Three-photon energy-time entanglement

Entangled quantum particles have correlations stronger than those allowed by classical physics. These correlations are at the heart of deep foundational questions in quantum mechanics, and form the basis of many emerging quantum technologies. Although the discrete variables of up to 14 ions and the continuous variables between three intense optical beams have been entangled, it has remained an open challenge to entangle the continuous properties of three or more individual particles. Here we experimentally demonstrate genuine tripartite continuous-variable entanglement between three separated particles. In our set-up the three particles are photons created directly from a single input photon; the creation process leads to quantum correlations between the energies and emission times of the photons. The entanglement between our photons is the three-party generalization of the Einstein-Podolsky-Rosen correlations for continuous variables, and could serve as a valuable resource in a wide variety of quantum information tasks.