A novel system for ultra-long-distance quantum key distribution in optical fiber, incorporating ultra-low-noise transition-edge
sensor (TES) photodetectors, is described. Integration of the TES detectors into the system was facilitated with a
unique optically switched interferometer design. The performance of the system over 101 km of dark, single-mode fiber
at 1550 nm and a clock rate of 1 MHz is described. Secret-key bits were produced after error correction and privacy
amplification when using mean photon numbers of 0.01, 0.0148, 0.02, 0.0304, and 0.2 photons/pulse at the output of the
transmitter. At a mean photon number of 0.1 photons per pulse at the transmitter, a transmission line loss of 29.92 dB,
roughly equivalent to 150 km of optical fiber, could be tolerated and secret bits extracted from the transmitted key.