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arXiv:2507.12167 · Physical Review A

Hyperfine van der Waals repulsion between open-shell polar molecules

Etienne F. Walraven, Tijs Karman

Published Sep 2, 2025• 0 citations

Abstract

We describe a novel type of interaction between open-shell polar molecules at sub-millikelvin temperatures. This hyperfine van der Waals interaction occurs between two molecules in two rotational states that differ by one quantum. Normally, this induces resonant dipolar interactions that lead to rapid collisional loss. For specific hyperfine states, however, selection rules prevent this. One can effectively turn off the dipolar interaction by merely flipping a nuclear spin. The resulting van der Waals interaction can be repulsive and can suppress collisional loss. We focus on laser-coolable CaF, but show this effect occurs universally for open-shell molecules, including MgF, SrF, BaF and YO. We propose that this effect could be measured by merging molecules in optical tweezers, where flipping a spin in one of the tweezers enables tuning of collision rates by five orders of magnitude.

DOI: 10.1103/z2bv-9gw7

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We discuss laser-dressed dipolar and van der Waals interactions between atoms and polar molecules, so that a cold atomic gas with laser admixed Rydberg levels acts as a designed reservoir for both elastic and inelastic collisional processes. The elastic scattering channel is characterized by large elastic scattering cross sections and repulsive shields to protect from close encounter collisions. In addition, we discuss a dissipative (inelastic) collision where a spontaneously emitted photon carries away (kinetic) energy of the collision partners, thus providing a significant energy loss in a single collision. This leads to the scenario of rapid thermalization and cooling of a molecule in the mK down to the μK regime by cold atoms.

Dipolar relaxation happens when one or both colliding atoms flip their spins exothermically inside a magnetic (

B

) field. This work reports precise measurements of dipolar relaxation in a Bose-Einstein condensate of ground state

^{87}

Rb atoms together with in-depth theoretical investigations. Previous perturbative treatments fail to explain our observations except at very small

B

-fields. By employing quantum defect theory based on analytic solutions of asymptotic van der Waals interaction

-C_6/R^6

(

R

being interatomic spacing), we significantly expand the applicable range of perturbative treatment. We find the

B

-dependent dipolar relaxation lineshapes are largely universal, determined by the coefficient

C_6

and the associated

s

-wave scattering lengths

a_{\rm sc}

of the states before and after spin flips. This universality, which applies generally to other atomic species as well, implicates potential controls of dipolar relaxation and related cold chemical reactions by tuning

a_{\rm sc}

.

Dipolar molecules in optical traps are a versatile platform for studying many-body phases of quantum matter in the presence of strong and long-range interactions. The dipolar interactions in such setups can be enabled by microwave driving opposite parity rotational levels of the molecules. We find that the regime where the

N=0,J=1/2,F=1

state is coupled to the

N=1,J=3/2,F=2

manifold with circularly polarized microwaves, in the presence of a small magnetic field, can lead to spin-1 quantum magnetic Hamiltonians, due to the decoupling between electron spin and orbit, that is unique to the

^2Σ

ground state molecules. We demonstrate that in one dimension, the phase diagram associated with this Hamiltonian, computed via tensor network methods, hosts the celebrated Haldane phase. We find that the Haldane phase persists even in the presence of SU(3) correction terms that break the SU(2) algebra of the Hamiltonian. We discuss the feasibility of the proposed scheme for

^2Σ

molecules with large rotational constants such as the directly laser cooled molecule MgF for future experiments.