Sep 12, 2016
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).
High and low frequencies
Both amplitude and phase variations cause noise problems, but amplitude variations cause more problems at low frequencies, whereas phase variations cause problems principally at high frequencies. The idea, therefore, is to squeeze either the amplitude or the phase of the vacuum photons, depending on the frequency one is scanning from gravitational waves. The result is that quantum vacuum noise causes fewer problems than it otherwise would across the entire spectrum of interest (around 10 Hz to 5 kHz).
The best way to squeeze vacuum photons is a technique called optical parametric amplification. This uses a laser to pump a nonlinear crystal inside an optical resonant cavity, with each laser photon producing two "daughter" photons. The vacuum photons interact with the laser photons and, as a result, the variance of choice is squeezed in the emergent photons while the other is antisqueezed. The amount of squeezing that is possible is limited by both optical loss and noise in the apparatus. In 2010 researchers at the Max Planck institute for Gravitational Physics in Hannover set a world record by squeezing the amplitude and phase variances (separately) by a factor of 19.
The Hannover team has now improved several aspects of its instrumentation. Most significantly, they have used a new, doubly resonant cavity: "You need two wavelengths to generate the squeezed light and we had a resonator that was resonant for both," explains team member Moritz Mehmet. In addition, says his teammate Henning Vahlbruch, they upgraded several other features: "We used the best available materials, a different cavity topology and custom-made photodetectors." The researchers broke their own record, squeezing vacuum photons by a factor of 32.
"The observation of squeezing at the level they're seeing it is very, very significant", James Hough, University of Glasgow.
A version of the researchers' squeezing scheme is planned for GEO600 – a gravitational wave detector near Hannover. This instrument has two 600 m arms and has used squeezed light since 2010. In addition, installation of an apparatus for creating squeezed vacuum states on LIGO is planned – "probably in the next year", says Valbruch. By doing so, it should be possible to reduce the quantum noise in the interferometer readout, allowing fainter signals to be discerned from events occurring farther away in the universe.
The research could also be useful for calibrating the efficiency of photodetectors, says Mehmet. The efficiency of the detectors used is crucial to the amount of squeezing detected. The researchers used custom-made, ultra-efficient photodetectors, but one could also use the amount of squeezing detected as an absolute measurement of an arbitrary photodetector's efficiency.
"It's very, very high quality work," says James Hough of the University of Glasgow, who specializes in gravitational wave detectors. "The observation of squeezing at the level they're seeing it is very, very significant." He says the next challenge is to extend the frequency range over which the squeezing is possible and adds, "I'm sure people in Hannover will soon be working on that."
The research is described in Physical Review Letters.