Observation of eight-photon entanglement

The creation of increasingly large multipartite entangled states is not only a fundamental scientific endeavour in itself1–3, but is also the enabling technology for quantum information4,5. Tremendous experimental effort has been devoted to generat-ing multiparticle entanglement with a growing number of qubits6–16. So far, up to six spatially separated single photons10–14 have been entangled based on parametric down-conversion17. Multiple degrees of freedom of a single photon have been exploited to generate forms of hyper-entangled states18,19. Here, using new ultra-bright sources of entangled photon pairs20, an eight-photon interferometer and post-selec-tion detection, we demonstrate for the first time the creation of an eight-photon Schrödinger cat state1 with genuine multipar-tite entanglement. The ability to control eight individual photons represents a step towards optical quantum compu-tation, and will enable new experiments on, for example, quantum simulation21,22, topological error correction23 and testing entanglement dynamics under decoherence24. In our experiment, we aim to create eight-photon Schrödinger cat states1, also known as Greenberger–Horne–Zeilinger states (GHZ)25, which can be written in the for