Environmental quantum states trigger emission in nonlinear photonics

Light-matter interactions are traditionally governed by two fundamental paradigms: spontaneous and stimulated radiation. However, in a nonlinear bath with photon-photon interactions, these paradigms break down, revealing new possibilities for photon emission. Here, we show a mechanism, termed triggered emission, in which an emitter, largely detuned from single-photon states, is triggered by the environment’s quantum state to emit a highly correlated photon pair, a doublon. By identifying two critical conditions, energy matching and wavefunction overlap, we demonstrate that the dynamics of the emitter are profoundly shaped by the environment’s quantum state. Furthermore, by engineering the initial environment photonic state and constructing a quasi-giant emitter, we realize partial unidirectional emission which evolves into a superposition state comprising a localized single-photon and a unidirectionally propagating, strongly correlated two-photon wavepacket. Our findings not only deepen the understanding of nonlinear emitter dynamics but also provide a versatile platform for quantum computing and quantum information processing.