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Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC)

A Municipal Quantum Network Link with Entangled Infrared-Photons Emitted from Strontium Ions

Norbert M. Linke

Published Jun 23, 2025• 0 citations

Abstract

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).

DOI: 10.1109/cleo/europe-eqec65582.2025.11109938

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Kilometer-Scale Ion-Photon Entanglement with a Metastable

^{88}

Sr

^{+}

Qubit

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.

Trapped ions are one of the leading candidates for scalable and long-distance quantum networks because of their long qubit coherence time, high-fidelity single- and two-qubit gates, and their ability to generate photons entangled with the qubit state of the ion. One method for creating ion-photon entanglement is to exploit optical transitions from the 6 P 1/2 to 6 S 1/2 levels, which naturally emit spin-photon entangled states. But these optical transitions typically lie in the ultraviolet and visible wavelength regions of the spectrum. These wavelengths exhibit significant fiber-optic propagation loss, thereby limiting the transfer of quantum information to tens of meters. Quantum frequency conversion is essential to convert these photons to telecom wavelengths so that they can propagate over long distances in fiber-based networks, as well as for compatibility with the vast number of telecom-based optoelectronic components. Here, we generate O-band telecom photons via a low-noise quantum frequency conversion scheme from photons emitted from the 6 P 1/2 to 6 S 1/2 dipole transition of a trapped barium ion. We use a two-stage quantum frequency conversion scheme to achieve a frequency shift of 375.4 THz between the input visible photon and the output telecom photon, achieving a conversion efficiency of 11%. We attain a signal-to-background ratio of over 100 for the converted O-band telecom photon with background noise of less than 15 counts/s. These results are an important step toward achieving trapped ion quantum networks over long distances for distributed quantum computing and quantum communication.

Fiber-based quantum networks require photons at telecommunications wavelengths to interconnect qubits separated by long distances. Trapped ions are leading candidates for quantum networking with high-fidelity two-qubit gates, long coherence times, and the ability to readily emit photons entangled with the ion's internal qubit states. However, trapped ions typically emit photons at wavelengths incompatible with telecommunications fiber. Here, we demonstrate frequency conversion of visible photons, emitted from the S–P dipole transition of a trapped Ba+ ion into the telecommunications C-band. These results are an important step toward enabling a long-distance trapped ion quantum internet.

Future quantum networks will require the ability to produce matter-photon entanglement at photon frequencies not naturally emitted from the matter qubit. This allows for a hybrid network architecture, where these photons can couple to other tools and quantum technologies useful for tasks such as multiplexing, routing, and storage, but which operate at wavelengths different from that of the matter qubit source, while also reducing network losses. Here, we demonstrate entanglement between a trapped ion and a 780-nm photon, a wavelength that can interact with neutral-Rb-based quantum networking devices. A single barium ion is used to produce 493-nm photons, entangled with the ion, which are then frequency converted to 780 nm while preserving the entanglement. We generate ion-photon entanglement with fidelities $\ensuremath{\ge}0.93$(2) and $\ensuremath{\ge}0.84$(2) for 493-nm and 780-nm photons respectively with the fidelity drop arising predominantly from a reduction in the signal-noise of our detectors at 780 nm compared with at 493 nm.

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.

Long-range quantum information processing will require the integration of different technologies to form hybrid architectures combining the strengths of multiple quantum systems. In this work, we propose a hybrid networking architecture designed to improve entanglement rates in quantum networks based on trapped ions. Trapped ions are excellent candidates as network nodes, but photon losses make long-distance networking difficult. To overcome some losses and extend the range of trapped-ion-based networks, we propose including neutral-atom-based nondestructive single-photon detection and single-photon storage in between networking nodes, forming a hybrid network. This work builds on recently demonstrated optical frequency conversion of single photons emitted by trapped ions. We derive the average two-node entanglement rate for this proposed network architecture as a function of distance. Using reasonable experimental parameters, we show that this proposed quantum network can generate remote entanglement rates up to a factor of 100 larger than that of an equivalent homogeneous network at both near-IR and C-band wavelengths for distances up to 50 km.