When particles with integer spin accumulate at low temperature and high
density, they undergo Bose–Einstein condensation (BEC). Atoms, magnons,
solid-state excitons, surface plasmon polaritons and excitons coupled to
light exhibit BEC, which results in high coherence due to massive occupation
of the respective system’s ground state. Surprisingly, photons were shown
to exhibit BEC recently in organic-dye-flled optical microcavities, which—
owing to the photon’s low mass—occurs at room temperature. Here we
demonstrate that photons within an inorganic semiconductor microcavity
also thermalize and undergo BEC. Although semiconductor lasers are
understood to operate out of thermal equilibrium, we identify a region of
good thermalization in our system where we can clearly distinguish laser
action from BEC. Semiconductor microcavities are a robust system for
exploring the physics and applications of quantum statistical photon
condensates. In practical terms, photon BECs ofer their critical behaviour
at lower thresholds than lasers. Our study shows two further advantages:
the lack of dark electronic states in inorganic semiconductors allows these
BECs to be sustained continuously; and quantum wells ofer stronger
photon–photon scattering. We measure an unoptimized interaction
parameter ( g̃ ≳ 10–3), which is large enough to access the rich physics of
interactions within BECs, such as superfuid light.