  Quantum key distribution (QKD) can provide unconditional secure communication
between two distant parties. Although the significance of QKD is undisputed,
its feasibility has been questioned because of certain limitations in the
practical application of real-life QKD systems. It is a common belief the lack
of perfect single-photon source and the existence of detection loss will
handicap the feasibility of QKD by creating security loopholes and distance
limitations. The measurement device independent QKD (MDIQKD) with decoy-state
method removes the security threats from both the imperfect single-photon
source and the detection loss. Lengthening the distance and improving the key
rate of QKD with such a superior method is thus the central issue in the
practical application of QKD. Here, we report the results of MDIQKD over 404 km
of ultralow-loss optical fibre and 311 km of standard optical fibre by
employing an optimized four-intensity decoy-state method. This record-breaking
implementation of MDIQKD method not only provides a new distance record for
both MDIQKD and all types of QKD systems, more significantly, it achieves a
distance that the traditional BB84 QKD would not be able to achieve with the
same detection devices even with ideal single-phone sources. For the first
time, our work demonstrates that with the MDIQKD method, imperfect devices can
achieve better results than what ideal sources could have achieved. This work
represents a significant step towards proving and developing a feasible
long-distance QKD.
