  Photon blockade is the result of the interplay between the quantized nature
of light and strong optical nonlinearities, whereby strong photon-photon
repulsion prevents a quantum optical system from absorbing multiple photons. We
theoretically study a single atom coupled to the light field, described by the
resonantly driven Jaynes--Cummings model, in which case the photon blockade
breaks down in a second order phase transition at a critical drive strength. We
show that this transition is associated to the spontaneous breaking of an
anti-unitary PT-symmetry. Within a semiclassical approximation we calculate the
expectation values of observables in the steady state. We then move beyond the
semiclassical approximation and approach the critical point from the disordered
(blockaded) phase by reducing the Lindblad quantum master equation to a
classical rate equation that we solve. The width of the steady-state
distribution in Fock space is found to diverge as we approach the critical
point with a simple power-law, allowing us to calculate the critical scaling of
steady state observables without invoking mean-field theory. We propose a
simple physical toy model for biased diffusion in the space of occupation
numbers, which captures the universal properties of the steady state. We list
several experimental platforms where this phenomenon may be observed.
