PT Journal AU Hadfield, RH Habif, JL Schlafer, J Schwall, RE Nam, SW TI Quantum key distribution at 1550 nm with twin superconducting single-photon detectors SO Applied Physics Letters JI Appl. Phys. Lett. PY 2006 BP 241129 VL 89 IS 24 DI 10.1063/1.2405870 DE SSPD; quantum cryptography ER