  An individual excited two level system decays to its ground state by emitting
a single photon in a process known as spontaneous emission. In accordance with
quantum theory the probability of detecting the emitted photon decreases
exponentially with the time passed since the excitation of the two level
system. 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
microwave cavity with a fast decay rate. 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.
