Excited-state quantum phase transition and the quantum-speed-limit time

We investigate the influence of an excited-state quantum phase transition on the quantum-speed-limit time of an open quantum system. The system consists of a central qubit coupled to a spin environment modeled by a Lipkin-Meshkov-Glick Hamiltonian. We show that when the coupling between qubit and environment is such that the latter is located at the critical energy of the excited-state quantum phase transition, the qubit evolution shows a remarkable slowdown, marked by an abrupt peak in the quantum-speed-limit time. Interestingly, this slowdown at the excited-state quantum phase transition critical point is induced by the singular behavior of the qubit decoherence rate, being independent of the Markovian nature of the environment.