codes module fixes (capacity estimation and logical operators) - #577
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…tract Monte-Carlo helpers Decode subsystem codes against their stabilizer generators in the code-capacity logical error rate estimators: - CSSCode: compute syndromes from the stabilizer generators (the matrices the decoders are built to invert), fixing a crash on subsystem codes, whose number of stabilizer generators differs from the number of gauge generators (parity checks). - QuditCode: build the decoder and syndromes from get_stabilizer_ops() instead of the full gauge matrix. For non-subsystem codes this is unchanged; for subsystem codes it decodes against the measurable stabilizer generators rather than the non-commuting gauge generators. - Keep the syndrome matrix as a field array so that get_decoder can select a decoder appropriate to the field; code-capacity estimation now works for codes over non-binary fields. Also: - get_syndrome_subgraphs: include every check present in the Tanner graph, so a check whose support overlaps no other check still appears in a returned subgraph. - concatenate: operate on a copy of the outer code so that the caller's logical operators are not modified. - CSSCode capacity: decide whether the X-type and Z-type decoders can be shared using the fully merged decoder arguments. Move the Monte-Carlo helpers (ErrorRateFunc, sample allocation, error-weight probabilities, and decode-and-erasure) into codes/_monte_carlo.py and re-export them, with dedicated unit tests. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
- get_distance_bound (CSSCode): make pauli keyword-only, matching the base signature's num_trials-first convention and removing a misleading positional-pauli affordance. - num_qubits: emit a single-line error message with a real class name (previously a raw string left a literal "\n" and an unfriendly class repr). - from_strings (QuditCode): reject an empty collection of parity checks with a clear error instead of an IndexError, and report the mismatched lengths when checks differ. - Rename the per-weight sampling helpers to reflect that they return failure and discard counts (not a variance), and to use consistent names across the code classes. - _get_distance_exact (CSSCode): drop a "no cover" pragma from a reachable fallback and cover it with a bare-CSSCode distance test. - set_logical_ops_x / set_logical_ops_z (QuditCode): drop an untested branch for single-type-support input; the documented input is a full symplectic (k, 2n) matrix. - get_destabilizer_ops: document that a pauli argument selects the destabilizers dual to the stabilizer generators of the opposite type. - Document that CSSCode's promise_equal_distance_xz is trusted rather than verified. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
set_logical_ops previously checked only that the X-type and Z-type logical operators had the correct cross-type commutation relations (Lx Ω Lz.T = I). It now checks the full symplectic Gram matrix Lx Ω L.T against the block anti-diagonal [[0, I], [-I, 0]], which additionally requires that the X-type logicals mutually commute and the Z-type logicals mutually commute. A basis that satisfies the cross-type relations but has, for example, two anticommuting X-type logicals is now rejected rather than silently accepted. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
…ss instance QuditCode.get_logical_ops and CSSCode.get_logical_ops cache logical operators in the same self._logical_ops attribute but in different formats: the base implementation stores the symplectic (mixed-support) form, while CSSCode stores the block-diagonal single-type form. Invoking the base implementation directly on a CSSCode instance (as circuits/transversal.py does) populated the shared cache with a base-format basis, after which CSSCode.get_logical_ops(Pauli.X, symplectic=True) misread it and returned X-type logical operators with spurious Z-type support. Guard the base implementation so that, when it is called on an instance whose type overrides get_logical_ops, it defers to that override. The cache is then always read and written in the format of the most-derived implementation. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
At error_rate == 1, _get_error_probs_by_weight assigns the unit mass to weight == block_length only when that weight lies within the array, and otherwise leaves the probabilities at zero (fully truncated), rather than slicing past the end of the array or marking impossible weights. The zero-sample divisor guard is factored into ErrorRateFunc._as_divisor and applied to both the infidelity and discard-rate paths, so a weight with no (kept) samples yields a zero rate and variance instead of nan or inf. The optimistic zero-kept-samples behavior and its link to the deferred Jeffreys error-bar fix are documented in place. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
…e set_logical_ops shapes - Look up logical operators through the code's own get_logical_ops in the transversal-circuit corrections, so CSS and non-CSS codes read the shared cache in a consistent format, and drop the base-method dispatch guard (and its test) that this made unnecessary. - Accept a width-n single-type matrix in QuditCode.set_logical_ops_x/z alongside the full width-2n symplectic form, and validate the operator count and shape up front via a shared _validate_logical_ops_shape used by both QuditCode and CSSCode, so a wrong count or a 1-D input raises a clear error rather than a cryptic linear-algebra or indexing failure. - Restore the positional pauli argument on CSSCode.get_distance_bound. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
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AI-generated summary
Correctness and robustness fixes in
codes/common.py, plus extraction of the Monte-Carlo capacity helpers into a newcodes/_monte_carlo.py. No public API signatures are added or removed; the behavioral changes are bug fixes and tightened/loosened input validation on existing methods.Capacity estimation
ErrorRateFunc, sample allocation, error-weight probabilities, decode-and-erasure) intocodes/_monte_carlo.py, re-exported fromcodes, with dedicated unit tests; rename the per-weight sampler to reflect that it returns failure and discard counts.ErrorRateFunc: aterror_rate == 1place the probability mass atblock_lengthonly when it is in range (no out-of-bounds slice or impossible weights), and share one zero-sample divisor guard across the infidelity and discard-rate paths.Logical operators
set_logical_ops: validate the full symplectic Gram matrix, so a basis with anticommuting same-type logicals is rejected rather than silently accepted.set_logical_ops_x/set_logical_ops_z: accept a width-nsingle-type matrix as a convenience alongside the full width-2nsymplectic form, and validate the operator count and shape up front, so a wrong number of operators (or a 1-D array) raises a clear error instead of a cryptic linear-algebra or indexing failure. The check is shared by theQuditCodeandCSSCodeimplementations.get_logical_ops, so CSS and non-CSS codes read the shared logical-operator cache in a consistent format.Other
from_strings: reject an empty collection of parity checks with a clear error instead of anIndexError, and report the mismatched lengths when checks differ._get_distance_exact: cover a reachable fallback with a bare-CSSCodedistance test.