The electric field in the z direction is controlled by external gate voltage V_g. At low V_g (direct regime), the spatially direct exciton is the lowest energy state, while at high V_g (indirect regime) the indirect exciton composed of electron and hole in different layers is the lowest energy state. The transition from the direct to the indirect regimes is determined by the ratio between the one-particle symmetric-antisymmetric splittings and the exciton binding energies. For a given CQW sample, this ratio and the direct-to-indirect crossover can be controlled by magnetic fields.

The existence of both direct and indirect regimes in one and the same sample allows distinguishing phenomena in cold exciton gases. Phenomena in cold exciton gases are observed only for long life indirect excitons, which cool down essentially to the lattice temperature, and are not observed for direct excitons, which cannot thermalize down to low temperatures within their short lifetime.