Kossakowski-form dissipator

[david-pl]

Adds the general GKSL form
D*(O) = Σ_nm K_nm (A_n† O A_m − ½{A_n† A_m, O}) to ppvm-lindblad,
alongside the existing jump-operator form, with Python bindings, tests
and benchmarks.

Replaces #179, which was stacked on the pre-split #178 and could not be
reopened usefully: its branch had never been rebased, so its diff against
the split stack was 160 files and reverted work that #180–#182 had since
landed. This branch is the same feature ported onto split/3 and cleaned
up; the original commits are preserved on kossakowski-dissipator.

Stacked on #182 — review after that one settles, see the note below.

What's here

Commit
7b52cf6b	core dissipator: kossakowski.rs, PairShape, add_kossakowski
e6266b52	superradiance + dipolar-relaxation benchmarks, profiling example
dc8c6234	Lindbladian(..., kossakowski=(ops, K)) Python surface
b52d22cc	dense-reference and orbit-representative tests

Conjugate pairs (n,m)/(m,n) are folded into a single upper-triangle
entry — both sandwiches produce the same output words with conjugate
phase and the action keeps only the real part — halving the pair count.
PairShape distinguishes the diagonal entry from a folded off-diagonal
one and carries the term list only the folded case needs, so the
off-diagonal-only path is unreachable on a diagonal pair.

Against the equivalent eigenmode-jump decomposition on a superradiant
chain, the pair-matrix path costs 27.7 ms vs 103 ms at n=10 and 96 ms vs
1.71 s at n=30: the per-string action scales with the number of nonzero
K_nm entries instead of carrying an extra factor of M.

Differences from #179

Ported into the spec/basis/step/algebra layout split/2 and
split/3 introduced, rather than appended to lib.rs:

• New kossakowski.rs module. spec.rs grows 65 lines, not 520.
• add_kossakowski (147 lines) split into validate_k / parse_ops /
  compile_pair / compile; the ~90-line action arm became
  Pair::accumulate + the one-sided and both-sided halves.
• PairShape enum replaces an off_diag bool that drove three separate
  branches in the hot path. Benchmarked both shapes: the deltas
  (+2.4% / −1.3% / +2.2%, n30 p=0.17) are smaller than the drift on the
  eigenmode_* control benchmarks (+3.4% to +6.6%) in the same run, so
  this is noise, not a regression.
• 7 clippy allows removed, 0 added. split/3 had none in
  ppvm-lindblad; three needless_range_loop became iterator loops and
  four type_complexity became two named aliases.
• precompute_ldagger_l generalized to precompute_adag_b(a, b), moved
  to algebra.rs beside PauliTerm and a named COEFF_DROP_TOL (was a
  1e-14 literal in five places).
• Dropped a dead let _ = i;, and row-length errors now report which row.
• support_mask lifted out of the pair loop, where it was being rebuilt
  per iteration.
• Deleted the k_ops field — LindbladSpec state that never outlived
  the constructor.
• Did not port the original's :func:/:meth: docstrings; split/3
  had already converted those to backticks per AGENTS.md.

One thing to check

test_orbit_dissipative.py failed on arrival. d8313e0e on split/3
("orbit-rep evolution on stabilized orbits") made rep coefficients member
coefficients on every orbit, where stabilized orbits were previously
divided by |G|; the tests encoded the old behaviour and c_I came out
exactly 6× the asserted value on a 6-site ring.

I rewrote the reconstruction as Σ |orbit(rep)| · c_rep and updated the
identity-bookkeeping expectation to coeff_I. The new values match the
closed form exactly. If d8313e0e was not meant to change that
convention, the bug is there and not in these tests — worth confirming
while reviewing #182.

Verification

• cargo test -p ppvm-lindblad — 8 passed
• pytest test/lindblad/ — 35 passed, 1 skipped
• cargo clippy -p ppvm-lindblad --all-targets — zero warnings
• pre-commit green on all four commits

🤖 Generated with Claude Code

[github-actions]

PR Preview Action v1.8.1
Preview removed because the pull request was closed.
2026-09-28 13:46 UTC