  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 for its first sample. Further samples are
collected by using the first sample as a baseline for vectorized batches of
Pauli frames propagating through the circuit.
