263 research outputs found
Cold collisions between atoms in optical lattices
We have simulated binary collisions between atoms in optical lattices during
Sisyphus cooling. Our Monte Carlo Wave Function simulations show that the
collisions selectively accelerate mainly the hotter atoms in the thermal
ensemble, and thus affect the steady state which one would normally expect to
reach in Sisyphus cooling without collisions.Comment: 4 pages, 1 figur
Quantum-Enhanced continuous-wave stimulated Raman spectroscopy
Stimulated Raman spectroscopy has become a powerful tool to study the
spatiodynamics of molecular bonds with high sensitivity, resolution and speed.
However, sensitivity and speed of state-of-the-art stimulated Raman
spectroscopy are currently limited by the shot-noise of the light beam probing
the Raman process. Here, we demonstrate an enhancement of the sensitivity of
continuous-wave stimulated Raman spectroscopy by reducing the quantum noise of
the probing light below the shot-noise limit by means of amplitude squeezed
states of light. Probing polymer samples with Raman shifts around 2950
with squeezed states, we demonstrate a quantum-enhancement of the
stimulated Raman signal-to-noise ratio (SNR) of 3.60 dB relative to the
shot-noise limited SNR. Our proof-of-concept demonstration of quantum-enhanced
Raman spectroscopy paves the way for a new generation of Raman microscopes,
where weak Raman transitions can be imaged without the use of markers or an
increase in the total optical power.Comment: 6 pages, 6 figure
Stabilization of the number of Bose-Einstein condensed atoms in evaporative cooling via three-body recombination loss
The dynamics of evaporative cooling of magnetically trapped Rb atoms
is studied on the basis of the quantum kinetic theory of a Bose gas. We carried
out the quantitative calculations of the time evolution of conventional
evaporative cooling where the frequency of the radio-frequency magnetic field
is swept exponentially. This "exponential-sweep cooling" is known to become
inefficient at the final stage of the cooling process due to a serious
three-body recombination loss. We precisely examine how the growth of a
Bose-Einstein condensate depends on the experimental parameters of evaporative
cooling, such as the initial number of trapped atoms, the initial temperature,
and the bias field of a magnetic trap. It is shown that three-body
recombination drastically depletes the trapped Rb atoms as the system
approaches the quantum degenerate region and the number of condensed atoms
finally becomes insensitive to these experimental parameters. This result
indicates that the final number of condensed atoms is well stabilized by a
large nonlinear three-body loss against the fluctuations of experimental
conditions in evaporative cooling.Comment: 7 pages, REVTeX4, 8 eps figures, Phys. Rev A in pres
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
Loop structure of the lowest Bloch band for a Bose-Einstein condensate
We investigate analytically and numerically Bloch waves for a Bose--Einstein
condensate in a sinusoidal external potential. At low densities the dependence
of the energy on the quasimomentum is similar to that for a single particle,
but at densities greater than a critical one the lowest band becomes
triple-valued near the boundary of the first Brillouin zone and develops the
structure characteristic of the swallow-tail catastrophe. We comment on the
experimental consequences of this behavior.Comment: 4 pages, 7 figure
Modulational instability in cigar shaped Bose-Einstein condensates in optical lattices
A self consistent theory of a cigar shaped Bose-Einstein condensate (BEC)
periodically modulated by a laser beam is presented. We show, both
theoretically and numerically, that modulational instability/stability is the
mechanism by which wavefunctions of soliton type can be generated in cigar
shaped BEC subject to a 1D optical lattice. The theory explains why bright
solitons can exist in BEC with positive scattering length and why condensate
with negative scattering length can be stable and give rise to dark solitary
pulses.Comment: Submitted, 4 pages, 3 figures. Revised versio
Bose-Einstein condensates in standing waves: The cubic nonlinear Schroedinger equation with a periodic potential
We present a new family of stationary solutions to the cubic nonlinear
Schroedinger equation with a Jacobian elliptic function potential. In the limit
of a sinusoidal potential our solutions model a dilute gas Bose-Einstein
condensate trapped in a standing light wave. Provided the ratio of the height
of the variations of the condensate to its DC offset is small enough, both
trivial phase and nontrivial phase solutions are shown to be stable. Numerical
simulations suggest such stationary states are experimentally observable.Comment: 4 pages, 4 figure
Modulated Amplitude Waves in Bose-Einstein Condensates
We analyze spatio-temporal structures in the Gross-Pitaevskii equation to
study the dynamics of quasi-one-dimensional Bose-Einstein condensates (BECs)
with mean-field interactions. A coherent structure ansatz yields a
parametrically forced nonlinear oscillator, to which we apply Lindstedt's
method and multiple-scale perturbation theory to determine the dependence of
the intensity of periodic orbits (``modulated amplitude waves'') on their wave
number. We explore BEC band structure in detail using Hamiltonian perturbation
theory and supporting numerical simulations.Comment: 5 pages, 4 figs, revtex, final form of paper, to appear in PRE
(forgot to include \bibliography command in last update, so this is a
correction of that; the bibliography is hence present again
Bogoliubov sound speed in periodically modulated Bose-Einstein condensates
We study the Bogoliubov excitations of a Bose-condensed gas in an optical
lattice. Of primary interest is the long wavelength phonon dispersion for both
current-free and current-carrying condensates. We obtain the dispersion
relation by carrying out a systematic expansion of the Bogoliubov equations in
powers of the phonon wave vector. Our result for the current-carrying case
agrees with the one recently obtained by means of a hydrodynamic theory.Comment: 16 pages, no figure
- …