As the cavity supports multiple photon spatial modes and because the light–matter coupling can be comparable to the energy splitting of these modes, the composition of the supermode polariton is changed by the light–matter coupling on condensation.
These polaritons are formed from a superposition of cavity photon eigenmodes (a supermode), coupled to atomic density waves of a quantum gas. Here we observe and study a non-equilibrium phase transition, the condensation of supermode-density-wave polaritons. By placing cold atoms in optical cavities and inducing strong coupling between light and excitations of the atoms, one can experimentally study phase transitions of open quantum systems. Phase transitions, where observable properties of a many-body system change discontinuously, can occur in both open and closed systems.
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