New research suggests a way to capture physicists’ most sought-after particle: gravitons

A team led by Stevens Professor Igor Pikovski has just shown how to detect single gravitons, considered the quantum building blocks of gravity. They say making this experiment a reality should be possible with quantum technology in the near future.

“This is a fundamental experiment that was long considered impossible, but we think we have found a way to do it,” says Igor Pikovski, a professor of physics at Stevens, also affiliated with Stockholm University.

Pikovski led a team of first-year graduate students Germain Tobar and Thomas Beitel and postdoctoral researcher Sreenath Manikandan. Their results on “Detection of single gravitons by quantum sensing” were published in Nature Communications.

Elusive particles that build the cosmic fabric

Gravity just works. Objects fall, planets orbit them. More than a hundred years ago, Einstein revolutionized our understanding of gravity by explaining it through changes in space and time. Many previously unimaginable effects of gravity have now been confirmed: time dilation, gravitational waves, and black holes.

But gravity has another peculiarity: we have so far only seen its “classical” version, while all other forces are explained by quantum theory. One of the holy grails of physics has long been to connect gravity to quantum mechanics, but this problem remains. In any quantum theory of gravity, we expect certain single, indivisible particles to appear.

Physicists have dubbed these elusive particles gravitons. They should be thought of as the building blocks of gravity, just as atoms are the building blocks of matter. In theory, the gravitational waves that frequently pass through Earth following colossal cosmic events such as black hole collisions are made up of a huge number of these gravitons.

Impressive detectors like LIGO can now confirm the existence of such gravitational waves. Yet no gravitons have ever been detected in history; the very idea of ​​spotting one was long considered impossible.

But that may have just changed.

Pikovski’s team proposed a solution that involves coupling existing physical sensing technology (an acoustic resonator, essentially a heavy cylinder) and equipping it with improved methods of energy state detection (also called quantum sensing).

“Our solution is similar to the photoelectric effect that led Einstein to the quantum theory of light,” Pikovski explains, “but gravitational waves replace electromagnetic waves. The key is that energy is exchanged between matter and waves only in discrete steps: individual gravitons are absorbed and emitted.”

But how can we detect them?

“We need to cool the material and then monitor how the energy changes in a single step, and this can be achieved through quantum sensing,” says Manikandan, a postdoctoral researcher at the Nordic Institute for Theoretical Physics in Stockholm.

“By observing these quantum jumps in matter, we can infer that a graviton has been absorbed,” adds Tobar, now a graduate student at Stockholm University. “We call this the gravito-phononic effect.”

One of the innovations proposed by the team is to use data available from LIGO, an American observatory composed of two installations that recently confirmed the existence of gravitational waves.

“The LIGO observatories are very good at detecting gravitational waves, but they can’t detect isolated gravitons,” notes Beitel, a doctoral student at Stevens. “But we can use their data to cross-correlate with our proposed detector to isolate isolated gravitons.”

Cosmic collisions, heavy cylinders, quantum sensors

How did Pikovski’s team come up with this ingenious experiment? A lot of math and creativity, plus a little help from recent technological advances.

“Many physicists have pondered this question over the years, but the answer was always the same: it’s impossible,” Pikovski says. “It was impossible to imagine quantum experiments that went beyond a few atoms, and they hardly interact with gravitons.”

But the game has changed: Scientists have recently begun to create and observe quantum effects in macroscopic objects. Pikovski realized that these macroscopic quantum objects are ideal for observing individual gravitational signatures: they interact much more strongly with gravity, and we can detect how these objects absorb and emit energy in discrete steps.

The team began thinking about a possible experiment. Using gravitational wave data already measured on Earth, such as those that arrived in 2017 from a collision of two distant Manhattan-sized (but super-dense) neutron stars, they calculated parameters that would optimize the probability of absorbing a single graviton.

“It turns out that this measurement can be done,” Manikandan explains, “for example, using a device similar to the Weber bar.”

Weber rods are thick, heavy (up to a ton) cylindrical rods named after their inventor, New Jersey native Joseph Weber. These rods have fallen out of favor recently due to the proliferation of optical detection technologies, but they would actually be very useful for a graviton-hunting expedition led by a physicist.

This is because they can absorb and emit gravitons, in direct analogy to what Einstein called the “stimulated emission and absorption” of photons, the smallest building blocks of light.

A newly designed quantum detector would be cooled to its lowest energy, then set into a very slight vibration by a passing gravitational wave. Ultra-sensitive energy sensors could then theoretically capture how these vibrations evolve in discrete steps. Each discrete change (also called a quantum jump) would indicate a single gravitational event.

Of course, there’s a catch to capturing gravitons: the necessary detection technology doesn’t yet exist.

“Quantum leaps have been observed in materials recently, but not yet in the quantities we need,” Tobar points out. “But the technology is advancing very quickly and we have more and more ideas to make it easier.”

“We are confident that this experiment would work,” Thomas enthuses. “Now that we know that gravitons can be detected, it gives us even more motivation to further develop the appropriate quantum detection technology. Hopefully, we will soon be able to capture single gravitons.”

But while new quantum technologies are crucial, the inspiration for this result came from elsewhere. “We know that quantum gravity is not yet solved and that it is too difficult to test it in all its glory,” Pikovski explains, “but now we can take the first steps, just as scientists did more than a hundred years ago with light quanta.”