In a landmark experiment, physicists have captured the first direct evidence of a long-theorized quantum phenomenon known as anyon tunneling. The research, conducted by a team at Purdue University, confirms decades-old predictions about the behavior of exotic quasiparticles in two-dimensional systems, resolving previous ambiguities between theory and experimental results and solidifying a key model of quantum physics.
The experiment precisely measured the movement of anyons—collective excitations of electrons that act like particles with fractional charge—between the edges of a specially designed semiconductor device. By observing a universal characteristic in how these anyons tunnel, the researchers validated the chiral Luttinger liquid theory, a foundational framework developed in the 1990s to describe the one-dimensional motion of these strange particles. The findings provide a more complete picture of the fractional quantum Hall effect and demonstrate a powerful new platform for probing the fundamental properties of topological states of matter.
A Decades-Old Quantum Prediction
The theoretical basis for the experiment dates back to the discovery of the fractional quantum Hall effect, an exotic state of matter that emerges when electrons are confined to a two-dimensional plane and subjected to a powerful magnetic field and extremely low temperatures. In this state, electrons cease to act individually and instead behave collectively, giving rise to quasiparticles called anyons. These anyons are remarkable because they possess only a fraction of an electron’s charge and exhibit unique quantum statistics, unlike the familiar fermions and bosons that govern the three-dimensional world.
In the early 1990s, physicist X.-G. Wen and his collaborators developed the chiral Luttinger liquid theory to describe the behavior of these quasiparticles. The theory predicted that anyons would move along the one-dimensional edges of the 2D material in so-called “edge modes.” It further predicted that if two of these edge modes were brought close together, anyons could “tunnel” from one to the other, and that the rate of this tunneling would follow a universal, predictable scaling law. For years, however, experimental evidence to conclusively support this specific prediction remained elusive.
Designing the Anyon Experiment
To test the theory, the Purdue University team, led by senior author Michael Manfra, engineered a sophisticated device capable of measuring the incredibly faint signals produced by tunneling anyons. The core of the setup was an electronic Fabry-Pérot interferometer, a device that allowed the researchers to precisely control and measure the behavior of quantum particles. The experiment was conducted under extreme conditions, with the device cooled to milliKelvin temperatures—just a fraction of a degree above absolute zero—and placed in a powerful magnetic field of approximately 10 Tesla.
The Interferometer Setup
Within the interferometer, quantum point contacts acted as “beam splitters” for the edge modes, analogous to how mirrors split beams of light in an optical interferometer. This structure gave the researchers fine control over the system, allowing them to bring two edge modes, moving in opposite directions, into very close proximity. When the edges are close enough, anyons can quantum-mechanically tunnel across the gap, creating a tiny electrical current. Measuring this current was the primary goal of the experiment.
A Novel Material Structure
A key innovation in the experiment was the use of a newly developed semiconductor heterostructure featuring a “screening well” design. This design was crucial for creating a very sharp and clean confinement for the electrons at the edges of the material. According to the researchers, this sharp confinement was essential for making the properties predicted by the chiral Luttinger liquid theory experimentally observable. The device itself was fabricated on a gallium arsenide (GaAs) chip mounted on a printed circuit board.
Observing the Tunneling Particles
With the two counterpropagating edge modes of the n=1/3 fractional quantum Hall state positioned next to each other, the team focused on detecting the tunneling events. The challenge was immense, as the expected tunneling current was on the order of picoamps—a trillionth of an ampere. Using highly sensitive amplifiers, the researchers were able to successfully measure these minuscule currents.
The team then systematically studied how the tunneling conductance—a measure of how easily the anyons could tunnel—changed in response to variations in voltage and magnetic field. This analysis was critical because the chiral Luttinger liquid theory predicts a very specific relationship, or scaling law, between the tunneling conductance and these parameters. This relationship is defined by a universal number, a scaling exponent, which acts as a unique signature of the underlying physics.
Results Validate Foundational Theory
The experimental results provided a stunning confirmation of the decades-old theory. The researchers determined that the scaling exponent for anyon tunneling was g=1/3. This value was precisely what Wen’s chiral Luttinger liquid theory had predicted for this specific fractional quantum Hall state. This direct measurement of a universal constant offered the first unambiguous experimental evidence of universal anyon tunneling, a cornerstone of the theory.
Manfra noted that this result settles previous questions and demonstrates that the complex interactions in this quantum system are indeed governed by this elegant theoretical framework. The experiment effectively showed that the collective behavior of thousands of electrons in the fractional quantum Hall liquid could be distilled down to this single, universal number describing how its edge-mode quasiparticles behave.
Implications for Quantum Physics
The validation of this key prediction has significant implications for the study of topological states of matter. These are phases of matter where the bulk material is an insulator, but the edges or surfaces are conductive, with properties that are robust against local disturbances. The connection between the physics of the bulk material and the behavior of its edges is known as the bulk-boundary correspondence, a central concept in the field.
The success of the Purdue experiment demonstrates that the complete topological order of a fractional quantum Hall state can be characterized using a single, versatile device. Manfra stated that his team has now used its Fabry-Pérot interferometer platform to measure the three essential properties that define the topological order at the n=1/3 state: the anyon’s fractional charge, its unique braiding statistics, and now, the universal scaling exponent of its tunneling behavior. This comprehensive measurement capability marks a significant advance, providing a powerful tool to explore and understand even more complex and potentially non-Abelian anyonic systems, which are of great interest for developing fault-tolerant quantum computers.
