Stabilizing an optical cavity containing a bulk diamond crystal at millikelvin temperatures in a cryogen-free dilution refrigerator

We successfully stabilized a Fabry–Pérot optical cavity incorporating a bulk diamond crystal at millikelvin temperatures in a cryogen-free dilution refrigerator with the pulse-tube cryocooler running. In stark contrast to previous demonstrations where lasers were locked to the cavities, our setup locks the cavity to the laser, thereby ensuring that emitted photons remain at a stable, predetermined frequency. This stability is advantageous for efficient quantum networking applications (e.g., remote entanglement generation) that require indistinguishable photons. Our measurements of cavity length fluctuation suggest that the setup could stabilize a cavity up to a finesse of 1.3 × 104 without the diamond and 5.3 × 103 with the diamond crystal. The finesse with a diamond crystal of ∼90 is primarily limited by the absorption loss inside the diamond.