Generating absolutely secure shared secret keys

Quantum key distribution (QKD) is a procedure to generate a secure bit string using very weak light and a public communication channel. The secure bit string supports many types of secure communication, and the security of QKD has already been theoretically proven. However, in the existing theories, some severe conditions are required, which may not always hold under realistic circumstances. Consequently, users of a QKD system cannot but trust its manufacturer, in the sense that they themselves cannot check whether or not the system satisfies those strict conditions. Here, we present our recent theoretical progress regarding relaxation of those conditions. We believe that this is a significant step toward a QKD system that allows users themselves to transparently verify its secureness without solely depending on a third party.

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