Quantum Key Distribution with Multi Letter Phase-Shift Keying

We present a new protocol for quantum key distribution using discrete phase-shift encoding with continuous variables. The novelty of the protocol is multi letter alphabets represented by coherent states of light with fixed amplitude and variable phase. Information is encoded in the phase of a coherent states which can be chosen from a regular discrete set consisting of an arbitrary number of letters. We evaluate the security of the protocol against the beam splitting attack. As a results we show the advantage of the proposed protocol over the standard two letter coherent state protocol, especially in the case when losses in the communication channel are low.