Groundbreaking Discovery: Scientists Uncover Graviton-like Particle in Quantum Material

Scientists from Columbia, Nanjing University, Princeton, and the University of Munster have found the first experimental evidence of a graviton-like particle, called chiral graviton modes (CGMs), in a semiconducting material. This discovery could help bridge the gap between quantum mechanics and Einstein’s theories of relativity. The team discovered the particle in a type of condensed matter known as a fractional quantum Hall effect (FQHE) liquid. The research continues the work of late Columbia professor Aron Pinczuk, who dedicated his career to studying FQHE liquids. The findings were published in the journal Nature.

Discovery of Graviton-like Particle in Quantum Material

A collaborative team of scientists from Columbia, Nanjing University, Princeton, and the University of Munster have reported the first experimental evidence of collective excitations with spin, known as chiral graviton modes (CGMs), in a semiconducting material. This discovery, published in the journal Nature, is significant as a CGM bears resemblance to a graviton, an elementary particle hypothesized to be responsible for gravity, one of the fundamental forces in the universe. The graviton, however, remains undiscovered and the ultimate cause of gravity is still a mystery.

The ability to study graviton-like particles in a laboratory setting could potentially bridge the gap between quantum mechanics and Einstein’s theories of relativity, thereby resolving a significant conundrum in physics and enhancing 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.

The Role of Fractional Quantum Hall Effect Liquids

The particle was discovered in a type of condensed matter known as a fractional quantum Hall effect (FQHE) liquid. FQHE 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, 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.

The Legacy of Aron Pinczuk

The late Columbia physicist Aron Pinczuk dedicated much of his career to studying the mysteries of FQHE liquids and developing experimental tools to probe such complex quantum systems. Pinczuk, who joined Columbia from Bell Labs in 1998 and was a professor of physics and applied physics, passed away in 2022, but his lab and its alumni across the globe have continued his legacy. Those alumni include article authors Ziyu Liu, who graduated with his PhD in physics from Columbia last year, and former Columbia postdocs Du, now at Nanjing University, and Ursula Wurstbauer, now at the University of Münster.

Techniques and Collaborations

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 on the Nature paper adapted the technique to use what’s known as 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.

The research 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.

Future Implications and Applications

The discovery of CGMs and their shared characteristics with gravitons, a still-undiscovered particle predicted to play a critical role in gravity, could 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.

In future work, Liu says the polarized light technique should be straightforward to apply to FQHE liquids at higher energy levels than they explored in the current paper. It should also apply to additional types of quantum systems where quantum geometry predicts unique properties from collective particles, such as superconductors. “For a long time, there was this mystery about how long wavelength collective modes, like CGMs, could be probed in experiments. We provide experimental evidence that supports quantum geometry predictions,” said Liu.