Ion-Photon Entanglement Across a City-Scale Quantum Network

Abstract

The scaling of quantum computers is an outstanding challenge for all experimental platforms. One promising avenue for scaling is through the construction of a quantum network, which makes use of distributed entanglement to connect separate quantum systems. Trapped ions are an appealing choice for quantum networks due to their long coherence times and high-fidelity gates. A cost-effective approach to the rapid scaling of such systems is by utilizing existing, deployed optical fibers. However, most atomic transitions produce photons in the ultraviolet or visible range, which experience high attenuation in silica fiber, a common choice for deployed telecommunications (telecom) fiber. To address this shortcoming, many schemes use quantum frequency conversion (QFC) to convert photons to telecom-compatible wavelengths, but this process is lossy and requires additional infrastructure. In this work, we develop a novel scheme that leverages the 1092 nm transition in the Strontium ion, a frequency which experiences low attenuation in telecom fiber. This dissertationdescribes the design and construction of a Strontium trapped ion quantum network and the demonstration of the first direct telecom transmission of ion-photon entanglement over a kilometer-scale distance. We achieve entanglement fidelities of 94.9(4)% and 92.9(5)% at rates of 350(4)/s and 15.9(4)/s with laboratory and deployed fibers respectively. These results are obtained by using a polarization encoding of the photonic qubit with no active stabilization of any optical fibers, illustrating the robustness of the scheme. This work demonstrates the viability of Strontium ions for medium distance quantum networking applications.