  We have demonstrated a proof-of-principle experiment of
reference-frame-independent phase coding quantum key distribution (RFI-QKD)
over an 80-km optical fiber. After considering the finite-key bound, we still
achieve a distance of 50 km. In this scenario, the phases of the basis states
are related by a slowly time-varying transformation. Furthermore, we developed
and realized a new decoy state method for RFI-QKD systems with weak coherent
sources to counteract the photon-number-splitting attack. With the help of a
reference-frame-independent protocol and a Michelson interferometer with
Faraday rotator mirrors, our system is rendered immune to the slow phase
changes of the interferometer and the polarization disturbances of the channel,
making the procedure very robust.
