Cold binary atomic collisions in a light field (original) (raw)

Optical-Bloch-equation method for cold-atom collisions: Cs loss from optical traps

Physical Review A, 1992

We develop an optical-Bloch-equation approach for modeling ultracold-atom collisions in optical traps. The method incorporates a molecular picture of the atomic collision, laser-field dressing of the molecular states participating in the dynamics, and decay of the population and polarization due to spontaneous emission of excited states. The last is important to incorporate because the duration of the cold collisions is longer than the excited-state lifetimes. The relative motion of the atoms during the course of the collision is treated semiclassically with corrections for the time-dependent relative motion of the atoms in the various channels. An application of the method to Cs trap loss due to fine-structurechanging collisions is presented. Good agreement with experiment is obtained.

Quantum and semiclassical calculations of cold-atom collisions in light fields

Physical Review A, 1998

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 timedependent 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.

Spontaneous emission of atoms via collisions of Bose-Einstein condensates

The widely used Gross-Pitaevskii equation treats only coherent aspects of the evolution of a Bose-Einstein condensate. However, inevitably some atoms scatter out of the condensate. We have developed a method, based on the field theory formulation, describing the dynamics of incoherent processes which are due to elastic collisions. We can therefore treat processes of spontaneous emission of atoms into the empty modes, as opposed to stimulated processes, which require non-zero initial occupation.

Atom loss and the formation of a molecular Bose-Einstein condensate by Feshbach resonance

Physical Review A, 2000

, large loss rates were observed when a time-varying magnetic field was used to tune a molecular Feshbach resonance state near the state of a pair of atoms in the condensate. A collisional deactivation mechanism affecting a temporarily formed molecular condensate [see V. A. Yurovsky, A. Ben-Reuven, P. S. Julienne and C. J. Williams, Phys. Rev. A 60, R765 (1999)], studied here in more detail, accounts for the results of the slow-sweep experiments. A best fit to the MIT data yields a rate coefficient for deactivating atom-molecule collisions of 1.6 × 10 −10 cm 3 /s. In the case of the fast sweep experiment, a study is carried out of the combined effect of two competing mechanisms, the three-atom (atom-molecule) or fouratom (molecule-molecule) collisional deactivation vs. a process of two-atom trap-state excitation by curve crossing [F.

Laser-Driven Collisions between Atoms in a Bose-Einstein Condensed Gas

Physical Review Letters, 1996

We have determined the rate of loss of atoms from a Bose-Einstein condensed gas due to binary processes in the presence of a far-detuned laser field. In this limit, the binary loss rate spectrum is markedly different from, and can greatly exceed, the basic atomic loss rate. We suggest that measurements of the loss rate spectrum can be used to determine the nature of atom interactions in a condensate. [S0031-9007(96)00919-2]

Collisional losses from a light-force atom trap

Physical review letters, 1989

We have studied the collisional loss rates for very cold cesium atoms held in a spontaneous-force optical trap. In contrast with previous work, we find that collisions involving excitation by the trapping light fields are the dominant loss mechanism. We also find that ...

Photoassociation dynamics in a Bose-Einstein condensate

Physical Review A, 2004

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.

Two-body transients in coupled atomic-molecular Bose-Einstein condensates

Physical review letters, 2008

We discuss the dynamics of an atomic Bose-Einstein condensate when pairs of atoms are converted into molecules by single-color photoassociation. Three main regimes are found and it is shown that they can be understood on the basis of time-dependent two-body theory. In particular, the so-called rogue dissociation regime [Phys. Rev. Lett., 88, 090403 (2002)], which has a density-dependent limit on the photoassociation rate, is identified with a transient regime of the two-atom dynamics exhibiting universal properties. Finally, we illustrate how these regimes could be explored by photoassociating condensates of alkaline-earth atoms.

Thermalization of coupled atom-light states in the presence of optical collisions

Physical Review A, 2010

The interaction of a two-level atomic ensemble with a quantized single-mode electromagnetic field in the presence of optical collisions is investigated both theoretically and experimentally. The main focus is on achieving thermal equilibrium for coupled atom-light states (in particular dressed states). We propose a model of atomic dressed-state thermalization that accounts for the evolution of the pseudo-spin Bloch vector components and characterize the essential role of the spontaneous emission rate in the thermalization process. Our model shows that the time of thermalization of the coupled atom-light states depends strictly on the ratio of the detuning to the resonant Rabi frequency. The predicted time of thermalization is in the nanosecond domain at full optical power and about 10 times shorter than the natural lifetime in our experiment. Experimentally we investigate the interaction of the optical field with rubidium atoms in an ultrahigh-pressure buffer gas cell under the conditions of large atom-field detuning comparable to the thermal energy in frequency units. In particular, an observed asymmetry of the saturated lineshape is interpreted as evidence of thermal equilibrium of coupled atom-light states.

Calculations of collisions between cold alkaline-earth-metal atoms in a weak laser field

Physical Review A, 2001

We calculate the light-induced collisional loss of laser-cooled and trapped magnesium atoms for detunings up to 50 atomic linewidths to the red of the 1 S0-1 P1 cooling transition. We evaluate loss rate coefficients due to both radiative and nonradiative state-changing mechanisms for temperatures at and below the Doppler cooling temperature. We solve the Schrödinger equation with a complex potential to represent spontaneous decay, but also give analytic models for various limits. Vibrational structure due to molecular photoassociation is present in the trap loss spectrum. Relatively broad structure due to absorption to the Mg2 1 Σu state occurs for detunings larger than about 10 atomic linewidths. Much sharper structure, especially evident at low temperature, occurs even at smaller detunings due to of Mg2 1 Πg absorption, which is weakly allowed due to relativistic retardation corrections to the forbidden dipole transition strength. We also perform model studies for the other alkaline earth species Ca, Sr, and Ba and for Yb, and find similar qualitative behavior as for Mg.