How do you measure one third of an electron?

Electrons are indivisible, yet in a strong magnetic field they can move as if they carried only a fraction of their charge. Mitali Banerjee’s group at EPFL has built a bilayer-graphene antidot—a small electrically defined energy hill—that measures those fractions directly. Quasiparticles circle the hill and tunnel at regular intervals as the magnetic field or gate voltage is swept. The spacing of the resulting conductance oscillations gives the charge. The team recorded e/3 at filling factors...