  Classical simulations are essential for the development of quantum computing,
and their exponential scaling can easily fill any modern supercomputer. In this
paper we consider the performance and energy consumption of large Quantum
Fourier Transform (QFT) simulations run on ARCHER2, the UK's National
Supercomputing Service, with QuEST toolkit. We take into account CPU clock
frequency and node memory size, and use cache-blocking to rearrange the
circuit, which minimises communications. We find that using 2.00GHz instead of
2.25GHz can save as much as 25% of energy at 5% increase in runtime. Higher
node memory also has the potential to be more efficient, and cost the user
fewer CUs, but at higher runtime penalty. Finally, we present a cache-blocking
QFT circuit, which halves the required communication. All our optimisations
combined result in 40% faster simulations and 35% energy savings in 44 qubit
simulations on 4,096 ARCHER2 nodes.
