title: Nano-Kelvin thermometry and temperature control: beyond the thermal noise limit.
abstract: We demonstrate thermometry with a resolution of 80  nK/Hz using an isotropic crystalline whispering-gallery mode resonator based on a dichroic dual-mode technique. We simultaneously excite two modes that have a mode frequency ratio that is very close to two (±0.3  ppm). The wavelength and temperature dependence of the refractive index means that the frequency difference between these modes is an ultrasensitive proxy of the resonator temperature. This approach to temperature sensing automatically suppresses sensitivity to thermal expansion and vibrationally induced changes of the resonator. We also demonstrate active suppression of temperature fluctuations in the resonator by controlling the intensity of the driving laser. The residual temperature fluctuations are shown to be below the limits set by fundamental thermodynamic fluctuations of the resonator material.
authors.affiliation: School of Physics, University of Western Australia, Western Australia 6009, Australia and Institute for Photonics and Advanced Sensing and School of Chemistry and Physics, University of Adelaide, South Australia 5005, Australia.
authors.forename: Wenle
authors.initials: W
authors.lastname: Weng
authors.affiliation: School of Physics, University of Western Australia, Western Australia 6009, Australia and Institute for Photonics and Advanced Sensing and School of Chemistry and Physics, University of Adelaide, South Australia 5005, Australia.
authors.forename: James D
authors.initials: JD
authors.lastname: Anstie
authors.affiliation: School of Mathematics and Physics, University of Queensland, Brisbane, Queensland 4072, Australia.
authors.forename: Thomas M
authors.initials: TM
authors.lastname: Stace
authors.affiliation: Department of Quantum Science, Australian National University, Australian Capital Territory 0200, Australia.
authors.forename: Geoff
authors.initials: G
authors.lastname: Campbell
authors.affiliation: School of Physics, University of Western Australia, Western Australia 6009, Australia.
authors.forename: Fred N
authors.initials: FN
authors.lastname: Baynes
authors.affiliation: School of Physics, University of Western Australia, Western Australia 6009, Australia and Institute for Photonics and Advanced Sensing and School of Chemistry and Physics, University of Adelaide, South Australia 5005, Australia.
authors.forename: Andre N
authors.initials: AN
authors.lastname: Luiten