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Symmetry-merged evolution: Trotter merging + momentum-sector CTPP#182

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Symmetry-merged evolution: Trotter merging + momentum-sector CTPP#182
AlexSchuckert wants to merge 11 commits into
split/2-ctpp-corefrom
split/3-symmetric-evolution

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PR 3 of 4 splitting #178 (stacked on #181). Full history: branch continuous-time-pauli-propagation.

The two consumers of the translation-symmetry primitive from PR 1:

  • Trotter path: PauliSum.symmetry_merge (k = 0) and PauliSum.momentum_merge (k ≠ 0, complex operator carried as a real pair; character-weighted fold rescaled by |G| to the summing projector, so merging after every gate layer is idempotent). TranslationGroup, canonicalize_basis_arr{,_complex}, and check_momentum_sector_arr exposed via ppvm._core. All inputs validated at the FFI boundary (ValueError, not panics).
  • CTPP path: pc_step_orbit_rep — per-step adaptive evolution entirely in orbit-representative form (complex coefficients, phase-aware action, same cached-CSC expm engine and PcStepConfig truncation policy as the real-space step). The live basis is ~|G|× smaller than full-basis evolution, and the reduction persists through every step; observables come out per momentum mode.

Tests: momentum-merge correctness incl. k-resolved Trotter convergence vs exact diagonalization; orbit-rep evolution vs full-basis-then-project (exact match); real/complex equivalence at k = 0. Regression gates rerun on this exact tree: L=5 momentum sanity bit-identical to the recorded reference; the τ_add real-space cell reproduces median 5.19e-3 / peak basis 77,820 exactly.

🤖 Generated with Claude Code

Two consumers of the translation-symmetry primitive:

Trotter path: PauliSum.symmetry_merge (k=0) and PauliSum.momentum_merge
(k≠0 carried as a real pair, character-weighted fold, |G|-rescaled to
the summing projector so merging after every step is idempotent), with
TranslationGroup and canonicalize_basis_arr{,_complex} /
check_momentum_sector_arr exposed to Python. Inputs validated at the
boundary (ValueError, not panics).

CTPP path: pc_step_orbit_rep — per-step evolution entirely in
orbit-representative form (complex coefficients, phase-aware action,
cached-CSC expm; basis ~|G|× smaller than full-basis evolution,
persisting through every step), with the same PcStepConfig truncation
policy incl. displacement admission.

Split 3/4 of the CTPP work; full history on branch
continuous-time-pauli-propagation.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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