Rare quantum state reveals particles with quarter-electron charge

An electron's charge is normally fixed, like a coin you can't break into pieces. But if they are cooled close to absolute zero and trapped in a two-dimensional layer under a powerful magnetic field, electrons organize into a collective state of “quasiparticles” that seem to hold only a fraction of an electron's charge.

This state is known as the “fractional quantum Hall effect”. A small number of these, the “even-denominator states”, have drawn attention because some theories predict that they could contain unusual quasiparticles called “non-Abelian anyons”.

Why are these interesting? Because their quantum properties make them great candidates for storing and processing information in fault-tolerant (error-resistant) topological quantum computers.

Now, scientists from the groups of Mitali Banerjee at EPFL, Moty Heiblum at the Weizmann Institute of Science, and Mansour Shayegan at Princeton University have studied one such state, known as the ν = 1/2 fractional quantum Hall state, which forms in a wide layer of gallium arsenide, which is used in optoelectronics, wireless communication, and even solar panels. Their work is published in Physical Review Letters.

“For the first time in the history of this field, two different groups (Weizmann and EPFL) have measured same values of fractional charge,” says Banerjee.

The researchers set out to answer a basic question about the ν = 1/2 state: what charge do its quasiparticles carry? For this, they studied electrons confined within a 70-nanometre-wide layer of gallium arsenide, each device containing a narrow constriction called a “quantum point contact”.

The constriction allowed the researchers to partially scatter quasiparticles moving through the device and measure the resulting “shot noise”: When current flows through the quantum point contact, individual charge carriers pass through in a random, stop-and-start way, like rain hitting a roof. The size of the resulting electrical "noise" reveals how much charge each carrier is moving.

The researchers built the point contact using a distinctive etching method, and then deposited metal gates around it to fine-tune how much current could pass through. They tested two nearly identical devices in two different labs, (one at EPFL and one at the Weizmann), and first confirmed their method worked by measuring known states with charges of a full electron and two-thirds of an electron.

When they applied the same technique to the ν = 1/2 state, both devices gave the same answer: the particles carried very close to a quarter of an electron's charge (0.250 ± 0.013 in one device and 0.249 ± 0.018) in the other. Both match one quarter of an electron’s charge, or e/4.

“This is important because the quantum Hall state that was studied here is special as it survives till a few degrees Kelvin, and is supposed to be only a second such known state in gallium arsenide to possess special non-Abelian properties that can eventually enable a topological quantum computer,” says Banerjee.

“Non-Abelian states act like a structural blueprint, where information is stored into the global geometry rather than a single point, ensuring immunity from local structural defects or noise. By simply moving such particles around each other we can create error-free quantum computers.”

Other contributors

EPFL Center for Quantum Science and Engineering