Qiskit / qiskit-fermions / 33872240997

98%

push

github

refactor(circuit): extract the Evolution product formula into a synthesizer (#311)

`Evolution._define` hardcoded a first-order product formula, so there was no way
to ask for anything else -- notably not a higher-order formula that respects the
operator's groups. The qubit side has had such a knob for a while (via
`MapperFnEvolutionSynthesis.product_formula`), but that acts after the mapping,
where the grouping is already gone.

Extract the existing decomposition verbatim into a `FermionicLieTrotter` behind a
new `FermionicSynthesis` interface, and let callers select it through a
keyword-only `synthesis` argument, mirroring `PauliEvolutionGate.synthesis`.
`_define` becomes a one-liner.

This is deliberately behaviour-preserving groundwork: the default reproduces the
previous decomposition exactly, so every existing gate-count expectation still
holds unchanged. It settles the plumbing -- package layout, docs registration,
pickling, argument position -- against an empty behavioural surface, so that the
methods landing on top are reviewable on their own terms.

The docs are deliberate about two things. "Synthesis" here means the
fermion-to-fermion level, and that level is *optional*: an `Evolution` can be
handed straight to the fermion-to-qubit stage no matter how many terms it holds,
so decomposing it first is a choice made to obtain cheaper factors or to expose
structure the later stage can exploit. And the interface is described in terms of
synthesis methods rather than product formulas, since a future implementation
need not be a Trotter formula at all.

`synthesis` is exposed as a read-only property over a private attribute. A gate's
definition is built once and then cached, so a synthesis method assigned after the
fact could never take effect; making the attribute read-only rules that out
structurally rather than warning about it. The constructor argument is
keyword-only and comes after `time` because several guides pass `time`
positionally, and i... (continued)

37 of 37 new or added lines in 4 files covered. (100.0%)

13447 of 13694 relevant lines covered (98.2%)

481681.9 hits per line