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| The LIGO gravitational wave detector in Livingston Louisiana. LIGO detectors could soon be using a new and more efficient scheme for squeezing light |
The quantum state of light has been squeezed more than ever before by physicists in Germany, who have developed a new low-loss technique. Squeezed light has been used to increase the sensitivity of gravitational wave detectors, and scientists are planning to deploy the new method on the GEO600 and LIGO gravitational wave detectors.
Detecting gravitational waves – the ripples in spacetime caused by energetic events in the Universe – relies on splitting a laser beam using an interferometer and sending the two halves back and forth along two orthogonal arms. When the two halves of the beam recombine, all the light normally comes out of one port of the interferometer. A passing gravitational wave will change the relative lengths of the two arms, creating an interference pattern and directing some of the light out of the "dark" port. However, by the time they reach Earth, gravitational waves from even the most dramatic events have tiny amplitudes, so sensitivity is crucial. The first confirmed discovery of a gravitational wave, announced by LIGO in February, was produced by the collision and merger of two black holes and changed the 4.2 km arm lengths by barely 10–19m (see "LIGO detects first ever gravitational waves – from two merging black holes").
At such extreme sensitivity, one of the main noise sources in such detectors is uncorrelated photons emerging from the quantum vacuum as a result of its zero-point energy – the energy that Heisenberg's uncertainty principle dictates can never be removed from a system. But, amazingly, even this source of noise can be minimized. The uncertainty principle puts a lower limit on the product of the variance in the amplitude (or number) of photons and the variance in the phase. Vacuum photons naturally have equal variance in both amplitude and phase. It is, however, possible to create a "squeezed state" of light, in which either one of these quantities is minimized (squeezed) and the other is allowed to increase (antisqueezed).
Physics World:
Squeezed light shatters previous record for manipulating quantum uncertainty
Tim Wogan
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