QND Measurement of Large-Spin Ensembles by Dynamical Decoupling

TL;DR

This paper demonstrates that dynamical decoupling enables effective quantum non-demolition measurements in large-spin atomic ensembles, overcoming challenges posed by Raman coherence, and achieves enhanced measurement sensitivity.

Contribution

Theoretical and experimental demonstration that dynamical decoupling restores QND measurement capabilities in large-spin systems affected by Raman coherence.

Findings

Achieved a sensitivity 5.7 dB better than spin projection noise.

Demonstrated dynamical decoupling using a two-polarization probing technique.

Showed that naive QND techniques fail for large spins without decoupling.

Abstract

Quantum non-demolition (QND) measurement of collective variables by off-resonant optical probing has the ability to create entanglement and squeezing in atomic ensembles. Until now, this technique has been applied to real or effective spin one-half systems. We show theoretically that the build-up of Raman coherence prevents the naive application of this technique to larger spin atoms, but that dynamical decoupling can be used to recover the ideal QND behavior. We experimentally demonstrate dynamical decoupling by using a two-polarization probing technique. The decoupled QND measurement achieves a sensitivity 5.7(6) dB better than the spin projection noise.