Observation of a three-photon bound state

Bound states of massive particles, be it in the form of nucleons, atoms or molecules, are ubiquitous, and constitute the bulk of the visible world around us. In contrast, photon-photon interactions are weak and need to be specifically engineered in the form of nonlinear optical media. Here we report the observation of a three-photon bound state inside a quantum nonlinear optical medium. The strong photon-photon interaction is achieved by coupling the light to highly excited, strongly interacting Rydberg states in a cold atomic gas. The photonic trimer, which can be viewed as a quantum soliton, is observed via bunching and a strongly nonlinear phase in the three-photon correlation function of the emerging light. The observations are quantitatively described by an effective field theory of Rydberg-induced photon-photon interactions, and agree with direct numerical simulations. This work paves the way towards the realization, understanding, and control of strongly interacting quantum gases of light.