Secure direct message exchange based on simultaneous usage of two different degrees of freedom

Abstract

In the era of information explosion supplied with a veryhigh level of technology as of today, the loss or leakage of information is one of the most severe problems. Security in communication has thus become more important than ever. In this paper, two protocols for quantum secure direct exchange of messages are proposed using hyperentangled photon pairs as resource for exchanging informative bits and single photons in hyperstates as resource for detecting eavesdroppers. In both the protocols the photons need to be transmitted only once from one to the other communicating party. However, the ways to transmit photons are different in the two proposed protocols. The first protocol employs block transmission of photons. Although its security is unconditional, it compulsorily requires quantum memories at both communicating stations. Contrasting to the first protocol, in the second protocol single photons are transmitted one by one and subject to immediate processing without the need of any quantum memories. The security of the second protocol is asymptotic in the sense that the chance for eavesdroppers to pass is approaching zero in the limit of long messages. Comparison with other protocols is also given.