A frequency-coded quantum key distribution scheme

A frequency-coded quantum key distribution scheme, what we propose here, is that using encoded qubit in different frequency of a photon in four kinds of states. These states satisfy requirements of BB84 protocol and could be produced with the recent advances in technology. Comparing with the scheme proposed in [Bloch et al., Opt. Lett. 32 (2007) 301], our scheme has no intrinsic deficiency that the measurement of one kind of the states will get wrong result with a little probability, and is a perfect BB84 protocol. The characters and feasibility of the scheme are discussed in detail.