One of the biggest components of current gravitational-wave detectors is the vacuum chamber where all the interferometer optics are housed — a component that, according to the new study, could be removed from future gravitational-wave interferometers.>
"What we demonstrate in our paper was a turbulence-free double-slit interferometer, which isn't exactly ideal for gravitational-wave detection, but the mechanism that produces the turbulence-free interference can theoretically be applied to any interferometer," said lead author Thomas Smith, a postgraduate from the University of Maryland, Baltimore County (UMBC).
"Our future steps are to look at those interferometers that are ideal for gravitational-wave detection by their layout and see if we can apply this mechanism to those," Smith added. "We could, theoretically, have a gravitational-wave detector out in the open air."
In the study, Smith and Yanhua Shih, a physics professor also at UMBC, argue that the quantum properties of light could be used as a powerful and, potentially, revolutionary new tool for dealing with interferometer noise and detecting even the weakest gravitational-wave signals.
The experimental setup proposed by Smith and Shih is based on an 1801 double-slit experiment devised by English physicist Thomas Young to demonstrate the wave theory of light. It was later used to demonstrate the wave-particle duality concept in quantum mechanics in the early 20th century. In its most basic form, the experiment consists of a light source that illuminates a plate, which has two slits for light to pass through. Behind the plate is a screen. As the light passes through the slits, the waves constructively and destructively interfere, creating a classic pattern of light and dark bands on the screen.
But what if you were to fire just one photon at the slits? Well, through the weirdness of quantum physics, that photon is just as probable to travel through slit A as it is slit B and can therefore interfere with itself to create an interference pattern on the screen. (For an in-depth explanation, read this classic Space.com piece.) This is a useful demonstration of quantum physics, but it would be useless in a gravitational-wave detector; single photons will be affected by air turbulence, so the paths to slit A and slit B will vary, jumbling the photons' phase and blurring out any interference pattern.
So, Smith and Shih suggested that, instead of the one-photon interference, two-photon interference patterns can be measured, and the noise that muddles the one-photon interference pattern will simply disappear from the equation. Much of the detail behind this method is buried in complex math, but it could provide a surprisingly elegant solution for laser interferometers, the researchers said.
Two-photon interference "is kind of a new concept in physics in the past few decades, and it seems to be catching hold," Smith told Space.com. "You can have two potential paths for the two photons to travel. And when you scan your two detectors in approximately the same location, these two potential paths for the two photons overlap. And, because of that overlap, both potential paths experience the same turbulence, the same phase shifts. And, because they're experiencing the same phase shifts, the interference pattern is unaffected … Within the math, the turbulence cancels."
The researchers even demonstrated this method with a tabletop experiment incorporating a laser light source, double slits, and a toaster oven in between to create air turbulence. When the oven is turned on, the classic interference pattern abruptly disappears; the air turbulence disrupts the photons, preventing interference from occurring. But if you carefully position two detectors to precisely measure the two-photon interference pattern and then turn the oven on, the interference pattern persists as if there were no turbulence at all.
"The classic interference pattern disappeared [when the oven was switched on], but the interference pattern we measured from the intensity fluctuation correlation in this turbulence-free interferometer remained at nearly 100 percent — still very clear," Shih said.
In this situation, the researchers can measure the pattern generated when pairs of photons interfere with themselves after traveling the same path from the coherent light source to the slits. In effect, these pairs of photons experience the exact same turbulence during their journey, like two passengers sitting next to each other on a roller-coaster ride. Sure, the two passengers will experience a lot of ups, downs, loops, and wobbles, but they will arrive at the end of the ride having traveled along the exact same path. Like the roller-coaster passengers, to the pairs of photons (and the interference pattern they create), it's as if the air turbulence weren't even there.
By the researchers' logic, if this system could be scaled up and somehow incorporated into the optical system of gravitational-wave interferometers, these detectors could function in the open air, and highly efficient vacuum systems would no longer be required. And once this complex system was removed, the possibilities would become very exciting.
"We could have one station on the surface of Earth and others on satellites … We could make a much bigger interferometer that would be much more sensitive than the ones we currently have," Shih said. The bigger the interferometer, he said, the more sensitive the detector would become to weaker and lower-frequency gravitational waves. [The Universe: Big Bang to Now in 10 Easy Steps]