QuEST is developed by Simon Benjamin’s Quantum Technology Theory Group (qtechtheory) and the e-Research center (oerc) at the University of Oxford. Development is currently led by Tyson Jones.
QuEST’s whitepaper was Scientific Reports’ 11th most downloaded Physics paper of 2019, and has collected over 100 citations.
QuEST is a full-state strong simulator, representing pure quantum states by statevectors, and mixed states by density matrices, with precise complex amplitudes in the Z-basis.
$$|\psi\rangle = \sum\limits_n^{2^\text{qubits}} \alpha_n |n\rangle \;\;\; \rightarrow \;\;\; \{ \; \alpha_1, \;\;\; \alpha_2, \;\;\; \dots, \;\;\; \alpha_{2^{\text{qubits}}} \; \} $$
$$ \rho = \sum\limits_i^{2^{\text{qubits}}}\sum\limits_j^{2^{\text{qubits}}} \beta_{ij} \;|i\rangle\langle j | \;\;\; \rightarrow \;\;\; \begin{pmatrix} \beta_{1,1} & \beta_{1,2} & \dots & \beta_{1,2^{\text{qubits}}} \\ \beta_{2,1} \\ \vdots & & \ddots\\ \beta_{2^{\text{qubits}},1} \end{pmatrix} $$
This means that every individual amplitude is precisely known, queryable, and modifiable during a computation. It also means quantities like inner products, probabilities, fidelities, and expectation values can be calculated to numerical precision. While the complexity of simulating a circuit is linear in the number of gates, it is exponential in the number of qubits. Using $2 \, b$ bytes for each floating-point complex amplitude (typically, $b = 8$ at double precision), the memory cost of a state-vector is
$$ b \; 2^{\text{qubits} – 29} \;\;\; \text{(GiB)} $$
These costs mean that changing from a modest 10 qubit to a 30 qubit simulation could in principle be over a million times slower – and a 40 qubit simulation would require 16 TiB of memory! Parallelisation techniques like multithreading, GPU acceleration and distribution are essential, but which to use depends on the simulation size. QuEST supports them all behind a seamless interface – the same simple code can run on laptops, desktops with GPUs, and networked supercomputers. Write once, run anywhere!
Website by Tyson Jones