Cavity-assisted measurement and coherent control of collective atomic spin oscillators

TL;DR

This paper demonstrates how a high-finesse optical cavity can be used to continuously measure and coherently control the collective spin of an atomic ensemble, enabling stabilization in quantum states despite ongoing measurement back-action.

Contribution

It introduces a method for cavity-assisted measurement and feedback control of atomic spins, achieving stabilization in quantum states through autonomous optical feedback.

Findings

Observation of cavity-induced amplification, damping, and optical spring effects.

Achievement of spin stabilization in high- or low-energy states.

Preparation of the spin ensemble in a nearly pure quantum state despite continuous measurement.

Abstract

We demonstrate continuous measurement and coherent control of the collective spin of an atomic ensemble undergoing Larmor precession in a high-finesse optical cavity. The coupling of the precessing spin to the cavity field yields phenomena similar to those observed in cavity optomechanics, including cavity amplification, damping, and optical spring shifts. These effects arise from autonomous optical feedback onto the atomic spin dynamics, conditioned by the cavity spectrum. We use this feedback to stabilize the spin in either its high- or low-energy state, where, in equilibrium with measurement back-action heating, it achieves a steady-state temperature, indicated by an asymmetry between the Stokes and anti-Stokes scattering rates. For sufficiently large Larmor frequency, such feedback stabilizes the spin ensemble in a nearly pure quantum state, in spite of continuous measurement by the cavity field.