  We propose a prepare-and-measure scheme for quantum key distribution with
2-bit quantum codes. The protocol is unconditionally secure under whatever type
of intercept-and-resend attack. Given the symmetric and independent errors to
the transmitted qubits, our scheme can tolerate a bit error rate up to 26% in
4-state protocol and 30% in 6-state protocol, respectively. These values are
higher than all currently known threshold values for prepare-and-measure
protocols. A specific realization with linear optics is given.
