20-Mode Universal Quantum Photonic Processor

Integrated photonics is an essential technol-ogy for optical quantum computing. Universal, phase-stable, reconfigurable multimode inter-ferometers (quantum photonic processors) en-able manipulation of photonic quantum states and are one of the main components of pho-tonic quantum computers in various architec-tures. In this paper, we report the realiza-tion of the largest quantum photonic proces-sor to date. The processor enables arbitrary unitary transformations on its 20 input modes with an amplitude fidelity of FHaar = 97.4% and FPerm = 99.5% for Haar-random and permu-tation matrices, respectively, an optical loss of 2.9 dB averaged over all modes, and high-visibility quantum interference with VHOM = 98%. The processor is realized in Si3N4 waveg-uides and is actively cooled by a Peltier ele-ment.