Sub-atom shot noise Faraday imaging of ultracold atom clouds

TL;DR

This paper demonstrates a minimally destructive dispersive Faraday imaging technique that measures ultracold atom cloud numbers with a precision surpassing the atom shot noise level, enabling multiple in situ measurements.

Contribution

The authors develop a noise model and experimental method that achieves sub-atom shot noise measurement precision in ultracold atom clouds using Faraday imaging.

Findings

Achieved measurement precision more than twice below the atom shot noise level.

Developed a parameter-free noise model based on image fluctuations.

Enabled multiple in situ measurements of the same ultracold atom cloud.

Abstract

We demonstrate that a dispersive imaging technique based on the Faraday effect can measure the atom number in a large, ultracold atom cloud with a precision below the atom shot noise level. The minimally destructive character of the technique allows us to take multiple images of the same cloud, which enables sub-atom shot noise measurement precision of the atom number and allows for an in situ determination of the measurement precision. We have developed a noise model that quantitatively describes the noise contributions due to photon shot noise in the detected light and the noise associated with single atom loss. This model contains no free parameters and is calculated through an analysis of the fluctuations in the acquired images. For clouds containing $N \sim 5 \times 10^6$ atoms, we achieve a precision more than a factor of two below the atom shot noise level.