  we experimentally implement a fault-tolerant quantum key distribution
protocol with two photons in a decoherence-free subspace (DFS). It is
demonstrated that our protocol can yield good key rate even with large bit-flip
error rate caused by collective rotation, while the usual realization of BB84
protocol cannot produce any secure final key given the same channel. Since the
experiment is performed in polarization space and does not need the calibration
of reference frame, important applications in free-space quantum communication
are expected. Moreover, our method can also be used to robustly transmit an
arbitrary two-level quantum state in a type of DFS.
