Quantum Frequency Conversion for Future Quantum Networks
Unprecedented quantum frequency conversion for future quantum networks by scientists
An international research team demonstrated a highly efficient and compact quantum frequency converter that can bridge the gap between high-performance quantum emitters and conventional telecommunications infrastructure, advancing the "quantum internet" concept. The study, led by Mathis Cohen and Université Côte d'Azur, Quandela SAS, and Université Paris-Saclay, describes a system that converts single photons from the near-infrared (NIR) spectrum to the telecommunication C-band with unparalleled end-to-end efficiency and quantum integrity preservation.
Challenge of Wavelength Incompatibility
The main obstacle to long-distance quantum resource distribution is wavelength mismatch. Current high-performance deterministic single-photon emitters, such as those based on semiconductor quantum dots (QDs), atoms, or color centers, operate in the near-infrared range (780–950 nm). However, typical silicon optical fibers lose signal when transmitting these wavelengths long distances.
The telecommunication C-band (about 1550 nm) has the lowest absorption in contemporary fiber networks and is the “natural flying qubit carrier” for long-distance communication. Quantum Frequency Conversion (QFC) adjusts a photonic state's wavelength while preserving its quantum properties, such as unicity and indistinguishability.
An Innovative Efficiency Standard
Researchers use a fiber-pigtailed semiconductor quantum dot single-photon source and a fiber-coupled nonlinear optical Lithium Niobate waveguide in their coherent frequency converter approach. This integrated system converted 925.7-nm photons to 1560-nm.
The team achieved 48.4% end-to-end efficiency. Because quantum information processing is susceptible to photon loss, this measure is crucial. The device maintained an in-fiber single-photon rate of 2.8 MHz (3.7% brightness) at the quantum frequency conversion stage's input, resulting in 1.3 MHz after maximum efficiency processing. A commercial fiber-pigtailed source sets a new photon rate record at the end of a quantum frequency conversion system.
Quantum Fidelity Maintenance
Efficiency is lost if quantum information is confused during conversion. The team prioritized indistinguishability (ensuring photons are the same in all physical modes) and unicity (releasing only one photon at a time).
Using HBT interferometry, researchers assessed the second-order autocorrelation coefficient (g(2)(0)). Single-photon purity was nearly 100%, with values of 0.044 before conversion and 0.051 after.
Hong-Ou-Mandel (HOM) interference tests also assessed indistinguishability. Conversion raised system corrected indistinguishability from 79.3% to 80.0%. These results demonstrate that the interface maintains coherence and that quantum dot photons are not degraded during conversion.
Designed for Real Life
This new system is fiber-integrated, unlike many previous demonstrations that used massive, “free-space” optical structures, which are vibration-sensitive and difficult to scale. For this wavelength shift, the conversion step uses a 4 cm periodically poled lithium niobate waveguide (PPLN/WG).
A 400 by 400 mm breadboard houses the pump preparation, conversion, and filtering stages, making the layout extremely small. The system is portable and can be used in real-world quantum network applications and out-of-lab deployment.
Another significant quality is spectral tunability. A configurable pump laser and crystal temperature allowed the researchers to tailor the conversion process over a 10 nm range. This aligns telecom wavelengths from different quantum dots, which is necessary in actual quantum networks to synchronize many sources.
Noise Reduction and Technical Precision
The conversion technique uses difference-frequency generation (DFG) powered by a strong 2272 nm continuous-wave pump laser. Noise—especially Raman scattering and residual pump light—is a major technical challenge in such systems. A filtering stage was built using four telecom dense-wavelength demultiplexers (DWDM) to address this. The system had excellent noise reduction with an SNR above 400 in the ideal working range.
Future Impact
This work enables several cutting-edge quantum technologies:
Quantum Key Distribution (QKD): Single-photon qubit carriers offer secure communication channels.
Quantum Repeaters: These devices can teleport quantum states over long distances, transcending laboratory limits.
Distributed Quantum Computing: Smoothly connecting quantum information processing units at different wavelengths.
Even if the researchers want to reduce filtering losses and improve fiber pigtailing, the existing technology achieves quantum frequency conversion excellence.
