Port PauliEvolutionGate to Rust

[davidfcohen]

Summary

We'd like a Rust-native data model that fills the same role as Python's PauliEvolution gate.

I used SparseObservable to represent the hermitian operator and Param to represent the time. My implementation considers Param::Obj an invariant for time.

The caller can inspect the operator and time with getter methods. The caller can also take ownership of operator and time with SparseObservable::into_parts.

PauliEvolution is PartialEq such that two instances are equal if both operations have an equivalent operator and time. We can compare time without considering Param::Obj because it's an invariant.

PauliEvolution implements Operation and CustomOperation so that it can operate in a circuit. We've skipped the matrix and label members because there's an ongoing effort to decide how those should behave.

Based on the issue description, it appears this module is intended to be used independent of Python, with a Python API coming later. FWIW, I believe it should be relatively straightforward to wrap this in a PyPauliEvolution using PyO3.

closes #15450

AI/LLM disclosure

• I didn't use LLM tooling, or only used it privately.
• I used the following tool to help write this PR description:
• I used the following tool to generate or modify code:

[Cryoris]

Not sure whether you're already looking for reviews, but given that you opened this as draft PR here are some 🙂

[Cryoris]

I would just use Param for the time parameter, that'll also give the compatibility with Python and C for free.

[Cryoris]

Oh I just saw 05d314a now - what's the problem with Param?

[davidfcohen]

If we want to use Param as the internal representation, we can create struct ComparableParam(Param); and implement PartialEq manually. What do you think?

[Cryoris]

Param has eq implemented, why can't we use Param directly?

[davidfcohen]

Param is not PartialEqwhich is a trait requirement for ComparableOp which is a trait requirement for Operation.

[qiskit-bot]

One or more of the following people are relevant to this code:

• @Qiskit/terra-core

[coveralls]

Coverage Report for CI Build 33200982562

Coverage decreased (-0.01%) to 87.734%

Details

• Coverage decreased (-0.01%) from the base build.
• Patch coverage: 39 uncovered changes across 1 file (57 of 96 lines covered, 59.38%).
• 2 coverage regressions across 2 files.

Uncovered Changes

File	Changed	Covered	%
crates/circuit_library/src/pauli_evolution_gate.rs	96	57	59.38%

Coverage Regressions

2 previously-covered lines in 2 files lost coverage.

File	Lines Losing Coverage	Coverage
crates/circuit/src/parameter/parameter_expression.rs	1	90.32%
crates/qasm2/src/lex.rs	1	93.06%

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Coverage Stats

Relevant Lines:	131175
Covered Lines:	115085
Line Coverage:	87.73%
Coverage Strength:	979360.12 hits per line

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💛 - Coveralls