Low-noise frequency downconversion for long-distance distribution of entangled atomic qubits

Distribution of quantum resources such as entanglement over distances beyond a few fiber attenuation lengths requires realization of quantum repeaters that utilize entanglement swapping to extend the distance between the entangled qubit pairs [1]. A quantum repeater is a small quantum computer capable of generating entangled qubit pairs with its neighboring repeaters and storing them in stable quantum memories. Once the entangled pairs are generated, it will perform Bell basis measurements, classical communication with appropriate repeaters, and single qubit gates necessary for entanglement swapping. For high fidelity operation, it might perform local operations and classical communications (LOCC) such as entanglement distillation [2] and/or quantum error correction [3,4]. Generation of remote entangled ion pairs mediated by photonic qubits has been demonstrated [5] but their reach is limited since the photons used in the experiment are in the UV part of the spectrum. Coherent conversion of the photons emitted by Yb ion at 369.5nm to a photon at 1310 nm would enable entanglement generation over long distances [6].