  Quantum computing experiments are moving into a new realm of increasing size
and complexity, with the short-term goal of demonstrating an advantage over
classical computers. Boson sampling is a promising platform for such a goal,
however, the number of involved single photons was up to five so far, limiting
these small-scale implementations to a proof-of-principle stage. Here, we
develop solid-state sources of highly efficient, pure and indistinguishable
single photons, and 3D integration of ultra-low-loss optical circuits. We
perform an experiment with 20 single photons fed into a 60-mode interferometer,
and, in its output, sample over Hilbert spaces with a size of $10^{14}$ $-$over
ten orders of magnitude larger than all previous experiments. The results are
validated against distinguishable samplers and uniform samplers with a
confidence level of 99.9%.
