  Measurement-device-independent quantum key distribution (MDIQKD) protocol is
immune to all attacks on detection and guarantees the information-theoretical
security even with imperfect single photon detectors. Recently, several
proof-of-principle demonstrations of MDIQKD have been achieved. Those
experiments, although novel, are implemented through limited distance with a
key rate less than 0.1 bps. Here, by developing a 75 MHz clock rate
fully-automatic and highly-stable system, and superconducting nanowire single
photon detectors with detection efficiencies more than 40%, we extend the
secure transmission distance of MDIQKD to 200 km and achieve a secure key rate
of three orders of magnitude higher. These results pave the way towards a
quantum network with measurement-device-independent security.
