Quantum key secure communication protocol via enhanced superdense coding

In this study, an improvement, and a generalization to more than two bits of the superdense coding protocol are presented. Based on both innovations, a novel quantum key secure communication protocol is developed, which uses an N-bit key, optical multiplexers and demultiplexers, and quantum repeaters, which work thanks to entanglement swapping. In this way, the new protocol allows the simultaneous transmission of N-bits encrypted through optical channels. This study incorporates implementations on two platforms: the Quirk simulator, and the 16-qubits Melbourne processor of the IBM Q Experience program. Errors in terms of the number of quantum repeaters are studied. Finally, this study is completed by analyzing the advantages of the optical link, with which the protocol works, versus the commonly used electromagnetic link for quantum communication between submerged submarines in the presence of a third party that acts as an eavesdropper.