  Magneto-optical microscopies, including optical measurements of magnetic
circular dichroism, are increasingly ubiquitous tools for probing spin-orbit
coupling, charge-carrier g-factors, and chiral excitations in matter, but the
minimum detectable signal in classical magnetic circular dichroism measurements
is fundamentally limited by the shot-noise limit of the optical readout field.
Here, we use a two-mode squeezed light source to improve the minimum detectable
signal in magnetic circular dichroism measurements by 3 dB compared with
state-of-the-art classical measurements, even with relatively lossy samples
like terbium gallium garnet. We also identify additional opportunities for
improvement in quantum-enhanced magneto-optical microscopies, and we
demonstrate the importance of these approaches for environmentally sensitive
materials and for low temperature measurements where increased optical power
can introduce unacceptable thermal perturbations.
