Feedback-cooling of an atomic spin ensemble

TL;DR

This paper presents a measurement-and-feedback method to deterministically prepare low-entropy states of atomic spin ensembles, achieving significant spin noise reduction and potential applications in quantum state engineering.

Contribution

It introduces a novel feedback technique combining quantum non-demolition measurement and incoherent optical feedback for controlling atomic spin states.

Findings

Achieved 12 dB spin noise reduction

Demonstrated multi-stage feedback advantages

Theoretical model agrees with experimental results

Abstract

We describe a measurement-and-feedback technique to deterministically prepare low-entropy states of atomic spin ensembles. Using quantum non-demolition measurement and incoherent optical feedback, we drive arbitrary states in the spin-orientation space toward the origin of the spin space. We observe 12 dB of spin noise reduction, or a factor of 63 reduction in phase-space volume. We find optimal feedback conditions and show that multi-stage feedback is advantageous. An input-output calculation of quantum noise incorporating realistic quantum noise sources and experimental limitations agrees well with the observations. The method may have application to generation of exotic phases of ultracold gases, for example macroscopic singlet states and valence-bond solids.