Simple protocol of quantum cryptography based on two truly orthogonal states

Abstract

One of the simplest protocols of quantum cryptography proposed so far is the one based on two nonorthogonal states. In this protocol, the receiver cannot determine every bit because there is no way to distinguish nonorthogonal states completely. Another simple protocol uses two orthogonal states, and the receiver can always discriminate between the two states that represent the bit values 0 and 1. In this protocol, however, the sender must mix the third states (vacuum states) in the bit stream to prevent eavesdropping. Thus the following question arises: do more simple protocols exist in which the sender switches between only two states and the receiver can obtain every bit value? Here we show that such a protocol does exist.