Qiskit / qiskit-fermions / 29844319920

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feat(linalg): add rectangular Givens decomposition for Slater prep (#197)

The square `givens_decomposition` realizes a full n×n orbital rotation, but
preparing a Slater determinant only needs its m occupied orbitals. Decomposing
the m×n occupied-coefficient matrix instead yields a diamond-shaped pattern of
at most m·(n-m) Givens rotations — far fewer than the n·(n-1)/2 brick-wall —
and carries no diagonal phases, since a global phase and any rotation within
the occupied space leave the prepared determinant unchanged. This unlocks the
same gate-count reduction ffsim's `givens_decomposition_slater` provides for
initial state preparation.

Closes #195

Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

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