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arXiv:physics/0202043 · Physical Review A

Optical shielding of cold collisions in blue-detuned near-resonant optical lattices

Jyrki Piilo, Kalle‐Antti Suominen

Published Jul 1, 2002• 6 citations
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Abstract

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.

DOI
10.1103/physreva.66.013401

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We report Monte Carlo wave-function simulation results for two colliding atoms in a blue-detuned near-resonant J=1\ensuremath→J=1J=\stackrel{\ensuremath{\rightarrow}}{1}J=1J=1\ensuremath→​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.
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Collisional loss rates of ultracold atoms in a laser trap (formed by red-detuned laser beams) can be suppressed by using a sufficiently intense blue-detuned optical shielding laser to repel the atoms before they collide. We present calculations that reveal an additional mechanism for collisional loss in the presence of a blue-detuned laser that reduces optical shielding as the blue-detuned laser intensity increases, and even causes enhancement of collisional loss effects with increased blue-detuned intensity. This mechanism can strongly affect the saturation of the optical shielding.
Inelastic collision rates between two very cold ground-state atoms can be strongly decreased by using an optical shielding laser to repel the colliding atoms at very large internuclear separation so that they never get close to each other. This can be achieved by using a sufficiently intense laser to excite a repulsive state of the quasimolecule formed by the two colliding atoms. We study a simple two-state model and compare the predictions of fully quantum-mechanical close coupling and time-dependent wave packet methods with semiclassical approximations based on the concept of a Landau-Zener curve crossing of field-dressed states. The effect of spontaneous emission in the excited state is treated by the Monte Carlo wave-function method. Calculations are given for a model sodium system at 250 \ensuremath{\mu}K. Strong optical shielding, without significant heating due to the quasimolecular excitation, can be achieved at relatively modest laser power. We find that Landau-Zener models give near quantitative agreement with the fully quantum calculations in spite of the extremely low temperature.
Research focusing on the formation of ultracold atomic and molecular quantum gases is a continuously expanding field due to its envisioned applications such as quantum-controlled chemistry or quantum simulation. The aim of our theoretical work is to find ways to suppress inelastic or reactive processes between colliding particles in ultracold quantum gases. Using a laser field detuned to the blue of a relevant transition, we propose to couple the initial colliding particle state to a repulsive excited one, thus preventing the particles to come close to each other. This "blueshielding" technique is applied for ultracold quantum gases, close to quantum degeneracy. We investigate the optical shielding in ultracold39K-Cs collision by using close-coupling channel method. As the (3)3Σ+and (2)3Π+molecular states are repulsive in long range[1], one verified effective optical shielding can be realized via driving the transition from the ground α3Σ+state to these excited states in the presence of linear-polarized light. We demonstrate that a strong field with a small blue detuning to the atomic transition is to the benefit of shielding since the Condon point is farther away from the inner region of ground triplet state. The spontaneous emission effect is taken into account by considering excited optical potentials with the modified formula of shielding rate [2]. We expect the shielding efficiency could be improved by using circular-polarized light [3].
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.
We have simulated the dynamics of interacting atoms in optical lattices. The periodic lattice structure is produced with laser beams detuned a few linewidths to the red from the atomic transition. The atoms localize in lattice sites as a steady state is reached during the cooling, but they can still to some extent move between the sites. Our Monte Carlo Wave Function simulations show that in such situations the interactions can lead to evaporative cooling of the atoms as the hotter ones are ejected from the lattice.