Negative-mass instability of the spin and motion of an atomic gas driven by optical cavity backaction

TL;DR

This paper demonstrates a negative-mass instability in an ultracold atomic gas system, where cavity-mediated spin-motion coupling causes exponential growth of a hybrid mode, with observable effects on cavity field and system covariance.

Contribution

It introduces a novel realization of spin-orbit interaction mediated by an optical cavity, leading to the observation of negative-mass instability in a cold atomic gas.

Findings

Observation of exponential growth of a hybrid mode

Imprinting of growth on cavity field modulations

Estimation of the two-mode state's covariance during decay

Abstract

We realize a spin-orbit interaction between the collective spin precession and center-of-mass motion of a trapped ultracold atomic gas, mediated by spin- and position-dependent dispersive coupling to a driven optical cavity. The collective spin, precessing near its highest-energy state in an applied magnetic field, can be approximated as a negative-mass harmonic oscillator. When the Larmor precession and mechanical motion are nearly resonant, cavity mediated coupling leads to a negative-mass instability, driving exponential growth of a correlated mode of the hybrid system. We observe this growth imprinted on modulations of the cavity field and estimate the full covariance of the resulting two-mode state by observing its transient decay during subsequent free evolution.