Bounds on the validity of the Lindblad equation for open systems

While it is well established that the dynamics of an open system is described well by the Lindblad master equation if the coupling to the bath is either in the weak or in the singular limit, it is not known whether this description can be extended to other coupling strengths. Here we use the exact solution of a microscopic system coupled to baths, to prove that Lindbladian dynamics only holds in these two limits for a pure decay system. This rules out the possibility of a non-Hermitian dynamics for any other couplings, for such open systems. Furthermore, we argue that exceptional points can only occur in the singular coupling limit, never in the weak coupling limit.