Tailored single-atom collisions at ultra-low energies

TL;DR

This paper demonstrates precise control and observation of ultra-low-energy atomic collisions involving cesium and rubidium, revealing new Feshbach resonances and reaction pathways that enable advanced quantum control and molecular potential measurements.

Contribution

It reports the first observation of ultra-low-energy Feshbach resonances and a new spin-exchange reaction pathway in atom-atom collisions, with high experimental control.

Findings

Detection of Feshbach resonances below 300 mG separated by 15 kHz

Observation of a new reaction path involving two quanta of angular momentum transfer

Control over ultra-low-energy collision features for quantum applications

Abstract

We employ collisions of individual atomic cesium (Cs) impurities with an ultracold rubidium (Rb) gas to probe atomic interaction with hyperfine- and Zeeman-state sensitivity. Controlling the Rb bath's internal state yields access to novel phenomena observed in inter-atomic spin-exchange. These can be tailored at ultra-low energies, owing to the excellent experimental control over all relevant energy scales. First, detecting spin-exchange dynamics in the Cs hyperfine state manifold, we resolve a series of previously unreported Feshbach resonances at magnetic fields below 300 mG, separated by energies as low as $h\times 15$ kHz. The series originates from a coupling to molecular states with binding energies below $h\times 1$ kHz and wave function extensions in the micrometer range. Second, at magnetic fields below $\approx 100\,$mG, we observe the emergence of a new reaction path for alkali atoms, where in a single, direct collision between two atoms two quanta of angular momentum can be transferred. This path originates from the hyperfine-analogue of dipolar spin-spin relaxation. Our work yields control of subtle ultra-low-energy features of atomic collision dynamics, opening new routes for advanced state-to-state chemistry, for controlling spin-exchange in quantum many-body systems for solid state simulations, or for determination of high-precision molecular potentials.