Witnessing quantum discord for secure quantum key distribution

Abstract The study of quantum information processing seeks to characterize the resources that enable quantum information processing to perform tasks that are unfeasible or inefficient for classical information processing. Quantum cryptography is one such task, and researchers have identified entanglement as a sufficient resource for secure key generation. However, quantum discord, another type of quantum correlation beyond entanglement, has been found to be necessary for guaranteeing secure communication. Despite this, it remains a long-standing problem to clearly understand the role that discord plays and how to effectively utilize discord for analyzing security in specific quantum cryptography protocols. Here, we strictly prove that discord determines the secrecy and show its practical interest by considering a three-state quantum key distribution protocol. Its robustness against imperfections in qubit sources and measurements as well as basis misalignment, results in better key rates than BB84 protocol. Those advantages are experimentally demonstrated via photonic phase encoding systems.