Quantum Frequency Conversion in Quantum Networks

Quantum frequency conversion allows quantum states encoded in photons at one wavelength to be coherently translated to another without degrading their non-classical properties. In emerging quantum networks, disparate nodes—such as trapped ions, solid-state spins and quantum dots—often operate at distinct optical frequencies, which hampers direct photonic interconnection. By employing nonlinear optical processes, including sum-frequency generation and four-wave mixing, quantum frequency conversion bridges these spectral gaps, enabling efficient coupling to low-loss telecommunication-band fibres. This spectral interface is pivotal for extending entanglement distribution over metropolitan and intercontinental scales, for implementing quantum repeaters, and for integrating memories with photonic channels. Advances in material platforms such as silicon-vacancy centres in diamond, lithium niobate waveguides and photonic crystal fibres have enhanced conversion efficiencies, reduced noise, and preserved coherence. These developments underpin the global drive towards scalable, heterogeneous quantum networks capable of synchronising quantum information across long distances for applications in secure communication, distributed quantum computing and precision metrology.

Research from Nature Portfolio

Recent studies have demonstrated bidirectional quantum frequency conversion of photonic qubits emitted by silicon-vacancy centres in nanophotonic diamond cavities to the telecommunication O-band, enabling entanglement between remote nuclear spin memories over tens of kilometres of deployed fibre. The conversion process preserved high fidelity and heralded storage times of seconds, marking a significant advance towards practical quantum repeaters. Complementing this, nonlinear sum-frequency generation has been harnessed to upconvert one photon of an entangled pair carrying high-dimensional spatial information, establishing a quantum channel for arbitrary photonic spatial modes. This approach merges structured light control with quantum processes and may be extended to other degrees of freedom. Additionally, long-distance entanglement distribution between trapped atomic nodes has been realised over more than 30 km of fibre by converting near-infrared photons to the telecom band with polarisation-preserving converters, guaranteeing heralded entanglement over urban and metropolitan links and laying the groundwork for device-independent quantum key distribution and repeater protocols.

Research from all publishers

In non-Nature outlets, low-noise, bidirectional frequency conversion has been demonstrated between visible-band single photons from a silicon-vacancy quantum memory and the telecom C-band, achieving sub-0.1 noise and high indistinguishability, and preserving entanglement fidelity across 50 km of deployed fibre. A complementary route exploits Bragg-scattering four-wave mixing in doped photonic crystal fibre engineered for dispersion matching between 920 nm and 1550 nm, achieving internal conversion efficiencies approaching 80 % and enabling interfacing of InAs quantum dot sources with telecom infrastructure. These platforms highlight the versatility of fibre-based nonlinear conversion for scalable, room-temperature quantum network components, supporting both wavelength translation and bandwidth manipulation to harmonise disparate quantum emitters with existing optical communication channels.

Quantum Frequency Conversion in Quantum Networks publication trend

The graph below shows the total number of articles in quantum frequency conversion in quantum networks across all publications each year (not limited to Nature Index journals).

Technical terms

Quantum frequency conversion (QFC): A nonlinear optical process that coherently shifts the frequency of a single photon while preserving its quantum state.

Quantum network: An interconnected set of quantum nodes that share and process quantum information via photonic channels.

Quantum memory: A device that stores quantum states of light, typically based on atomic, ionic or solid-state systems.

Sum-frequency generation: A second-order nonlinear process combining two photons into one at their sum frequency.

Four-wave mixing: A third-order nonlinear interaction in which two photons exchange energy to generate two new photons at different frequencies.

Telecommunication band: The spectral region around 1,300–1,600 nm with minimal loss in optical fibres, used for long-distance data transmission.

References

1. Entanglement of nanophotonic quantum memory nodes in a telecom network. Nature (2024).
2. Quantum transport of high-dimensional spatial information with a nonlinear detector. Nature Communications (2023).
3. Entangling single atoms over 33 km telecom fibre. Nature (2022).
4. Telecom Networking with a Diamond Quantum Memory. PRX Quantum (2024).
5. Tunable frequency conversion in doped photonic crystal fiber pumped near degeneracy. Optica (2024).