# Ion-photon entanglement and quantum frequency conversion with trapped Ba$^+$ ions

## TL;DR

This paper presents a method for generating ion-photon entanglement with Ba$^+$ ions, enhancing fidelity and entanglement probability, and demonstrates quantum frequency conversion to telecom and 780 nm wavelengths for long-distance quantum networking.

## Contribution

It introduces a novel shelving technique for Ba$^+$ ions to improve entanglement fidelity and probability, and outlines a quantum frequency conversion approach for hybrid quantum networks.

## Key findings

- Projected ion-photon entanglement fidelity of approximately 89%.
- Comparison of entanglement generation methods based on collection optics.
- Feasibility of converting emitted photons to telecom and 780 nm wavelengths.

## Abstract

Trapped ions are excellent candidates for quantum nodes, as they possess many desirable features of a network node including long-lifetimes, on-site processing capability and produce photonic flying qubits. However, unlike classical networks in which data may be transmitted in optical fibers and the range of communication readily extended with amplifiers, quantum systems often emit photons that have limited propagation range in optical fibers and, by virtue of the nature of a quantum state, cannot be noiselessly amplified. Here, we first describe a method to extract flying qubits from a Ba$^+$ trapped ion via shelving to a long lived, low-lying D-state with higher entanglement probabilities compared with current strong and weak excitation methods. We show a projected fidelity of $\approx$89% of the ion-photon entanglement. We compare several methods of ion-photon entanglement generation and show how the fidelity and entanglement probability varies as a function of the photon collection optic's numerical aperture. We then outline an approach for quantum frequency conversion of the photons emitted by the Ba$^+$ ion to the telecom range for long-distance networking and to 780 nm, for potential entanglement with Rubidium based quantum memories. Our approach is significant for extending the range of quantum networks and for development of hybrid quantum networks compromised of different types of quantum memories.