Temperature measurement with nano-Kelvin resolution is demonstrated at room temperature, based on the thermal dependence of an optical crystal anisotropy in a high quality whispering gallery resonator. As the resonator's TE and TM modes frequencies have different temperature coefficients, their differential shift provides a sensitive measurement of the temperature variation, which is used for active stabilization of the temperature.
@article{arxiv.1105.6350,
title = {Temperature measurement and stabilization in a birefringent whispering gallery resonator},
author = {D. V. Strekalov and R. J. Thompson and L. M. Baumgartel and I. S. Grudinin and N. Yu},
journal= {arXiv preprint arXiv:1105.6350},
year = {2011}
}