  Integrated photonics is an essential technology for optical quantum
computing. Universal, phase-stable, reconfigurable multimode interferometers
(quantum photonic processors) enable manipulation of photonic quantum states
and are one of the main components of photonic quantum computers in various
architectures. In this paper, we report the realization of the largest quantum
photonic processor to date. The processor enables arbitrary unitary
transformations on its 20 input modes with an amplitude fidelity of
$F_{\text{Haar}} = 97.4\%$ and $F_{\text{Perm}} = 99.5\%$ for Haar-random and
permutation matrices, respectively, an optical loss of 2.9 dB averaged over all
modes, and high-visibility quantum interference with $V_{\text{HOM}}=98\%$. The
processor is realized in $\mathrm{Si_3N_4}$ waveguides and is actively cooled
by a Peltier element.
