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Kilometer-Scale Ion-Photon Entanglement with a Metastable $^{88}$Sr$^{+}$ Qubit
Mika A. Zalewski, Denton Wu, Ana Luiza Ferrari, Yuanheng Xie, Norbert M. Linke·June 12, 2025·DOI: 10.1103/nc84-zjp3
Quantum PhysicsAtomic Physics
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Abstract
We demonstrate entanglement between the polarization of an infrared photon and a metastable $^{88}$Sr$^+$ ion qubit. This entanglement persists after transmitting the photon over a $2.8\:$km long commercial fiber deployed in an urban environment. Tomography of the ion-photon entangled state yields a fidelity of $0.949(4)$ within the laboratory and $0.929(5)$ after fiber transmission, not corrected for readout errors. Our results establish the Strontium ion as a promising candidate for metropolitan-scale quantum networking based on an atomic transition at $1092\:$nm, a wavelength compatible with existing telecom fiber infrastructure.