Inelastic collisions in an exactly solvable two-mode Bose-Einstein Condensate

Inelastic collisions occur in Bose-Einstein condensates, in some cases, producing particle loss in the system. Nevertheless, these processes have not been studied in the case when particles do not escape the trap. We show that such inelastic processes are relevant in quantum properties of the system such as the evolution of the relative population, the self trapping effect and the probability distribution of particles. Moreover, including inelastic terms in the model of the two-mode condensate allows for an exact analytical solution. Using this solution, we show that collisions favor the generation of entanglement between the modes of the condensate as long as the collision rate does not exceed the natural frequency of the system

Atom loss from Bose-Einstein condensates due to Feshbach resonance

In recent experiments on Na Bose-Einstein condensates [S. Inouye et al, Nature 392, 151 (1998); J. Stenger et al, Phys. Rev. Lett. 82, 2422 (1999)], large loss rates were observed when a time-varying magnetic field was used to tune a molecular Feshbach resonance state near the state of pairs of atoms belonging to the condensate many-body wavefunction. A mechanism is offered here to account for the observed losses, based on the deactivation of the resonant molecular state by interaction with a third condensate atom.Comment: LaTeX, 4 pages, 4 PostScript figures, uses REVTeX and psfig, submitted to Physical Review A, Rapid Communication

Influence of a tight isotropic harmonic trap on photoassociation in ultracold homonuclear alkali gases

The influence of a tight isotropic harmonic trap on photoassociation of two ultracold alkali atoms forming a homonuclear diatomic is investigated using realistic atomic interaction potentials. Confinement of the initial atom pair due to the trap leads to a uniform strong enhancement of the photoassociation rate to most, but also to a strongly suppressed rate for some final states. Thus tighter traps do not necessarily enhance the photoassociation rate. A further massive enhancement of the rate is found for strong interatomic interaction potentials. The details of this interaction play a minor role, except for large repulsive interactions for which a sharp window occurs in the photoassociation spectrum as is known from the trap-free case. A comparison with simplified models describing the atomic interaction like the pseudopotential approximation shows that they often provide reasonable estimates for the trap-induced enhancement of the photoassociation rate even if the predicted rates can be completely erroneous.Comment: 19 pages, 17 figure

Collisions of cold magnesium atoms in a weak laser field

We use quantum scattering methods to calculate the light-induced collisional loss of laser-cooled and trapped magnesium atoms for detunings up to 30 atomic linewidths to the red of the 1S_0-1P_1 cooling transition. Magnesium has no hyperfine structure to complicate the theoretical studies. We evaluate both the radiative and nonradiative mechanisms of trap loss. The radiative escape mechanism via allowed 1Sigma_u excitation is dominant for more than about one atomic linewidth detuning. Molecular vibrational structure due to photoassociative transitions to bound states begins to appear beyond about ten linewidths detuning.Comment: 4 pages with 3 embedded figure

Loading Bose condensed atoms into the ground state of an optical lattice

We optimize the turning on of a one-dimensional optical potential, V_L(x,t) = S(t) V_0 cos^2(kx) to obtain the optimal turn-on function S(t) so as to load a Bose-Einstein condensate into the ground state of the optical lattice of depth V_0. Specifically, we minimize interband excitations at the end of the turn-on of the optical potential at the final ramp time t_r, where S(t_r) = 1, given that S(0) = 0. Detailed numerical calculations confirm that a simple unit cell model is an excellent approximation when the turn-on time t_r is long compared with the inverse of the band excitation frequency and short in comparison with nonlinear time \hbar/\mu where \mu is the chemical potential of the condensate. We demonstrate using the Gross-Pitaevskii equation with an optimal turn-on function S(t) that the ground state of the optical lattice can be loaded with very little excitation even for times t_r on the order of the inverse band excitation frequency

Photoassociation spectroscopy of cold alkaline earth atoms near the intercombination line

The properties of photoassociation (PA) spectra near the intercombination line (the weak transition between $^{1}S_{0}$ and $^{3}P_{1}$ states) of group II atoms are theoretically investigated. As an example we have carried out a calculation for Calcium atoms colliding at ultra low temperatures of 1 mK, 1 $\mu$K, and 1 nK. Unlike in most current photoassociation spectroscopy the Doppler effect can significantly affect the shape of the investigated lines. Spectra are obtained using Ca--Ca and Ca--Ca$^*$ short-range {\it ab initio} potentials and long-range van der Waals and resonance dipole potentials. The similar van der Waals coefficients of ground $^{1}S_{0} + ^{1}S_{0}$ and excited $^{1}S_{0} + ^{3}P_{1}$ states cause the PA to differ greatly from those of strong, allowed transitions with resonant dipole interactions. The density of spectral lines is lower, the Condon points are at relatively short range, and the reflection approximation for the Franck-Condon factors is not applicable, and the spontaneous decay to bound ground-state molecules is efficient. Finally, the possibility of efficient production of cold molecules is discussed

Photoassociation dynamics in a Bose-Einstein condensate

A dynamical many body theory of single color photoassociation in a Bose-Einstein condensate is presented. The theory describes the time evolution of a condensed atomic ensemble under the influence of an arbitrarily varying near resonant laser pulse, which strongly modifies the binary scattering properties. In particular, when considering situations with rapid variations and high light intensities the approach described in this article leads, in a consistent way, beyond standard mean field techniques. This allows to address the question of limits to the photoassociation rate due to many body effects which has caused extensive discussions in the recent past. Both, the possible loss rate of condensate atoms and the amount of stable ground state molecules achievable within a certain time are found to be stronger limited than according to mean field theory. By systematically treating the dynamics of the connected Green's function for pair correlations the resonantly driven population of the excited molecular state as well as scattering into the continuum of non-condensed atomic states are taken into account. A detailed analysis of the low energy stationary scattering properties of two atoms modified by the near resonant photoassociation laser, in particular of the dressed state spectrum of the relative motion prepares for the analysis of the many body dynamics. The consequences of the finite lifetime of the resonantly coupled bound state are discussed in the two body as well as in the many body context. Extending the two body description to scattering in a tight trap reveals the modifications to the near resonant adiabatic dressed levels caused by the decay of the excited molecular state.Comment: 27 pages revtex, 16 figure

Radio-frequency dressing of multiple Feshbach resonances

We demonstrate and theoretically analyze the dressing of several proximate Feshbach resonances in Rb-87 using radio-frequency (rf) radiation. We present accurate measurements and characterizations of the resonances, and the dramatic changes in scattering properties that can arise through the rf dressing. Our scattering theory analysis yields quantitative agreement with the experimental data. We also present a simple interpretation of our results in terms of rf-coupled bound states interacting with the collision threshold.Comment: 4+ pages, 3 figures, 1 table; revised introduction & references to reflect published versio