PT Journal AU Takesue, H Nam, SW Zhang, Q Hadfield, RH Honjo, T Tamaki, K Yamamoto, Y TI Quantum key distribution over a 40-dB channel loss using superconducting single-photon detectors SO Nature Photonics JI Nat. Photon. PY 2007 BP 343 EP 348 VL 1 DI 10.1038/nphoton.2007.75 DE quantum cryptography; SSPD; QKD; DSP ER