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 such as 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 for Gravitational Physics in Hannover, Germany, 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.

Vahlbruch et al., Physical Review Letters, 7 July 2006
Contact Henning Vahlbruch, Max Planck Institute for Gravitational Physics
henning.vahlbruch@aei.mpg.de
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