Improving the total efficiency of quantum key distribution by comparing Bell states

A protocol for quantum key distribution by comparing Bell states [G. Gao, Opt. Commun. 281 (2008) 876] is recently proposed by Gan Gao. Gan Gao claimed that his protocol has the advantage of high total efficiency and its total efficiency is 50%. In this paper, by giving a little modification to the original one, we introduce an improved version of Gao's protocol, which can make the total efficiency of the communication come up to 100%.