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arXiv · 2609.25795

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

Jonas Meiers·Christian Haen·Max Bergerhoff·Pascal Baumgart·Tobias Bauer·Christoph Becher·Jürgen Eschner
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Abstract

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 $25\,\mathrm{km}$ of spooled telecom fiber. A single trapped $^{40}\mathrm{Ca}^{+}$ 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 $|S|=2.75^{+0.16}_{-0.15}$, exceeding the threshold $|S|=2.362$ 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.

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Jonas Meiers, Christian Haen, Max Bergerhoff, Pascal Baumgart, Tobias Bauer, Christoph Becher, Jürgen Eschner. 2026-09-22. An asymmetric atom-photon architecture for device-independent quantum key distribution over 25\,km. https://arxiv.org/abs/2609.25795

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