{"id":5188,"date":"2011-09-14T07:14:05","date_gmt":"2011-09-14T14:14:05","guid":{"rendered":"http:\/\/dabacon.org\/pontiff\/?p=5188"},"modified":"2011-09-14T07:14:05","modified_gmt":"2011-09-14T14:14:05","slug":"entangled-ligo","status":"publish","type":"post","link":"https:\/\/dabacon.org\/pontiff\/2011\/09\/14\/entangled-ligo\/","title":{"rendered":"Entangled LIGO"},"content":{"rendered":"


\n\"\"<\/a>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<\/a> 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.
\nThe 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.
\nBut 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<\/a> by Carl Caves, who showed that using squeezed states<\/a> of light could reduce the uncertainty in an interferometer.
\nWhat’s amazing is that in a\u00a0new paper<\/a>, the LIGO collaboration has actually succeeded for the first time\u00a0in 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.\"\"<\/a>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.
\nIt looks like quantum entanglement might play a very important role in the eventual detection of gravitational waves. Tremendously exciting stuff.<\/p>\n","protected":false},"excerpt":{"rendered":"

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 … <\/p>\n

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