PT -symmetric photonic quantum systems with gain and loss do not exist

We discuss the impact of gain and loss on the evolution of photonic quantum states and find that PT -symmetric quantum optics in gain/loss systems is not possible. Within the framework of macroscopic quantum electrodynamics we show that gain and loss are associated with non-compact and compact operator transformations, respectively. This implies a fundamentally different way in which quantum correlations between a quantum system and a reservoir are built up and destroyed.