Vol. 3, Issue 8, Part K (2017)

Abstract
We have seen that secure and efficient quantum communication is possible using arbitrary, orthogonal multi-partite quantum state. The orthogonal-state-based protocols described in the previous chapter are extremely interesting as they are fundamentally different from the existing conjugate-coding-based BB84-type protocols. In this chapter, we aim to propose another orthogonal-state-based protocol of DSQC. More precisely, the obstacles are placed in the lower arm of a Mach-Zehnder interferometer and a single photon is sent through the input port. The presence of the obstacle disrupts the destructive interference that would otherwise occur, thereby revealing the presence of the obstacle. This type of interaction-free measurements is referred to as counterfactual and aquantum cryptographic protocol that uses interaction-free measurement to avoid the transmission of information carrying qubits over the quantum channel accessible to Eve is referred to as a counterfactual protocol. However, till the recent past orthogonal-state-based protocols were only a theoretical concept and were limited to QKD. Only recently, orthogonal-state-based protocols of QKD are experimentally realized and extended to cryptographic tasks beyond QKD. This paper aims to briefly review the orthogonal-state-based protocols of secure quantum communication that are recently introduced by our group and other researchers. The protocol is shown to be unconditionally secure. Interestingly, it is found that the protocol is not maximally efficient. The proposed orthogonal-state-based protocol may have many useful applications in experimental quantum cryptography, as it provides a wide choice of quantum states that can be generated with nowadays technology.