Characterization of an ultrastable sapphire optical cavity operated at 10 K*

The frequency stability of optical clocks is often limited by the linewidth of the probe laser, where the highest stability cavity-stabilized lasers are dominated by thermal noise in the cavity mirrors [1]. We present an optical reference cavity with a calculated thermal noise limit near Δf/f = 3x10^-18, which is compatible with reaching the ultimate stability limit due to quantum projection noise for clocks based on the ¹S₀-³P₀ transition in ²⁷Al⁺ (8 mHz natural linewidth at 267 nm) [2]. The cavity is constructed of single-crystal sapphire with GaAs/AlGaAs crystalline mirror coatings and is operated at 10 K using a closed-cycle cryocooler. We measure environmental and technical noise, including temperature fluctuations and vibrations, and characterize their impact on cavity stability. The cavity stability is found to be near 10^-16 by comparing to room-temperature ULE cavities.

[1] A. D. Ludlow, M. M. Boyd, J. Ye, E. Peik and P. O. Schmidt, Optical atomic clocks, Rev. Mod. Phys. 87, 637 (2015).

[2] S. M. Brewer, J.-S. Chen, A. M. Hankin, E. R. Clements, C. W. Chou, D. J. Wineland, D. B. Hume, and D. R. Leibrandt Phys. Rev. Lett. 123, 033201 (2019)