fix(propagator)!: 🐛 number an incremental Heisenberg axis like the equivalent circuit - #346
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…uivalent circuit `build_graph` grew a graph in two opposite directions when extending. Layers were prepended, which is right: in the Heisenberg picture, conjugating an already-conjugated operator by one more gate makes that gate act on the reference state first. Parameter indices were appended unconditionally. So `build_graph(a); build_graph(b)` built the circuit `b + a` but numbered it as if it had built `a + b`, and parameter 0 drove the gate that comes last. Nothing raised; the expectation value stayed plausible and simply belonged to a different state. Schrodinger appends both, so it was already consistent. The axis now follows the picture, so an incremental build and the one-call build of the equivalent composition agree parameter for parameter. The renumbering is a *cyclic* rotation of the index values rather than a shift, because a shift would leave index 0 unused mid-way and `validate_parameter_mapping` rejects a non-contiguous mapping -- which also means no caller-side workaround could place the index directly. `seed_parameters` is read on the same post-call axis and un-rotated before it reaches the engine, which still reads it against the axis the graph has at build time. BREAKING CHANGE: parameter indices change on every Heisenberg `build_graph` extension, so anything caching an index or a gradient vector across such a call changes meaning. A downstream that rotates `seed_parameters` itself to compensate must drop that rotation in the same bump, or it will double-rotate silently -- only `lower_atol`/`upper_atol` runs are affected, so nothing raises. An axis-order workaround that renumbers in layer order is idempotent and becomes a verified no-op. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
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Included review availability: Your plan provides up to 1 included review per hour; 0 remain after this review. 📝 WalkthroughWalkthroughIncremental ChangesIncremental graph construction
Estimated code review effort: 3 (Moderate) | ~20 minutes Suggested reviewers: Sequence Diagram(s)sequenceDiagram
participant Caller
participant MonomialPropagator
participant CppEngine
Caller->>MonomialPropagator: build_graph with post-call seed_parameters
MonomialPropagator->>MonomialPropagator: reorder Heisenberg seeds
MonomialPropagator->>CppEngine: extend graph
MonomialPropagator->>MonomialPropagator: remap existing parameter indices
MonomialPropagator-->>Caller: updated parameter_mapping
Merge Risk: ⚪ Minimal · up to Incremental Heisenberg graph construction now aligns parameter axes and gradients with equivalent circuit ordering, with updated seed semantics and regression coverage. No merge-blocking risk remains. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
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Inline comments:
In `@docs/content/docs/features/evaluation.mdx`:
- Line 46: Update the documentation sentence describing seed_parameters so it
states that the parameter is needed only when extending a non-empty graph, while
preserving that an explicit value affects coefficient-informed truncation during
the first build_graph call.
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VERSIONcpp/include/monoprop/MonomialPropagator.hdocs/content/docs/concepts/simulation_modes.mdxdocs/content/docs/features/evaluation.mdxsrc/monoprop/circuit.pysrc/monoprop/majorana_propagator.pysrc/monoprop/monomial_propagator.pysrc/monoprop/pauli_propagator.pytests/test_circuit.py
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robertodr
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Sep 9, 2026
Codecov Report✅ All modified and coverable lines are covered by tests. Additional details and impacted files@@ Coverage Diff @@
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adamglos92
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Sep 9, 2026
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It seems that it would be cleaner if the fix would be done on the C++ side.
Only the *parameter* axis is renumbered on a Heisenberg extension. Gate arrival
order is not, so the same per-gate mapping wires an incremental build and the
one-call build of its equivalent circuit differently, even where their per-layer
mappings agree:
per-layer: incremental [0,0,1,1,2,2,3,3] == single [0,0,1,1,2,2,3,3]
then per-gate [0,1,2,3]:
incremental -> layers [2,2,3,3,0,0,1,1] E = -0.030621345425
single -> layers [0,0,1,1,2,2,3,3] E = -0.879951499811
`evaluation.mdx` called the two builds "interchangeable ... same expectation
value and gradient anywhere", which is true of the parameter axis alone and
contradicted the warning on `parameter_mapping`. Both now say the same thing.
The gate axis is offset in C++ (`g += gate_offset`), so closing that gap is a
follow-up there.
Also fix the stated reason for the rotation being cyclic. Contiguity is not an
engine constraint -- `set_parameter_mapping` checks only length. A shift fails
because it moves one block and not the other: the existing indices would collide
with the block just appended and that block's numbering would stay wrong.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
adamglos92
approved these changes
Sep 10, 2026
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Can I merge this or wait? |
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I approved, but I would still prepare if one of @Algorithmiq/software-engineers would review it |
ludmilaasb
reviewed
Sep 17, 2026
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| The parameter axis follows that equivalent circuit, not the order the calls were made in, so | ||
| the incremental build and the one-call build of the composition agree on | ||
| [parameter_mapping][monoprop.monomial_propagator.MonomialPropagator.parameter_mapping] and on | ||
| the expectation value and gradient at any parameter vector. In Heisenberg each extension | ||
| therefore takes the low indices and lifts the ones already in the graph by | ||
| `circuit.n_parameters`. | ||
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| Gate *arrival* order is the exception: it is not renumbered, so the two builds number their | ||
| gates differently even though their parameter axes match. That only matters if you re-wire the | ||
| graph by assigning a per-gate mapping — see the warning on | ||
| [parameter_mapping][monoprop.monomial_propagator.MonomialPropagator.parameter_mapping]. |
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This AI phrasing is very annoying. "It is this, not that".
I suggest changing for more natural writing style.
Comment on lines
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| # A cyclic rotation, because both blocks move: the existing indices go up by | ||
| # `num_new`, and the block just appended wraps from the top of the axis down to | ||
| # 0..num_new-1. Shifting only the existing ones would collide with the new block and | ||
| # leave its numbering untouched, which is the bug. Assigning the per-layer form | ||
| # (length graph_layers) picks that reading unambiguously; the rotated mapping is | ||
| # still contiguous, so this only skips the property setter's redundant validation. |
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Summary
build_graphgrew a graph in two opposite directions when extending. Layers wereprepended, which is correct — in the Heisenberg picture, conjugating an already-conjugated
operator by one more gate makes that gate act on the reference state first, and
Circuit.__add__already acknowledged this ("incremental multi-call building, whoseordering is picture-dependent"). Parameter indices were appended unconditionally
(
mapping = [self._n_params + m for m in circuit.resolved_mapping]).So
build_graph(a); build_graph(b)built the circuitb + abut numbered it as if it hadbuilt
a + b: parameter 0 drove the gate that comes last. Nothing raised. The expectationvalue stayed plausible and simply belonged to a different state. Schrödinger appends both
layers and indices, so it was already consistent.
The invariant that should hold, where
==means the same parameter vector yields the sameexpectation value:
The graph satisfied this in both pictures; the axis only in Schrödinger. It now holds for
both, for the axis, the value and the gradient.
Two details worth flagging in review:
leave index 0 unused mid-way and
validate_parameter_mappingrejects a non-contiguousmapping — which is also why no caller-side workaround can place the index directly, and
the fix has to live here.
seed_parametersis now read on the post-call axis and un-rotated before it reaches theengine, which still reads it against the axis the graph has at build time. This is the
half that needs coordination — see Breaking change below.
Changes
monomial_propagator.py: rotate the per-layer mapping by(m + num_new) % n_paramsaftera Heisenberg extension, and un-rotate
seed_parametersbefore the engine build. Assignedvia the simulator's per-layer mapping (length
graph_layers), so the reading isunambiguous and
_n_paramsstays correct.majorana_propagator.py/pauli_propagator.py: both overridebuild_graphwith theirown docstring — the pages users of the public classes actually read — and both still said
indices "are shifted up onto the accumulated parameter axis automatically". Updated with
the picture rule and the seed's axis.
parameter_mapping: documented the two API hazards found along the way. The setter'sdocstring never reaches the docs site (griffe renders a property from its getter only), so
the warning lives on the getter, where it renders as a callout: the per-layer and per-gate
forms are told apart by length alone but indexed differently (equivalent-circuit order vs
gate-arrival order), they collide for any all-single-monomial graph, and
list(range(n))is not a neutral reset. Making the two forms structurally unambiguous would mean new API on
both the Python and C++ surfaces, so it is left out of this PR.
MonomialPropagator.h: note that C++ callers own the numbering — the header takesparameter_mappingas given, so there is no implicit assignment to fix on that side.evaluation.mdx: new Extending a graph across several calls section, cross-linked fromsimulation_modes.mdx.tests/test_circuit.py: five tests, below.Verification
Every new test fails on
mainand passes here — reverted the fix and re-ran to confirm: 6failures, all Heisenberg legs, Schrödinger legs passing either way.
test_incremental_build_wires_the_picture_s_equivalent_circuit— the invariant per pictureacross all three fixtures, asserted on the axis, the value and the gradient at angles
different from the build values. The existing tests compared energies at the build
parameters, where the misnumbering is invisible; that is why this shipped. The LiH fixture
supplies mixed-arity gates, so
n_gates != graph_layers.test_incremental_heisenberg_moves_each_block_to_the_front_of_the_axis— blocks of width2, 1, 3, so
n_new > 1. Two guards assert the forward composition and a block reversedinside itself both give different answers, so the equality cannot pass vacuously.
test_incremental_reindex_keeps_a_multi_monomial_gate_on_one_index— pins[0, 0, 1, 1, 2]across an extension.
test_extend_seed_parameters_are_read_on_the_post_call_axis— atlower_atol=0.1acorrectly seeded extension reproduces the one-call build's
graph_size,sizeandvalue exactly, while a pre-call-axis seed differs. Holds at every atol probed from 0.05 to
0.3. My first attempt at this test asserted that the appended block's seed entries are
unreachable, which is false — they seed the new layers too — and it passed without the fix;
worth knowing if this area is touched again.
test_extend_without_seed_builds_structurallymoved with the contract (c1 + c2→c2 + c1). Note it was passing vacuously: its observable expectation is exactly 0 forevery gate order.
Beyond the suite: 589 Python tests, 267 C++ tests,
just build-docsclean,prekclean onthe changed files. Checked by hand that an empty extension, an identity gate carrying an
explicit index, and extending after an in-place
contract_partiallyall behave, and that theinvariant holds for
PauliPropagatortoo (shared engine).The reporter validated the same approach as a
build_graphwrapper against Algorithmiq'saurora_stateADAPT loop, which extends a Heisenberg graph one gate per iteration: fullsolver suite with its own workaround disabled at 123 passed / 95 skipped (6 failing before
any fix), and machine precision against exact statevector energies on three systems.
Breaking change
Parameter indices change on every Heisenberg
build_graphextension, so anything caching anindex or a gradient vector across such a call changes meaning.
The axis half is safe to land alone:
aurora_state's workaround (renumber the axis in layerorder) is idempotent, so it becomes a verified no-op and nothing breaks if it stays. The
seed half is not.
aurora_statecurrently rotatesseed_parametersitself to compensate,and with the upstream rotation added it would double-rotate silently — it only affects
lower_atol/upper_atolruns, so it does not raise and no test catches it; it just feedscoefficient-informed truncation the wrong coefficients. That rotation has to come out in the
same coordinated bump (
aurora_statepinsmonoprop==0.9.1a0).Fixing only the axis would avoid the coordination but leave an axis in natural order with a
seed in append order — a worse contract. This PR takes the coordinated option, as recommended
in the report.
If your release process owns
VERSIONbumps (history shows them as standalonechore:commits), the
VERSIONline is the part to drop from this PR.Checklist
docs/,CONTRIBUTING.md) if neededCHANGELOG/ release notes updated if applicableAI/LLM disclosure
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