New Advances in Multichannel and Three-Dimensional Theories of Ultracold Atomic Collision Dynamics

Laurent Sanchez-Palencia

J. Weiner

Sérgio Ricardo Muniz, Luís Gustavo Marcassa, Reginaldo de Jesus Napolitano +4 more

We report the observation of hyperfine-changing collisions and the suppression of their rates in a sample of ultracold sodium atoms. We show that the suppression scheme used in the study of photoassociative ionization of Na [Phys. Rev. Lett. 76, 2033 (1996)] is also able to suppress the hyperfine-changing collisions, producing suppression up to 80%. The experimental results are compared with predictions of a simple Landau-Zener model and of a three-dimensional, multichannel close-coupling calculation. Both theories agree at low intensity, but at high intensity the experiment shows a saturation behavior that is qualitatively described only by the three-dimensional multichannel theory.

Masato Morita, Paul W. Brumer, Timur V. Tscherbul

We extend the powerful formalism of multichannel quantum defect theory combined with a frame transformation to ultracold atom-molecule collisions in magnetic fields. By solving the coupled-channel equations with hyperfine and Zeeman interactions omitted at short range, the extended theory enables a drastically simplified description of the intricate quantum dynamics of ultracold molecular collisions in terms of a small number of short-range parameters. We apply the formalism to ultracold Mg+NH collisions in a magnetic field, achieving a 10^{4}-fold reduction in computational effort.

Paul S. Julienne, Robert Heather, J. Vigué

Alyson Laskowski, Nirav P. Mehta

Multichannel quantum-defect theory (MQDT) provides a powerful toolkit for describing and understanding collisions of cold alkali-metal atoms. Various MQDT approximations differ primarily in how they characterize the so-called short-ranged $K$ matrix ${\mathbf{K}}_{\text{sr}}$, which encapsulates the short-ranged physics into a handful of low-energy parameters that exhibit simple and smooth dependence on energy and field. Here, we compare three different methods for computing ${\mathbf{K}}_{\text{sr}}$ for homonuclear collisions of alkali-metal atoms, from lithium to cesium. The MQDT calculations are benchmarked against numerically converged coupled-channels calculations that use a log-derivative propagator out to the asymptotic region. We study how well these approximations reproduce positions of $s$-wave magnetic Feshbach resonances, comparing with experiment where possible, and identify the limitations of various approximations.