Qiskit / qiskit-fermions / 30361090251

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feat(transpiler): add `filter_trivial` to `QDriftTrotterization` (#222)

* feat(transpiler): add filter_trivial to QDriftTrotterization

Excitations sampled by qDRIFT that act entirely within the known-occupied
or known-unoccupied mode set cannot change the sampled bitstrings, so
including them only wastes one of the num_terms slots. filter_trivial
rejects and re-draws such terms, tracking the occupied/unoccupied mode
sets from any preceding InitializeModes gate(s).

Keep the common (non-filtering) sampling path on a single batched
rng.choice(size=num_terms) draw rather than num_terms separate scalar
draws, since rejection sampling is only needed once filtering is
actually active.

Closes #194

Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

* feat(transpiler): support PrepareSlaterDeterminant in filter_trivial

Since PrepareSlaterDeterminant is the fused composition of an
InitializeModes reference occupation and an OrbitalRotation,
QDriftTrotterization's filter_trivial now treats it the same way it
would treat those two gates back-to-back: seed the
occupied/unoccupied mode sets from its occupation, then immediately
mark every mode it touches as "uncertain" because of the rotation it
also carries.

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Co-authored-by: Claude Sonnet 5 <noreply@anthropic.com>

69 of 69 new or added lines in 1 file covered. (100.0%)

10167 of 10375 relevant lines covered (98.0%)

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