The fractional quantum Hall effect, where plateaus in the Hall resistance at values of h/ν e2 coexist with zeros in the longitudinal resistance, results from electron correlations in two dimensions under a strong magnetic field. (Here h is Planck's constant, ν the filling factor and e the electron charge.) Current flows along the sample edges and is carried by charged excitations (quasiparticles) whose charge is a fraction of the electron charge. Although earlier research concentrated on odd denominator fractional values of ν, the observation of the even denominator ν = 5/2 state sparked much interest. This state is conjectured to be characterized by quasiparticles of charge e/4, whose statistics are 'non-abelian'—in other words, interchanging two quasiparticles may modify the state of the system into a different one, rather than just adding a phase as is the case for fermions or bosons. As such, these quasiparticles may be useful for the construction of a topological quantum computer. Here we report data on shot noise generated by partitioning edge currents in the ν = 5/2 state, consistent with the charge of the quasiparticle being e/4, and inconsistent with other possible values, such as e/2 and e. Although this finding does not prove the non-abelian nature of the ν = 5/2 state, it is the first step towards a full understanding of these new fractional charges. A quantum computer relies on 'qubits' (or quantum bits) which can be in a quantum state that implies the qubit is both '0' and '1' at the same time. But for conventional qubit designs, typically based on a single particle such as an electron or photon, these quantum states are vulnerable to disturbances and can be easily lost. There is a hypothetical alternative: the topological qubit, which would hold information as a series of 'braids' formed of two-dimensional quasiparticles. This new form of computing comes a step closer with the publication of compelling evidence for the existence of 'e/4' quasiparticles of the type that could transform topological quantum computers from the hypothetical to the proof-of-principle stage. In a News Feature, Liesbeth Venema explains some of the background to this exciting field. This paper reports data of shot noise generated by the 5/2 fractional state in an ultraclean two-dimensional electron gas that compellingly points in the direction of the e/4 quasiparticles. It is believed that this observation is a first step towards understanding new fractional charges.

Observation of a quarter of an electron charge at the ν = 5/2 quantum Hall state