Naoaki Koyama

Device-independent conference key agreement (DI-CKA) enables secure multipartite communication without relying on the internal structure or reliability of the devices used. However, realizing DI-CKA protocols is extremely challenging because observing loophole-free Bell violations demands exceptionally high detection efficiency. In this study, we incorporate noisy preprocessing—a technique that deliberately introduces noise into raw key bits to reduce an eavesdropper's information—into a DI-CKA protocol based on the Holz inequality to relax the required detection efficiency. We evaluate the performance of the protocol by calculating a lower bound on the key rate using a numerical optimization method, alongside an analytical upper bound derived from a specific eavesdropping attack. Our results demonstrate that the introduction of noisy preprocessing successfully relaxes the required detection efficiency even in the Holz-type DI-CKA. Furthermore, we find that the analytical upper bounds coincide with the numerical lower bounds with very high precision, demonstrating the tightness of the evaluated key rates.