Protected subspace Ramsey spectroscopy

TL;DR

This paper analyzes a modified Ramsey spectroscopy method that uses sub-radiant states to suppress decay effects, leading to enhanced frequency sensitivity and precision beyond single atom linewidth in dense atomic ensembles.

Contribution

It provides an in-depth analytical and numerical study of protected subspace Ramsey spectroscopy, demonstrating improved sensitivity scaling using sub-radiant states.

Findings

Sub-radiant states enable better than 1/√N sensitivity scaling.

The technique achieves precision beyond single atom linewidth.

Numerical simulations confirm enhanced metrological performance.

Abstract

We study a modified Ramsey spectroscopy technique employing slowly decaying states for quantum metrology applications using dense ensembles. While closely positioned atoms exhibit superradiant collective decay and dipole-dipole induced frequency shifts, recent results [Ostermann, Ritsch and Genes, Phys. Rev. Lett. \textbf{111}, 123601 (2013)] suggest the possibility to suppress such detrimental effects and achieve an even better scaling of the frequency sensitivity with interrogation time than for noninteracting particles. Here we present an in-depth analysis of this 'protected subspace Ramsey technique' using improved analytical modeling and numerical simulations including larger 3D samples. Surprisingly we find that using sub-radiant states of $N$ particles to encode the atomic coherence yields a scaling of the optimal sensitivity better than $1/\sqrt{N}$. Applied to ultracold atoms in 3D optical lattices we predict a precision beyond the single atom linewidth.