DIRECT OBSERVATION OF A FRACTIONAL CHARGE

Since Millikan's famous oil-drop experiments(1), it has been web known that electrical charge is quantized in units of the charge of an elec tron, e. For this reason, the theoretical prediction(2,3) by Laughlin of the existence of fractionally charged 'quasiparticles'-proposed as an explanation for the fractional quantum Hall (FQH) effect-is very co unterintuitive. The FQH effect is a phenomenon observed in the conduct ion properties of a two-dimensional electron gas subjected to a strong perpendicular ; magnetic field. This effect results from the strong i nteraction between electrons, brought about by the magnetic held, givi ng rise to the aforementioned fractionally charged quasiparticles whic h carry the current. Here we report the direct observation of these co unterintuitive entities by using measurements of quantum shot noise. Q uantum shot noise results from the discreteness of the current-carryin g charges and so is proportional to both the charge of the quasipartic les and the average current. Our measurements of quantum shot noise sh ow unambiguously that current in a two-dimensional electron gas in the FQH regime is carried by fractional charges-e/3 in the present case-i n agreement with Laughlin's prediction.