Experimental Simulation of Symmetry-Protected Higher-Order Exceptional Points with Single Photons

Exceptional points (EPs) of non-Hermitian (NH) systems have recently attracted increasing attention due to their rich phenomenology and intriguing applications. Compared to the predominantly studied second-order EPs, higher-order EPs have been assumed to play a much less prominent role since they generically require the tuning of more parameters. Here we experimentally simulate two-dimensional topological NH band structures using single-photon interferometry, and observe topologically stable third-order EPs obtained by tuning only two real parameters in the presence of symmetry. Focusing on two-dimensional NH system, we demonstrate on two types of third-order EPs, respectively protected by parity-time symmetry and chiral symmetry, and four-fold degeneracies, composed by the non-defective two-fold degeneracies and second-order EPs. Our work reveals the abundant and conceptually richer higher-order EPs protected by symmetries, and offers a versatile platform for further research on topological NH systems.