Significance of Bell States Over Four-Qubit Entangled States in Quantum Bidirectional Direct Communication Protocols

The controller-independent quantum bidirectional direct communication (QBDC) protocol using Bell states is widely studied. In recent times, protocols have been proposed using four-qubit entangled states with the intention of increasing security. In this work, we compare the controller-independent QBDC protocol using Bell states with that of a protocol that exploits four-qubit entangled states. We observe that the same two-qubit protocol can be run on a four-qubit system without information leakage. The security offered is the same in both cases, but the quantum efficiency of the protocol is reduced in the case of four-qubit entangled states. We highlight the significance of the protocol using Bell states from the physical implementation point of view.