Despite the tremendous progress of quantum cryptography, efficient quantum communication over long distances (>1000km) remains an outstanding challenge due to fiber attenuation and operation errors accumulated over the entire communication distance. Quantum repeaters (QRs), as a promising approach, can overcome both photon loss and operation errors, and hence significantly speedup the communication rate. Depending on the methods used to correct loss and operation errors, all the proposed QR schemes can be classified into three categories (generations). The first generation QRs uses heralded entanglement generation for the correction of erasure errors and entanglement purification for the correction of operation errors. The second generation QRs uses heralded entanglement generation for the correction of erasure errors and quantum error correction for the correction of operation errors. The third generation QRs uses quantum error correction for the correction of both erasure and operation errors respectively. Here, quantum information can be faithfully transmitted via a noisy channel without the use of long distance teleportation, thus eliminating the need to establish remote entangled links and consequently making them ultrafast.
We perform a systematic comparison of three generations of quantum repeaters by evaluating the cost of both temporal and physical resources, and identify the optimized quantum repeater architecture for a given set of experimental parameters. Our work provides a roadmap for the experimental realizations of highly efficient quantum networks over transcontinental distances.