PHYSICS NEWS UPDATE

The American Institute of Physics Bulletin of Physics News

Number 784 July 7, 2006 by Phillip F. Schewe, Ben Stein,
and Davide Castelvecchi www.aip.org/pnu

SQUEEZED LIGHT AND GRAVITY WAVES. A proven method for reducing the
noise in high-precision optical measurements will soon be applied to
the search for gravitational waves. The most likely way such waves
will be detected is by observing their subtle effects on suspended
mirrors in detectors like the Laser Interferometer
Gravitational-wave Observatory (LIGO). At LIGO, laser light is
split into two beams which reflect many times from mirrors suspended
at the ends of two long pipes positioned at right angles. The two
beams are brought back together to form an interference pattern.
This procedure is adjusted so that a photodetector is positioned at
a null in the pattern; that is, it normally sees no photons coming
its way. The plan is that a passing gravity wave would ever so
slightly move the suspended mirrors in the two pipes
(which are otherwise insulated from ordinary kinds of vibration)
relative to each other, which in turn would disturb the interference
pattern. Suddenly the photodetector would record photons, heralding
a gravity wave. One problem with this scheme is "shot noise," the
quantum-based uncertainty in our knowledge of how many photons are
present in a laser beam at any moment. Fluctuations in photon
number could trigger a false positive reading. Physicists at the
Max Planck Institute (Hannover) and the University of Hannover are
hoping to reduce the quantum noise inherent in this interferometric
approach to gravity wave detection by squeezing light. Squeezed
light is produced when quantum noise in one or the other of two
complementary variables describing a light beam (such as phase and
amplitude) is greatly reduced at the expense of the other by sending
the light through (a series of) special optical crystals. The use
of squeezed light reduces quantum noise in a number of
optoelectronic applications. Usually the squeezed light approach is
applied at megahertz frequencies, but the Hannover researchers have
for the first time gotten it to work at all the detection
frequencies pertinent for LIGO including frequencies below a hundred
hertz, the expected frequency range of gravitational waves arriving
from some distant coalescing black holes in the universe. According
to Henning Vahlbruch(henning.vahlbruch@aei.mpg.de) a squeezed-light
control scheme would help reduce noise and raise the sensitivity of
gravity wave detectors. (Valbruch et al., Physical Review Letters,
7 July 2006; website at

http://www.geo600.uni-hannover.de/~schnabel/ )

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