Generation of macroscopic singlet states in atomic ensembles

TL;DR

This paper explores how quantum non-demolition measurements can generate highly entangled, spin-squeezed states in unpolarized atomic ensembles, approaching many-body singlet states with potential applications in high-resolution magnetometry and quantum memory.

Contribution

It introduces a Gaussian approach to unpolarized spin states and demonstrates the feasibility of near-perfect spin squeezing in non-polarized ensembles.

Findings

Achieves near-perfect spin squeezing with negligible back-action.

States approach many-body singlet states with macroscopic entanglement.

Potential applications in high-resolution magnetometry and quantum memories.

Abstract

We study squeezing of the spin uncertainties by quantum non-demolition (QND) measurement in non-polarized spin ensembles. Unlike the case of polarized ensembles, the QND measurements can be performed with negligible back-action, which allows, in principle, perfect spin squeezing as quantified by [G. Toth et al., Phys. Rev. Lett. 99, 250405 (2007)]. The generated spin states approach many-body singlet states, and contain a macroscopic number of entangled particles, even when individual spin is large. We introduce the Gaussian treatment of unpolarized spin states and use it to estimate the achievable spin squeezing for realistic experimental parameters. Our proposal might have applications for magnetometry with a high spatial resolution or quantum memories storing information in decoherence free subspaces.