Deterministic Squeezed States with Joint Measurements and Feedback

TL;DR

This paper demonstrates the creation of entangled, spin-squeezed states in a large atomic ensemble using joint measurements and real-time feedback, achieving significant quantum phase variance reduction below the standard quantum limit.

Contribution

It introduces a method to deterministically generate spin-squeezed states with large atom numbers via joint measurement and feedback, achieving record entanglement enhancements.

Findings

Achieved 5.5(8) times variance reduction below SQL with feedback.

Observed up to 59(8) times improvement in quantum phase variance without feedback.

Reported one of the largest entanglement enhancements in any system.

Abstract

We demonstrate the creation of entangled, spin-squeezed states using a collective, or joint, measurement and real-time feedback. The pseudospin state of an ensemble of $N= 5\times 10^4$ laser-cooled $^{87}$Rb atoms is deterministically driven to a specified population state with angular resolution that is a factor of 5.5(8) [7.4(6)~dB] in variance below the standard quantum limit for unentangled atoms --- comparable to the best enhancements using only unitary evolution. Without feedback, conditioning on the outcome of the joint premeasurement, we directly observe up to 59(8) times [17.7(6)~dB] improvement in quantum phase variance relative to the standard quantum limit for $N=4\times 10^5$ atoms. This is one of the largest reported entanglement enhancements to date in any system.