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.
Through the many centuries that we have been trying to understand our universe, the development of precise measurement devices has always served to expand our understanding. Telescopes, radio receivers, lasers, atomic clocks, and a host of other inventions have revolutioned the way in which we live. The ways in which we have used those devices were hardly anticipated by their inventors, but they have made their impact felt nonetheless. Today we are facing the fundamental quantum mechanical and thermodynamic limits of matter as we try to probe deeper into the nature of the very small and also use these devices to probe the universe on its largest scales. The most sensitive force sensors (such as micro fabricated cantilevers) are able to quickly search for contaminations in the air, the most well polished mirrors allow us to see further out into the cosmos than ever before, and the most precise lasers and interferometers allow us to observe the stretching and squeezing of space itself to see if Einstein was right about relativity after all. This project was designed to push the envelope on just how well one can measure the displacement of a large object. By using ultra pure silicon and cooling it down to -150 C, we seek to see just what the limit is for these measurements. Will we be limited by thermodynamic motion of the mirrors? Will we be limited by the quantum forces from the laser beam? These are the types of questions that this experiment is set to explore. At the end of the funding period we have assembled and cooled down the system. We are at present able to see the thermodynamic noise from the Silicon pieces at room temperature and will be spending the coming year trying to understand their behavior as they are cooled to cryogenic temperatures. If we are able to achieve the expected low temperature performance, this should lead to great advances in atomic spectroscopy, astromical reach of new telescopes, and much which is unexpected.
