  We report here the experimental observation of a dynamical quantum phase
transition in a strongly interacting open photonic system. The system studied,
comprising a Jaynes-Cummings dimer realized on a superconducting circuit
platform, exhibits a dissipation driven localization transition. Signatures of
the transition in the homodyne signal and photon number reveal this transition
to be from a regime of classical oscillations into a macroscopically
self-trapped state manifesting revivals, a fundamentally quantum phenomenon.
This experiment also demonstrates a small-scale realization of a new class of
quantum simulator, whose well controlled coherent and dissipative dynamics is
suited to the study of quantum many-body phenomena out of equilibrium.
