  The condensation of excitons, bound electron-hole pairs in a solid, into a
coherent collective electronic state was predicted over 50 years ago. Perhaps
surprisingly, the phenomenon was first observed in a system consisting of two
closely-spaced parallel two-dimensional electron gases in a semiconductor
double quantum well. At an appropriate high magnetic field and low temperature,
the bilayer electron system condenses into a state resembling a superconductor,
only with the Cooper pairs replaced by excitons comprised of electrons in one
layer bound to holes in the other. In spite of being charge neutral, the
transport of excitons within the condensate gives rise to several spectacular
electrical effects. This article describes these phenomena and examines how
they inform our understanding of this unique phase of quantum electronic
matter.
