The rotation and movement of polar molecules in an ultracold gas are intertwined with each other through dipolar interactions between the molecules, giving rise to rich, tunable dynamics. This molecular platform could advance the understanding of electron-transport phenomena in condensed-matter systems and be used for quantum sensing. Dipolar interactions between rotating molecules in motion can be tuned to give rise to rich dynamics.

NatureFeb 1, 20230 citations

Exploring the quantum frontier of spin dynamics

P. Bruno

Bulletin of the American Physical SocietyMar 22, 20110 citations

arXiv:2208.02216 · Nature

Tunable itinerant spin dynamics with polar molecules

Jun-Ru Li, Kyle Matsuda, Calder Miller, Annette N. Carroll, William G. Tobias, Jacob S. Higgins, Jun Ye

Published Feb 1, 2023• 68 citations

arXiv HTML PDF

DOI: 10.1038/s41586-022-05479-2

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We propose encoding a pseudo-spin-

1/2

system in the ground (

v=0

) and first excited (

v=1

) vibrational states of polar molecules. Double microwave shielding simultaneously shields molecules in both states, suppressing two-body losses by orders of magnitude while strictly avoiding three-body recombination. The microwave dressing is state-dependent and results in highly tunable, long-range dipolar Ising exchange (

J_z

), density-density (

V

), and spin-density (

W

) interactions. These interaction length scales readily exceed the typical interparticle spacing, pushing the molecules deep into the strongly interacting regime. In bulk gases, this enables the exploration of itinerant quantum magnetism and quantum droplets with novel anisotropic spin textures; in optical lattices, it naturally realizes extended Hubbard and

t

J_z

models, opening new directions in quantum simulation.

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-two-dimensional 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 we 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.