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A new laboratory device converts 393-nanometer ultraviolet photons into light in the telecom C band, a range better suited to travel through optical fiber. In a paper published in Physical Review Letters on October 13, 2026, researchers report 28.8% external photon-conversion efficiency and background noise of 35 counts per second. The result could help connect ultraviolet-emitting quantum systems to fiber links, but it is not a demonstration of a deployed or long-distance quantum network.
Why do quantum networks need to change a photon’s wavelength?
Quantum networks link stationary quantum systems by sending photons between them. Some systems, including certain trapped-ion platforms, emit ultraviolet or blue photons. Those wavelengths are poorly suited to long-distance transmission through standard optical fiber compared with telecom wavelengths. As co-author Qiang Zhang put it, “For those wavelength bands, fiber attenuation is severe.”
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsQuantum frequency conversion shifts a photon’s frequency toward a more fiber-friendly wavelength while aiming to preserve its quantum information. The new device targets the gap between 393 nm ultraviolet light and the telecom C band. If such conversion can be integrated reliably with quantum emitters and network equipment, it could make photons from those emitters more practical to send through fiber. The paper establishes a converter result, not the performance of a complete network.
What did the researchers build?
Yi Yang and colleagues report a single-stage quantum frequency converter made with a periodically poled, thin-film lithium-niobate waveguide. The waveguide’s patterned structure supports the nonlinear optical process needed to convert 393 nm photons to the telecom C band. The authors also analyze phase matching in the presence of domain defects and describe a noise-control approach based on differential tuning of difference-frequency generation and spontaneous parametric down-conversion.
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The paper reports three figures that describe different aspects of the device:
- 839%/(W cm²) normalized conversion efficiency: reported for the fundamental guided mode. This is a normalized efficiency metric, not the external efficiency of the complete conversion setup.
- 28.8% external photon-conversion efficiency: the reported external efficiency for conversion from 393 nm to the telecom C band.
- 35 counts per second background noise: the reported noise level for the converter.
These numbers should not be treated as interchangeable. In particular, the much larger normalized-efficiency figure does not mean that 839% of input photons were converted; it is defined with power and device length in the denominator. For a practical system, external efficiency and noise both matter because conversion losses reduce the number of usable photons, while background counts can make genuine quantum signals harder to distinguish.
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Did the team connect the converter to a quantum system?
Phys.org reports that the researchers interfaced the optimized module with a 40Ca+ trapped-ion system and operated at very low excitation rates. That is an experimental interface between the converter and a quantum emitter, an important step beyond characterizing a converter in isolation. It does not show that the device has carried quantum information over a long-distance fiber link or operated as part of a deployed network.
How does this result compare with earlier converters?
A 2024 TU Delft repository record describes a different visible-to-telecom converter used with photons from nitrogen-vacancy-center qubits. Its abstract reports 43% external efficiency and noise of 2 s−1/pm (17 s−1/GHz). Those figures do not by themselves show that the earlier device performs better than the 2026 ultraviolet converter: the source platforms and wavelength ranges differ, the converter implementations are different, and the noise figures are reported with bandwidth units rather than as the same unqualified counts-per-second quantity.
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A meaningful comparison would need to align input and output wavelengths, quantum-emitter platform, external-efficiency definition, and noise rate over a clearly specified bandwidth. It should also distinguish an isolated converter characterization from a test that interfaces the converter to a quantum system. The 2026 result’s significance is its particular ultraviolet-to-telecom conversion and reported low background level, not a proven across-the-board performance lead.
Separately, a 2025 Physical Review A paper discusses time-bin-entanglement-based frequency conversion as a possible way to connect quantum networks, including a role for entanglement swapping. That work provides broader network context; it is not the same converter or experiment as Yang and colleagues’ 2026 device.
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What does the converter mean for quantum communications?
The work addresses a concrete compatibility problem: a quantum emitter may produce photons at wavelengths that do not travel well through fiber, while network links favor telecom wavelengths. A waveguide-based converter that can shift those photons with limited added noise could become one component in a future link architecture.
For now, the evidence supports a laboratory demonstration with an experimental trapped-ion interface. It does not establish end-to-end communication distance, network reliability, or deployment readiness. Those system-level results cannot be inferred from conversion efficiency and background noise alone.
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Sources
- Yi Yang et al., “High-Performance Quantum Frequency Conversion from Ultraviolet to Telecom Band,” Physical Review Letters, October 13, 2026.
- Phys.org, “New converter could bridge light wavelength gap in quantum communications,” October 27, 2026.
- TU Delft Repository, “Low-noise short-wavelength pumped frequency downconversion for quantum frequency converters,” 2024.
- “Multiuser quantum communication network via time-bin-entanglement-based frequency conversion and Bell-state measurement,” Physical Review A, May 12, 2025.
Frequently Asked Questions
What is the telecom C band used for?
It is a wavelength range used in optical communications, where fiber links can carry light more effectively than at the ultraviolet wavelengths emitted by some quantum systems. The converter shifts photons toward that range; the report does not specify a particular fiber-network deployment.
Does frequency conversion preserve a photon’s quantum state?
Preserving the encoded quantum information is the goal of quantum frequency conversion, but the reported efficiency and noise figures alone do not establish every state-preservation or network-level measure. Those require additional characterization beyond the figures summarized here.
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Could this device be used with other trapped-ion wavelengths?
The reported device is specifically described as converting 393 nm photons to the telecom C band. The available evidence does not establish conversion performance for other ion transitions or input wavelengths.
