A 24-mode laser-written universal photonic processor in a glass-based platform

Photonic integrated circuits are increasingly used in quantum communications, computing, and sensing. Among them, universal photonic processors (UPPs) are attracting a lot of attention from both the academic and industrial world for their ability to perform any unitary transformation on a quantum photonic state, akin to optical FPGAs. These processors employ meshes of programmable Mach-Zehnder interferometers and are fabricated on platforms like silicon nitride, silicon and glass using various techniques. One such technique, femtosecond laser writing (FLW), is versatile and cost-effective, producing waveguides in glass substrates with high quality and low losses over a wide wavelength range, a feature essential for quantum applications. FLW also allows for the manufacturing of microstructures, such as 2D and 3D void trenches, which improve thermal isolation and reduce power dissipation and thermal crosstalk in thermo-optic phase shifters. With this work, we introduce the first 24-mode FLW-UPP, the most complex UPP reported in the literature. The device is optimized for operation with quantum dot sources emitting at 925 nm and features average total insertion losses of only 4.35 dB, making it suitable for the most demanding multi-photon quantum experiments. Thanks to the realization of suspended waveguides, it operates efficiently with a total power consumption of less than 14W, requiring only a simple thermo-electric cooler for temperature regulation. Preliminary calibration indicates high reproducibility on the splitting ratio distribution of the directional couplers, with amplitude fidelity exceeding 98 % on average for Haar-random unitary transformations. These results highlight the potential of FLW-UPPs in advancing and scaling quantum photonic technologies.