472 research outputs found
Role of Bose enhancement in photoassociation
We discuss the role of Bose enhancement of the dipole matrix element in
photoassociation, using stimulated Raman adiabatic passage as an example. In a
nondegenerate gas the time scale for coherent optical transients tends to
infinity in the thermodynamic limit, whereas Bose enhancement keeps this time
scale finite in a condensate. Coherent transients are therefore absent in
photoassociation of a thermal nondegenerate gas, but are feasible if the gas is
a condensate.Comment: 14 pages, 2 figure
Mean-Field Theory of Feshbach-Resonant Interactions in 85Rb Condensates
Recent Feshbach-resonance experiments with 85Rb Bose-Einstein condensates
have led to a host of unexplained results: dramatic losses of condensate atoms
for an across-resonance sweep of the magnetic field, a collapsing condensate
with a burst of atoms emanating from the remnant condensate, increased losses
for decreasing interaction times-- until short times are reached, and seemingly
coherent oscillations between remnant and burst atoms. Using a simple yet
realistic mean-field model, we find that rogue dissociation, molecular
dissociation to noncondensate atom pairs, is strongly implicated as the
physical mechanism responsible for these observations.Comment: v2: numbers changed, not conclusions; 5 pages, 3 figures, submitted
to PR
Rate limit for photoassociation of a Bose-Einstein condensate
We simulate numerically the photodissociation of molecules into noncondensate
atom pairs that accompanies photoassociation of an atomic Bose-Einstein
condensate into a molecular condensate. Such rogue photodissociation sets a
limit on the achievable rate of photoassociation. Given the atom density \rho
and mass m, the limit is approximately 6\hbar\rho^{2/3}/m. At low temperatures
this is a more stringent restriction than the unitary limit of scattering
theory.Comment: 5 pgs, 18 refs., 3 figs., submitted to Phys. Rev. Let
Mean-field stationary state of a Bose gas at a Feshbach resonance
We study the steady state of a zero-temperature Bose gas near a Feshbach or
photoassociation resonance using a two-channel mean-field model that
incorporates atomic and molecular condensates, as well as correlated atom pairs
originating from dissociation of molecules into pairs of atoms. We start from a
many-body Hamiltonian for atom-molecule conversion, and derive the time
dependent version of the mean-field theory. The stationary solution of the time
dependent model is rendered unique with an approximation that entails that all
noncondensate atoms are correlated, as if emerging from dissociation of
molecules. The steady state is solved numerically, but limiting cases are also
found analytically. The system has a phase transition in which the atomic
condensate emerges in a nonanalytic fashion. We quantify the scaling of the
observable quantities, such as fractions of atomic and molecular condensates,
with the detuning and the atom-molecule conversion strength. Qualitatively, the
dependence on detuning rounds out with increasing coupling strength. A study of
the thermodynamics shows that the pressure of the atom-molecule system is
negative, even on the molecule side of the resonance. This indicates the
possibility of mechanical instability
Cooling trapped atoms in optical resonators
We derive an equation for the cooling dynamics of the quantum motion of an
atom trapped by an external potential inside an optical resonator. This
equation has broad validity and allows us to identify novel regimes where the
motion can be efficiently cooled to the potential ground state. Our result
shows that the motion is critically affected by quantum correlations induced by
the mechanical coupling with the resonator, which may lead to selective
suppression of certain transitions for the appropriate parameters regimes,
thereby increasing the cooling efficiency.Comment: 4 pages, 3 figures; version published in PR
Dynamic splitting of a Bose-Einstein Condensate
We study the dynamic process of splitting a condensate by raising a potential
barrier in the center of a harmonic trap. We use a two-mode model to describe
the phase coherence between the two halves of the condensate. Furthermore, we
explicitly consider the spatial dependence of the mode funtions, which varies
depending on the potential barrier. This allows to get the tunneling coupling
between the two wells and the on-site energy as a function of the barrier
height. Moreover we can get some insight on the collective modes which are
excited by raising the barrier. We describe the internal and external degrees
of freedom by variational ansatz. We distinguish the possible regimes as a
function of the characteristic parameters of the problem and identify the
adiabaticity conditions.Comment: 17 pages, 8 figure
Non-destructive optical measurement of relative phase between two Bose condensates
We study the interaction of light with two Bose condensates as an open
quantum system. The two overlapping condensates occupy two different Zeeman
sublevels and two driving light beams induce a coherent quantum tunneling
between the condensates. We derive the master equation for the system. It is
shown that stochastic simulations of the measurements of spontaneously
scattered photons establish the relative phase between two Bose condensates,
even though the condensates are initially in pure number states. These
measurements are non-destructive for the condensates, because only light is
scattered, but no atoms are removed from the system. Due to the macroscopic
quantum interference the detection rate of photons depends substantially on the
relative phase between the condensates. This may provide a way to distinguish,
whether the condensates are initially in number states or in coherent states.Comment: 26 pages, RevTex, 8 postscript figures, 1 MacBinary eps-figur
Pairing mean-field theory for the dynamics of dissociation of molecular Bose-Einstein condensates
We develop a pairing mean-field theory to describe the quantum dynamics of
the dissociation of molecular Bose-Einstein condensates into their constituent
bosonic or fermionic atoms. We apply the theory to one, two, and
three-dimensional geometries and analyze the role of dimensionality on the atom
production rate as a function of the dissociation energy. As well as
determining the populations and coherences of the atoms, we calculate the
correlations that exist between atoms of opposite momenta, including the column
density correlations in 3D systems. We compare the results with those of the
undepleted molecular field approximation and argue that the latter is most
reliable in fermionic systems and in lower dimensions. In the bosonic case we
compare the pairing mean-field results with exact calculations using the
positive- stochastic method and estimate the range of validity of the
pairing mean-field theory. Comparisons with similar first-principle simulations
in the fermionic case are currently not available, however, we argue that the
range of validity of the present approach should be broader for fermions than
for bosons in the regime where Pauli blocking prevents complete depletion of
the molecular condensate.Comment: 16 pages, 10 figure
Directional `superradiant' collisions: bosonic amplification of atom pairs emitted from an elongated Bose-Einstein condensate
We study spontaneous directionality in the bosonic amplification of atom
pairs emitted from an elongated Bose-Einstein condensate (BEC), an effect
analogous to `superradiant' emission of atom-photon pairs. Using a simplified
model, we make analytic predictions regarding directional effects for both
atom-atom and atom-photon emission. These are confirmed by numerical mean-field
simulations, demonstrating the the feasibility of nearly perfect directional
emission along the condensate axis. The dependence of the emission angle on the
pump strength for atom-atom pairs is significantly different than for
atom-photon pairs
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