Evolution

Breaking this gate down in bosonic space (into a circuit of smaller Evolution gates) is optional, and it is what the synthesis attribute governs. The gate can equally be handed straight to the boson-to-qubit stage, which maps and synthesizes it whole, however many terms its operator holds. See synthesis.

When it is decomposed (by decompose(), for instance) synthesis decides how. It defaults to BosonicLieTrotter, which splits the evolution group-by-group when the operator has groups assigned (see Group operator terms: use the operator structure), and term-by-term otherwise (exact only when the individual factors mutually commute). Another method can be chosen, for a whole circuit, with the B2BSynthesis transpiler pass; BosonicQDrift, for example, replaces the evolution with a randomized sample of its terms. The gates it emits are atomic, so synthesis no longer applies to them.

This gate carries no occupation cutoff. It is defined purely in terms of the bosonic operator, whose Fock space is infinite-dimensional. The truncation is introduced later, by the boson-to-qubit encoding chosen during transpilation – which is precisely what lets one ansatz be evaluated at several truncation levels without rebuilding it. See occupation_encoding().