Generation of coherent quantum light from a single impurity-bound exciton

Impurity-bound excitons in II-VI semiconductors are promising optically active solid-state spin qubits that combine exceptional optical quantum efficiency with a low-noise spin environment. Previous studies on single impurities in these materials relied on incoherent optical excitation to generate photons. However, many quantum applications require resonant driving of quantum emitters to precisely control optical transitions and maintain coherence of the emission. Here, we demonstrate coherent optical emission of quantum light from a resonantly driven single impurity-bound exciton in ZnSe. The resonantly driven emitter exhibits bright quantum emission that preserves the phase of the resonant drive. It also features an intensity-dependent nonlinear phase shift at low photon numbers. Resonant excitation enables direct measurement of the Debye-Waller factor, determined to be 0.94, which indicates high efficiency emission to the zero-phonon line. Time-resolved resonance fluorescence measurements reveal a fast optically driven ionization process, along with a slower spontaneous ionization process having a lifetime of 21 μ s due to charge tunneling from the impurity. These results pave the way for coherent generation of quantum light and low-photon-number nonlinear optics through resonant excitation of impurity-bound excitons.