Observation of Dicke superradiance for two artificial atoms in a cavity with high decay rate.

An individual excited two-level system decays to its ground state in a process known as spontaneous emission. The probability of detecting the emitted photon decreases exponentially with the time passed since its excitation. In 1954, Dicke first considered the more subtle situation in which two emitters decay in close proximity to each other. He argued that the emission dynamics of a single two-level system is altered by the presence of a second one, even if it is in its ground state. Here, we present a close to ideal realization of Dicke's original two-spin Gedankenexperiment, using a system of two individually controllable superconducting qubits weakly coupled to a fast decaying microwave cavity. The two-emitter case of superradiance is explicitly demonstrated both in time-resolved measurements of the emitted power and by fully reconstructing the density matrix of the emitted field in the photon number basis.