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The Journal of Physical Chemistry A

Channel Interference in Optical Collisions of Cold Atom Beams

Vladimir Yurovsky, A. Ben‐Reuven

Published Nov 1, 1998• 16 citations
Publisher

Abstract

Experiments were conducted recently (by Weiner and co-workers) on the optical shielding (suppression) of atomic collisions in cold-atom beams, and its variation with the angle between the polarization direction of the shielding light and the direction of approach of the beam. This case is shown here to be a typical example of an optical collision in which quantum interference may persevere between two incident collision partial waves leading to the same output state. This effect depends on the relative collisional phase shift of the two interfering channels, as well as on the angle of approach, and will vanish when averaging over the latter (as with collisions in the bulk). The extent of variation of this interference effect with the relative phase shift is quite broad, and may lead, under favorable conditions, to almost complete shielding at a finite value of the shielding-laser power. The latter observation leaves open the possibility of exerting coherent control over the interference effect in order to optimize the shielding.

DOI
10.1021/jp9819902

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Experiments were conducted recently (by Weiner and co-workers) on the optical shielding (suppression) of atomic collisions in cold-atom beams, and its variation with the angle between the polarization direction of the shielding light and the direction of approach of the beam. This case is shown here to be a typical example of an optical collision in which quantum interference may persevere between two incident collision partial waves leading to the same output state. This effect depends on the relative collisional phase shift of the two interfering channels, as well as on the angle of approach, and will vanish when averaging over the latter (as with collisions in the bulk). The extent of variation of this interference effect with the relative phase shift is quite broad, and may lead, under favorable conditions, to almost complete shielding at a finite value of the shielding-laser power. The latter observation leaves open the possibility of exerting coherent control over the interference effect in order to optimize the shielding.
Experiments were conducted recently (by Weiner and co-workers) on the optical shielding (suppression) of atomic collisions in cold-atom beams, and its variation with the angle between the polarization direction of the shielding light and the direction of approach of the beam. This case is shown here to be a typical example of an optical collision in which quantum interference may persevere between two incident collision partial waves leading to the same output state. This effect depends on the relative collisional phaseshift of the two interfering channels, as well as on the angle of approach, and will vanish when averaging over the latter (as with collisions in the bulk). The extent of variation of this interference effect with the relative phaseshift is quite broad, and may lead, under favorable conditions, to almost complete shielding at a finite value of the shielding-laser power. The latter observation leaves open the possibility of exerting coherent control over the interference effect in order to optimize the shielding.
A simple two-channel model, based on the semiclassical Landau-Zener (LZ) approximation, with averaging over angle-dependent exponents, is proposed as a fast means for accounting for the incomplete optical shielding of collisions, as observed in recent experiments conducted by Weiner and co-workers on ultracold sodium-atom traps, and its dependence on the laser polarization. The model yields a reasonably good agreement with the recent quantum close-coupling calculations of Julienne and co-workers. The remaining discrepancy between both theories and the data is qualitatively attributed to a partial overlap of the collision ranges at which loss processes and optical shielding occur.
Two recent developments in ultracold atomic collision physics are discussed. The first is the “optical shielding” of ultracold atomic collisions by optical radiation that is tuned to the blue of an atomic resonance. This radiation couples a pair of colliding atoms to repulsive excited states and prevents their close approach. This effect may substantially suppress inelastic collision rates, and is an interesting example of the control of a cold collision process by a weak perturbing field. The second topic is ultracold atom photoassociation spectroscopy. In this work, high resolution free‐bound absorption spectra are obtained which provide detailed information on the long‐range states of the two interacting atoms. The physics of these states has an appealing simplicity, and in addition is of importance for a complete understanding of the effects of atomic interactions in cold atom experiments. The photoassociation spectra also provide a unique probe of cold collisions.
Abstract Ultracold temperatures in dilute quantum gases opened the way to an exquisite control of matter at the quantum level. Here we focus on the control of ultracold atomic collisions using a laser to engineer their interactions at large interatomic distances. We show that the entrance channel of two colliding ultracold atoms can be coupled to a repulsive collisional channel by the laser light so that the overall interaction between the two atoms becomes repulsive: this prevents them to come close together and to undergo inelastic processes, thus protecting the atomic gases from unwanted losses. We illustrate such an optical shielding (OS) mechanism with 39 K and 133 Cs atoms colliding at ultracold temperature (<1 μK). The process is described in the framework of the dressed-state picture and we then solve the resulting stationary coupled Schrödinger equations. The role of spontaneous emission and photoinduced inelastic scattering is also investigated as possible limitations of the shielding efficiency. We predict an almost complete suppression of inelastic collisions over a broad range of Rabi frequencies and detunings from the 39 K D2 line of the OS laser, both within the [0, 200 MHz] interval. We found that the polarization of the shielding laser has a minor influence on this efficiency. This proposal could easily be formulated for other bialkali-metal pairs as their long-range interaction are all very similar to each other.
We consider the interference of atoms caused by the wave nature of their motion. Coherently interfering beams of particles may be obtained by using the diffraction of atoms from separated standing waves resonant with the transition between the ground and excited states of the atoms. Interference arises with and without spatial separation of the diffracted atomic beams. A problem concerning two-quantum perturbation of the wave function of an atom in the ground state by a standing-wave field has been solved. Both amplitude and phase perturbation are taken into account. We show that in spite of the absence of coherence in the incident beam and a rapid decay of the coherence induced by the standing wave for scattered spatially overlapping beams, the interference can be observed for the conditions of an echo. We first consider the phenomenon of an echo under the quantum-mechanical action of light on the translational degrees of freedom. We show that owing to this phenomenon phase memory can be transferred through the ground state over unlimitedly large distances. Owing to the interference of the density harmonics, a periodic structure in the spatial distribution of the atomic density with a period of less than one wavelength of the light is localized at distances of the order of the distance between the fields. The harmonic amplitude is calculated for instantaneous excitation in the case in which the duration of the excitation is comparable with the backward Doppler linewidth in the beam and for scattering under Bragg conditions. In the latter case we show that the effective temperature of the atoms participating in the interference is less than one recoil energy.
We propose a method of controlling two-atom interaction using both magnetic and laser fields. We analyse the role of quantum interference between magnetic and optical Feshbach resonances in controlling cold collision. In particular, we demonstrate that this method allows us to suppress inelastic and enhance elastic scattering cross sections. Quantum interference is shown to modify significantly the threshold behaviour and resonant interaction of ultracold atoms. Furthermore, we show that it is possible to manipulate not only the spherically symmetric s-wave interaction but also the anisotropic higher partial-wave interactions which are particularly important for high-temperature superfluid or superconducting phases of matter.