  Bose-Einstein condensation has in the last two decades been observed in cold
atomic gases and in solid-state physics quasiparticles, exciton-polaritons and
magnons, respectively. The perhaps most widely known example of a bosonic gas,
photons in blackbody radiation, however exhibits no Bose-Einstein condensation,
because the particle number is not conserved and at low temperatures the
photons disappear in the system's walls instead of massively occupying the
cavity ground mode. This is not the case in a small optical cavity, with a
low-frequency cutoff imprinting a spectrum of photon energies restricted to
values well above the thermal energy. The here reported experiments are based
on a microscopic optical cavity filled with dye solution at room temperature.
Recent experiments of our group observing Bose-Einstein condensation of photons
in such a setup are described. Moreover, we discuss some possible applications
of photon condensates to realize quantum manybody states in periodic photonic
lattices and photonic Josephson devices.

Quantum Gases