Tunable coherent and dissipative light-matter interactions enabled by chiral exceptional points of a microcavity

The ability to harness the ubiquitous dissipation in realistic quantum systems holds great promise for developing advanced quantum technologies. In particular, the presence of dissipation violates the fundamental premise of Hermiticity and results in the non-Hermitian singularities associated with coalesced eigenstates in parameter space known as exceptional points (EPs), providing a new strategy to manipulate quantum states in both dynamical evolution and stationary limit. In this work, we show that for a microcavity supporting chiral EPs, light-matter interaction between the cavity photons and a quantum emitter can be flexibly switched from coherent to fully dissipative by tuning the phase factor of a complex coupling rate, an additional degree of freedom offered by the chiral EPs of a microcavity. The tunable coherent and dissipative light-matter interactions are evidenced by the distinguishing quantum dynamics and optical spectra, where the dispersion of a coupled quantum system changes from level repulsion to level attraction. While dissipation is generally considered fatal for quantum effects and should be suppressed, here we find that the EP-enabled dissipative light-matter interaction shows a unique advantage in achieving high-efficiency single-photon emission in a weak-coupling regime through the non-Hermitian photon blockade, which originates from the anharmonicity in the losses instead of in the eigenenergy levels for the conventional photon blockade (CPB). A giant enhancement of more than 5 orders of magnitude is found for both photon correlations and intensity compared to that of CPB in coherent light-matter interaction. We expect our work to enrich the understanding of light-matter interaction in open photonic resonators and provide insight into the manipulation of quantum states and quantum correlations in a wide variety of quantum systems by utilizing the non-Hermicity of electromagnetic environments.