EAGER: Low Noise Cryogenic Optical Resonator

California Institute Of Technology, Pasadena CA

Investigators

Abstract

High precision measurements in several areas at the forefront of experimental physics today are limited by Brownian thermal fluctuations. This includes torsion pendulums used for measurements of the equivalence principle and the gravitational inverse-square law, investigations of macroscopic quantum mechanical oscillators, cryogenic sapphire oscillators for use in the timing of pulsars for radio astronomy, the pendulums and mirrors of laser interferometer gravitational wave detectors, and reference cavities in atomic clocks and spectroscopy. For example, the large scale interferometers which are now being built in the U.S. as part of the Advanced LIGO project, will have an unprecedented sensitivity to gravitational waves from violent astrophysical phenomena but are limited in their sensitivity by thermal noise in their optical coatings. In the future, making further dramatic improvements in the astrophysical reach of gravitational wave detectors will require reducing the thermal noise in the interferometers mirrors. This Brownian motion of the mirror surface also limits the achievable frequency stability of modern stabilized lasers. These lasers are stabilized to rigid Fabry-Perot resonator cavities with mirrors very similar to those in the LIGO interferometers. The best atomic clocks today are limited by the frequency fluctuations of the lasers used to interrogate them. Improving the stability of the optical cavities would allow researchers to push the stability of the atomic clocks by orders of magnitude. This EAGER award supports preliminary research aimed at making more than an order of magnitude improvement in the line-widths of these reference cavities by attempting to use, for the first time, cryogenic silicon as an optical reference cavity. Since the thermal energy in the mirrors and their coatings is proportional to temperature, there could be a direct improvement by going to low temperature although the noise in such a cavity has never been measured. The scheme to be used is untested and an unanticipated noise source may be uncovered. However, the potential benefits which can be reaped by using high quality, cryogenic materials are tremendous and would have far reaching, possibly transformative, repercussions in leading edge physics measurements. An ultra-stable laser wavelength is an essential tool in a variety of fields including gravitational-wave detection, atomic clocks, and tests of General Relativity. The stability of the laser wavelength is important enough in these fields to have stimulated a long program of research in constructing increasingly stable reference cavities. In addition to understanding the thermal noise behavior at low temperatures, this research could eventually produce a new level of frequency stability for lasers and precision for atomic clocks.