516 research outputs found
Population trapping due to cavity losses
In population trapping the occupation of a decaying quantum level keeps a
constant non-zero value. We show that an atom-cavity system interacting with an
environment characterized by a non-flat spectrum, in the non-Markovian limit,
exhibits such a behavior, effectively realizing the preservation of
nonclassical states against dissipation. Our results allow to understand the
role of cavity losses in hybrid solid state systems and pave the way to the
proper description of leakage in the recently developed cavity quantum
electrodynamic systems.Comment: 4 pages, 3 figures, version accepted for publication on Phys. Rev.
Quantum and Semiclassical Calculations of Cold Atom Collisions in Light Fields
We derive and apply an optical Bloch equation (OBE) model for describing
collisions of ground and excited laser cooled alkali atoms in the presence of
near-resonant light. Typically these collisions lead to loss of atoms from
traps. We compare the results obtained with a quantum mechanical complex
potential treatment, semiclassical Landau-Zener models with decay, and a
quantum time-dependent Monte-Carlo wave packet (MCWP) calculation. We formulate
the OBE method in both adiabatic and diabatic representations. We calculate the
laser intensity dependence of collision probabilities and find that the
adiabatic OBE results agree quantitatively with those of the MCWP calculation,
and qualitatively with the semiclassical Landau-Zener model with delayed decay,
but that the complex potential method or the traditional Landau-Zener model
fail in the saturation limit.Comment: 21 pages, RevTex, 7 eps figures embedded using psfig, see also
http://www.physics.helsinki.fi/~kasuomin
Sudden death and sudden birth of entanglement in common structured reservoirs
We study the exact entanglement dynamics of two qubits in a common structured
reservoir. We demonstrate that, for certain classes of entangled states,
entanglement sudden death occurs, while for certain initially factorized
states, entanglement sudden birth takes place. The backaction of the
non-Markovian reservoir is responsible for revivals of entanglement after
sudden death has occurred, and also for periods of disentanglement following
entanglement sudden birth.Comment: 4 pages, 2 figure
Environment-dependent dissipation in quantum Brownian motion
The dissipative dynamics of a quantum Brownian particle is studied for
different types of environment. We derive analytic results for the time
evolution of the mean energy of the system for Ohmic, sub-Ohmic and super-Ohmic
environments, without performing the Markovian approximation. Our results allow
to establish a direct link between the form of the environmental spectrum and
the thermalization dynamics. This in turn leads to a natural explanation of the
microscopic physical processes ruling the system time evolution both in the
short-time non-Markovian region and in the long-time Markovian one. Our
comparative study of thermalization for different environments sheds light on
the physical contexts in which non-Markovian dissipation effects are dominant.Comment: 10 pages, 6 figures, v2: added new references and paragraph
Nonlinear level crossing models
We examine the effect of nonlinearity at a level crossing on the probability
for nonadiabatic transitions . By using the Dykhne-Davis-Pechukas formula,
we derive simple analytic estimates for for two types of nonlinear
crossings. In the first type, the nonlinearity in the detuning appears as a
{\it perturbative} correction to the dominant linear time dependence. Then
appreciable deviations from the Landau-Zener probability are found to
appear for large couplings only, when is very small; this explains why the
Landau-Zener model is often seen to provide more accurate results than
expected. In the second type of nonlinearity, called {\it essential}
nonlinearity, the detuning is proportional to an odd power of time. Then the
nonadiabatic probability is qualitatively and quantitatively different from
because on the one hand, it vanishes in an oscillatory manner as the
coupling increases, and on the other, it is much larger than . We
suggest an experimental situation when this deviation can be observed.Comment: 9 pages final postscript file, two-column revtex style, 5 figure
Atomic collision dynamics in optical lattices
We simulate collisions between two atoms, which move in an optical lattice
under the dipole-dipole interaction. The model describes simultaneously the two
basic dynamical processes, namely the Sisyphus cooling of single atoms, and the
light-induced inelastic collisions between them. We consider the J=1/2 -> J=3/2
laser cooling transition for Cs, Rb and Na. We find that the hotter atoms in a
thermal sample are selectively lost or heated by the collisions, which modifies
the steady state distribution of atomic velocities, reminiscent of the
evaporative cooling process.Comment: 17 pages, 15 figure
Weisskopf-Wigner model for wave packet excitation
We consider a laser induced molecular excitation process as a decay of a
single energy state into a continuum. The analytic results based on
Weisskopf-Wigner approach and perturbation calculations are compared with
numerical wave packet results. We find that the decay model describes the
excitation process well within the expected parameter region.Comment: 14 pages, Latex2.09, 9 Postscript figures embedded using psfig, see
also http://www.physics.helsinki.fi/~kasuomin
Cavity losses for the dissipative Jaynes-Cummings Hamiltonian beyond Rotating Wave Approximation
A microscopic derivation of the master equation for the
Jaynes-Cummings model with cavity losses is given, taking into account the
terms in the dissipator which vary with frequencies of the order of the vacuum
Rabi frequency. Our approach allows to single out physical contexts wherein the
usual phenomenological dissipator turns out to be fully justified and
constitutes an extension of our previous analysis [Scala M. {\em et al.} 2007
Phys. Rev. A {\bf 75}, 013811], where a microscopic derivation was given in the
framework of the Rotating Wave Approximation.Comment: 12 pages, 1 figur
Counterintuitive transitions in multistate curve crossing involving linear potentials
Two problems incorporating a set of horizontal linear potentials crossed by a
sloped linear potential are analytically solved and compared with numerical
results: (a) the case where boundary conditions are specified at the ends of a
finite interval, and (b) the case where the sloped linear potential is replaced
by a piecewise-linear sloped potential and the boundary conditions are
specified at infinity. In the approximation of small gaps between the
horizontal potentials, an approach similar to the one used for the degenerate
problem (Yurovsky V A and Ben-Reuven A 1998 J. Phys. B 31,1) is applicable for
both problems. The resulting scattering matrix has a form different from the
semiclassical result obtained by taking the product of Landau-Zener amplitudes.
Counterintuitive transitions involving a pair of successive crossings, in which
the second crossing precedes the first one along the direction of motion, are
allowed in both models considered here.Comment: LaTeX 2.09 using ioplppt.sty and psfig.sty, 16 pages with 5 figures.
Submitted to J. Phys.
The validity of the Landau-Zener model for output coupling of Bose condensates
We investigate the validity of the Landau-Zener model in describing the
output coupling of Bose condensates from magnetic traps by a chirped
radiofrequency field. The predictions of the model are compared with the
numerical solutions of the Gross-Pitaevskii equation. We find a dependence on
the chirp direction, and also quantify the role of gravitation.Comment: 4 pages, Late
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