Measuring the Minuscule

About a year ago, the same Weizmann team, working in the Joseph H. and Belle R. Braun Center for Submicron Research, measured a charge one-third the charge of an electron. Yet this finding was so revolutionary that another experiment was conducted, using a different electronic system. Prof. Mordehai Heiblum of the Condensed Matter Physics Department headed the research team, whose other members were Drs. Michael Reznikov, Rafi de Picciotto, Tim Griffith, Vladimir Umansky, Diana Mahalu, and engineer Gregory Bunin.

Ever since the American physicist Robert Millikan first measured the charge of an electron 80 years ago, this value has been widely regarded as the smallest basic unit of electric charge. However, in 1982 the American physicist Robert Laughlin proposed a very simple way to describe highly complex interactions between electrons: His theory made the bizarre assumption that the highly interacting electrons behave as if they were noninteracting imaginary particles with odd-denominator fractions of electronic charges (i.e., one-third, one-fifth, one-seventh, etc., of an electron).

The first piece of evidence affirming Laughlin's theory came when the Weizmann Institute team measured an electronic charge one-third the charge of an electron. This proof played an important part in the decision to award Robert Laughlin, Horst L. Stormer, and Daniel C. Tsui the 1998 Nobel Prize in Physics.

However, proof had been obtained using an electron system whose conductance was one-third of quantum conductance, a fact that raised some doubts (owing to the possibility that the measurement somehow reflected the conductance and not the electronic charge).

 

To remove these doubts, the Weizmann team carried out a further experiment in which they succeeded in measuring an even smaller electronic charge, equal to one-fifth of the charge of a single electron.

 

This measurement was obtained with an electron system whose conductance was equal to two-fifths of quantum conductance. The dissimilarity between the electronic charge and quantum conductance proved that the measurement refers to the electronic charge itself, regardless of the conductance.