GPU numerical accuracy: compensated sums, norm-aware results, optional f64

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Evaluación

Dificultad
4/5
Tiempo estimado
3-5 días
Aptitud para principiantes
52/100
Tipo de issue
Error
Claridad
Bastante claro
Estado de actividad
Activo
Stack tecnológico
rust
Área
backend, performance

Línea de trabajo

Start with qubit_probability.wgsl, src/backend.rs around measure_qubit, and src/pauli.rs to trace probability and expectation calculations. Check how shaders are compiled and how the GPU adapter requests features before assessing the optional f64 path. Done means branch renormalisation uses each branch's measured norm, expectation values account for state norm, and f64 remains optional on supported adapters.

Escrito por el modelo de indexación a partir del texto del issue.

Descripción

enhancement
What you want to do

The GPU backend works in f32. Three things make its error bigger than it needs to be:

  1. qubit_probability.wgsl adds up to about 32 thousand terms per thread one after another in f32
    (at 30 qubits), and measure_qubit (src/backend.rs:153) renormalises by 1/√p from that sum.
    Errors add up over many mid-circuit measurements in one shot.
  2. The outcome-0 branch is scaled by 1/√(1 − p1), which is only right if the state's norm is
    exactly 1.
  3. Pauli::expectation (src/pauli.rs) assumes ‖ψ‖ = 1 and does not divide by the norm, so norm
    drift becomes a bias in the expectation value.
Implementation notes
  • Use Kahan (compensated) summation in each thread's loop in qubit_probability.wgsl. Return both
    the probability of 1 and the total norm from the same pass, and renormalise each branch by its
    own sum.
  • Divide expectation values by Σ|a|², which is a cheap extra sum in the same loop.
  • f64: request wgpu::Features::SHADER_F64 when the adapter offers it and compile a second
    shader. Most consumer GPUs run f64 at 1/16–1/64 of f32 speed, so it should only be opt-in.
What it could look like
No API change for 1–3. Optionally `backend="gpu64"` on adapters that support `SHADER_F64`.
Contributing
  • I would like to open a pull request for this (see CONTRIBUTING.md).
Lenguaje dominante
Rust
Estrellas
1
Forks
0
Merge medio
12 min
PR fusionados (30 d)
4

Preparar el entorno

Primeros pasos

  1. Lee el issue completo y luego la guía de contribución del proyecto.
  2. Comenta en el issue que vas a ocuparte — evita que dos personas hagan lo mismo.
  3. Haz un fork del repositorio y trabaja en una rama.
  4. Abre un pull request que haga referencia al número del issue.

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