  We review the path integral method wherein quantum systems are mapped with
Feynman's path integrals onto a classical system of "ring-polymers" and then
simulated with the Monte Carlo technique. Bose or Fermi statistics correspond
to possible "cross-linking" of polymers. As proposed by Feynman, superfluidity
and Bose condensation result from macroscopic exchange of bosons. To map
fermions onto a positive probability distribution, one must restrict the paths
to lie in regions where the fermion density matrix is positive. We discuss a
recent application to the two-component electron-hole plasma. At low
temperature excitons and bi-excitons form. We have used nodal surfaces
incorporating paired fermions and see evidence of a Bose condensation in the
energy, specific heat and superfluid density. In the restricted path integral
picture, pairing appears as intertwined electron-hole paths. Bose condensation
occurs when these intertwined paths wind around the periodic boundaries.
