group_wise

group_wise(mapper_fn)

Wraps a mapper function so that it maps an operator one group at a time.

Rather than mapping the operator in one go, the wrapped function maps each of its groups separately and sums the results (see Group operator terms: use the operator structure). The sum is the same operator either way, but its Pauli terms come out grouped: the terms of one group are adjacent instead of interleaved with everybody else’s.

That matters because a product formula synthesizes the terms in the order it receives them. Where the terms of a group act on disjoint qubits, having them adjacent lets them be scheduled in parallel, which can reduce the two-qubit depth substantially at an unchanged gate count.

Use it to wrap any MapperFunction:

MapperFnEvolutionSynthesis(group_wise(jordan_wigner))

An operator without groups is mapped in one go, exactly as mapper_fn would on its own.

Parameters

mapper_fn (Callable[[OperatorTrait, int], SparseObservable]) – the mapper function to wrap.

Returns

A mapper function that maps mapper_fn over the operator’s groups and sums the results.

Return type

Callable[[OperatorTrait, int], SparseObservable]