Physicists Just Measured a Quasiparticle With One-Quarter of an Electron’s Charge

Don't you just love quantum physics?

An electron carries a fixed amount of electric charge. You can’t go around slicing it half and get half its electric charge. But if you put many electrons into an extreme quantum state, something much stranger can emerge: collective excitations that move through the material as if they carry only a fraction of an electron’s charge.

Physicists have now measured such quasiparticles carrying almost exactly one-quarter of an electron’s charge in an unusual state of matter known as the “ν = 1/2 fractional quantum Hall state”.

Two devices, tested independently at the Weizmann Institute of Science and EPFL, confirmed the result, basically showing that these strange ripples in the electron system carry one-quarter of an electron’s charge.

And this isn’t just a random search for bizarre quantum behavior. This type of phenomenon could pave the way for more fault-tolerant quantum computers.

How an electron’s charge can appear to split

The experiment starts with the fractional quantum Hall effect, a phenomenon that appears when electrons are confined to an extremely thin, effectively two-dimensional layer, cooled to very low temperatures and exposed to a powerful magnetic field.

Under those conditions, the electrons don’t behave as independent particles anymore. Their interactions produce collective quantum states with new properties.

One consequence is the appearance of quasiparticles. These aren’t fundamental particles like electrons; in fact, they’re not particles at all (hence the ‘quasi’). Technically, quasiparticles are disturbances involving many interacting electrons that move through the material. However, they behave like real particles in physical experiments and mathematical equations.

And they can carry fractional electric charge.

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The researchers studied electrons trapped in a 70-nanometer-wide layer of gallium arsenide, or GaAs, a semiconductor widely used in electronics. They applied a magnetic field of about 12 tesla — roughly four to eight times stronger than the field inside a typical hospital MRI scanner. They then cooled the system to temperatures extremely close to absolute zero (−273.15 °C or −459.67 °F).

Their target was a state called ν = 1/2. The Greek letter ν, pronounced “nu,” describes how electrons fill the quantum energy levels created by the magnetic field.

The odd non-odd state

Most familiar fractional quantum Hall states have odd numbers in the denominator, such as 1/3. States such as 1/2 and 5/2 belong to a rarer class called even-denominator states.

Physicists care about them because some theories predict that they can host non-Abelian anyons — another quasiparticle with unusually complex quantum behavior.

Anyons themselves are already unusual. Unlike ordinary particles such as electrons and photons, exchanging two anyons can change the quantum state of the system. For the non-Abelian variety, the order in which particles move around one another matters.

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That property has made them candidates for topological quantum computing, an approach that would encode information in the collective state of several particles rather than in one fragile local quantum state. In principle, that could make quantum information less vulnerable to some kinds of errors.

But before researchers can ask how these quasiparticles behave when exchanged, they need to establish something more basic: exactly what electrical charge do they carry?

The answer’s in the noise

The thing is, you can’t just place these quasiparticles on a scale. Instead, the team measured their charge through tiny fluctuations in an electrical current.

They used a narrow bottleneck known as a quantum point contact. Quasiparticles moving along the edge of the quantum Hall system reached this constriction, where most continued through while a small number scattered backward.

Because those scattering events happen individually and randomly, the electrical current fluctuates slightly. Physicists call this shot noise. The effect is similar, in principle, to hearing individual raindrops strike a roof instead of listening to a perfectly continuous stream of water. The statistical pattern of the noise contains information about the size of the individual packets passing through.

Here, those packets were quasiparticles.

The first experiment measured an effective charge of 25% ± 0.013 times the electron charge. The second, carried out independently, produced 0.249 ± 0.018.

Basically, both show the charge is a quarter of that of an electron.

Quarter charge is an important clue

The good news is that 25% of an electron charge, validated through two different experiments, is promising. But finding this charge doesn’t prove that the ν = 1/2 state contains non-Abelian anyons.

Thing is, physicists still don’t know exactly how this quantum state is organized. One possibility, known as the Halperin 331 state, would produce relatively simple anyons: moving them around one another changes the quantum state in a predictable way, but the order of those exchanges does not fundamentally matter. Another possibility, a Pfaffian-like state, could produce non-Abelian anyons, where the order in which particles are moved around one another changes the final quantum state. That extra layer of “memory” is what makes non-Abelian anyons especially interesting for topological quantum computing.

The trouble is that both possibilities predict particles carrying one-quarter of an electron’s charge. So, the new measurement can’t yet tell physicists which of the two versions of the quantum state they have created (or whether it’s something completely new).

But it does remove one major uncertainty. The researchers have now shown that the charge behaves exactly as the leading theories predict. The next experiments will have to probe something subtler: how these particles interact when they move around one another.

For now, physicists have confirmed the charge. The next challenge is to find out exactly what kind of quantum particle is carrying it.

Journal Reference: Tomer Alkalay et al, Observation of e/4 Charge at ν = 1/2 in a Wide GaAs Quantum Well, Physical Review Letters (2026). DOI: 10.1103/c73x-q4z7. You can also read the pre-print for free on arXiv: arxiv.org/abs/2602.08468