If electron pairs can condense into a macroscopic superconducting state, can’t electron-hole pairs—excitons—similarly condense to form a neutral superfluid? Such was the speculation of several theorists in the 1960s. Experimenters have been searching ever since for excitonic condensates. They have focused the search on the electron-hole pairs created by shining light onto a semiconductor. Such studies have turned up some intriguing behavior, but still no conclusive proof of a condensate.
The long-sought evidence for an excitonic condensate has now surfaced in a different and unexpected quarter: a quantum Hall bilayer. The bilayer is composed of two slabs of doped semiconductors separated by a thin insulating region. Each slab functions as a two-dimensional electron (or hole) gas. A strong magnetic field is applied perpendicular to the layers, and the charges move in quantized orbits about the field lines.
Because its two layers are doped with the same charge, a quantum Hall...
