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An exciton is an electron-hole bound state and behaves approximately as a boson when the gas is dilute compared with the inverse exciton volume. A condensation experiment needs a long enough lifetime for cooling, high phase-space density and evidence of spontaneous coherence; excessive density causes ionization or a Mott transition instead.

Bulk Cu₂O offers long-lived excitons, but heating, losses and Auger recombination complicate interpretation. Spectral narrowing or anomalous transport alone is insufficient: early claims required reinterpretation. Coupled quantum wells separate electron and hole layers, increasing lifetime and enabling cold indirect-exciton gases, though disorder, trapping and nonequilibrium flow must be distinguished from condensation.^[ref]

In a semiconductor microcavity, coherent strong coupling mixes an exciton with a light cavity photon to form polaritons with very small effective mass. Nonresonant pumping supplies particles without imposing the condensate's phase. The 2006 CdTe experiment reported a threshold, strong low-momentum occupation, increased temporal coherence and long-range spatial coherence. These combined signatures support an exciton-polariton condensate; a threshold or bright emission alone does not distinguish it from photon lasing. Check that strong coupling persists above threshold. Finite lifetime and pumping make this an open, driven system, so thermalization must be demonstrated rather than inferred merely from the word condensate.^[ref]

For positive gain and nonlinear-loss coefficients, a constant-density exciton-polariton condensate with physical velocity $v\hat{x}$ has $\psi =\sqrt{\gamma /\Gamma }\exp [i(mvx-\mu t)/\hslash ]$. Its chemical potential obeys the displayed relation. For prescribed real $\mu$, a real velocity requires $\mu \ge g\gamma /\Gamma$. If chemical potential is determined by the solution, any real velocity is allowed. Linear density-phase perturbations in the comoving frame satisfy $\lambda (\lambda +2\gamma /\hslash )+{ϵ}_k({ϵ}_k+2g{n}_{\ast })/{\hslash }^2=0$, where ${ϵ}_k={\hslash }^2{k}^2/(2m)$. Repulsive coupling has no growing modes in this simple model; attractive coupling has a long-wavelength instability even though the plane wave exists.