In the fascinating world of quantum physics, where particles defy conventional logic, subatomic entities sometimes behave in ways that seem almost magical. The rules of classical physics, which govern the everyday world, do not apply at this minuscule scale. In the quantum realm, particles can exist in multiple places simultaneously, pass through impenetrable barriers, and even communicate instantaneously over vast distances. These phenomena, once thought impossible, have spurred physicists to delve deeper into the strange and puzzling properties of quantum mechanics.
A recent study by physicists at Brown University has made a groundbreaking discovery in this mysterious realm. The research team has identified a new class of quantum particles known as fractional excitons. These particles behave in ways that have never been observed before, challenging existing paradigms and offering new avenues for exploration in the world of quantum physics.
Led by Jia Li, an associate professor of physics at Brown University, the team’s discovery highlights the emergence of particles that defy the classical classifications of matter. “Our findings point toward an entirely new class of quantum particles that carry no overall charge but follow unique quantum statistics,” said Li. “The most exciting part is that this discovery unlocks a range of novel quantum phases of matter, presenting a new frontier for future research, deepening our understanding of fundamental physics, and even opening up new possibilities in quantum computation.”
Li, along with three graduate students—Naiyuan Zhang, Ron Nguyen, and Navketan Batra—worked with Dima Feldman, a professor of physics at Brown, to investigate the nature of these fractional excitons. The research team’s work was published in Nature on January 8, 2025.
The core of the team’s discovery revolves around a phenomenon known as the fractional quantum Hall effect. This effect builds on the classical Hall effect, in which a magnetic field applied to a material carrying an electric current causes a perpendicular voltage to appear. Under normal conditions, this voltage increases in a linear fashion. However, at extremely low temperatures and high magnetic fields, something unusual happens: the voltage increases in distinct, quantized steps. This is known as the quantum Hall effect.
In the fractional quantum Hall effect, the steps are not whole numbers; instead, they occur in fractional amounts, which is where the term “fractional” comes from. This fractional change in voltage corresponds to a fraction of an electron’s charge, creating an entirely new class of particles that challenge traditional concepts of charge and matter.
In their experiment, the research team utilized a sophisticated setup involving two thin layers of graphene—an atomically thin material with remarkable electrical properties—separated by an insulating crystal of hexagonal boron nitride. This setup allowed the researchers to manipulate the movement of electrical charges with precision and also facilitated the creation of excitons. Excitons are quasiparticles formed by the combination of an electron and a hole, the absence of an electron in an atom’s structure.
By applying magnetic fields millions of times stronger than Earth’s magnetic field, the team was able to observe the behavior of these novel fractional excitons. What they found was striking: the fractional excitons displayed a behavior that was unlike anything seen before in physics.
Typically, fundamental particles fall into two categories: bosons and fermions. Bosons are particles that can occupy the same quantum state simultaneously, meaning that multiple bosons can exist in the same space without any restrictions. In contrast, fermions follow the Pauli exclusion principle, which states that no two fermions can occupy the same quantum state at the same time. Electrons, protons, and neutrons are examples of fermions.
The fractional excitons observed by the Brown University team, however, did not neatly fit into either of these categories. While they displayed the fractional charges predicted by the theory, their behavior exhibited characteristics of both bosons and fermions, making them appear as a hybrid of the two. This dual nature of the particles is what makes them resemble anyons, a class of particles that exists somewhere between fermions and bosons. However, the fractional excitons went even further, exhibiting behaviors that set them apart from anyons as well.
“This unexpected behavior suggests that fractional excitons could represent an entirely new class of particles with unique quantum properties,” said Naiyuan Zhang, one of the co-first authors of the study. “We show that excitons can exist in the fractional quantum Hall regime and that some of these excitons arise from the pairing of fractionally charged particles, creating fractional excitons that don’t behave like bosons.”
The implications of this discovery could be profound, particularly in the realm of quantum computation. Quantum computers rely on the principles of quantum mechanics to process information in ways that classical computers cannot. The unique properties of fractional excitons could potentially be harnessed to develop faster, more reliable quantum computers by exploiting the unusual behaviors of these particles. “We’ve essentially unlocked a new dimension for exploring and manipulating this phenomenon,” said Li. “And we’re only beginning to scratch the surface.”
The team’s experiment marks the first time these types of fractional excitons have been observed experimentally, making it a major milestone in the understanding of quantum mechanics. This discovery opens up new possibilities for future research into quantum matter and quantum computation, and it could lead to entirely new quantum phases of matter that were previously unimaginable.
Looking forward, the researchers plan to continue their investigations into the properties of fractional excitons. One of the next steps in their research will be to explore how these particles interact with one another and whether their behaviors can be controlled for practical applications. Understanding how to manipulate these particles will be crucial for any future advancements in quantum technologies.
“This feels like we have our finger right on the knob of quantum mechanics,” said Dima Feldman, another member of the research team. “It’s an aspect of quantum mechanics that we didn’t know about or, at least, we didn’t appreciate before now.”
This discovery is just the beginning. The team is excited about the potential for future experiments and the possibilities that may emerge from the study of fractional excitons. The quantum world, with all its strange and often counterintuitive properties, continues to surprise and inspire scientists, offering glimpses of a deeper understanding of the universe and the potential to revolutionize technology.
Reference: Naiyuan J. Zhang et al, Excitons in the fractional quantum Hall effect, Nature (2025). DOI: 10.1038/s41586-024-08274-3. www.nature.com/articles/s41586-024-08274-3