Researchers at the École Normale Supérieure and Université Pierre et Marie Curie in Paris reported cooling a mirror with laser light. Brownian motion from internal thermal noise is the greatest impediment to the accuracy of interferometric gravity-wave detectors.

The team measured the Brownian motion by the fluctuations in phase of an 810-nm beam from a Ti:sapphire laser injected into a high-finesse cavity behind a mirror. A feedback loop using the cavity signal modulated an auxiliary 500-mW beam to reduce phase noise.

The scientists' results appeared in the Oct. 18 issue of Physical Review Letters. They said the system could quiet thermal noise in an interferometer by a factor of 20 without affecting the instrument's sensitivity.