95 research outputs found
Decay process of quantum open system at finite-temperature
Starting from the formal solution to the Heisenberg equation, we revisit an
universal model for a quantum open system with a harmonic oscillator linearly
coupled to a boson bath. The analysis of the decay process for a Fock state and
a coherent state demonstrate that this method is very useful in dealing with
the problems in decay process of the open system. For finite temperature, the
calculations of the reduced density matrix and the mean excitation number for
the open system show that an initial coherent state will evolve into a
temperature-dependant coherent state after tracing over the bath variables.
Also in short-time limit, a temperature-dependant effective Hamiltonian for the
open system characterizes the decay process of the open system
Emergence of pulled fronts in fermionic microscopic particle models
We study the emergence and dynamics of pulled fronts described by the
Fisher-Kolmogorov-Petrovsky-Piscounov (FKPP) equation in the microscopic
reaction-diffusion process A + A A$ on the lattice when only a particle is
allowed per site. To this end we identify the parameter that controls the
strength of internal fluctuations in this model, namely, the number of
particles per correlated volume. When internal fluctuations are suppressed, we
explictly see the matching between the deterministic FKPP description and the
microscopic particle model.Comment: 4 pages, 4 figures. Accepted for publication in Phys. Rev. E as a
Rapid Communicatio
Dynamics of quantum entanglement in the reservoir with memory effects
The non-Markovian dynamics of quantum entanglement is studied by the
Shabani-Lidar master equation when one of entangled quantum systems is coupled
to a local reservoir with memory effects. The completely positive reduced
dynamical map can be constructed in the Kraus representation. Quantum
entanglement decays more slowly in the non-Markovian environment. The
decoherence time for quantum entanglement can be markedly increased by the
change of the memory kernel. It is found out that the entanglement sudden death
between quantum systems and entanglement sudden birth between the system and
reservoir occur at different instants.Comment: 14 pages, 3 figure
Quantum anti-Zeno effect in artificial quantum systems
In this paper, we study a quantum anti-Zeno effect (QAZE) purely induced by
repetitive measurements for an artificial atom interacting with a structured
bath. This bath can be artificially realized with coupled resonators in one
dimension and possesses photonic band structure like Bloch electron in a
periodic potential. In the presence of repetitive measurements, the pure QAZE
is discovered as the observable decay is not negligible even for the atomic
energy level spacing outside of the energy band of the artificial bath. If
there were no measurements, the decay would not happen outside of the band. In
this sense, the enhanced decay is completely induced by measurements through
the relaxation channels provided by the bath. Besides, we also discuss the
controversial golden rule decay rates originated from the van Hove's
singularities and the effects of the counter-rotating terms.Comment: 12 pages, 8 figure
Non-Markovian dynamics for an open two-level system without rotating wave approximation: Indivisibility versus backflow of information
By use of the two measures presented recently, the indivisibility and the
backflow of information, we study the non-Markovianity of the dynamics for a
two-level system interacting with a zero-temperature structured environment
without using rotating wave approximation (RWA). In the limit of weak coupling
between the system and the reservoir, and by expanding the time-convolutionless
(TCL) generator to the forth order with respect to the coupling strength, the
time-local non-Markovian master equation for the reduced state of the system is
derived. Under the secular approximation, the exact analytic solution is
obtained and the sufficient and necessary conditions for the indivisibility and
the backflow of information for the system dynamics are presented. In the more
general case, we investigate numerically the properties of the two measures for
the case of Lorentzian reservoir. Our results show the importance of the
counter-rotating terms to the short-time-scale non-Markovian behavior of the
system dynamics, further expose the relations between the two measures and
their rationality as non-Markovian measures. Finally, the complete positivity
of the dynamics of the considered system is discussed
Non-Markovian dynamics in a spin star system: The failure of thermalization
In most cases, a small system weakly interacting with a thermal bath will
finally reach the thermal state with the temperature of the bath. We show that
this intuitive picture is not always true by a spin star model where non-Markov
effect predominates in the whole dynamical process. The spin star system
consists a central spin homogeneously interacting with an ensemble of identical
noninteracting spins. We find that the correlation time of the bath is
infinite, which implies that the bath has a perfect memory, and that the
dynamical evolution of the central spin must be non- Markovian. A direct
consequence is that the final state of the central spin is not the thermal
state equilibrium with the bath, but a steady state which depends on its
initial state.Comment: 8 page
Exact quantum jump approach to open systems in Bosonic and spin baths
A general method is developed which enables the exact treatment of the
non-Markovian quantum dynamics of open systems through a Monte Carlo simulation
technique. The method is based on a stochastic formulation of the von Neumann
equation of the composite system and employs a pair of product states following
a Markovian random jump process. The performance of the method is illustrated
by means of stochastic simulations of the dynamics of open systems interacting
with a Bosonic reservoir at zero temperature and with a spin bath in the strong
coupling regime.Comment: 4 pages, 2 figure
Fronts with a Growth Cutoff but Speed Higher than
Fronts, propagating into an unstable state , whose asymptotic speed
is equal to the linear spreading speed of infinitesimal
perturbations about that state (so-called pulled fronts) are very sensitive to
changes in the growth rate for . It was recently found
that with a small cutoff, for ,
converges to very slowly from below, as . Here we show
that with such a cutoff {\em and} a small enhancement of the growth rate for
small behind it, one can have , {\em even} in the
limit . The effect is confirmed in a stochastic lattice model
simulation where the growth rules for a few particles per site are accordingly
modified.Comment: 4 pages, 4 figures, to appear in Rapid Comm., Phys. Rev.
Spin dynamic simulations of solid effect DNP: the role of the relaxation superoperator
Relaxation plays a crucial role in the spin dynamics of dynamic nuclear polarisation. We review here two different strategies that have recently been used to incorporate relaxation in models to predict the spin dynamics of solid effect dynamic nuclear polarisation. A detailed explanation is provided how the Lindblad-Kossakowski form of the master equation can be used to describe relaxation in a spin system. Fluctuations of the spin interactions with the environment as a cause of relaxation are discussed and it is demonstrated how the relaxation superoperator acting in Liouville space on the density operator can be derived in the Lindblad-Kossakowski form by averaging out non-secular terms in an appropriate interaction frame. Furthermore we provide a formalism for the derivation of the relaxation superoperator starting with a choice of a basis set in Hilbert space. We show that the differences in the prediction of the nuclear polarisation dynamics that are found for certain parameter choices arise from the use of different interaction frames in the two different strategies. In addition we provide a summary of different relaxation mechanism that need to be considered to obtain more realistic spin dynamic simulations of solid effect dynamic nuclear polarisation
Ac Stark Effects and Harmonic Generation in Periodic Potentials
The ac Stark effect can shift initially nonresonant minibands in
semiconductor superlattices into multiphoton resonances. This effect can result
in strongly enhanced generation of a particular desired harmonic of the driving
laser frequency, at isolated values of the amplitude.Comment: RevTeX, 10 pages (4 figures available on request), Preprint
UCSBTH-93-2
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