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Long-distance entanglement distribution is the key task for quantum networks, enabling applications such as secure communication and distributed quantum computing. In this work, we take a crucial step toward this task by sharing entanglement over long optical fibers between a single 87Rb atom and a single photon. High fidelity of the atomic state could be maintained during long flight times through such fibers by prolonging the coherence time of the single atom to 10 ms based on encoding in long-lived states. In addition, the attenuation in the fibers is minimized by converting the wavelength of the photon to the telecom S band via polarization-preserving quantum frequency conversion. These improvements enable us to observe entanglement between the atomic quantum memory and the emitted photons transmitted through standard spooled telecom fibers with a length of 101 km with a fidelity of 70.8±2.4%. This fidelity is comparable to recent demonstrations over 20 km, despite the channel loss now significantly exceeding 20 dB. In fact, now the reduction in fidelity is due to detector dark counts rather than loss of coherence of the atom or photon, proving the suitability of our platform to realize city-to-city-scale quantum network links.2 MoreReceived 7 August 2023Revised 23 December 2023Accepted 6 March 2024DOI:https://doi.org/10.1103/PRXQuantum.5.020307Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasLight-matter interactionQuantum communicationQuantum opticsQuantum Information, Science & TechnologyAtomic, Molecular & Optical
Zhou et al. (Wed,) studied this question.