Native QEC Program IR

QecProgram in src/qec/mod.rs is a measurement-record IR for QEC workloads that need detectors, logical observables, postselection, expectation metadata, and Pauli-noise annotations before sampler lowering. It is separate from Circuit so measurement-record programs do not need to fit final-measurement OpenQASM semantics.

QecOp stores gates, basis measurements, MPP-style Pauli-product measurements, resets, detector rows, observable includes, expectation-value metadata, postselection predicates, feed-forward corrections, noise annotations, and tick separators. Record references can be absolute indices or rec[-k] style lookbacks. Construction validates qubit bounds, gate arity, finite coordinates and coefficients, finite probabilities, and measurement-record scope. Detector, observable, and postselection rows can be resolved to absolute measurement indices for later compilation into packed samplers.

Feed-forward

QecOp::Feedforward conditions a body on the parity of a record list against an expected value, the shape a detector already has. It is a consumer of the guarded-region contract rather than a second conditional mechanism: the QEC record space and the classical bit vector are one address space, because the reference runner writes record i to classical bit i, so a resolved record index is directly the bit a ClassicalCondition::Parity reads. The op executes as the guarded instruction circuit::guarded picks for its body: a Region through Backend::apply_region, or a Conditional when the body is one gate.

The body admits gates and resets only. Detectors and observables index the record space absolutely, so a measurement whose execution depended on a record would make every later index depend on the shot.

The compiled QEC sampler evaluates a static affine map from random bits to outcomes, which a record-conditioned branch makes depend on the sample. It rejects by name and points at run_qec_program_reference, as do the deferred lowering and the density-matrix estimator. The detector-error-model derivation inherits the deferred lowering's rejection rather than carrying its own. The op is built through QecProgram::feedforward; the native text format does not spell it.

Parsing

parse_qec_program and QecProgram::from_text parse the native QEC text subset used by current benchmark planning: H, S, S_DAG, T, T_DAG, CX, CZ, R/RX/RY, M/MX/MY, MR variants, MPP, DETECTOR, OBSERVABLE_INCLUDE, POSTSELECT, EXP_VAL, Pauli-noise instructions, TICK, QUBIT_COORDS, SHIFT_COORDS, and flattened REPEAT blocks. The parser resolves rec[-k] references while building the program. Numeric arguments on basis measurements, such as M(0.001), lower to pre-measurement Pauli flips that affect the measurement record.

Lowering

compile_qec_program_rows lowers basis measurements and MPP records into the same packed X/Z Pauli row representation used by the compiled sampler internals. It also carries detector, observable, and postselection rows forward as absolute measurement-record indices. Detector, observable, and postselection projection uses PackedShots::parity_rows, so the QEC layer reuses the existing packed parity engine instead of maintaining a second one. This is a sampler-lowering artifact, not an execution engine. Gate, reset, and noise execution lives in run_qec_program; EXP_VAL has no packed-row representation, so the row compiler rejects it and run_qec_program routes such programs to the estimator paths described below.

Execution

run_qec_program lowers Clifford-compatible programs into the packed compiled sampler, compiles Pauli-noise annotations into sensitivity rows XORed into the records, and routes EXP_VAL programs to estimator paths. run_qec_program_reference is the per-shot state-vector correctness oracle. The runner routing, the compiled and noisy sampling paths, the circuit lowerings, and the result shape are covered in QEC program execution.

Expectation values

EXP_VAL(c) P1*...*Pk estimates c * <P> for the Pauli product P in the program's final state and returns one QecObservableEstimate per op, in op order, in QecSampleResult::expectation_values. The placement rules, the estimator paths, and the analytical strategy ladder are defined in QEC program execution.