275 research outputs found
Shortcuts to adiabaticity from linear response theory
A shortcut to adiabaticity is a finite-time process that produces the same
final state as would result from infinitely slow driving. We show that such
shortcuts can be found for weak perturbations from linear response theory. With
the help of phenomenological response functions a simple expression for the
excess work is found -- quantifying the nonequilibrium excitations. For two
specific examples, the quantum parametric oscillator and the spin-1/2 in a
time-dependent magnetic field, we show that finite-time zeros of the excess
work indicate the existence of shortcuts. Finally, we propose a degenerate
family of protocols, which facilitate shortcuts to adiabaticity for specific
and very short driving times.Comment: 9 pages, 8 figure; published versio
Thermodynamic control -- an old paradigm with new applications
Tremendous research efforts have been invested in exploring and designing
so-called shortcuts to adiabaticity. These are finite-time processes that
produce the same final states that would result from infinitely slow driving.
Most of these techniques rely on auxiliary fields and quantum control
techniques, which makes them rather costly to implement. In this Perspective we
outline an alternative paradigm for optimal control that has proven powerful in
a wide variety of situations ranging from heat engines over chemical reactions
to quantum dynamics -- thermodynamic control. Focusing on only a few, selected
milestones we seek to provide a pedagogical entry point into this powerful and
versatile framework.Comment: 7 pages, 1 figure; Short review paper intended as Perspective in EPL
(Europhys. Lett
Holevo's bound from a general quantum fluctuation theorem
We give a novel derivation of Holevo's bound using an important result from
nonequilibrium statistical physics, the fluctuation theorem. To do so we
develop a general formalism of quantum fluctuation theorems for two-time
measurements, which explicitly accounts for the back action of quantum
measurements as well as possibly non-unitary time evolution. For a specific
choice of observables this fluctuation theorem yields a measurement-dependent
correction to the Holevo bound, leading to a tighter inequality. We conclude by
analyzing equality conditions for the improved bound.Comment: 5 page
Assessing performance of quantum annealing with non-linear driving
Current generation quantum annealers have already proven to be successful
problem-solvers. Yet, quantum annealing is still very much in its infancy, with
suboptimal applicability. For instance, to date it is still an open question
which annealing protocol will universally cause the fewest diabatic
excitations, and even whether there is a universally optimal strategy.
Therefore, in the present work, we report analytical and numerical studies of
the diabatic excitations arising from non-linear protocols applied to the
transverse field Ising chain, the exactly solvable model that serves as quantum
annealing playground. Our analysis focuses on several driving schemes that
inhibit or facilitate the dynamic phases discussed in a previous work. Rather
remarkably, we find that the paradigmatic Kibble-Zurek behavior can be
suppressed with "pauses" in the evolution, both for crossing and stopping at
the quantum critical point of the system
Quantum fluctuation theorems in the strong damping limit
We consider a driven quantum particle in the strong friction regime described
by the quantum Smoluchowski equation. We derive Crooks and Jarzynski type
relations for the reduced quantum system by properly generalizing the entropy
production to take into account the non-Gibbsian character of the equilibrium
distribution. In the case of a nonequilibrium steady state, we obtain a quantum
version of the Hatano-Sasa relation. We, further, propose an experiment with
driven Josephson junctions that would allow to investigate nonequilibrium
entropy fluctuations in overdamped quantum systems.Comment: 6 pages, 2 figures, with simplified derivation and examples adde
Quantum Fluctuation Relations for the Lindblad Master Equation
An open quantum system interacting with its environment can be modeled under
suitable assumptions as a Markov process, described by a Lindblad master
equation. In this work, we derive a general set of fluctuation relations for
systems governed by a Lindblad equation. These identities provide quantum
versions of Jarzynski-Hatano-Sasa and Crooks relations. In the linear response
regime, these fluctuation relations yield a fluctuation-dissipation theorem
(FDT) valid for a stationary state arbitrarily far from equilibrium. For a
closed system, this FDT reduces to the celebrated Callen-Welton-Kubo formula
Measurement of inclusive D*+- and associated dijet cross sections in photoproduction at HERA
Inclusive photoproduction of D*+- mesons has been measured for photon-proton
centre-of-mass energies in the range 130 < W < 280 GeV and a photon virtuality
Q^2 < 1 GeV^2. The data sample used corresponds to an integrated luminosity of
37 pb^-1. Total and differential cross sections as functions of the D*
transverse momentum and pseudorapidity are presented in restricted kinematical
regions and the data are compared with next-to-leading order (NLO) perturbative
QCD calculations using the "massive charm" and "massless charm" schemes. The
measured cross sections are generally above the NLO calculations, in particular
in the forward (proton) direction. The large data sample also allows the study
of dijet production associated with charm. A significant resolved as well as a
direct photon component contribute to the cross section. Leading order QCD
Monte Carlo calculations indicate that the resolved contribution arises from a
significant charm component in the photon. A massive charm NLO parton level
calculation yields lower cross sections compared to the measured results in a
kinematic region where the resolved photon contribution is significant.Comment: 32 pages including 6 figure
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