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arXiv:2506.06268

Design Tradeoffs in Photonically Linked Qubit Networks

Ely Novakoski, Jungsang Kim

Published Jun 6, 2025• 0 citations
arXivHTMLPDF

Abstract

Quantum networking can be realized by distributing pairs of entangled qubits between remote quantum processing nodes. Devoted communication qubits within each node can naturally interface with photons which bus quantum information between nodes. With the introduction of CQED to enhance interactions between communication qubits and photons, advanced protocols capable of achieving high entanglement distribution rates with high fidelity become feasible. In this paper, we consider two such protocols based on trapped ion communication qubits strongly coupled to small optical cavities. We study the rate and fidelity performance of these protocols as a function of critical device parameters and the photonic degree of freedom used to carry the quantum information. We compare the performance of these protocols with the traditional two-photon interference scheme, subjecting all protocols to the same experimentally relevant constraints. We find that adoption of the strong-coupling protocols could provide substantial distribution rate improvements of $30-75\%$ while maintaining the high-fidelities $\mathcal{F}\gtrsim99\%$ of the traditional scheme.

DOI
10.48550/arxiv.2506.06268

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Quantum networking can be realized by distributing pairs of entangled qubits between remote quantum processing nodes. Devoted communication qubits within each node can naturally interface with photons which bus quantum information between nodes. With the introduction of CQED to enhance interactions between communication qubits and photons, advanced protocols capable of achieving high entanglement distribution rates with high fidelity become feasible. In this paper, we consider two such protocols based on trapped ion communication qubits strongly coupled to small optical cavities. We study the rate and fidelity performance of these protocols as a function of critical device parameters and the photonic degree of freedom used to carry the quantum information. We compare the performance of these protocols with the traditional two-photon interference scheme, subjecting all protocols to the same experimentally relevant constraints. We find that adoption of the strong-coupling protocols could provide substantial distribution rate improvements of 30−75%30-75\%30−75% while maintaining the high-fidelities F≳99%\mathcal{F}\gtrsim99\%F≳99% of the traditional scheme.
We demonstrate remote entanglement of trapped-ion qubits via a quantum-optical fiber link with fidelity and rate approaching those of local operations. Two ^{88}Sr^{+} qubits are entangled via the polarization degree of freedom of two spontaneously emitted 422 nm photons which are coupled by high-numerical-aperture lenses into single-mode optical fibers and interfere on a beam splitter. A novel geometry allows high-efficiency photon collection while maintaining unit fidelity for ion-photon entanglement. We generate heralded Bell pairs with fidelity 94% at an average rate 182 s^{-1} (success probability 2.18×10^{-4}).
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Trapped ions coupled to optical cavities can be used to build up quantum interfaces between stationary and flying qubits in a quantum network. Shared entangled states between different network nodes have proven to be an essential resource for various applications of a quantum network, such as distributed quantum computation. At a first quantum network node, we have trapped ions in a linear Paul trap and coupled them to an optical cavity two centimeter in length. We have demonstrated entanglement of a single ion with a single photon, and used this high-fidelity operation to entangle two ions in a heralded fashion [1,2]. However, the speed of these operations is intrinsically limited by the ion-cavity coupling strength, which is predetermined by the length of the optical cavity. Fiber-based optical cavities have been coupled to single ions and it has been shown that these microscopic cavities allow access to the strong coupling regime [3]. Operating in this regime would enable quantum communication protocols to be carried out over long distances with enhanced fidelity and efficiency. With this goal, we have designed and constructed a novel ion-cavity system which incorporates a fiber cavity. In my talk, I will introduce basic building blocks of quantum networks based on trapped ions coupled to optical cavities and will present recent results, including simulation and characterization of our fiber-based ion-cavity system. [1] A. Stute et al., Nature. 485, 482 (2012) [2] B. Casabone et al., Phys. Rev. Lett. 111, 100505 (2013) [4] H. Takahashi et al., arXiv:1808.04031 (2018)
Trapped ions are a promising platform for local quantum information processing. In order to distribute this quantum information over long distances, we can take advantage of optical cavities, which ofier a coherent interface between matter and light, enabling the transfer of quantum information from stationary qubits such as ions onto photons. We demonstrate such an interface by coupling trapped ions to a cavity and have recently shown that a quantum state can be faithfully transferred from a single ion onto a single photon. In particular, this transfer can be improved by taking advantage of a collective effect between multiple ions, namely, superradiant emission into the cavity. In this proof-of-principle experiment, we tune the phase of a two-ion entangled state between sub- and superradiance. The superradiant coupling is then used to enhance the transfer of quantum information onto a photon from a logical qubit encoded in the two ions. Finally, prospects for linking together distant ions in cavities via a quantum network are discussed. Toward this goal, we outline a fiber-based ion-cavity experiment which allows access to the single-ion strong-coupling regime.