How to catch a graviton

Gravitons, the quantum particles of gravity, were thought to be impossible to observe. Scientists have now worked out how they can be detected by using quantum sensing technology.

Scientists worked out how single gravitons can be detected. Igor Pikovski Group
Scientists worked out how single gravitons can be detected, using quantum sensing of gravitational waves. Image credit: Pikovski research group.

Gravity is one of the fundamental forces of nature, but it is by far the weakest one. Gravity shows when large masses are involved, but at the microscopic level, it plays hardly any role at all. However, if we believe in quantum theory, even gravity must be made of tiny, quantized particles – gravitons. Just as photons are the particles that make up light, gravitons are particles that make up gravitational waves. And even though there is no complete theory of quantum gravity (the sought after “theory of everything”), all attempts for such a theory agree on one fact: the existence of gravitons. But these elusive particles were assumed to be too hard to observe.

Scientists just overturned this conviction. A team led by Igor Pikovski, Professor at Stockholm University and Stevens Institute of Technology, worked out how to build a single-graviton-detector. Pikovski, together with PhD students Germain Tobar, Thomas Beitel and postdoctoral researcher Sreenath K. Manikandan, showed how quantum sensing enables the detection of gravitons, and that it can be achieved in the near future. The results are published in Nature Communications.

 

Einstein’s insights to the rescue

Single gravitons barely interact with anything at all. They pass nearly all matter as they cross the universe. Detecting them seemed impossible. Even gravitational waves as predicted by Einstein were just recently confirmed in the LIGO detectors, after decades of technology development. And these waves consist of about 1036 gravitons.

“Detecting single gravitons seemed like an impossible task” says Pikovski, “but we think we found a way.”

The inspiration for this new result came from the very early days of quantum theory. In 1905, Einstein conjectured that light must consist of indivisible quanta – particles that we today call photons. He applied this insight to the photoelectric effect, where he predicted that the energy between light and matter is exchanged only in discrete amounts. Despite much resistance in the physics community, this new quantum theory of light and matter was eventually accepted, and the photoelectric effect that Einstein described played a central role.

“Our solution mimics the photoelectric effect, but we use acoustic resonators and gravitational waves that pass Earth”, says Tobar. “We call it the ‘gravito-phononic’ effect.”

A passing gravitational wave slightly distorts space, which is mostly unnoticeable. But if one measures precisely enough, one can detect how objects are stretched and squeezed periodically. LIGO detects these changes by measuring distances with a laser. There is also another approach: using massive objects that vibrate. “If we use heavy cylinders that resonate with the waves, then for a sufficiently strong wave some energy can get deposited” points out Manikandan, researcher in theoretical physics at Nordita in Stockholm. “The trick is to use quantum sensing to observe single quantum jumps in energy whenever single gravitons are absorbed or emitted.”

 

Quantum signals from classical waves

To detect even a single event, the gravitational waves have to be extremely energetic. And there is no way to create gravitational waves on demand.

“We can solve both problems by using existing gravitational wave observatories” explains Beitel. “We wait until LIGO detects a passing gravitational wave and observe how it produces quantum jumps in our detector at the same time”.

The chances of such an event are small, so the researchers computed the optimal parameters for already detected waves. And the results are promising. A neutron-star merger as observed in 2017, they computed, would produce just the right number of gravitons such that one would be absorbed with high probability in a realistic device.
Scientists were hoping to find quantum hints of gravity in gravitational waves, but so far there has been no conceivable experiment. Frank Wilczek, Professor of Physics at Stockholm University, Arizona State University and MIT, pioneered this research area. Wilczek recently showed that gravitational waves cause quantum noise in gravitational wave detectors, but that this statistical noise is too small to be observed. Pikovski’s team, which is affiliated with Wilczek’s group at Stockholm University, focused instead on quantum sensing in massive quantum devices. In their solution, a large detector made of a special material like superfluid Helium is cooled to its lowest energy level, and quantum sensing is used to monitor how its energy changes in discrete steps. This has been achieved before, but the technology must now be scaled up to larger masses, a challenge for quantum technology development.   

“Our proposed experiment is not easy” says Pikovski. “But quantum gravity is the biggest puzzle in modern physics, and our result shows that real experiments can soon test some of its predictions.”

Publication:
G. Tobar, S. K. Manikandan, T. Beitel and I. Pikovski.
Nature Communications 15, 7229 (2024)
https://doi.org/10.1038/s41467-024-51420-8