Experimental Progress Toward Producing a Highly Polarization Squeezed Beam for Quantum Metrology

In this work we investigate a modified spin-exchange-relaxation-free (SERF) magnetometer which acts as a source of highly polarization-squeezed light. An off-resonant probe beam propagating through a dense atomic vapor experiences both a Faraday rotation and an AC Stark shift, thus entangling the polarization state of the probe with the coherent spin state of the atoms. If we pass the emerging probe beam through a quarter-wave plate and propagate through the sample again from an orthogonal direction, we may map the Faraday rotation due to quantum polarization fluctuations back onto the coherent spin state with negative feedback. In other words, the AC Stark shift causes the collective atomic spin to precess in such a way as to suppress quantum polarization fluctuations in one polarization quadrature (while amplifying them in another). The polarization-squeezing can now be detected with a projective measurement. In this scheme, the gain associated with Faraday detection of the atomic spin state should allow for unprecedentedly high squeezing in the SERF band, DC to 10 kHz [1]. We report on experimental progress achieved toward this goal.

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