Long-lived quantum memory enabling atom-photon entanglement over 101 km telecom fiber

TL;DR

This paper demonstrates a long-distance quantum entanglement between a single atom and photon over 101 km of telecom fiber, using a long-lived atomic qubit and quantum frequency conversion to enable scalable quantum networks.

Contribution

The authors developed a method to extend atom-photon entanglement over 101 km by enhancing atomic coherence time and converting photon wavelength to telecom band, achieving high-fidelity entanglement over long distances.

Findings

Entanglement fidelity over 70% after 101 km fiber

Atomic coherence time extended to 7 ms

Photon wavelength converted to telecom S-Band

Abstract

Long-distance entanglement distribution is the key task for quantum networks, enabling applications such as secure communication and distributed quantum computing. Here we report on novel developments extending the reach for sharing entanglement between a single $^{87}$Rb atom and a single photon over long optical fibers. To maintain a high fidelity during the long flight times through such fibers, the coherence time of the single atom is prolonged to 7 ms by applying a long-lived qubit encoding. In addition, the attenuation in the fibers is minimized by converting the photon's wavelength to the telecom S-Band via polarization-preserving quantum frequency conversion. This enables to observe entanglement between the atomic quantum memory and the emitted photon after passing 101 km of optical fiber with a fidelity better than 70.8$\pm$2.4%. The fidelity, however, is no longer reduced due to loss of coherence of the atom or photon but in the current setup rather due to detector dark counts, showing the suitability of our platform to realize city-to-city scale quantum network links.