Gibbs ensemble and (iii) the generalised
Gibbs ensemble (GGE), reflecting further constants of motion in integrable
models. For each effective description, we investigate notions of entropy
production, the validity of the minimal work principle and properties of
optimal work extraction protocols. While we keep the discussion general, much
room is dedicated to the discussion of paradigmatic non-interacting fermionic
quantum many-body systems, for which we identify significant differences with
respect to the role of the minimal work principle. Our work not only has
implications for experiments with cold atoms, but also can be viewed as
suggesting a mindset for quantum thermodynamics where the role of the external
heat baths is instead played by the system itself, with its internal degrees of
freedom bringing coarse-grained observables to equilibrium.Comment: 22 pages, 4 figures, improvements in presentatio
Strong coupling corrections in quantum thermodynamics
Quantum systems strongly coupled to many-body systems equilibrate to the
reduced state of a global thermal state, deviating from the local thermal state
of the system as it occurs in the weak-coupling limit. Taking this insight as a
starting point, we study the thermodynamics of systems strongly coupled to
thermal baths. First, we provide strong-coupling corrections to the second law
applicable to general systems in three of its different readings: As a
statement of maximal extractable work, on heat dissipation, and bound to the
Carnot efficiency. These corrections become relevant for small quantum systems
and always vanish in first order in the interaction strength. We then move to
the question of power of heat engines, obtaining a bound on the power
enhancement due to strong coupling. Our results are exemplified on the
paradigmatic situation of non-Markovian quantum Brownian motion.Comment: 20 pages, 3 figures, version two is substantially revised and
contains new result
Iron Displacements and Magnetoelastic Coupling in the Spin-Ladder Compound BaFe2Se3
We report long-range ordered antiferromagnetism concomitant with local iron
displacements in the spin-ladder compound BaFeSe. Short-range magnetic
correlations, present at room temperature, develop into long-range
antiferromagnetic order below T = 256 K, with no superconductivity down to
1.8 K. Built of ferromagnetic Fe plaquettes, the magnetic ground state
correlates with local displacements of the Fe atoms. These iron displacements
imply significant magnetoelastic coupling in FeX-based materials, an
ingredient hypothesized to be important in the emergence of superconductivity.
This result also suggests that knowledge of these local displacements is
essential for properly understanding the electronic structure of these systems.
As with the copper oxide superconductors two decades ago, our results highlight
the importance of reduced dimensionality spin ladder compounds in the study of
the coupling of spin, charge, and atom positions in superconducting materials
Orbital Selective Magnetism in the Spin-Ladder Iron Selenides BaKFeSe
Here we show that the 2.80(8) {\mu}B/Fe block antiferromagnetic order of
BaFe2Se3 transforms into stripe antiferromagnetic order in KFe2Se3 with a
decrease in moment to 2.1(1) {\mu}B/Fe. This reduction is larger than expected
from the change in electron count from Ba to K, and occurs with
the loss of the displacements of Fe atoms from ideal positions in the ladders,
as found by neutron pair distribution function analysis. Intermediate
compositions remain insulating, and magnetic susceptibility measurements show a
suppression of magnetic order and probable formation of a spin-glass. Together,
these results imply an orbital-dependent selection of magnetic versus bonded
behavior, driven by relative bandwidths and fillings.Comment: Final versio
Work Fluctuations in Slow Processes: Quantum Signatures and Optimal Control
This is the final version. Available from American Physical Society via the DOI in this recordAn important result in classical stochastic thermodynamics is the work fluctuation-dissipation relation (FDR), which states that the dissipated work done along a slow process is proportional to the resulting work fluctuations. We show that slowly driven quantum systems violate this FDR whenever quantum coherence is generated along the protocol, and we derive a quantum generalization of the work FDR. The additional quantum terms in the FDR are found to lead to a non-Gaussian work distribution. Fundamentally, our result shows that quantum fluctuations prohibit finding slow protocols that minimize both dissipation and fluctuations simultaneously, in contrast to classical slow processes. Instead, we develop a quantum geometric framework to find processes with an optimal trade-off between the two quantities.Engineering and Physical Sciences Research Council (EPSRC)National Science FoundationRoyal Societ
The second law and beyond in microscopic quantum setups
The Clausius inequality (CI) is one of the most versatile forms of the second
law. Although it was originally conceived for macroscopic steam engines, it is
also applicable to quantum single particle machines. Moreover, the CI is the
main connecting thread between classical microscopic thermodynamics and
nanoscopic quantum thermodynamics. In this chapter, we study three different
approaches for obtaining the CI. Each approach shows different aspects of the
CI. The goals of this chapter are: (i) To show the exact assumptions made in
various derivations of the CI. (ii) To elucidate the structure of the second
law and its origin. (iii) To discuss the possibilities each approach offers for
finding additional second-law like inequalities. (iv) To pose challenges
related to the second law in nanoscopic setups. In particular, we introduce and
briefly discuss the notions of exotic heat machines (X machines), and "lazy
demons".Comment: As a chapter of: F. Binder, L. A. Correa, C. Gogolin, J. Anders, and
G. Adesso (eds.), "Thermodynamics in the quantum regime - Recent Progress and
Outlook", (Springer International Publishing). v1 does not include references
to other book chapter
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