Support code-declared inlined feedback in memory circuits

[kvmto]

Problem

memory_circuit hardcodes the record→detector combination rule: every detector is "same record slot, two consecutive rounds" plus fixed boundary closures. This assumes each returned ancilla measurement is already a clean stabilizer value. Measurement schemes whose readout carries record-conditioned byproducts — e.g. superdense (paired-ancilla) extraction, where one ancilla's outcome heralds a Pauli byproduct on part of the plaquette — cannot express their true detector parities. Today such codes must apply the correction as a physical gate (an extra noise location — on hardware this correction is a classical frame update, not a gate) or abandon the framework path entirely.

Stim solves this with record-controlled Paulis plus with_inlined_feedback(); the CUDA-Q kernel language has no equivalent, and dem_from_kernel rejects measurement-dependent branching. This PR provides the cudaqx-level resolution.

Design

Make the record→detector combination rule a declarative property of the code, with the identity default preserving today's behavior exactly:

extract records ──► inlined feedback (declared by code; identity default) ──► detectors ──► decoder

The code declares data, not behavior (device kernels cannot call host callbacks mid-trace; the data rides the same rail as the parity matrices):

• virtual cudaqx::tensor<uint8_t> code::get_inlined_feedback() const — shape [numCols × numCols], numCols = num_ancilla_qubits, [Z][X] per-round record order. Entry (j, k) = 1: the cross-round detector for record j additionally XORs record k of the earlier round; the final boundary detector for record j additionally XORs record k of the last round. Default: empty (identity).
• virtual cudaqx::tensor<uint8_t> code::get_observable_inlined_feedback() const — shape [num_observables × numCols]; entry (m, k) = 1: observable m additionally XORs record k of every round. Default: empty.

experiments.cpp validates and threads the flattened data into every memory_circuit invocation; inside the kernel, empty feedback keeps the existing code paths verbatim (the legacy cudaq::detectors(prev, curr) line and observable emission are untouched under the default), while non-empty feedback emits per-record cudaq::detector parities, extended boundary detectors, and per-round observable record collection (one logical_observable call per observable, order unchanged). Python plugins simply define the two methods returning numpy arrays; the nanobind bridge forwards them.

Wrong declarations fail loudly: stim's determinism analysis inside dem_from_kernel validates every declared parity on each call.

Tests

• Identity guarantee: all existing C++ suites (test_qec 46, test_qec_stim 7, test_decoders 43, test_dem_sampling 36) and Python test_code.py pass unchanged — with empty feedback the emitted circuit structure is bit-identical to main.
• Behavioral proof (C++): a 2-qubit toy code with a paired-ancilla superdense round carrying an explicit uncorrected byproduct. With declared feedback ([[0,1],[0,0]] / [[0,1]] — derivation documented in-code; verified unique by exhaustive search): noiseless sample_memory_circuit yields all-zero syndromes and deterministic logical readout. Negative control: the identical circuit without the declaration throws stim's "non-deterministic detectors" — proving the feature does the work.
• Python bridge: the same toy as a @qec.code plugin (positive + negative), exercising the trampoline and handle paths.

Notes for reviewers

• The Python toy pins the stim target: while testing we found that the qpp target does not preserve cross-round mid-circuit measurement correlations when sampling Python kernels (reproduced with a feedback-free minimal circuit: two consecutive XX ancilla measurements agree in only ~50% of shots on qpp-cpu vs 100% on stim). This is a pre-existing cuda-quantum defect, orthogonal to this PR; we plan to file it upstream with the reproduction.
• Motivating case: a triangular color code plugin (Add triangular color code plugin with superdense memory circuit #658) registering a superdense stabilizer_round — with this feature its byproduct correction moves from a physical CX into the detector definitions, making the framework-path DEM noise-faithful to the classical-feedforward convention.
• Natural follow-ups (out of scope here): a batch post-processing helper for raw-record consumers and the realtime/streaming variant of the same declaration; a code-owned detector-emitter kernel for fully custom layouts; per-detector coordinate metadata (color annotations for color-code decoders).

Update: rule extraction (second + third commits)

The initial commit computed the record-composition arithmetic inline in the kernel. Two follow-up commits restructure this into the intended data → rule → emission split, with no user-visible change (all suites bit-identical):

• Rule (device) — new internal header lib/device/inlined_feedback.h: named, documented __qpu__ record builders (cross_round_detector_records, boundary_detector_records, observable_feedback_offsets, collect_observable_feedback_round, observable_support_records). The memory_circuit kernel is now a thin emitter: state prep, rounds, and cudaq::detector/cudaq::logical_observable calls over the vectors these helpers return. Its signature is unchanged (the two flattened matrices — the declared data, mirroring the getters).
• Rule (host) — cudaq::qec::build_inlined_feedback_layout in detector_error_model: the same composition rule computed host-side as a CSR inlined_feedback_layout, for record-stream consumers that cannot run a kernel (the planned realtime D_sparse augmentation and batch record folds). Unit-tested standalone.
• Lockstep guarantee — a conformance test (HostLayoutMatchesKernelM2D) derives the expected measurement→detector matrix from the host layout and asserts it equals the M2D that dem_from_kernel extracts from the kernel's emissions on the toy. The device fold and the host rule cannot drift apart silently.

Emission stays in-kernel deliberately: stim builds the probabilistic DEM and runs its determinism gate only over kernel-declared detectors.

Test tally after the refactor: test_qec 55 (49 + 5 layout unit tests + 1 conformance), test_qec_stim 7, test_decoders 43, test_dem_sampling 36, Python test_code.py 21.

Update: basis-split observable feedback (fourth commit)

Working the motivating case (#658) through this API surfaced a defect in the original design: observable inlined feedback is basis-dependent, while the single get_observable_inlined_feedback() was threaded identically into Z- and X-basis memory experiments. In the superdense color code, the heralding records must be folded into the Z-basis logical observable but not the X-basis one; with a single basis-blind matrix, every X-basis run fails stim's determinism gate (confirmed independently by GF(2) analysis of the record recursion and by noiseless sampling).

The fix mirrors the existing basis-selected rail for the observable data matrix (is_z_prep ? get_observables_z() : get_observables_x()): the getter is split into get_observable_inlined_feedback_z() / _x() (empty defaults), and each driver queries only the selected getter — pinned by a selection test in both directions. The detector-feedback matrix remains a single basis-neutral getter, which #658's validation confirms is sufficient (its detector matrix serves both bases unchanged).

Status of the motivating case: with this API, the color-code plugin (#658) declares its matrices and its framework-path DEM matches an independently constructed reference DEM bit-exactly in both bases (mechanism sets and probabilities), replacing the physical correction gate entirely.

Status

Draft for design feedback on the API shape and semantics.

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