Multi-block exceptional points in open quantum systems

Abstract: Open quantum systems can be approximately described by non-Hermitian Hamiltonians (NHHs) and Lindbladians. The two approaches differ by quantum jump terms corresponding to a measurement of the system by its environment. We analyze the emergence of exceptional points (EPs) in NHHs and Lindbladians. In particular, we show how EPs in NHHs relate to a novel type of EPs -- multi-block EPs -- in the non-Hermitian Liouvillian, i.e. the Lindbladian in absence of quantum jump terms. We further analyze how quantum jump terms modify the multi-block structure. To illustrate our general findings, we present two prime examples: qubits and qutrits coupled to additional ground state levels that serves as sinks of the population. In those examples, we can navigate through the EP block structure by a variation of physical parameters. We analyze how the dynamics of the population of the states is affected by the order of the EPs. Additionally, we demonstrate that the quantum geometric tensor serves as a sensitive indicator of EPs of different kinds.