Ultracompact 3D integrated photonic chip for high-fidelity high-dimensional quantum gates

Spatial modes of photons offer a rich encoding resource for high-dimensional quantum information processing. Multiplane light conversion (MPLC) enables spatial mode transformation and is applicable in both classical and quantum optics. Here, we demonstrate a polymer-based MPLC device, fabricated via femtosecond laser three-dimensional printing, that realizes high-dimensional quantum logic gates in an ultracompact format. Specifically, we design a three-dimensional Hadamard gate by training a diffractive neural network to generate the required phase distribution. The device is evaluated through quantum process tomography at the single-photon level within spatial modes, achieving a fidelity of 90%. Compared to traditional spatial light modulator-based implementations, our approach integrates spatial mode manipulation into a miniaturized photonic platform. These results highlight the feasibility of polymer-based MPLC for compact quantum logic and open possibilities for scalable, high-dimensional quantum information processing on integrated photonic chips.