Direct Observation of Fractional Charge

(a) Quantum Antidot of area S can be populated either by electrons or by Laughlin quasiparticles. The particles are attracted by the electric field proportional to the voltage applied to the back gate, which forms a parallel plate capacitor with the 2D electrons. The change in the number of particles is monitored by tunneling between the two front gates, via the antidot:
change by one particle is seen as a peak in tunneling conductance.

(b) It takes the same electric field to attract one electron as three Laughlin quasiparticles; thus the charge of each quasiparticle is e* = e/3.

The experiment is conducted in very strong magnetic fields and at a very low temperature, so that two-dimensional electrons condense into new states of matter: integer (i =1) or fractional (f = 1/3) quantum Hall liquids.  The quantum numbers i  or f  label the quantum Hall states.