Squeezed light for gravitational waves

There is little doubt that gravitational waves exist. Indirect evidence for them is strong. But they have yet to be directly observed, despite teams of researchers looking for them with a network of extraordinarily sensitive interferometers. And those interferometers are running up against the shot-noise limit: Their main source of noise, at least at the high-frequency end of their bandwidth, comes from the quantum fluctuations of light, which can be interpreted as the zero-point electromagnetic fluctuations of the vacuum itself. Now, researchers led by Roman Schnabel (University of Hanover, Germany) have beaten the shot-noise limit of the GEO 600 interferometer near Hanover. They did it by using squeezed light, an optical state in which the electric-field uncertainty at part of the electromagnetic wave is reduced at the expense of the uncertainty at another part. As shown in the figure, they injected a squeezed vacuum—a squeezed electromagnetic state with alternating high and low uncertainty, but with no underlying bright wave—into the interferometer’s signal output port. The squeezed vacuum, combined with the high-power laser light injected at the other side of the beamsplitter, altered the quantum fluctuations of the light traversing the 600-m arms and ultimately reduced the noise of the interferometer’s output. Because squeezing degrades rapidly as light is attenuated, squeezing the input vacuum by an experimentally challenging factor of 10 reduced the output variance by just a factor of 2. A planned upgrade to GEO 600 to reduce optical losses should make the squeezing even more effective. (LIGO Scientific Collaboration, Nat. Phys., in press, doi:10.1038/nphys2083 .)—Johanna Miller