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arXiv:2606.12313

Quantum repeater segment with free-space coupled co-trapped ions using telecom photon interference

Max Bergerhoff, Pascal Baumgart, Christian Haen, Jonas Meiers, Tobias Bauer, Jonas Haferkamp, Christoph Becher, Jürgen Eschner

quant-phSubmitted Jun 10, 2026• Announced Jun 11, 2026• 0 citations
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Abstract

A quantum repeater segment is a basic building block of a quantum repeater, generating buffered entanglement of quantum memories to connect quantum repeater cells. It also enables the connection between quantum computers. In the implementation we present here, photons emitted from two co-trapped free-space coupled $^{40}$Ca$^+$ ions are converted to the telecom-C band and interfered after transmission over 440$\,$m of optical fiber (220$\,$m per arm), where a photonic Bell measurement is performed to create entanglement between the memories. With this scheme we generate an entangled $\left|Ψ^+\right\rangle$ Bell state with $\ge 68(8)\,$% fidelity, highlighting trapped $^{40}$Ca$^+$ ions as a promising quantum repeater hardware platform.

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11 pages, 7 figures

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A quantum repeater segment is a basic building block of a quantum repeater, generating buffered entanglement of quantum memories to connect quantum repeater cells. It also enables the connection between quantum computers. In the implementation we present here, photons emitted from two co-trapped free-space coupled 40^{40}40Ca+^++ ions are converted to the telecom-C band and interfered after transmission over 440 \,m of optical fiber (220 \,m per arm), where a photonic Bell measurement is performed to create entanglement between the memories. With this scheme we generate an entangled ∣Ψ+⟩\left|Ψ^+\right\rangle∣Ψ+⟩ Bell state with ≥68(8) \ge 68(8)\,≥68(8)% fidelity, highlighting trapped 40^{40}40Ca+^++ ions as a promising quantum repeater hardware platform.
Quantum repeater cells are crucial for overcoming loss in long-distance quantum networks. Here, the authors present a trapped-ion implementation of a quantum repeater cell, using two 40{}^{40}40Ca+{}^{+}+ ions that act as quantum memories and are coupled to free-space photonic channels. With this setup they are able to demonstrate asynchronous generation of atom-photon and photon-photon entanglement.
Trapped atomic ions constitute one of the leading physical platforms for building the quantum repeater nodes to realize large-scale quantum networks. In a long-distance trapped-ion quantum network, it is essential to have crosstalk-free dual-type qubits: one type, called the communication qubit, to establish entangling interface with telecom photons; and the other type, called the memory qubit, to store quantum information immune from photon scattering under entangling attempts. Here, we report the first experimental implementation of a telecom-compatible and crosstalk-free quantum network node based on two trapped 40^{40}40Ca+^{+}+ ions. The memory qubit is encoded on a long-lived metastable level to avoid crosstalk with the communication qubit encoded in another subspace of the same ion species, and a quantum wavelength conversion module is employed to generate ion-photon entanglement over a 12 12\,12km fiber in a heralded style. Our work therefore constitutes an important step towards the realization of quantum repeaters and long-distance quantum networks.
Trapped atomic ions constitute one of the leading physical platforms for building the quantum repeater nodes to realize large-scale quantum networks. In a long-distance trapped-ion quantum network, it is essential to have crosstalk-free dual-type qubits: one type, called the communication qubit, to establish an entangling interface with telecom photons; and the other type, called the memory qubit, to store quantum information immune from photon scattering under entangling attempts. Here, we report the first experimental implementation of a telecom-compatible and crosstalk-free quantum network node based on two trapped ^{40}Ca^{+} ions. The memory qubit is encoded on a long-lived metastable level to avoid crosstalk with the communication qubit encoded in another subspace of the same ion species, and a quantum wavelength conversion module is employed to generate heralded ion-photon entanglement over a 12 km fiber. Our work therefore constitutes an important step toward the realization of quantum repeaters and long-distance quantum networks.
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