Qiskit / qiskit-fermions / 30100745176

98%

push

github

refactor(ucj): assign disjoint-support groups to diagonal Coulomb terms (#211)

The diagonal Coulomb operator was built as a coefficient-accumulating dict
keyed by term, guarding against key collisions that can never actually occur
(each (block, i, j) triple is a distinct term). It also carried no group
indices, so `Evolution` synthesis emitted one gate per term with no
parallelism.

Build the terms directly (no collision-handling dict) and tag each with a
`groups` index so that same-group terms have mutually disjoint support --
hence each group synthesizes as a single parallel layer of two-mode gates.

The grouping is a closed-form circle-method (round-robin) edge coloring rather
than a greedy pass: the interaction graph is a doubled complete graph on the
modes, whose optimal edge coloring is a direct formula of the two mode indices.
This is assigned on the fly (no collected state) and is layer-optimal -- it
uses exactly the maximum mode degree for an even mode count (always the case
for a spinful register) and one more for an odd count, the latter being an
unavoidable property of odd complete graphs. A greedy coloring, by contrast,
wastes several extra layers for larger odd `norb`.

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

17 of 17 new or added lines in 1 file covered. (100.0%)

10096 of 10304 relevant lines covered (97.98%)

321.59 hits per line