Quantum dialogue with one qubit to represent two bits

Two communicants can make use of quantum dialogue (QD) to exchange their private data in a secure efficient manner without preparing keys ahead. In order to increase the capacity of quantum communication, there were several QD protocols taking a photon in double degrees of freedom as their quantum resources recently. However, although they adopt different approaches to develop such a photon, these QD protocols have a common point—all making one degree of freedom represent only one bit. This paper proposes a novel method for one degree of freedom to symbolize two bits. Thus, its information density is twice that of these existing protocols. This will bring about a new QD of high information density. In a certain degree, its information-theoretical efficiencies can reach as high as 83.33%, more than the current maximum—66.67%. Moreover, the proposed method can also improve on cost and efficiency. Finally, the presented QD protocol is analysed turning out secure and without information leakage. Consequently, this work provides us a desirable QD alternative.