  Reference-frame-independent quantum key distribution (RFI QKD) protocol can
reduce the requirement on the alignment of reference frames in practical
systems. However, comparing with the Bennett-Brassard (BB84) QKD protocol, the
main drawback of RFI QKD is that Alice needs to prepare six encoding states in
the three mutually unbiased bases (X, Y, and Z), and Bob also needs to measures
the quantum state with such three bases. Here, we show that the RFI QKD
protocol can be secured in the case where Alice sends fewer states. In
particular, we find that transmitting three states (two eigenstates of the Z
basis and one of the eigenstates in the X basis) is sufficient to obtain the
comparable secret key rates and the covered distances, even when the security
against coherent attacks with statistical fluctuations of finite-key size is
considered. Finally, a proof-of-principle experiment based on time-bin encoding
is demonstrated to show the feasibility of our scheme, and its merit to
simplify the experimental setup.
