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arXiv:physics/0207105 · Journal of the Optical Society of America B

Collision rates in near-resonant optical lattices

Jyrki Piilo

Published May 1, 2003• 3 citations
arXivHTMLPDF

Abstract

We present a simple method for calculating the binary collision rate between atoms in near-resonant optical lattices. The method is based on Monte Carlo wave-function simulations, and the collision rate is obtained by monitoring the quantum flux beyond the average distance between the atoms. To illustrate the usefulness of the method, we calculate the collision rates for a wide range of occupation densities and various modulation depths of the lattice. The method presented here, combined with the semiclassical calculations accounting for intrawell collisions, can simplify the study of the effects of binary collisions on the dynamics of atomic clouds trapped in near-resonant optical lattices.

DOI
10.1364/josab.20.001135

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We present a simple method for calculating the binary collision rate between atoms in near-resonant optical lattices. The method is based on Monte Carlo wave-function simulations, and the collision rate is obtained by monitoring the quantum flux beyond the average distance between the atoms. To illustrate the usefulness of the method, we calculate the collision rates for a wide range of occupation densities and various modulation depths of the lattice. The method presented here, combined with the semiclassical calculations accounting for intrawell collisions, can simplify the study of the effects of binary collisions on the dynamics of atomic clouds trapped in near-resonant optical lattices.
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We report Monte Carlo wave-function simulation results for two colliding atoms in a blue-detuned near-resonant $J=\stackrel{\ensuremath{\rightarrow}}{1}J=1$ optical lattice. Our results show that complete optical shielding of collisions can be achieved within the lattice with suitably selected and realistic laser field parameters. More importantly, our results demonstrate that the shielding effect does not interfere with the actual trapping and cooling process, and it is produced by the lattice lasers themselves, without the need to use additional laser beams.
This thesis gives an account of work done on semi-classical and quantum Monte Carlo simulations in far-detuned optical lattices. Firstly, the basic principles of laser cooling of atoms are presented including a short introduction to optical lattices in the near and far-detuned regime. A detailed analysis is made of the band-structure of optical lattices, using the Bloch formalism, and of the bound-state population distribution appropriate for a thermal sample of trapped atoms. Secondly, a general overview is given of the quantum Monte Carlo method for simulating the dynamics of atom-light interactions. This is followed by a detailed study of the concept of Raman cooling, which is a useful tool to prepare atoms in the ground motional state of the lattice and an important first step to achieving quantum state control with ultra-cold atoms. A simplified model of Raman cooling is introduced and simulated using the quantum Monte Carlo wave-function approach. Then the implementation of simulations of resolved-sideband Raman cooling based on this model is discussed as is how the results were used to optimize the experimental work done by our group. The results of these simulations show for the first time that the quantum Zeno effect has a crucial impact on the efficiency of Raman cooling experiments. Also the experimental measurements of the temperature of Raman sideband cooled atoms for a range of parameters are compared with theoretical results and shows a good qualitative agreement. Thirdly, the results of semi-classical numerical simulations of parametric excitation in optical lattices are presented. It is shown that the modulation of the potential can result in selective parametric excitation of trapped atoms. The theoretical results show good qualitative agreement with experiment. The thesis is concluded with a description of possible avenues for future studies on quantum state control in optical lattices.
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.
Ionizing collisional loss rate of the metastable argon and krypton atoms in an optical lattice is compared with the rate of atoms in free space as a function of lattice laser detuning. Contrary to a simple picture of the isolation of atoms by the lattice potential, our experimental result shows that the collision rate in optical lattice is always larger than that in free space except at a large laser detuning of ∼10 3 transition linewidth. Comparing the experimental result with the theoretical values that are derived from the classical trajectory calculation of the atom, we found that the optical pumping to non-trapped adiabatic potential surface prevents the collision suppression because it keeps the number of non-trapped atoms to a finite value even at low excitation rate.