Fix #880: return simplified QubitOperator from symmetry_conserving_bravyi_kitaev

[stark256-spec]

Closes #880.

Problem

get_sparse_operator raises on the operator produced by symmetry_conserving_bravyi_kitaev:

from openfermion import get_sparse_operator, FermionOperator, symmetry_conserving_bravyi_kitaev

fermion_op = FermionOperator("0^ 1^")
qubit_op = symmetry_conserving_bravyi_kitaev(fermion_op, 4, 2)
get_sparse_operator(qubit_op)   # ValueError: axis 0 index 7 exceeds matrix dimension 4

Root cause

symmetry_conserving_bravyi_kitaev finishes by calling remove_indices, which shifts qubit indices. When two qubits are mapped onto the same new index, a term ends up with multiple Paulis acting on one qubit, e.g.:

((0, X), (1, Y), (1, X))

remove_indices writes these terms straight into the operator’s .terms dict, so they bypass the simplification QubitOperator normally performs on construction. qubit_operator_sparse assumes each qubit appears at most once per term (it grows the tensor product one factor per Pauli), so a repeated qubit makes the per-term matrix larger than the n_qubits Hilbert space and the assembly raises.

Fix

Rebuild the operator after remove_indices so every term is routed back through QubitOperator’s simplification, restoring canonical one-Pauli-per-qubit form (e.g. ((0, X), (1, Y), (1, X)) → ((0, X), (1, Z))). This keeps the operator mathematically identical — the existing eigenspectrum-based tests still pass — while satisfying the invariant that QubitOperators are simplified, which get_sparse_operator and other consumers rely on.

Tests

Added test_output_is_simplified_qubit_operator (the reporter’s reproducer), asserting:

1. every returned term is canonical (at most one Pauli per qubit);
2. get_sparse_operator no longer raises;
3. the resulting sparse operator matches the explicitly-simplified operator.

Verification (Python 3.12):

• pytest remove_symmetry_qubits_test.py — 6 passed (5 existing + new regression).
• black --check (line-length 100), pylint --rcfile=.pylintrc (3.3.9), and mypy all clean on the changed files.

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[gemini-code-assist]

Code Review

This pull request addresses issue #880 by ensuring that symmetry_conserving_bravyi_kitaev returns a simplified QubitOperator in canonical form (with at most one Pauli operator per qubit). This is achieved by rebuilding the operator term-by-term, which triggers QubitOperator's internal simplification and prevents errors when calling get_sparse_operator. A regression test has also been added to verify this behavior. There are no review comments, so I have no additional feedback to provide.