Fix order of noise application Pauli propagation

[david-pl]

Small mistake that was spread throughout large parts of the examples: for actual Heisenberg propagation, we need to apply a noise channel that comes after a gate in the circuit, before applying the gate when propagating an observable via PauliSum.

• The most prominent point where this was an issue is in the documentation example for the TFIM trotter, we definitely should fix that.
• Also fixed in everywhere else (even in tests etc. where it doesn't really matter) just to be consistent and so agents don't get confused.
• Adds a test that compares small circuits to the exact density matrix forward propagation using a simple hand-rolled DM simulator based on numpy.

[github-actions]

PR Preview Action v1.8.1
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2026-06-26 08:03 UTC

[Copilot]

Pull request overview

Fixes a widespread ordering mistake in Heisenberg-picture Pauli propagation: when a physical circuit is “gate → noise”, the corresponding PauliSum call order (which propagates observables backward) must be “noise → gate”. The PR updates examples/docs/benches accordingly and adds a Python test that cross-checks PauliSum against an exact numpy density-matrix + Kraus simulation.

Changes:

• Reordered noise-channel calls to precede their associated gates across docs, skills examples, Rust examples, and benchmarks.
• Updated the Trotter regression test circuit (and golden value) to reflect correct gate/noise ordering.
• Added a Python correctness test that validates ordering and rotation-angle conventions vs an exact density-matrix reference.

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
skills/ppvm-usage/SKILL.md	Fixes the noise/gate call order in the skill’s canonical PauliSum example.
skills/ppvm-usage/examples/python/noise_truncation.py	Updates the runnable skill example to apply noise before gates in Heisenberg order.
ppvm-python/test/test_heisenberg_ordering.py	Adds a density-matrix reference test to validate ordering + angle-sign conventions.
ppvm-python/test/benchmarks/test_trotter.py	Updates the Python benchmark’s Trotter op ordering to match correct Heisenberg semantics.
examples/trotter.rs	Updates the Rust example Trotter loop to apply noise before gates in propagation order.
docs/notebooks/trotter.py	Fixes the documentation notebook’s noisy Trotter step ordering (including loss/noise placement).
crates/ppvm-pauli-sum/tests/trotter.rs	Updates the end-to-end Trotter regression test sequence and golden constant for correct ordering.
crates/ppvm-pauli-sum/examples/trotter_storage_cliff.rs	Applies corrected noise-before-gate ordering in the example workload.
crates/ppvm-pauli-sum/examples/trotter_qubit_sweep.rs	Applies corrected noise-before-gate ordering in the sweep example.
crates/ppvm-pauli-sum/examples/hash_quality.rs	Applies corrected noise-before-gate ordering in the hash-quality investigation example.
crates/ppvm-pauli-sum/benches/trotter.rs	Fixes benchmark operation ordering and truncation comment consistency (“per operation”).
crates/ppvm-pauli-sum/benches/trotter-scaling.rs	Mirrors the corrected operation ordering in the scaling benchmark.

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