High-order exceptional point in a nanofiber cavity quantum electrodynamics system

We present an all-fiber emitter-cavity quantum electrodynamics (QED) system which consists of two two-level emitters and a nanofiber cavity. Our scheme makes it possible to observe the higher-order exceptional points based on the coupling between the emitters and the nanofiber cavity. The effective gain of this cavity can be obtained by weakly driven to the nanofiber cavity via two identical laser fields, which will realize coherent perfect absorption (CPA) in the implementation of the experiments. Under the experimental feasible parameters, the Hamiltonian of this system is in the condition of pseudo-Hermiticity, which means that its eigenvalues can be made of one real and a pair of complex conjugates, or be all real. By controllably tuned the ratio of the two emitter-cavity coupling strengths, and the ratio of the decay rates of the emitters, we can discover both the three-order exceptional point (EP3) and the second-order exceptional point (EP2) without parity-time symmetry in our emitter-cavity system. These results can also be demonstrated by the total output spectra and transmission spectra. We also find that the symmetric modes come into being when the coupling strength greater than the critical coupling strength at EP3 points. Our proposal will provide a new method to realize higher-order exceptional points.