Experimental measurements have revealed both low-energy and high-energy chiral gravitons in fractional quantum Hall states, providing long-sought evidence supporting parton theory. The findings introduce a new way to probe hidden quasiparticles in strongly correlated quantum matter and offer fresh insight into the geometry underlying these exotic electronic systems.
Electrons are often thought of as individual particles carrying negative electric charge. Yet under extraordinary conditions, they can behave collectively, producing entirely new forms of excitation that emerge from their coordinated motion rather than from any single electron. These collective excitations, known as quasiparticles, have become central to understanding some of the most unusual states of matter.
Researchers at Nanjing University and collaborating institutions have now reported the observation of multiple chiral gravitons—rare collective excitations predicted to arise in fractional quantum Hall (FQH) systems. Published in Nature Physics, the work provides experimental evidence supporting parton theory, a long-standing framework proposing the existence of quark-like quasiparticles within condensed matter systems.
The discovery also establishes a new experimental strategy for probing hidden quasiparticles that until now had largely remained theoretical.
A decades-old prediction gains experimental support
The quantum Hall effect appears when electrons are confined to an extremely thin layer, cooled to temperatures close to absolute zero, and subjected to a very strong magnetic field. Under these conditions, electrons no longer behave independently. Instead, their interactions generate collective quantum states with unusual properties.
To explain these states, physicists developed parton theory, which proposes that electrons in fractional quantum Hall systems can effectively behave as if they split into smaller, quark-like quasiparticles known as partons. These emergent partons should not be confused with quarks or gluons in particle physics, despite the similar terminology.
More recently, theoretical work suggested that tiny fluctuations in a system’s quantum metric—a quantity describing the “shape” of a quantum state—could generate collective spin-2 excitations called chiral gravitons.
Although these gravitons were predicted theoretically, obtaining convincing experimental evidence had proved difficult.
Two graviton modes reveal a more complex picture
Senior author Lingjie Du explained that previous experiments had identified only one type of chiral graviton.
“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 told Phys.org.
The latest experiments uncovered something unexpected. In fractional quantum Hall states around quarter filling, including filling factors v = 2/7 and 2/9, the researchers detected not only the familiar low-energy graviton but also a distinct high-energy graviton.
According to Du, this observation carries important implications.
“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.”
He explained that the team’s earlier work in 2024 had shown that graviton energy scales with the fractional charge associated with a fractional quantum Hall state. Detecting two graviton modes within the same quantum Hall state therefore points to the existence of two different fractional charges, a result that fits naturally within parton theory.
Detecting hidden quasiparticles through light scattering
Earlier experiments had successfully measured only the lower-energy graviton. Observing the higher-energy excitation remained an important objective because it could provide stronger evidence for hidden partons.
“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.
To search for these higher-energy excitations, the researchers employed circularly polarized resonant inelastic light scattering, a technique capable of revealing subtle excitations inside a material.
Their experiments examined two-dimensional electron gases confined within single quantum wells. Measurements were performed at approximately 50 millikelvin under magnetic fields reaching 14 tesla, conditions necessary for the fractional quantum Hall effect to emerge.
Du said the approach allowed the team to probe both the spin and energy of graviton modes in the previously inaccessible high-energy regime.
“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,” he said. “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.”
Evidence that partons possess real physical dynamics
The measurements ultimately revealed both low-energy and high-energy chiral gravitons.
These observations provided spectroscopic evidence for previously elusive high-energy partons, offering a new view of the geometric excitations that occur within fractional quantum Hall systems. According to the researchers, the findings indicate that emergent partons are not simply mathematical constructs used to describe complex behavior but quasiparticles with measurable geometric dynamics.
“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.”
Opening a new path for studying exotic quantum matter
Beyond supporting existing theory, the work introduces a practical method for investigating fractionalized quantum systems through graviton measurements.
According to Du, the experimental approach could eventually be extended to many other forms of exotic 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,” he said.
The researchers also see opportunities to expand their technique beyond the spin-2 chiral gravitons observed in the present study.
“There are many interesting directions to explore,” Du added. “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.”
He also noted that a superconducting instability resulting from paired neutral partons could produce a non-Abelian Moore-Read state, which might be identified through graviton measurements and is considered essential for topological quantum computation.
While those possibilities remain targets for future investigations, the current study demonstrates that measuring chiral gravitons can reveal hidden quasiparticles inside fractional quantum Hall matter, providing experimental support for theoretical ideas that have guided the field for years.
