18,574 research outputs found
Efficiency of harmonic quantum Otto engines at maximal power
Recent experimental breakthroughs produced the first nano heat engines that
have the potential to harness quantum resources. An instrumental question is
how their performance measures up against the efficiency of classical engines.
For single ion engines undergoing quantum Otto cycles it has been found that
the efficiency at maximal power is given by the Curzon-Ahlborn efficiency. This
is rather remarkable as the Curzon-Alhbron efficiency was originally derived
for endoreversible Carnot cycles. Here, we analyze two examples of
endoreversible Otto engines within the same conceptual framework as Curzon and
Ahlborn's original treatment. We find that for endoreversible Otto cycles in
classical harmonic oscillators the efficiency at maximal power is, indeed,
given by the Curzon-Ahlborn efficiency. However, we also find that the
efficiency of Otto engines made of quantum harmonic oscillators is
significantly larger.Comment: 6 pages, 2 figure
Solving spin quantum-master equations with matrix continued-fraction methods: application to superparamagnets
We implement continued-fraction techniques to solve exactly quantum master
equations for a spin with arbitrary S coupled to a (bosonic) thermal bath. The
full spin density matrix is obtained, so that along with relaxation and
thermoactivation, coherent dynamics is included (precession, tunnel, etc.). The
method is applied to study isotropic spins and spins in a bistable anisotropy
potential (superparamagnets). We present examples of static response, the
dynamical susceptibility including the contribution of the different relaxation
modes, and of spin resonance in transverse fields.Comment: Resubmitted to J. Phys. A: Math. Gen. Some rewriting here and there.
Discussion on positivity in App.D3 at request of one refere
Semiclassical instanton formulation of Marcus-Levich-Jortner theory
Marcus-Levich-Jortner (MLJ) theory is one of the most commonly used methods
for including nuclear quantum effects into the calculation of electron-transfer
rates and for interpreting experimental data. It divides the molecular problem
into a subsystem treated quantum-mechanically by Fermi's golden rule and a
solvent bath treated by classical Marcus theory. As an extension of this idea,
we here present a "reduced" semiclassical instanton theory, which is a
multiscale method for simulating quantum tunnelling of the subsystem in
molecular detail in the presence of a harmonic bath. We demonstrate that
instanton theory is typically significantly more accurate than the cumulant
expansion or the semiclassical Franck-Condon sum, which can give
orders-of-magnitude errors and in general do not obey detailed balance. As
opposed to MLJ theory, which is based on wavefunctions, instanton theory is
based on path integrals and thus does not require solutions of the
Schr\"odinger equation, nor even global knowledge of the ground- and
excited-state potentials within the subsystem. It can thus be efficiently
applied to complex, anharmonic multidimensional subsystems without making
further approximations. In addition to predicting accurate rates, instanton
theory gives a high level of insight into the reaction mechanism by locating
the dominant tunnelling pathway as well as providing information on the
reactant and product vibrational states involved in the reaction and the
activation energy in the bath similarly to what would be found with MLJ theory.Comment: 21 pages, 4 figure
Exact wave-packet decoherence dynamics in a discrete spectrum environment
We find an exact analytical solution of the reduced density matrix from the
Feynman-Vernon influence functional theory for a wave packet influenced by an
environment containing a few discrete modes. We obtain two intrinsic energy
scales relating to the time scales of the system and the environment. Different
relationship between these two scales alters the overall form of the solution
of the system. We also introduce a decoherence measure for a single wave packet
which is defined as the ratio of Schr\"odinger uncertainty over the
delocalization extension of the wave packet and characterizes the
time-evolution behavior of the off-diagonal reduced density matrix element. We
utilize the exact solution and the docherence measure to study the wave packet
decoherence dynamics. We further demonstrate how the dynamical diffusion of the
wave packet leads to non-Markovian decoherence in such a microscopic
environment.Comment: 12 pages, 2 figure
Quantum dynamics in strong fluctuating fields
A large number of multifaceted quantum transport processes in molecular
systems and physical nanosystems can be treated in terms of quantum relaxation
processes which couple to one or several fluctuating environments. A thermal
equilibrium environment can conveniently be modelled by a thermal bath of
harmonic oscillators. An archetype situation provides a two-state dissipative
quantum dynamics, commonly known under the label of a spin-boson dynamics. An
interesting and nontrivial physical situation emerges, however, when the
quantum dynamics evolves far away from thermal equilibrium. This occurs, for
example, when a charge transferring medium possesses nonequilibrium degrees of
freedom, or when a strong time-dependent control field is applied externally.
Accordingly, certain parameters of underlying quantum subsystem acquire
stochastic character. Herein, we review the general theoretical framework which
is based on the method of projector operators, yielding the quantum master
equations for systems that are exposed to strong external fields. This allows
one to investigate on a common basis the influence of nonequilibrium
fluctuations and periodic electrical fields on quantum transport processes.
Most importantly, such strong fluctuating fields induce a whole variety of
nonlinear and nonequilibrium phenomena. A characteristic feature of such
dynamics is the absence of thermal (quantum) detailed balance.Comment: review article, Advances in Physics (2005), in pres
A self-contained quantum harmonic engine
We propose a system made of three quantum harmonic oscillators as a compact
quantum engine for producing mechanical work. The three oscillators play
respectively the role of the hot bath, the working medium and the cold bath.
The working medium performs an Otto cycle during which its frequency is changed
and it is sequentially coupled to each of the two other oscillators. As the two
environments are finite, the lifetime of the machine is finite and after a
number of cycles it stops working and needs to be reset. We analyse the
entanglement and quantum discord generated during the strokes and show that
high work generation is always accompanied by large quantum correlations.Comment: Updated, published version. See also related but independent work
from Pozas-Kerstjens et al. arXiv:1708.0636
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