Sequential generation of multiphoton entanglement with a Rydberg superatom

Multiqubit entanglement is an indispensable resource for quantum information science. In particular, the entanglement of photons is of conceptual interest due to its implications in measurement-based quantum computing, communication and metrology. The traditional way of spontaneous parametric downconversion already demonstrates the entanglement of up to a dozen photons but is hindered by its probabilistic nature. Here we experimentally demonstrate an efficient approach for multiphoton generation with a Rydberg superatom, a mesoscopic atomic ensemble under Rydberg blockade. Using it as an efficient single-photon interface, we iterate the photon creation process that gives rise to a train of single photons entangled in the time-bin degree of freedom. Photon correlations verify entanglement up to six qubits. The overall efficiency to detect one photon is 9.4%. After correcting the measurement inefficiencies, we obtain a scaling factor of 27%, surpassing previous results and paving the way for larger-scale photonic entanglement. Up to six photons in a Greenberger–Horne–Zeilinger state are sequentially generated by using a Rydberg superatom—a mesoscopic atomic ensemble under the condition of strong Rydberg blockade. The efficiency scaling factor for adding one photon is 0.27.