qsim and qsimh are a collection of C++ libraries for quantum circuit simulation. These libraries provide powerful, low-cost tools for researchers to test quantum algorithms before running on quantum hardware.
qsim makes use of AVX/FMA vector operations, OpenMP multithreading, and gate fusion   to accelerate simulations. This performance is best demonstrated by the use of qsim in cross-entropy benchmarks here: .
This repository includes two top-level libraries for simulation:
- qsim is a Schrödinger state-vector simulator designed to run on a single machine. It produces the full state vector as output which, for instance, allows users to sample repeatedly from a single execution.
- qsimh is a hybrid Schrödinger-Feynman simulator built for parallel execution on a cluster of machines. It produces amplitudes for user- specified output bitstrings.
These libraries can be invoked either directly or through the qsim-Cirq interface to perform the following operations:
- Determine the final state vector of a circuit (qsim only).
- Sample results from a circuit. Multiple samples can be generated with minimal additional cost for circuits with no intermediate measurements (qsim only).
- Calculate amplitudes for user-specified result bitstrings. With qsimh, this is trivially parallelizable across several machines.
Circuits of up to 30 qubits can be simulated in qsim with ~16GB of RAM; each additional qubit doubles the RAM requirement. In contrast, careful use of qsimh can support 50 qubits or more.
 M. Smelyanskiy, N. P. Sawaya, A. Aspuru-Guzik, "qHiPSTER: The Quantum High Performance Software Testing Environment", arXiv:1601.07195 (2016).
 T. Häner, D. S. Steiger, "0.5 Petabyte Simulation of a 45-Qubit Quantum Circuit", arXiv:1704.01127 (2017).
, F. Arute et al, "Quantum Supremacy Using a Programmable Superconducting Processor", Nature 574, 505, (2019).