Entanglement between photons at telecommunication wavelengths and long-lived quantum memories is one of the fundamental requirements of long-distance quantum communication. Quantum memories featuring on-demand read-out and multimode operation are additional precious assets that will benefit the communication rate. In this work we report the first demonstration of entanglement between a telecom photon and a collective spin excitation in a multimode solid-state quantum memory. Photon pairs are generated through widely non-degenerate parametric down-conversion, featuring energy-time entanglement between the telecom-wavelength idler and a visible signal photon. The latter is stored in a Pr3+:Y2SiO5 crystal as a spin wave using the full Atomic Frequency Comb scheme. We then recall the stored signal photon and analyze the entanglement using the Franson scheme. We measure conditional fidelities of 92(2)% for excited-state storage, enough to violate a CHSH inequality, and 77(2)% for spin-wave storage. Taking advantage of the on-demand read-out from the spin state, we extend the entanglement storage in the quantum memory for up to 47.7~μs, which could allow for the distribution of entanglement between quantum nodes separated by distances of up to 10 km.
@article{arxiv.2106.05079,
title = {Entanglement between a telecom photon and an on-demand multimode solid-state quantum memory},
author = {Jelena V. Rakonjac and Dario Lago-Rivera and Alessandro Seri and Margherita Mazzera and Samuele Grandi and Hugues de Riedmatten},
journal= {arXiv preprint arXiv:2106.05079},
year = {2022}
}