@David Tanner (University of Florida)
@Seeking the unseen: gravitational waves with LIGO and axion dark matter with ADMX
I will discuss two experiments in which I participate. A common feature is the use of optical methods (interferometers, cavities, etc.) to search for signals of astrophysical import. A second common them is that neither experiment has as yet seen a signal. Third (eventually) is the use of cryogenic methods.

The LIGO experiment has as its goal the detection and study of gravitational waves. These waves were predicted 80 years ago by Einstein's general theory of relativity but never have been observed directly. The LIGO observatory consists of laser interferometers in Washington State and Louisiana. These are Michelson interferometers, with Fabry-Perot cavities in the arms to increase the sensitivity. The arms are 4 km in length, allowing precise measurement of the motion of test masses which make up the end mirrors of the interferometers. These interferometers have accumulated almost two years of observing and set important limits on the strength of signals from a variety of sources. The LIGO laboratory is about to begin the construction of advanced LIGO, which will upgrade the instrument to improve sensitivity tenfold. I will focus on some of the extremely demanding experimental aspects of the LIGO detector, including the input optics, the part of the detector built by the University of Florida LIGO group. The Axion Dark Matter eXperiment (ADMX) is an experiment to search for axions. This weakly-interacting dark matter particle could constitute the halo of our galaxy. In ADMX, halo axions flow through a microwave resonant cavity permeated by a static magnetic field, where some convert into microwave photons. These photons are detected by an ultralow-noise receiver.

The ADMX Collaboration has set limits on the axion-to-photon coupling and/or local axion halo mass density for axion mass between 1.9 and 3.9 micro-eV. Consideration of phase-space structure of the axion flow, which predicts extremely sharp peaks in the axion kinetic-energy spectrum, improves the limit. Presently underway is an upgrade to the experiment, using SQUID RF amplifiers, which will improve the sensitivity by about a factor of 20.