  This paper presents ``Stim", a fast simulator for quantum stabilizer
circuits. The paper explains how Stim works and compares it to existing tools.
With no foreknowledge, Stim can analyze a distance 100 surface code circuit (20
thousand qubits, 8 million gates, 1 million measurements) in 15 seconds and
then begin sampling full circuit shots at a rate of 1 kHz. Stim uses a
stabilizer tableau representation, similar to Aaronson and Gottesman's CHP
simulator, but with three main improvements. First, Stim improves the
asymptotic complexity of deterministic measurement from quadratic to linear by
tracking the {\em inverse} of the circuit's stabilizer tableau. Second, Stim
improves the constant factors of the algorithm by using a cache-friendly data
layout and 256 bit wide SIMD instructions. Third, Stim only uses expensive
stabilizer tableau simulation to create an initial reference sample. Further
samples are collected in bulk by using that sample as a reference for batches
of Pauli frames propagating through the circuit.
