Laser's radiation pressure quiets a mirror.

Mirrors play a crucial role in a variety of precision measurements. To detect extremely faint signals from space, for instance, researchers must minimize disturbances that deform a mirror's surface, down to the thermal noise caused by the constant jiggling of the mirror's atoms.

Physicists now report the first experimental evidence that they can reduce thermal-noise effects by using the pressure of laser light to counter a mirror's tiny movements. Antoine Heidmann and his collaborators at the Laboratoire Kastler Brossel in Paris describe their findings in the Oct. 18 PHYSICAL REVIEW LETTERS.

An elementary way to decrease thermal noise is to lower the temperature. For situations where such cooling isn't possible, Paolo Tombesi and his coworkers at the University of Camerino in Italy proposed a scheme last year for reducing thermal fluctuations by using feedback control. Heidmann's team applied their idea: Detect a mirror's movement, then respond by changing the intensity of a laser beam shining on the mirror to make it cancel out much of the motion.

The pressure exerted by the laser light can reduce thermal agitation to one-twentieth of its original intensity, in effect chilling the mirror, Heidmann and his colleagues discovered. In a demonstration of the technique's sensitivity, they also showed that a small alteration in the feedback loop would increase the amount of noise, causing the mirror to heat up.

Scientists are currently focusing on one application of this approach. "The cooling mechanism [we have demonstrated] may be useful to increase the sensitivity of gravitational-wave interferometers," Heidmann's group notes.

Theorists believe that interacting black holes and neutron stars can create extreme distortions in the geometry of the space around them. These space-time disturbances travel outward as gravitational waves, which slightly jostle any bodies in their paths (SN: 6/26/93, p. 408).

Reaching Earth, a gravitational wave might change the distance between two test masses 4 kilometers apart by just [10.sup.-16] centimeter, or one-100-millionth the diameter of a hydrogen atom.

Highly sensitive instruments for detecting such signals are now under construction. The Laser Interferometer Gravitational-Wave Observatory (LIGO) in Livingston, La., and Hanford, Wash., is scheduled to begin operating in the year 2001. Scientists in France and Italy are collaborating on another detector, called VIRGO, located near Pisa, Italy.

Researchers at the Massachusetts Institute of Technology and elsewhere are exploring the possibility of developing technologies that can increase the sensitivity of future gravitational-wave detectors. Finding novel ways to keep mirrors still is an important part of those efforts.

The main difficulty with a laser feedback system is that researchers must make sure that freezing the thermal noise doesn't also nullify the effects they intend to examine, Heidmann and his colleagues remark. The group is now investigating laser feedback schemes that would reduce the background noise without changing an interferometer's response to gravitational waves.

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