Long-Lived Quantum Memory Enabling Atom-Photon Entanglement over 101 km of Telecom Fiber

Abstract

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.