Observation of Genuine Tripartite Non-Gaussian Entanglement from a Multimode Superconducting Three-Photon Spontaneous Parametric Down-Conversion Source

The generation of entangled photons through Spontaneous Parametric Down-Conversion (SPDC) is a critical resource for many key experiments and technologies in the domain of quantum optics. The use of the strong nonlinearities in circuit quantum electrodynamics has recently enabled the observation of Three Photon SPDC (3P-SPDC). Despite great interest in the entanglement structure of the resultant states, entanglement between photon triplets produced by a 3P-SPDC source has still has not been confirmed. Here, we report on the observation of genuine tripartite non-Gaussian entanglement in the steady-state output field of a 3P-SPDC source consisting of a 2d multimode superconducting parametric cavity coupled to a transmission line. We study this non-Gaussian tripartite entanglement using an entanglement witness built from three-mode correlation functions, and observe a maximum violation of the bound by over 15 standard deviations of the statistical noise. Furthermore, we find strong agreement between the observed and theoretically predicted scaling of the entanglement witness. We then explore the impact of the temporal mode function used to define the photon mode on the observed value of the entanglement witness.