Introduction

qsv is a high-performance quantum statevector simulator written in Rust, built as a study in performance engineering. Off-the-shelf simulators already exist (qsim, Qiskit-Aer, QuEST, Yao.jl); qsv's purpose is not to be another one, but to demonstrate how one is optimized — cache-, SIMD-, and threading-aware design driven by profiling and benchmarked honestly against the established tools.

The thesis that organizes everything

Statevector simulation is memory-bandwidth-bound, not compute-bound.

Applying a 1-qubit gate streams the entire $2^N$-amplitude array while doing only ~2 complex multiplies per 16-byte amplitude — an arithmetic intensity of roughly 0.13 FLOP/byte, deep in the bandwidth-bound region of the roofline. Every optimization decision in qsv is justified by one question:

Does this reduce bytes moved per gate, or raise arithmetic intensity per byte moved?

This reframes "squeeze all the performance" as primarily a memory-traffic and cache problem. See How we optimize for the full argument and the evidence behind it.

What's here

• A tutorial for using qsv as a library.
• An architecture overview of the crate and its pluggable backends.
• How we optimize — the optimization strategy and milestone narrative (the centerpiece).
• The core kernel — the insert_zero_bit indexing trick every production simulator shares, explained from the ground up.
• Benchmarking & profiling — how we measure.
• Research notes — distilled findings from qsim, Qiskit-Aer, QuEST, Yao.jl, cuStateVec, spinoza, and Algorithms for Modern Hardware.

Status

Foundations and the first optimization milestones are in place: a Structure-of-Arrays statevector, the universal bit-shift kernel, a pluggable Backend trait with three implementations (a naive oracle plus two optimized backends), and a differential test suite that validates every kernel against the oracle. See the roadmap for the milestone-by-milestone plan.