  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 down-conversion
already demonstrates entanglement of up to a dozen photons but is hindered by
its probabilistic nature. Here, we experimentally demonstrate an efficient
approach for multi-photon 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
temporal photonic modes entangled in photon number degree. We detect the
multiphoton entanglement via converting the photon number degree to a time-bin
degree. Photon correlations verify entanglement up to 12 modes. The efficiency
scaling factor for adding one photon is 0.27, surpassing previous results, and
can be increased significantly without fundamental limitations.
