Transmission of light–matter entanglement over a metropolitan network

We report on the transmission of telecom photons entangled with a multimode solid-state quantum memory over a deployed optical fiber in a metropolitan area. Photon pairs were generated through spontaneous parametric downconversion, with one photon stored in a rare-earth-based quantum memory, and the other, at telecommunication wavelengths, traveling through increasing distances of optical fiber, first in the laboratory and then outside in a deployed fiber loop. We measured highly non-classical correlations between the stored and the telecom photons for storage times up to 25 µs and for a fiber separation up to 50 km. We also report light–matter entanglement with a two-qubit fidelity up to 88%, which remains constant within error bars for all fiber lengths, showing that the telecom qubit does not suffer decoherence during the transmission. Finally, we moved the detection stage of the telecom photons to a different location placed 16 km away, and confirmed the non-classical correlations between the two photons. Our system was adapted to provide the transmission of precise detection times and synchronization signals over long quantum communication channels, providing the first steps for a future quantum network involving quantum memories and non-classical states.