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arXiv:2506.11257

Kilometer-Scale Ion-Photon Entanglement with a Metastable $^{88}$Sr$^{+}$ Qubit

Zalewski, Mika A., Dong Wu, Ferrari, Ana Luiza, Yuanheng Xie, Norbert M. Linke

Published Jun 12, 2025• 0 citations
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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.

DOI
10.48550/arxiv.2506.11257

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Kilometer-Scale Ion-Photon Entanglement with a Metastable 88^{88}88Sr+^{+}+ Qubit
We demonstrate entanglement between the polarization of an infrared photon and a metastable 88^{88}88Sr+^++ ion qubit. This entanglement persists after transmitting the photon over a 2.8 2.8\:2.8km long commercial fiber deployed in an urban environment. Tomography of the ion-photon entangled state yields a fidelity of 0.949(4)0.949(4)0.949(4) within the laboratory and 0.929(5)0.929(5)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 1092\:1092nm, a wavelength compatible with existing telecom fiber infrastructure.
In analogy to the development of classical computing, the true potential of quantum devices at scale lies in their interconnection within a network. The “quantum internet” [1] is still a distant aspiration with the main limitation being the remote entanglement generation rate dropping off sharply with distance due to the fiber losses of the visible and UV photons involved. Still, there have been a number of successful demonstrations of remote entanglement with different platforms [2]–[5]. Ongoing efforts have focused on converting entangled photons emitted from atomic systems into fiber-compatible wavelengths, but this conversion process incurs efficiency loss, noise, and system complexity. In this conference paper, we present initial results from direct transmission of infrared ion-photons emitted from Strontium ions at 1,092 nm through a municipal dark fiber link. This wavelength falls into a low-loss window in fiber transmission that will allow for local area quantum networks without the frequency-conversion overhead. This comes at the cost of a smaller branching ratio, limiting the transmission rate. However, there is a distance range of a few kilometers where this is an advantageous choice, making this scheme highly promising for city-scale quantum networks. We stimulate trapped Strontium ions to emit single photons at 1,092 nm. After emission, the ion is left in one of two states, which form a metastable qubit (see diagram in figure 1).
Research into long-distance quantum memory-based networking to date has exclusively used dark fibers. This avoids the detector background from telecommunications (telecom) traffic, but as a result excludes many fibers deployed in the field. If memory-photon entanglement and telecom signals coexist on one fiber, the entire classical fiber infrastructure becomes available for quantum links. We present the first experimental demonstration of such coexistence. Ion-photon entanglement using 1092 nm photons emitted by a Strontium-88 ion is distributed over a deployed 2.8 km fiber loop which carries Ethernet and 5G traffic. All classical control signals required to coordinate the quantum transmitter and receiver systems co-propagate on the same fiber. These include fiber sensing for polarization stabilization. Our results demonstrate that memory-based quantum networks can be realized on active classical network infrastructure.
Concatenation of quantum repeater nodes has been proposed as a means to distribute entanglement efficiently across future long-distance networks. Here, we demonstrate a trapped-ion repeater node that establishes photonic entanglement over a 17 km fiber traversing a city, moving beyond laboratory-only prototypes. The node comprises two atomic ions coupled to an optical cavity. First, a detailed characterization is performed of the long fiber channel and of ion-photon entanglement distributed across it. Next, a repeater-node protocol is implemented, achieving entanglement between a local photon and a remote photon with a Bell-state fidelity of 81.6−1.4+1.9%81.6^{+1.9}_{-1.4}\%81.6−1.4+1.9​%. These results demonstrate that trapped-ion repeater-node technology can operate over existing city-scale fiber infrastructure and represent a step toward distributed quantum repeater networks.
A key enabling feature of future quantum networks is interoperability between platforms that operate at different wavelengths and with different qubit encodings. We demonstrate an interface that converts atom-photon entanglement from polarization encoding at an atomic wavelength to time-bin encoding in the telecom C-band. Atom-entangled photons at 854 nm are generated from a single 40^{40}40Ca+^++ ion. After quantum frequency conversion to 1550 nm, the photonic polarization qubit is converted into a time-bin qubit using a fiber-based Mach--Zehnder-like encoder. Full quantum tomography of the final state verifies that the process preserves entanglement with 96.3(4.2)% fidelity. Together with the independent work of Ferrari et al. [arXiv:2607.07805 (2026)], this is the first demonstration of polarization-to-time-bin conversion of photons entangled with a single atomic quantum memory. The telecom-compatible interface enables robust qubit transmission over optical fibers and provides a key building block for heterogeneous quantum networking architectures.
Entanglement between stationary quantum memories and photonic channels is the essential resource for future quantum networks. Together with entanglement distillation, it will enable efficient distribution of quantum states. We report on the generation and observation of entanglement between a ^{87}Rb atom and a photon at telecom wavelength transmitted through up to 20 km of optical fiber. For this purpose, we use polarization-preserving quantum frequency conversion to transform the wavelength of a photon entangled with the atomic spin state from 780 nm to the telecom S band at 1522 nm. We achieve an unprecedented external device conversion efficiency of 57% and observe an entanglement fidelity between the atom and telecom photon of ≥78.5±0.9% after transmission through 20 km of optical fiber, mainly limited by decoherence of the atomic state. This result is an important milestone on the road to distribute quantum information on a large scale.
Time-bin photonic qubits are well-suited for quantum network applications due to their robustness to polarization instability in fiber links and potential for heterogeneous networks. In this work, we implement the first entanglement-preserving polarization-to-time-bin conversion of a photon qubit in an entangled state with a matter qubit, together with the independent work of Haen \textit{et al.} \cite{haen2026telecom}. Photons initially generated with polarization encoding are converted to the time-bin basis through a polarization-discriminating asymmetric Mach--Zehnder interferometer. The photonic qubits are generated via the 109210921092~nm transition of a 88^{88}88Sr+^{+}+ ion. We measure state fidelity bounds of 0.906±0.011≤F≤0.934±0.0110.906 \pm 0.011 \le \mathcal{F} \le 0.934\pm 0.0110.906±0.011≤F≤0.934±0.011, with conversion error <0.028< 0.028<0.028, and find this fidelity shows no statistically significant reduction under depolarizing noise, even at full depolarization strength.