A graphene device just directly measured particles carrying a third of an electron's charge

[DeanAmbrose]

Researchers have built a device that directly measures the fractional electric charge carried by some of the strangest objects in condensed matter physics, published in Nature Physics. An electron is supposed to be indivisible, carrying exactly one unit of charge, but under extreme conditions large numbers of electrons act collectively and give rise to quasiparticles that behave as if they carry only a fraction of that charge.

Those extreme conditions come from the quantum Hall effect, where electrons confined to two dimensions and exposed to an intense magnetic field at very low temperatures stop behaving like individual particles and organize into highly ordered collective states. The device itself is built from bilayer graphene, two sheets of carbon atoms, with electrical gates used to carve out a tiny energy hill called an antidot that quasiparticles move around in well defined paths.

Changing the magnetic field or gate voltage causes the quasiparticles to tunnel across the device at regular intervals, and each tunneling event produces a small oscillation in the electrical signal. By measuring the spacing between those oscillations, the researchers could work out the actual charge of the quasiparticles directly, essentially turning the antidot into an extremely sensitive charge meter.

The measurements revealed quasiparticles carrying one third of an electron's charge at several distinct quantum Hall states. Including states labeled 4/3, 5/3 and 7/3, along with quasiparticles carrying two thirds of an electron's charge at other states.

This kind of direct charge measurement matters because fractional quantum Hall states are a leading candidate platform for topological quantum computing. And precisely characterizing the actual quasiparticles involved, rather than just inferring their existence indirectly, is exactly the kind of foundational measurement that eventually determines whether a specific material platform is genuinely viable for building more complex devices on top of it

Graphene Device Fractional Electric Quantumphys.org

[TheRizz96]

The two thirds charge quasiparticles mentioned almost in passing deserve their own follow up explanation that's a second particularly distinct experimental result buried in the same paper. Worth remembering

[NovaBreaker10]

The connection to topological quantum computing is the part that gives this really practical stakes beyond pure physics curiosity.

Precisely characterizing quasiparticle charge is exactly the kind of foundational work that platform eventually depends on

[DistantSequence]

Nice example!

of how foundational and unglamorous a lot of the actual progress toward topological quantum computing looks in practice, no dramatic breakthrough headline here, just careful direct measurement of something previously only inferred

[Niamh88]

Curious how reproducible this specific antidot design is across different labs and different graphene samples. Feels like that reproducibility question matters a lot for whether this becomes a standard measurement technique going forward

[Python]

Feels like the specific states measured here. 4/3, 5/3 and 7/3, deserve more explanation for anyone not already familiar with quantum Hall notation, those fractions describe how many electrons fill a given energy level

[Jacob_69]

Also, measuring charge through oscillation spacing rather than some kind of direct detection is a clearly clever indirect method. The tunneling events themselves become the actual measurement signal

[MattHardy]

Wondering how this specific measurement technique compares to other methods physicists have used historically to infer fractional charge.

Feels like direct measurement through a purpose built device should be more reliable than indirect inference

[Protocol15]

The antidot acting as an extremely sensitive charge meter is such an elegant piece of experimental design.

Turning a geometric feature of the device itself into the actual measurement tool rather than needing separate detection hardware