Highly polarization-squeezed light beam for quantum metrology*

Current state-of-the-art quantum-enhanced magnetometry uses a polarization-squeezed light beam, with fluctuations about 2 dB below the shot noise limit, to enhance the sensitivity by 17% [1]. Following a theoretical proposal by Sherson and Molmer [2] we seek to exploit off-resonant Faraday rotation in dense atomic vapor to create a polarization-squeezed beam, with unprecedentedly high squeezing in the frequency band from near-dc to about a kHz – this is the regime where spin-exchange-relaxation-free (SERF) magnetometers operate. Our proposed experiment maps the quantum polarization fluctuations in the light into effective magnetic field fluctuations via the AC Stark effect. This causes the atomic spins to alter the spin-dependent index of refraction in such a way as to cancel the original quantum polarization fluctuations, thus yielding polarization-squeezed light. We estimate that this method should generate squeezing nearly an order of magnitude stronger than that achieved in Ref. [1].

[1] C. Troullinou, R. Jiménez-Martînez, J. Kong, V. Lucivero, and M. Mitchell, PRL 127 (19), 193601 (2021)

[2] J. F. Sherson and K. Mølmer, PRL 97 (14), 143602 (2006)