Magneto-optical spectroscopies are increasingly powerful probes of spin excitations in quantum materials, but at cryogenic temperatures, the laser excitation can be highly non-perturbative. While balanced photodetection can be used to suppress classical noise sources, the photon shot noise limit fundamentally constrains the measurement sensitivity for a given laser power. Here, we have used a two-mode squeezed light source to suppress noise below the shot noise level for magnetic circular dichroism measurements, thus enabling lower power measurements with reduced photothermal effects. We also describe the fundamental sensitivity limits for quantum enhanced interferometric and intensity-difference magneto-optical Kerr effect and circular dichroism spectroscopies.
We describe the operation of a free-space confocal optical microscope operated in a dilution refrigerator. The microscope is installed on a cold insertable probe to enable fast sample exchange while the refrigerator is held at low temperatures. A vector magnet provides a 6 T field normal to the sample and 1 T fields at arbitrary angles. A variety of optical microscopies and spectroscopies, including photoluminescence, Raman, magneto-optical Kerr effect, and spin relaxometry measurements are described, and some of the challenges associated with performing these measurements at milliKelvin temperatures are explored.
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