Magneto Optical Sensing beyond the Shot Noise Limit

Magneto-optical sensors including spin noise spectroscopies and magneto-optical Kerr microscopies are now ubiquitous tools for materials characterization. Further improvements in sensitivity with classical resources require either increased laser power that can induce unwanted heating and perturbations or increased measurement times that are increasingly infeasible. 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 nrad/√Hz sensitivity is achievable with optical power as small as 1 μW such that a realistic T = 83 mK can be maintained in commercially available dilution refrigerators.