Einstein's general theory of relativity predicts that when massive astronomical events occur, such as the explosion of large stars or the collision of black holes, they should generate ripples in the very fabric of space known as gravitational waves. These waves have yet to be observed; however, a technique that uses the quantum mechanical nature of light to improve the sensitivity of working gravitation wave detectors is reported in Nature Physics this week.
As a gravitational wave passes by through a region of space, it is expected to cause the space itself to expand and contract — much like ripples on the surface of a pond. Roman Schabel and colleagues of the LIGO collaboration hope to detect these transient ripples in space with a global network of gravitational wave observatories. Each of these observatories measures tiny variations in the distance travelled by two halves of a laser beam that has been split along perpendicular arms of a kilometre-sized instrument called a Michelson interferometer.
Unfortunately, the magnitude of the change that a gravitational wave is predicted to induce in the output of such a device is so small that it is usually dwarfed by noise generated by quantum mechanical fluctuations of its light beams. To overcome this, the authors use so-called squeezed light, which exploits a loophole in the laws of quantum mechanics that enables them to reduce the fluctuations in one characteristic of their light by increasing the fluctuations in another.
By implementing this approach in the GEO600 interferometer located near Sarstedt, Germany, the authors reach significantly higher levels of sensitivity than previously achieved in the device.