Quantum Hall antidot acts as a fractional charge meter

Physicists have demonstrated a new type of extremely sensitive charge detector built on the quantum Hall effect, capable of resolving charge in fractional units of the electron’s charge. The device, described in a study published in Nature Physics, functions as a “fractional coulombmeter”—a meter for electric charge that operates not with whole electrons but with quasiparticles carrying fractions of an electron’s charge. The achievement opens a route to probing some of the most fragile and exotic states of matter known, and could prove essential for future experiments aiming to braid and read out anyons, the quasiparticles that underpin proposals for topological quantum computation.

The quantum Hall effect arises when a two-dimensional electron gas, typically formed at the interface of a semiconductor heterostructure, is cooled to cryogenic temperatures and subjected to a strong perpendicular magnetic field. Under these conditions the electronic spectrum collapses into highly degenerate Landau levels, and the Hall conductance becomes quantized. At fractional filling factors, interactions among electrons dominate and produce the fractional quantum Hall regime, in which the fundamental excitations carry fractions of the elementary charge, such as e/3 or e/5. Detecting these fractional charges directly has long been a challenge, because conventional electrometers and charge sensors are designed around ordinary electrons and often lack the sensitivity, bandwidth, or back-action characteristics needed for quasiparticle experiments.

The new device takes the form of an antidot: a small hole etched into the two-dimensional electron gas that acts as an artificial impurity in the surrounding quantum Hall fluid. Instead of passing straight through the sample, the quantized Hall current must circulate around the antidot along narrow edge channels. Crucially, the antidot region itself can trap a small, countable number of quasiparticles. Each time a quasiparticle tunnels onto or off the antidot island, the electrostatic potential of the island shifts, and this shift modulates the tunneling of quasiparticles around the perimeter. The result is a characteristic periodic oscillation in the measured conductance, with a period set by the ratio of the applied voltage to the quasiparticle charge. By reading out these oscillations, the researchers can convert voltage changes into charge changes—performing the function of a coulombmeter, but one calibrated in fractional units of e.

In their measurements, the team fabricated the antidot in a high-mobility gallium arsenide–based two-dimensional electron gas and tuned the filling factor of the surrounding fluid into a fractional quantum Hall state. By driving a radio-frequency excitation and monitoring the reflected signal, they performed a form of radio-frequency reflectometry, a technique borrowed from quantum-dot charge sensing that allows charge changes to be detected with microsecond-scale temporal resolution and exquisite charge sensitivity. The periodic conductance oscillations they observed directly reflected the accumulation of quasiparticles of definite fractional charge on the antidot, confirming that the device operates as a genuine fractional coulombmeter rather than merely as a sensitive conventional electrometer.

A key advantage of the antidot architecture is its versatility. The device can be operated in several distinct regimes simply by adjusting gate voltages and the magnetic field. In one regime it behaves as a precise charge meter, resolving individual tunneling events of fractionally charged quasiparticles. In another, it can act as a tunable source and detector of quasiparticles, injecting them into edge channels at controllable rates. This dual functionality is significant for the growing experimental program aimed at anyon interferometry, in which quasiparticles are made to travel around closed loops and acquire statistical phases that reveal their exotic quantum statistics. A device that both generates and senses single fractional quasiparticles greatly simplifies such experiments, which traditionally require multiple separately calibrated components.

The researchers also characterized the device’s sensitivity and back-action in detail. Charge sensitivity reached levels comparable to the best radio-frequency single-electron transistors and quantum-point-contact charge sensors, but with the crucial difference that the detected object carries a fraction of the electron charge. This means the effective resolving power with respect to quasiparticles is even more impressive, since the signal per tunneling event is proportionally smaller. Moreover, the coupling between the antidot and the surrounding edge channels can be tuned, allowing the experimenters to balance measurement strength against the disturbance introduced into the quantum Hall fluid—a critical consideration when the goal is to observe delicate interference phenomena or to preserve fragile quasiparticle states over extended periods.

Beyond its immediate utility for fundamental physics, the fractional coulombmeter addresses a pressing need in the emerging field of topological quantum computation. Certain fractional quantum Hall states, most famously the so-called 5/2 state, are predicted to host non-Abelian anyons—quasiparticles whose braiding operations act on a degenerate quantum state space and could therefore encode quantum information in a form intrinsically protected from local noise. Reading out the outcome of a braid operation typically amounts to detecting a change in quasiparticle number or charge on a localized island. An antidot-based fractional coulombmeter provides exactly this capability, offering a path toward the single-shot, high-fidelity readout that any practical topological qubit architecture will demand.

The work also refines our understanding of antidot physics itself. Decades of study have revealed that antidots host a rich variety of phenomena, including Coulomb-blockade-like charge quantization, resonant tunneling through localized states, and complex dynamics of quasiparticle exchange with the edge. By operating the antidot explicitly as a metrological device, the team has turned what was previously a source of experimental complications into a resource. The periodic charge oscillations serve as an in situ calibration of the quasiparticle charge, and the device could even be used to compare effective charges in different fractional states, testing theoretical predictions about the internal structure of the quantum Hall fluid and the nature of its quasiparticle excitations.

The demonstration is likely to stimulate a wave of follow-up experiments across several laboratories worldwide. Natural next steps include integrating the fractional coulombmeter with interferometric structures to perform single-quasiparticle statistics measurements, extending the technique to fractional states with even smaller quasiparticle charges, and translating the platform into materials such as graphene, where exceptionally clean fractional quantum Hall states—including even-denominator states—are now routinely observed. There are also longer-term ambitions: coupling the antidot detector to microwave resonators to reach quantum-limited sensing, and using arrays of antidots to build quasiparticle-based circuits that manipulate fractional charges with the same control that conventional electronics exercises over electrons.

What makes the result especially compelling is its conceptual simplicity. The coulombmeter, one of the oldest instruments in physics, has been reborn in a regime its inventors could scarcely have imagined: a device that measures charge in thirds and fifths of an electron, etched into a frozen quantum fluid and read out through the quantum interference of quasiparticles. As experiments on anyons and topological matter move from proof-of-principle demonstrations toward genuine quantum technologies, tools of this kind—sensitive, tunable, and natively fluent in the language of fractional charge—are likely to become as fundamental to quasiparticle physics as the electrometer once was to the study of the electron itself.