Researchers at Nanjing University and other institutes have reported the observation of multiple chiral gravitons, providing evidence for the parton theory of the FQH effect.

As published in Nature Physics, the team were seeking experiemental evidence of various chiral gravitons, to in turn further research in partons hidden in fractionalised quantum matter.

“Our experiments provide a route to resolving individual partons and their fractional quantum Hall phases through graviton measurements, which could be extended to a wide range of exotic phases of matter, including excitonic topological orders and fractional Chern insulators,” commented Lingjie Du, senior author of the paper.

What are chiral gravitons?

Negatively charged particles, also known as electrons, occasionally coordinate movements in such a way that certain collective excitations, known as quasiparticles.

This is observed in the quantum Hall effect, where electrons are confined to a very thin layer,, exposed to a very strong magnetic field and cooled to temperatures around 0 kelvin.

What is parton theory?

There are multiple theories to explain the collective excitations of quantum Hall states- one of which is the parton theory framework. This theory posits that emergent partons(quark-like quasiparticles in condensed matter physics, not to be confused with the quarks of particle physics) are responsible in some for the collective excitations of quantum Hall states.

Small fluctuations in a system’s quantum metric thoeretically produce collective spin-2 excitations known as chiral gravitons.

Low-energy gravitons were observed in FQH states

“In fractional quantum Hall (FQH) states around half filling, we observed only one kind of chiral graviton mode, now referred to as the low-energy graviton,” Du continued.

“Later, around quarter filling, at filling factors such as v = 2/7 and 2/9, we observed a high-energy graviton in addition to the low-energy one. This finding is significant. Our earlier experimental work in 2024 indicated that the graviton energy is proportional to the fractional charge associated with an FQH state.

“Therefore, the observation of two graviton modes within one FQH state points to the presence of two distinct fractional charges, which can be naturally understood within the parton theory of the FQH effect.”

In earlier studies, the team experimentally observed low-energy gravitons, but had not observed a high-energy one. Low-energy gravitons are quasiparticles emerging in the FQH effect that need less energy to emerge, whereas probing high-energy partons require higher energy excitations.

Du and the team are still hoping to observe high-energy partons, as it would offer more conclusive evidence for the parton theory of the FQH effect.

“The partons discussed here are fractionally charged, quark-like quasiparticles, distinct from anyons, which can also carry fractional charge but obey anyonic statistics,” Du explained.

“Fluctuations of the quantum metric can give rise to a long-wavelength spin-2 geometric excitation associated with high-energy partons, namely the high-energy graviton. In our new study, we used a method called circularly polarized resonant inelastic light scattering at ultra-low temperatures (around 50 mK) and in strong magnetic fields (up to 14 tesla) to probe the spin and energy of the graviton mode in the high-energy range, which enabled us to detect the high-energy graviton.”

Using circularly polarized resonant inelastic light scattering, measurements taken by the team confirmed the presence of both low and high-energy gravitons. This in turn provided spectroscopic evidence for high-energy partons, which had not been directly observed before this study.

“The observation of multiple gravitons, particularly the high-energy graviton, is significant for validating the geometric theory of the FQH effect,” Du said. “It also offers experimental evidence that FQH partons are bona fide quasiparticles in strongly correlated matter and provides long-sought evidence for the parton theory of the FQH effect.”

“There are many interesting directions to explore”

“For example, while the graviton modes we detected are chiral spin-2 modes, higher-spin modes, which may offer a possible connection to nonrelativistic string physics, could be detected using photons carrying orbital angular momentum,” Du added.

“A superconducting instability arising from the pairing of neutral partons could give rise to a non-Abelian Moore-Read state, which could potentially be identified through the detection of graviton modes and is essential for topological quantum computation.”