Spin Squeezing and Closed-Loop Magnetometry Below the Standard Quantum Limit

Abstract

Precision measurement of a collective atomic spin has many applications in sensing and metrology, including use in atomic clocks and magnetometers. Squeezing of the collective spin can improve the resolution of such measurements to levels significantly below the standard quantum limit. In this work we demonstrate spin squeezing on the collective spin of a million spin-4 Cs atoms. This squeezing is generated via quantum backaction from a dispersive quantum nondemolition measurement, using the Faraday interaction with an optical probe field passing through the atoms. By applying composite pulse techniques and operating inside a state-of-the-art magnetic shield that strongly attenuates background fields, we are able to achieve precise control of the atomic spins and accurately measure quantum projection noise and spin squeezing. We demonstrate 5 dB of metrologically relevant spin squeezing in this experiment. The atomic ensemble’s sensitivity to magnetic fields and the improved measurement resolution due to spin squeezing allow us to use the setup to perform precise measurements of the magnetic field environment. With the implementation of real-time feedback inside the shielded environment, we demonstrate closed-loop RF magnetometry with a sensitivity of 7.9 dB below the standard quantum limit. This work sets the stage for further explorations of quantum feedback and simulation, wherein feedback control can be used to drive more complicated quantum dynamics.