The quantum antidot electrometer: Direct observation of fractional charge

A Quantum Antidot electrometer has been used in the first direct observation of the fractionally quantized electric charge. In this paper, after introductory remarks, we review recent experiments performed on the integer i = 1, 2 and fractional f = 1/3 quantum Hall plateaus extending over a filling factor range of at least 27 %. On the integer plateaus we directly measure electron's charge, in Coulombs, with standard deviation of 0.7 % and absolute accuracy of 0.035e. We further find the charge of the f = 1/3 Laughlin quasiparticles to be invariantly e/3, with standard deviation of 1.2 %, independent of filling factor v, tunneling current, and temperature.