MergeEvolutions¶

class MergeEvolutions(atol=1e-12)¶

Bases: GenericPass[DAGCircuit, DAGCircuit]

A transpilation pass merging consecutive Evolution gates of the same operator.

Two evolutions under the same operator \(H\) satisfy

\[e^{-i t_2 H} e^{-i t_1 H} = e^{-i (t_1 + t_2) H},\]

because \(H\) commutes with itself. This pass finds neighboring Evolution gates of equivalent operators and replaces them by one gate whose evolution time is the sum of theirs. The rewrite is exact, so it changes neither the simulated state nor the Trotter error of the circuit; it only removes a gate, and with it the synthesized operations that gate would have produced.

Such neighbors arise wherever two sequences of factors meet on the same factor. Examples are the seams between the repetitions of FermionicCircuit.repeat(), between consecutive UCC or UCJ layers, or between consecutive samples of QDriftTrotterization. FermionicSuzukiTrotter already fuses the seams within its own output (see FermionicSuzukiTrotter.merge_adjacent); this pass is the general mechanism which also acts across gate boundaries.

Two gates merge only when all of the following hold:

they act on the identical wires, in the identical order, with nothing else in between;
their operators are of the same type and equivalent up to atol;
they are either both Evolution.atomic or both not, and in the latter case they are decomposed by the same kind of Evolution.synthesis with identical settings.

The merged gate keeps the operator, and thus the groups, of the first gate of the run.

Caution

This is an early development prototype. Beware of changes to its interface without warning during the pre-release development of this package.