On-chip non-Hermitian cavity quantum electrodynamics

Exceptional points (EPs) are singularities in non-Hermitian systems where at least two eigenstates coalesce. They provide additional control over light-matter interactions and can, for example, enhance radiation from ensembles of photonic emitters. Advanced control over the characteristics of single quantum emitters via EPs remains, however, elusive. Here we engineer the quantum vacuum, the lowest energy state of the electromagnetic field, via a chiral EP to shape the spontaneous emission of a single quantum emitter. We develop a heterogeneously integrated lithium niobate-GaAs photonic circuit comprising high-quality quantum emitters, low-loss photonic circuits, electro-optic modulators and piezoelectric actuators. We dynamically tune the clockwise-counterclockwise mode coupling to access EPs, thereby inducing anomalous spontaneous emission dynamics with a sevenfold lifetime modulation (120-850 ps) and tunable chirality. Furthermore, we shape the emission spectra at the single-photon level via an EP-controlled local density of states, generating squared-Lorentzian, Fano-asymmetric and EP-induced transparency emissions. The latter manifests as a suppression of photon emission at zero detuning, arising from the non-Lorentzian optical response characteristics inherent to EP systems. This work unveils uncommon cavity quantum electrodynamics unique to EPs and exemplifies how the concept of non-Hermitian quantum photonics may contribute towards high-performance topological quantum light sources.