States of an Ensemble of Two-Level Atoms with Reduced Quantum Uncertainty

TL;DR

This paper demonstrates generating entangled, spin-squeezed states of a large atomic ensemble via quantum nondemolition measurement, achieving sensitivity beyond the standard quantum limit for atomic clocks.

Contribution

It introduces a method to produce and verify spin-squeezed states in a large atomic ensemble using resonator-enhanced quantum nondemolition measurement.

Findings

Achieved up to 8.8 dB reduction in quantum projection noise.

Demonstrated 3.0 dB sensitivity beyond the standard quantum limit.

Produced entangled states in an ensemble of 5×10^4 atoms.

Abstract

We generate entangled states of an ensemble of 5*10^4 rubidium-87 atoms by optical quantum nondemolition measurement. The resonator-enhanced measurement leaves the atomic ensemble, prepared in a superposition of hyperfine clock levels, in a squeezed spin state. By comparing the resulting reduction of quantum projection noise (up to 8.8(8) dB) with the concomitant reduction of coherence, we demonstrate a clock input state with spectroscopic sensitivity 3.0(8) dB beyond the standard quantum limit.