Magneto Optical Sensing beyond the Shot Noise Limit

Magneto-optical sensors including spin noise spectroscopies and magneto-optical Kerr effect microscopies are now ubiquitous tools for materials characterization that can provide new understanding of spin dynamics, hyperfine interactions, spin-orbit interactions, and charge-carrier g-factors. Both interferometric and intensity-difference measurements can provide photon shot-noise limited sensitivity, but further improvements in sensitivity with classical resources require either increased laser power that can induce unwanted heating and electronic perturbations or increased measurement times that can obscure out-of-equilibrium dynamics and radically slow experimental throughput. Proof-of-principle measurements have already demonstrated quantum enhanced spin noise measurements with a squeezed readout field that are likely to be critical to the non-perturbative characterization of spin excitations in quantum materials that emerge at low temperatures. Here, we propose a truncated nonlinear interferometric readout for low-temperature magneto-optical Kerr effect measurements that is accessible with today's quantum optical resources. We show that 10 $\text{nrad}/\sqrt{\text{Hz}}$ sensitivity is achievable with optical power as small as 1 $\mu$W such that a realistic $T$ = 83 mK can be maintained in commercially available dilution refrigerators. The quantum advantage for the proposed measurements persists even in the limit of large loss and small squeezing parameters.