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.”
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So disturbances in one part of the universe can instantly affect distant other parts of the universe, mysteriously bypassing the ubiquitous speed-of-light barrier. Spooky!
There are lots of different ways particles can become entangled, but in every case, both particles must arise from a single “mother” process. It’s a bit like how identical twins emerge from a single fertilized egg, sharing the genetic material between them.
For instance, passing a single photon through a special kind of crystal can split that photon into two new “daughter” particles. We’ll call them “green” and “red.” Those particles will be entangled. Energy must be conserved, so both daughter particles have a lower frequency and energy than the original mother particle, but the total energy between them is equal to the mother’s energy.
We have no way of knowing which is the green one and which is the red. We just know that each daughter photon has a 50/50 chance of being one or the other color. But should we chance to see one of the particles and note that it is red, we can instantly conclude that the other must be green.
Entanglement is a tricky thing, and easily undone by even the slightest interference. That’s why it’s useful in quantum cryptography: the system can detect any “eavesdropper” immediately and know the transmission has been compromised. It now seems likely that gravitational waves could be detected just as easily, by leaving a telltale signature on any entangled particles they encounter.
Squeezing the Light
Physicists have been using light (photons) to probe the mysteries of nature for centuries. But at the quantum scale, uncertainty — a.k.a quantum noise — gets in the way of gleaning useful information.
Squeezing is a way to increase certainty in one quantity (e.g., position or speed) by trading a decrease in certainty in another complementary property. Using special crystals, this squeezing process creates quantum entangled photons between the interferometer’s mirrors, turning one photon into two.
Now you have highly sensitive entangled photons directly in the path of any gravitational waves that happen by. And LIGO scientists have successfully demonstrated that this does, indeed, result in more sensitive detectors, as evidenced in the plot above showing the noise at each frequency in one of the detectors. Per Bacon (again):
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.
“The strange thing is, when you look at it, there’s nothing there, yet this ‘nothing’ which is the vacuum fluctuation can be squeezed and we know it’s real, because it changes the sensitivity of the detector,” physicist David Blair told ABC Science. Blair is director of the Australian International Gravity Wave Research Centre at the University of Western Australia, part of the LIGO collaboration.
LIGO hasn’t reached its full sensitivity yet; that will happen once the planned upgrades for Advanced LIGO are complete. Hopefully, by then, this new “squeezed light” approach can be incorporated into those upgraded detectors. Gravitational waves are a prediction of general relativity. It would be strangely fitting if quantum mechanics ultimately helped detect them.
Image credits: LIGO