4,255 research outputs found
The multifragmentation of spectator matter
We present the first microscopic calculation of the spectator fragmentation
observed in heavy ion reactions at relativistic energies which reproduces the
slope of the kinetic energy spectra of the fragments as well as their
multiplicity, both measured by the ALADIN collaboration. In the past both have
been explained in thermal models, however with vastly different assumptions
about the excitation energy and the density of the system. We show that both
observables are dominated by dynamical processes and that the system does not
pass a state of thermal equilibrium. These findings question the recent
conjecture that in these collisions a phase transition of first order, similar
to that between water and vapor, can be observed.Comment: 7 page
Domain Growth in Random Magnets
We study the kinetics of domain growth in ferromagnets with random exchange
interactions. We present detailed Monte Carlo results for the nonconserved
random-bond Ising model, which are consistent with power-law growth with a
variable exponent. These results are interpreted in the context of disorder
barriers with a logarithmic dependence on the domain size. Further, we clarify
the implications of logarithmic barriers for both nonconserved and conserved
domain growth.Comment: 7 pages, 4 figure
Nakajima-Zwanzig versus time-convolutionless master equation for the non-Markovian dynamics of a two-level system
We consider the exact reduced dynamics of a two-level system coupled to a
bosonic reservoir, further obtaining the exact time-convolutionless and
Nakajima-Zwanzig non-Markovian equations of motion. The considered system
includes the damped and undamped Jaynes-Cummings model. The result is obtained
by exploiting an expression of quantum maps in terms of matrices and a simple
relation between the time evolution map and time-convolutionless generator as
well as Nakajima-Zwanzig memory kernel. This non-perturbative treatment shows
that each operator contribution in Lindblad form appearing in the exact
time-convolutionless master equation is multiplied by a different time
dependent function. Similarly, in the Nakajima-Zwanzig master equation each
such contribution is convoluted with a different memory kernel. It appears that
depending on the state of the environment the operator structures of the two
set of equations of motion can exhibit important differences.Comment: 12 pages, no figure
Amplification of Fluctuations in Unstable Systems with Disorder
We study the early-stage kinetics of thermodynamically unstable systems with
quenched disorder. We show analytically that the growth of initial fluctuations
is amplified by the presence of disorder. This is confirmed by numerical
simulations of morphological phase separation (MPS) in thin liquid films and
spinodal decomposition (SD) in binary mixtures. We also discuss the
experimental implications of our results.Comment: 15 pages, 4 figure
Controlling Entanglement Generation in External Quantum Fields
Two, non-interacting two-level atoms immersed in a common bath can become
mutually entangled when evolving with a Markovian, completely positive
dynamics. For an environment made of external quantum fields, this phenomenon
can be studied in detail: one finds that entanglement production can be
controlled by varying the bath temperature and the distance between the atoms.
Remarkably, in certain circumstances, the quantum correlations can persist in
the asymptotic long-time regime.Comment: 12 pages, to appear in J. Opt. B: Quantum Semiclass. Op
Controlling entanglement by direct quantum feedback
We discuss the generation of entanglement between electronic states of two
atoms in a cavity using direct quantum feedback schemes. We compare the effects
of different control Hamiltonians and detection processes in the performance of
entanglement production and show that the quantum-jump-based feedback proposed
by us in Phys. Rev. A {\bf 76} 010301(R) (2007) can protect highly entangled
states against decoherence. We provide analytical results that explain the
robustness of jump feedback, and also analyse the perspectives of experimental
implementation by scrutinising the effects of imperfections and approximations
in our model.Comment: 10 pages, 8 figures. To appear in PR
Driving-dependent damping of Rabi oscillations in two-level semiconductor systems
We propose a mechanism to explain the nature of the damping of Rabi
oscillations with increasing driving-pulse area in localized semiconductor
systems, and have suggested a general approach which describes a coherently
driven two-level system interacting with a dephasing reservoir. Present
calculations show that the non-Markovian character of the reservoir leads to
the dependence of the dephasing rate on the driving-field intensity, as
observed experimentally. Moreover, we have shown that the damping of Rabi
oscillations might occur as a result of different dephasing mechanisms for both
stationary and non-stationary effects due to coupling to the environment.
Present calculated results are found in quite good agreement with available
experimental measurements
Relaxation to equilibrium driven via indirect control in Markovian dynamics
We characterize to what extent it is possible to modify the stationary states
of a quantum dynamical semigroup, that describes the irreversible evolution of
a two-level system, by means of an auxiliary two-level system. We consider
systems that can be initially entangled or uncorrelated. We find that the
indirect control of the stationary states is possible, even if there are not
initial correlations, under suitable conditions on the dynamical parameters
characterizing the evolution of the joint system.Comment: revtex4, 7 page
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