Researchers claim to find particles with quarter-electron charge

NETANYA, Israel — Physicists at the Weizmann Institute (Rehovot, Israel) claim they have demonstrated, for the first time, the existence of 'quasiparticles' with one quarter the charge of an electron. The key thing is the quarter charge as particles with one-third and one-fifth charge have been detected before, the researchers said.

As well as being a negation of one of the major principles of atomic physics such quarter-charge particles could be a step towards creating exotic quantum computers that might be powerful, yet highly stable, the scientists said.

Particles with fractional electron charge were first predicted over 20 years ago under conditions existing alongside the so-called quantum Hall effect. Such quasiparticles were found by the Weizmann group some ten years ago. Although electrons are supposedly indivisible, if they are confined to a two-dimensional layer inside a semiconductor, chilled down to a fraction of a degree above absolute zero and exposed to a strong magnetic field that is perpendicular to the layer, they effectively behave as independent particles, called quasiparticles, with charges smaller than that of an electron. Until now, these charges had always been fractions with odd denominators: one third of an electron, one fifth, and so on.

The experiment done by research student Merav Dolev in the research group of Professor Moty Heiblum in collaboration with Vladimir Umansky and Diana Mahalu, and Professor Ady Stern, all of the Condensed Matter Physics Department, found the elusive quarter-charge quasiparticle.

The gallium arsenide material they produced for the semiconductor is claimed to be one of the purest in the world. The scientists tuned the electron density in the two-dimensional layer – in which about three billion electrons were confined in the space of a square millimeter. The device they created is shaped like a flattened hourglass, with a narrow 'waist' in the middle that allows only a small number of charge-carrying particles to pass through at a time. The 'Shot noise' produced when some quasiparticles passing through and others bounced back caused fluctuations in the current that are proportional to the passing charges, thus allowing the scientists to accurately measure the quasiparticles' charge.

Quarter-charge quasiparticles should act quite differently from odd fractionally charged particles, and this is why they have been sought as the basis of the theoretical 'topographical quantum computer' the researchers said. When particles such as electrons, photons, or even those with odd fractional charges change places with one another, there is little overall effect. In contrast, quarter-charge particle exchanges might preserve information on the particles' history, according to quantum theory.

To be useful for topologically-based quantum computers, the quarter-charge particles must be shown to have 'non-Abelian' properties. These subtle properties are extremely difficult to observe. Heiblum and his team are now working on devising experimental setups to test for these properties.