Graviton-like particle observed in quantum material for the first time

In what could turn out to be a revolutionary discovery, a team of scientists has presented the first experimental evidence of collective excitations with spin called chiral graviton modes (CGMs) in a quantum material.

Searching for gravitons in the quantum realm

CGMs appear to be similar to gravitons, hypothetical elementary particles that are believed to give rise to gravity, one of the fundamental forces in the universe.

Despite their crucial role in our understanding of the universe, gravitons have yet to be discovered, and the ultimate cause of gravity remains a mystery.

The ability to study graviton-like particles in the lab could help bridge the gap between quantum mechanics and Einstein’s theories of relativity, solving a major dilemma in physics and expanding our understanding of the universe.

“Our experiment marks the first experimental substantiation of this concept of gravitons, posited by pioneering works in quantum gravity since the 1930s, in a condensed matter system,” said Lingjie Du, a former Columbia postdoc and senior author on the paper.

Discovering CGMs in fractional quantum hall effect liquids

The team discovered the particle in a type of condensed matter called a fractional quantum Hall effect (FQHE) liquid.

These liquids are systems of strongly interacting electrons that occur in two dimensions at high magnetic fields and low temperatures.

They can be theoretically described using quantum geometry, which are emerging mathematical concepts that apply to the minute physical distances at which quantum mechanics influences physical phenomena.

Electrons in an FQHE are subject to a quantum metric that had been predicted to give rise to CGMs in response to light.

However, in the decade since the quantum metric theory was first proposed for FQHEs, limited experimental techniques existed to test its predictions.

Building on a legacy of quantum research

The late Columbia physicist Aron Pinczuk spent much of his career studying the mysteries of FQHE liquids and developing experimental tools to probe such complex quantum systems.

His lab and its alumni across the globe have continued his legacy, including article authors Ziyu Liu, Lingjie Du, and Ursula Wurstbauer.

“Aron pioneered the approach of studying exotic phases of matter, including emergent quantum phases in solid state nanosystems, by the low-lying collective excitation spectra that are their unique fingerprints,” commented Wurstbauer, a co-author on the current work.

“I am truly happy that his last genius proposal and research idea was so successful and is now published in Nature. However, it is sad that he cannot celebrate it with us. He always put a strong focus on the people behind the results,” he concluded.

Adapting techniques to reveal CGM properties

One of the techniques Pinczuk established was called low-temperature resonant inelastic scattering, which measures how light particles, or photons, scatter when they hit a material, thus revealing the material’s underlying properties.

Liu and his co-authors adapted the technique to use circularly polarized light, in which the photons have a particular spin.

When the polarized photons interact with a particle like a CGM that also spins, the sign of the photons’ spin will change in response in a more distinctive way than if they were interacting with other types of modes.

International collaboration in search for quantum gravitons

The new paper was an international collaboration. Using samples prepared by Pinczuk’s long-time collaborators at Princeton, Liu and Columbia physicist Cory Dean completed a series of measurements at Columbia.

They then sent the sample for experiments in low-temperature optical equipment that Du spent over three years building in his new lab in China.

They observed physical properties consistent with those predicted by quantum geometry for CGMs, including their spin-2 nature, characteristic energy gaps between its ground and excited states, and dependence on so-called filling factors, which relate the number of electrons in the system to its magnetic field.

Connecting subfields of physics with CGMs

CGMs share those characteristics with gravitons, a still-undiscovered particle predicted to play a critical role in gravity.

Both CGMs and gravitons are the result of quantized metric fluctuations, explained Liu, in which the fabric of spacetime is randomly pulled and stretched in different directions.

The theory behind the team’s results can therefore potentially connect two subfields of physics: high energy physics, which operates across the largest scales of the universe, and condensed matter physics, which studies materials and the atomic and electronic interactions that give them their unique properties.

“For a long time, there was this mystery about how long wavelength collective modes, like CGMs, could be probed in experiments,” Liu explained.

“We provide experimental evidence that supports quantum geometry predictions,” said Liu. “I think Aron would be very proud to see this extension of his techniques and new understanding of a system he had studied for a long time,” he concluded.

Continuing the search for CGMs and gravitons in quantum materials

The groundbreaking discovery of chiral graviton modes (CGMs) in a semiconducting material opens up new avenues for exploring the mysteries of gravity.

This discovery could eventually, in essence, bridge the gap between quantum mechanics and Einstein’s theories of relativity.

By adapting established techniques and collaborating across international borders, this team of brilliant scientists has provided the first experimental evidence supporting quantum geometry predictions.

This study honors the legacy of the late physicist Aron Pinczuk and paves the way for future research that could potentially connect high energy physics and condensed matter physics, leading to a deeper understanding of the universe and its fundamental forces.

The research, conducted by experts from Columbia, Nanjing University, Princeton, and the University of Munster, was recently published in the prestigious journal Nature.

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