Development of a Spin-Squeezed Scalar Optically Pumped Magnetometer

We report on the development of an atomic magnetometer with metrological advantage from spin-squeezing. The increased fragility of entangled states such as spin-squeezed ensembles largely precludes advantage in long-time measurements meant to achieve the greatest possible sensitivity. However, squeezing offers the potential to improve sensitivity for short interrogation times leading to increased bandwidth.

Our device is a pulsed scalar (total field) magnetometer based on rubidium vapor.  It is designed to address the three contributions to the quantum noise of such a device: First, back-action noise is avoided via quantum non-demolition measurement realized by an RF pulse which rotates the atomic spins into the plane perpendicular the magnetic field before readout. Second, the contribution of photon shot noise is ameliorated by a vapor cell that passes the probe beam multiple times through the vapor to decrease the photon shot noise relative to spin projection noise. Finally, the quantum non-demolition measurement of the precessing spins is split into two periods—first, spins are resolved along a selected direction to a certainty below the standard quantum limit. Second, the magnetic field is estimated with a subsequent additional non-demolition readout period that is conditioned on information gained in the preceding measurement.  In this way, the spins are conditionally squeezed in the selected direction via a form of measurement induced squeezing.