  This is an updated version of supplementary information to accompany "Quantum
supremacy using a programmable superconducting processor", an article published
in the October 24, 2019 issue of Nature. The main article is freely available
at https://www.nature.com/articles/s41586-019-1666-5. Summary of changes since
arXiv:1910.11333v1 (submitted 23 Oct 2019): added URL for qFlex source code;
added Erratum section; added Figure S41 comparing statistical and total
uncertainty for log and linear XEB; new References [1,65]; miscellaneous
updates for clarity and style consistency; miscellaneous typographical and
formatting corrections.
