Electric charge normally comes in an indivisible unit: the charge of an electron. Indeed, quarks were thought to be the only particles with fractional charge - and today they only exist in particles that have a integer charge. But last month, two groups of physicists revealed the first direct evidence that an electric current can be carried by quasiparticles with fractional charge."All the interacting electrons are there but they behave as if they are non-interacting quasiparticles with charges of one-third, " says Moty Heiblum of the Weizmann Institute of Science in Rehovot, Israel, who heads one of the groups.

The Israeli group, published its results in Nature, while a French group based at the CEA laboratory near Paris, published its results in Physical Review Letters.

Both groups measured a small electrical current in a two-dimensional electron gas sandwiched between two semiconductor layers. Fluctuations in the current - shot noise - were used to measure the electrical charge of the carrier particles. The sample was chilled to less than 1 K and a strong magnetic field applied at right angles to the layers. By analysing the shot noise in this regime, both groups reported evidence that the electric current is carried by quanta with charge one-third that of the electron. "Up until now, there was no evidence that current could be carried by a fractionally charged quasiparticle, " says Christian Glattli, who heads the French group.

The results agree with a theory which was formulated by Robert Laughlin in 1982 to explain the fractional quantum Hall effect. According to Laughlin, electrons in strong magnetic fields form an exotic new collective state, similar to the way in which collective states form in superfluid helium. A quantum of magnetic flux and an electron exist as a quasiparticle that carries the electric current.

So why did the researchers observe quasiparticles with charges of a third, rather than any other fraction? In Laughlin's theory, the denominator is always odd, so quasiparticles can carry one-third, one-fifth, one-seventh - or indeed, two-thirds, two-fifths or three-fifths - of the charge on an electron. "It is very difficult to explain intuitively - it is just how nature works, " says Heiblum.

"It is a beautiful result, " says Mark Fromhold of Nottingham University. "It is remarkable that electrical signals from individual quasi-particles can be detected and used directly to measure their fractional charge."