In the global race to build a quantum internet, photons are the main “currency” for information transit. However, creating these photons in a high-purity condition is crucial for future quantum technology, which will enable incredibly secure communication and vastly faster computers.

Pavia University and CSEM researchers produced “frequency-degenerate” photon pairs with unprecedented clarity. This breakthrough reduces noise by 10,000, overcoming a critical bottleneck that has long limited integrated quantum photonics' scalability.

The “Identical Twin” Photon Challenge

Two photons must be indistinguishable for quantum logic gates and QKD to work. To disrupt a quantum process, these particles must be “frequency-degenerate,” or nearly similar in colour and timing.

Spontaneous Parametric Down-Conversion (SPDC) splits a high-energy “pump” photon pair into two lower-energy “daughter” photons, the signal and the idler. The pump photon must be double the intended output frequency to ensure frequency-degenerate offspring.

This typical technique faces technological challenges from “parasitic” nonlinear effects caused by the high-energy pump light needed for operation. In a sea of classical light, these unwanted outcomes produce noise photons that are indistinguishable from the quantum signal, “drowning out” quantum information.

Quantum Noise “Cascaded” Solution

Olivia Hefti, Marco Clementi, and Enrico Melani's dual-pump technique overcomes single-pump problems. Instead of a high-energy source, the team cascaded inside a waveguide structure.

Sum-Frequency Generation (SFG) initially creates a higher-energy internal state by merging two lower-energy pump photons. SPDC instantly decays this internal state into the desired entangled photon pair. The initial pumps and final photons have different frequencies, so this two-step process is necessary. Consequently, the quantum signal is physically insulated from pump-generated “noise” or unwanted signals.

This invention claims to reduce noise by 40 dB compared to existing methods. The 10,000-times clearer signal produces pure photon pairs.

Why Thin-Film Lithium Niobate?

Thin-Film Lithium Niobate (TFLN) makes this gadget successful. Lithium niobite has long been used in telecommunications because to its remarkable nonlinear properties, which allow light to interact with itself. When researchers utilise thin-film waveguides to restrict light in extremely small spaces, these interactions are far more efficient.

The TFLN approach reduces noise and improves brightness, while other researchers have used spontaneous four-wave mixing on silicon nitride circuits to synthesise photon pairs at 1200 per milliwatt. The TFLN device achieved a brightness of 1.0×10 5 Hz per nanometre per square milliwatt. Scalability requires great efficiency since it generates enormous volumes of entanglement with low power consumption, which is needed to assembly hundreds of these sources on a CPU.

Integration with telecom infrastructure

Interoperability with fiber-optic infrastructure is a major benefit of this innovation. Designed to work in the "telecom band" at 1550 nanometres, the same frequency as worldwide internet data transport.

Producing frequency-degenerate photon pairs directly in this region eliminates complex and “lossy” frequency conversion processes. The system is “plug-and-play” for future quantum networks that must send quantum data via thousands of miles of subsurface cables.

The group has also simplified quantum source design by encapsulating the generation process from pumping to collecting in a waveguide structure, using mass-producible integrated photonic circuits instead of massive labs.

Towards Scalable Quantum Systems