A fully crystalline cryogenic reference cavity

To date, neutral atom optical lattice clocks have demonstrated the highest precision measurements of fractional frequency shifts. State-of-the-art optical oscillators employing cryogenic reference cavities have been used to push this frontier, enabling record-level stabilities and resulting in foundational advances in optical frequency metrology. The fundamental performance of cryogenic cavities utilizing crystalline spacers and substrates has been limited by the Brownian thermal noise associated with mechanical dissipation of the mirror coatings. We operate a cryogenic silicon cavity with crystalline coatings, a promising candidate for reduced thermal noise, and identify novel noise mechanisms associated with these coatings.