The Quantum Exact Simulation Toolkit is a high performance simulator of quantum circuits, state-vectors and density matrices. QuEST uses multithreading, GPU acceleration and distribution to run lightning first on laptops, desktops and networked supercomputers. QuEST just works; it is stand-alone, requires no installation, and is trivial to compile and run. QuEST hybridises OpenMP and MPI with huge compiler support to run on all sorts of multicore, multi-CPU and distributed hardware, uses HIP to run on AMD GPUs, integrates cuQuantum and Thrust for cutting-edge performance on modern NVIDIA GPUs, and has a custom kernel backend to run on older CUDA-compatible GPUs. And it hides these deployment modes behind a single, seamless interface.

QuEST is developed by the QTechTheory group at the University of Oxford, and these authors. To learn more:

see the tutorial
view the documentation
visit the website
see some examples
read the whitepaper, which featured in Scientific Report's Top 100 in Physics :trophy:

:tada: Introduction

QuEST has a simple interface, which is agnostic to its runtime environment, between CPUs, GPUs and over networks.

hadamard(qubits, 0);
 
controlledRotateX(qubits, 0, 1, angle);
 
double prob = calcProbOfOutcome(qubits, 0, outcome);

Yet, it is flexible

Vector v;
v.x = 1; v.y = .5; v.z = 0;
rotateAroundAxis(qubits, 0, angle, v);
 
ComplexMatrix2 u = {
    .real = {{.5, .5}, { .5,.5}},
    .imag = {{.5,-.5}, {-.5,.5}}};
unitary(qubits, 0, u);
 
mixDepolarising(qubits, 0, prob);

and extremely powerful

ComplexMatrixN u = createComplexMatrixN(5);
int ctrls[] = {0, 1, 2};
int targs[] = {5, 20, 15, 10, 25};
multiControlledMultiQubitUnitary(qubits, ctrls, 3, targs, 5, u);
 
ComplexMatrixN k1, k2, k3 = ...
mixMultiQubitKrausMap(qubits, targs, 5, {k1, k2, k3}, 3);
 
double val = calcExpecPauliHamil(qubits, hamiltonian, workspace);
 
applyTrotterCircuit(qubits, hamiltonian, time, order, repetitions);

:white_check_mark: Features

QuEST supports:

:ballot_box_with_check: density matrices for precise simulation of noisy quantum computers
:ballot_box_with_check: general unitaries with any number of control and target qubits
:ballot_box_with_check: general decoherence channels of any dimension
:ballot_box_with_check: general Hermitian operators in the Pauli basis
:ballot_box_with_check: many many operators, including even Pauli gadgets, analytic phase functions and Trotter circuits
:ballot_box_with_check: many tools to analyse quantum states, such as calculations of probability, fidelity, and expected value
:ballot_box_with_check: variable precision through a qreal numerical type which can use single, double or quad precision
:ballot_box_with_check: QASM output to verify simulated circuits
:ballot_box_with_check: direct access to amplitudes for rapid custom modification of the quantum state
:ballot_box_with_check: native compilation on MacOS, Linux and Windows, through Clang, GNU, Intel, and MSVC compilers

:book: Documentation

The tutorial includes instructions for
- compiling QuEST
- running QuEST locally and on supercomputers
- testing QuEST using the comprehensive unit tests
The documentation is divided into the following modules (collated here)
Additional utilities for debugging and testing are documented below

‍For developers: QuEST's doc is automatically regenerated when the master branch is updated via Github Actions. To locally regenerate the doc, run doxygen doxyconfig/config in the root directory, which generates html documentation in Doxygen_doc/html.

</blockquote>

:rocket: Getting started

To rocket right in, download QuEST with git at the terminal

git clone https://github.com/quest-kit/QuEST.git

cd QuEST

Compile the tutorial example (source) using cmake and make

mkdir build
cd build
cmake ..
make

then run it with

./demo

‍Windows users should install Build Tools for Visual Studio, and CMake, and run the above commmands in the Developer Command Prompt for VS, though using build commands
cmake .. -G "NMake Makefiles"

nmake

If using MSVC and NMake in this way fails, users can forego GPU acceleration, download MinGW-w64, and compile via
cmake .. -G "MinGW Makefiles"

make

:heart: Acknowledgements

We sincerely thank the following external contributors to QuEST.

Jakub Adamski for optimising distributed communication of max-size messages.
Bruno Villasenor Alvarez of AMD for porting the GPU backend to HIP, for compatibility with AMD GPUs.
HQS Quantum simulations for contributing mixDamping on CPU.
Kshitij Chhabra for patching some validation bugs.
Drew Silcock for patching the multithreaded build on MacOS.
Zach van Rijn for patching the multithreading code for GCC-9 OpenMP-5 compatibility.
SchineCompton for patching the GPU CMake release build.
Christopher J. Anders for patching the multithreading (when default off) and GPU builds (revising min cmake).
Gleb Struchalin for patching the cmake standalone build.
Milos Prokop for serial prototyping of initDiagonalOpFromPauliHamil.

QuEST uses the mt19937ar Mersenne Twister algorithm for random number generation, under the BSD licence. QuEST optionally (by additionally importing QuEST_complex.h) integrates the language agnostic complex type by Randy Meyers and Dr. Thomas Plum

:newspaper: Related projects

QuESTlink
a Mathematica package enabling symbolic circuit manipulation, analytic simulation, visualisation and high performance simulation with remote accelerated hardware.
pyQuEST
a python interface to QuEST, based on Cython, developed within the QTechTheory group. Please note, pyQuEST is currently in the alpha stage.
PyQuEST-cffi
a python interface to QuEST based on cffi developed by HQS Quantum Simulations. Please note, PyQuEST-cffi is currently in the alpha stage and not an official QuEST project.