Detecting the faint ripples in spacetime known as gravitational waves is the primary objective of the Laser Interferometer Gravitational Observatory (LIGO), a huge collaboration that has been searching space for gravitational waves since 2002. Now LIGO scientists have developed a new technique that almost doubles the sensitivity of these detectors by exploiting "squeezed light" and the phenomenon of quantum entanglement.
LIGO is essentially a giant interferometer. There is a very large mirror hung in such a way as to form an arm, with two more mirrors hung perpendicular to it to form an L-shape when viewed from above. Scientists then pass laser light through a beam splitter, thereby dividing the beam between those two arms, and let the light bounce back and forth a few times before returning to the beam splitter.
LIGO has three such detectors, since it needs to operate at least two detectors at the same time as a control, so they don't get false positives. A passing gravity wave will cause ripples in spacetime, which in turn will change the distance measured by a light beam; the amount of light falling on the strategically placed photodetector will vary slightly in response.
The resulting signal will tell scientists how the light hitting the photodector changes over time. LIGO scientists liken the instrument to "a microphone that converts gravitational waves into electrical signals."
Here's the biggest problem facing LIGO: any change in the beams caused by gravitational waves is so tiny, it's drowned out by a quantum effect called vacuum fluctuations. Per Ars Technica:
Basically, the place where we measure the light coming out of the interferometer is also a place where light enters the interferometer. So, we aren't adding two light fields together at the beamsplitter. No, we are adding four light fields together. Scientists are not so stupid as to accidentally allow stray light into this device, but nature has its own way of producing strays. The vacuum itself is seething with photons that pop into existence and then disappear again. On average, nothing is there. Unfortunately for LIGO, on average is not good enough.
So improving the sensitivity of LIGO's detectors is an ongoing quest. And according to physicist and blogger Dave Bacon (a.k.a. The Quantum Pontiff), there was a seminal paper published in 1981 by Carl Caves demonstrating that using so-called squeezed states of light could reduce the inherent uncertainty in interferometers by creating entangled photons between the two mirrors. In Bacon's words: "We can fight quantum with quantum!"
How To Entangle Photons
When subatomic particles collide, they can become invisibly connected, though they may be physically separated. Even at a distance, they are inextricably interlinked and act like a single object - hence the term "entanglement," or, as Einstein preferred to call it, "spooky action at a distance."
This is useful because if you measure the state of one, you will know the state of the other without having to make a second measurement, because the first measurement determines what the properties of the other particle must be as well. Cornell University physicist N. David Mermin has described entanglement as "the closest thing we have to magic."