An asymmetric atom-photon architecture for device-independent quantum key distribution over 25\,km

Device-independent quantum key distribution (DIQKD) can guarantee security without trusting the internal workings of the measurement devices, but extending it to fiber networks demands high-quality entanglement, reliable heralding, and faithful photon transmission simultaneously. Here, we address these requirements in an event-ready asymmetric atom-photon architecture implemented over $25\mathrm{k}\mathrm{m}$ of spooled telecom fiber. A single trapped ${}^{40}{\mathrm{C}\mathrm{a}}^{+}$ ion forms one party of the protocol while a single transmitted photon measured at the remote station forms the other. Double quantum frequency conversion and active polarization stabilization preserve the atom-photon quantum correlation across the full link. We obtain a Clauser-Horne-Shimony-Holt parameter of $\mathrm{\mid }S\mathrm{\mid }=2.75_{-0.15}^{+0.16}$, exceeding the threshold $\mathrm{\mid }S\mathrm{\mid }=2.362$ required for a positive asymptotic secret-key fraction under the DIQKD model used. Within the same asymptotic model, this corresponds to a conservative estimate of 69 secret-key bits out of 10908 detected Bell states. The demonstrated architecture establishes a route towards DIQKD in heterogeneous, repeater-compatible quantum networks.