Key Expanding in Measurement-Device-Independent Quantum Key Distribution

A common problem of a discrete-variable quantum key distribution (QKD) protocol is that related to its key rate. This work proposes a solution to this problem: the procedure key expanding. The key expanding not only increases the key size (respectively, key rate) of the final secret key, but also increases the efficiency of a QKD scheme. The contribution comes from using a part of the classical data exchanged in the QKD. This process can be implemented along with privacy amplification, thus not requiring any additional time resource. Also, it can be used in combination with the other solution to the mentioned problem, namely high-dimensionality. The combination allows key rates that have not been obtainable so far. The key expanding is verified to be a secure procedure—it does not leak information about the final key to an eavesdropper. Moreover, the paper presents the key expanding in the case of the well-known measurement-device-independent QKD protocol.