Theory of itinerant collisional spin dynamics in nondegenerate molecular gases

TL;DR

This paper develops a theoretical framework for understanding spin dynamics in ultracold nondegenerate molecular gases, revealing mechanisms for controlling decoherence and enabling coherent many-body spin phenomena.

Contribution

It introduces a detailed theory of collisional spin dynamics in nondegenerate molecular gases, including loss suppression techniques and implications for quantum many-body physics.

Findings

Loss-induced quantum autoselection suppresses collective spin decoherence.

Electric field tuning can nearly eliminate collisional loss in certain molecular species.

Coherent spin mixing dynamics can be realized in stable, nondegenerate molecular gases.

Abstract

We study the fully itinerant dynamics of ultracold but nondegenerate polar molecules with a spin-$1/2$ degree of freedom encoded into two of their electric field dressed rotational states. Center of mass molecular motion is constrained to two-dimensions via tight confinement with a one-dimensional optical lattice, but remains mostly unconstrained within the plane. The pseudospins can become entangled through ultracold dipolar collisions, for which the locality of interactions is greatly relaxed by free molecular motion. At the level of single-molecule observables, collision-induced entanglement manifests as spin decoherence, for which our theoretical calculations serve well to describe recent Ramsey contrast measurements of quasi-2D confined KRb molecules at JILA [A. Carroll et al., Science 388 6745 (2025)]. In presenting a more detailed theoretical analysis of the KRb experiment, we highlight a key finding that molecular loss enhanced by particle exchange symmetry can lead to a suppression of collective spin decoherence, a mechanism with refer to as ``loss-induced quantum autoselection". We then show that by utilizing bialkali species with sufficiently large dipole moments, loss can be near completely suppressed in all collision channels via electric field tunable confinement-induced collisional shielding. The afforded collisional stability permits fully coherent spin mixing dynamics, natively realizing unitary circuit dynamics with random all-to-all connectivity and U(1) charge conservation. This work establishes a bridge between the domains of ultracold molecular collisions and many-body spin physics, ultimately proposing the use of nondegenerate bulk molecular gases as a controllable platform for nonequilibrium explorations of itinerant quantum matter.