Entangled LIGO

The quest to observe gravitational waves has been underway for several years now, but as yet there has been no signal. To try to detect gravitational waves, the LIGO collaboration basically uses huge kilometer-scale Michaelson-type interferometers, one of which is seen in the aerial photo to the left. When a gravitational wave from, say, a supernova or in-spiraling pair of black holes arrives at the detector, the wave stretches and shrinks spacetime in the transverse directions, moving the test masses at the ends of the interferometer arms and hence changing the path length of the interferometer, creating a potentially observable signal.
The problem is, the sensitivity requirements are extreme. So extreme in fact, that within a certain frequency band the limiting noise comes from vacuum fluctuations of the electromagnetic field. Improving the signal-to-noise ratio can be achieved by a “classical” strategy of increasing the circulating light power, but this strategy is limited by the thermal response of the optics and can’t be used to further increase sensitivity.
But as we all know, the quantum giveth and the quantum taketh away. Or alternatively, we can fight quantum with quantum! The idea goes back to a seminal paper by Carl Caves, who showed that using squeezed states of light could reduce the uncertainty in an interferometer.
What’s amazing is that in a new paper, the LIGO collaboration has actually succeeded for the first time in using squeezed light to increase the sensitivity of one of its gravity wave detectors. Here’s a plot of the noise at each frequency in the detector.The red line shows the reduced noise when squeezed light is used. To get this to work, the squeezed quadrature must be in phase with the amplitude (readout) quadrature of the observatory output light, and this results in path entanglement between the photons in the two beams in the arms of the interferometer. The fluctuations in the photon counts can only be explained by stronger-than-classical correlation among the photons.
It looks like quantum entanglement might play a very important role in the eventual detection of gravitational waves. Tremendously exciting stuff.

7 Replies to “Entangled LIGO”

  1. Does this have any direct impact on the current sensitivity of LIGO or is it just proof of principle? Now that I’ve decohered I can ask the question and not go out and read the damn paper myself 😉

  2. Ah, from the abstract “GEO600 now operates with its best ever sensitivity, which proves the usefulness of quantum entanglement and the qualification of squeezed light as a key technology for future GW astronomy.” Cool!!!

  3. There’s a natural isomorphism between sensing and amplification (namely, high-gain power amplifiers can and do control classical meter-needles) and thus squeezed-photon sensing and squeezed-photon amplification are naturally dual.
    From this point-of-view, yet another seminal Carlton Caves reference is Quantum limits on noise in linear amplifiers (1982), which in turn includes multiple references to the maser literature dating back to the 1950s. These ideas have found practical fruition in (for example) the recent article by Tong et al. in Nature Photonics titled “Towards ultrasensitive optical links enabled by low-noise phase-sensitive amplifiers” (2011).
    The accompanying editorial commentary by Guifang Li titled “Optical communications: Amplifying to perfection” provides an in-depth and closely-reasoned summary of the practical implications of this work (with further references). In light of these well-known practical applications, the LIGO article’s claim “The increase of the GEO 600 sensitivity below its shot-noise limit by non-classical means is indeed the first practical application of this quantum technology” perhaps is an over-simplification, and surely deserves additional references.
    It’s absolutely true that LIGO’s noise-squeezing demonstration is a wonderful technical feat that points the way to very significant practical improvements, not only in gravity-wave detection, but also in many other fields, including in particular quantum spin microscopy (which is our own main professional interest). Congratulations to this fine LIGO team!

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