‘Squeezed Laser’ Boosts Interferometer Accuracy

Gravitational waves, though yet to be observed, were first predicted by Einstein in his theory of relativity. Observing these massive astronomical events, which generate tiny fluctuations and cause the fabric of space-time to expand and contract like ripples on the surface of a pond, requires extremely sensitive detectors. Until now, these have not been available.

The new squeezed light laser of GEO600. A highly complex laser system produces light which particularly quiet in the gravitational wave detector. (Image: Max Planck Institute for Gravitational Physics)

But scientists have discovered a technique that they hope will increase the sensitivity of a global network of gravitational wave observatories (LIGO, GEO600 and Virgo), each of which will measure tiny variations in the distance travelled by two halves of a laser beam that has been split along perpendicular arms of a kilometer-sized instrument called a Michelson interferometer.

3D visualization of gravitational waves produced by 2 orbiting black holes. (Image: Henze, NASA)

According to Roman Schnabel of Leibniz University, Hannover, and his colleagues at the Max Planck Society, detecting a gravitational wave in an interferometer is difficult, because the wave's signal is so small that it is usually dwarfed by the noise generated by the quantum mechanical fluctuations of the light beams. To make accurate measurements, physicists must rely on metrology techniques that are as free as possible from interference, such as the so-called shot noise. Shot noise is actually nothing more than uncertainty of the laser's intensity. Through a process called "squeezing," this uncertainty has been minimized, producing laser light with almost no fluctuations.

Using the squeezed light method, which generates a completely new quality of laser light, the researchers increased the measurement sensitivity of the GEO600 gravitational wave detector to 150 percent, which they say is an important step toward the direct sensing of the gravitational waves.

Artist's impression of gravitational waves from two orbiting black holes. (Image: T. Carnahan, NASA GSFC)

“We now feed the squeezed light into the interferometer, in addition to our normal laser light,” Schnabel said. “If the two light fields then superimpose, the resulting laser beam has a much more uniform intensity, compared to the original signal beam. We thus smooth out the irregularities caused by quantum physical effects in the detector signal.”

Since April of last year, the squeezed light laser has been undergoing a longer test phase at GEO600 and is currently being used in the search for gravitational waves.

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