Ultrasensitive Atomic Spin Measurements with a Nonlinear Interferometer

Understand

We study nonlinear interferometry applied to a measurement of atomic spin and demonstrate a sensitivity that cannot be achieved by any linear-optical measurement with the same experimental resources.

• We use alignment-to-orientation conversion, a nonlinear-optical technique from optical magnetometry, to perform a nondestructive measurement of the spin alignment of a cold $^{87}$Rb atomic ensemble.
• We observe state-of-the-art spin sensitivity in a single-pass measurement, in good agreement with covariance-matrix theory.
• Taking the degree of measurement-induced spin squeezing as a figure of merit, we find that the nonlinear technique's experimental performance surpasses the theoretical performance of any linear-optical measurement on the same system, including optimization of probe strength and tuning.