On-chip photon-pair sources in thin-film lithium niobate

Short answer: Photon pairs made on a lithium niobate chip are a building block for integrated quantum photonics. Two 2024 and 2025 Light: Science and Applications papers report a layer-poled waveguide with a normalized brightness of 3.1 x 10^6 Hz per nm per mW squared, and a reconfigurable circuit with two sources that reaches 99.0 percent interference visibility and Bell-state fidelities above 90 percent. These are laboratory devices.

Why an integrated photon-pair source?

The 2024 abstract says integrated photon-pair sources are crucial for scalable photonic quantum systems, and that thin-film lithium niobate is a promising platform for on-chip photon-pair generation by spontaneous parametric down-conversion (SPDC). The authors say the devices face practical challenges: periodically poled lithium niobate offers flexible quasi-phase matching but suffers from poor fabrication reliability and repeatability, while conventional modal phase matching gives limited efficiency because of inadequate mode overlap.

What did the layer-poled waveguide show?

The authors introduce a layer-poled lithium niobate nanophotonic waveguide. It uses layer-wise polarity inversion through electrical poling to break spatial symmetry and strengthen the nonlinear interaction for modal phase matching. They report a normalized second-harmonic conversion efficiency of 4615 percent per watt per square centimetre. Through a cascaded second-harmonic and SPDC process, they demonstrate photon-pair generation with a normalized brightness of 3.1 x 10^6 Hz per nm per mW squared in a 3.3 mm waveguide, which they say surpasses existing on-chip sources under similar operating configurations. They also say the waveguides have enhanced fabrication reliability and reduced sensitivity to geometric variations and temperature fluctuations compared with periodically poled devices. They expect the approach to become a promising solution for on-chip wavelength conversion and non-classical light generation, which is an expectation.

What did the 2025 reconfigurable circuit show?

The 2025 abstract begins by noting that photon pairs from nonlinear optical processes are excellent for preparing entangled states because of their long coherence times and compatibility with fibre networks, and that lithium niobate on insulator has become a leading platform for integrating sources into optical circuits. The authors present a reconfigurable photonic integrated circuit on this platform that combines two on-chip photon-pair sources with programmable interferometers. The sources reach a brightness of 26 MHz per nm per mW with a coincidence-to-accidental ratio above 100. They interfere the two sources with 99.0 plus or minus 0.7 percent visibility, which they say shows the indistinguishability needed to produce entanglement on-chip. They show preparation of any of the maximally entangled Bell states with fidelity above 90 percent, verified by quantum state tomography. They conclude that the platform is compelling and scalable for integrated quantum photonics, which is their view.

What about the third source?

An APL Photonics article on photon-pair generation using inverse-designed thin-film lithium niobate mode converters was also listed. Its abstract could not be read, so nothing is taken from it beyond its title.

How to read this

The two papers report brightness in different normalized units, so their numbers should not be compared directly. Brightness, visibility and fidelity measure different qualities of a source. This post is based on abstracts, and full texts were not checked. It makes no claim about products. For the broader platform, see our post on lithium niobate electro-optic quantum photonics.

FAQ

What is SPDC?

A nonlinear process in which one photon splits into a correlated pair, used in both papers.

How bright was the layer-poled source?

3.1 x 10^6 Hz per nm per mW squared, per the 2024 abstract.

How good was the on-chip interference?

99.0 plus or minus 0.7 percent visibility between two sources.

What Bell-state fidelity was reported?

Above 90 percent, verified by tomography.