Bidirectional Quantum Frequency Conversion enables the Transmission of Atom-Photon Entanglement over a 24-Kilometer Fiber Link –
Long-distance Quantum Networks offer enormous potential for the communication and processing of Quantum Information. By enabling the distribution of entanglement, they provide the basis for numerous Quantum Information Applications, including efficient Quantum Communication with Quantum Repeaters, scalable Quantum Computing by linking remote Quantum Processors into larger units, and Quantum Sensing. Entanglement distribution is therefore a central goal of Quantum Networks, as it enables the transmission of Quantum Information over long distances. While communication over long distances can make use of existing fiber-optic infrastructure, photons in the telecommunications band are required, as transmission losses are lowest at these wavelengths. However, modern Quantum Network Nodes often do not operate at these wavelengths, making Quantum Frequency Conversion necessary. This process converts the wavelength of photons from the visible or near-infrared range to the telecommunications range without altering their Quantum State. Unidirectional conversion is sufficient for implementations in which entanglement swapping takes place at an additional central node. For universally applicable Quantum Networks, however, a bidirectional solution is required. High-precision and efficient bidirectional Quantum Frequency Conversion for Quantum Communication over long distances remains technologically challenging. Researchers from the QR.N consortium at the Garching and Munich sites have now developed a new method to overcome this limitation. The results of their experiment were published in Physical Review Letters at the end of August 2026.
In the paper, the researchers describe a new method using two customized, low-noise frequency converters to transmit entanglement between a single atom and a photon in the near-visible spectral range over a 24-kilometer commercially deployed fiber link in the metropolitan area between Munich and Garching. Single photons offer numerous advantages for Quantum Networks, including long coherence times and the ability to selectively transmit and process Quantum States, making them a promising platform for Quantum Nodes. In the experiment, the researchers used bidirectional frequency conversion to interface the atomic qubit of a rubidium-87 atom, which emits at a wavelength of 780 nm, with telecommunications infrastructure operating at 1514 nm and subsequently convert it back to 780 nm. This resulted in a photon transmission efficiency of 1.7%, while the entanglement fidelity between the atom and photon was reduced by less than 1%. Through careful selection of the telecommunications wavelength, the use of narrowband filters, and thorough optimization of the conversion process, the researchers were able to ensure a high signal-to-noise ratio. As a result, they achieved an entanglement fidelity of F > 85% between the atom and photon at both wavelengths. Compared with the generated entangled state at a transmission distance of 0 kilometers, this corresponds to only a slight decrease.
The results represent an important step toward integrating atomic Quantum Nodes into existing fiber-optic networks and using such systems in Quantum Information Processing Protocols and free-space applications for long-distance Quantum Communication. In the longer term, they could pave the way for novel applications in Quantum Information Processing.
Source reference: https://journals.aps.org/prl/abstract/10.1103/94hz-xtht
