  Non-Hermitian spectral degeneracies, known as exceptional points (EPs),
feature simultaneous coalescence of both eigenvalues and the associated
eigenstates of a system. A host of intriguing EP effects and their applications
have been revealed in the classical realm, such as loss-induced lasing,
single-mode laser, and EP-enhanced sensing. Here we show that a purely quantum
effect, known as single-photon blockade, emerges in a Kerr microring resonator
due to EP-induced asymmetric coupling between the optical modes and the
nonlinearity-induced anharmonic energy-level spacing. A striking feature of
this photon blockade is that it emerges at two-photon resonance which in
Hermitian systems will only lead to photon-induced tunneling but not to photon
blockade. By tuning the system towards to or away from an EP, one can control
quantum correlations, implying the potential use of our system for frequency
tunable single-photon generation and an antibunching-to-bunching light switch.
Our work sheds new light on EP-engineered purely quantum effects, providing
unique opportunities for making and utilizing various single-photon quantum EP
devices.
