⚡️ Use Gate in AE phase_estimation path

[AxEquals-b]

Description

This PR fixes a performance issue in the ae benchmark by passing a Gate (instead of a QuantumCircuit) to phase_estimation.

What changed

• File: src/mqt/bench/benchmarks/ae.py
• One-line change:
  • from: pe = phase_estimation(num_eval_qubits, grover_operator)
  • to: pe = phase_estimation(num_eval_qubits, grover_operator.to_gate(label="Q"))

Why

qiskit.circuit.library.phase_estimation accepts both QuantumCircuit and Gate, and documents that passing a Gate can be more performant because control/power operations can use optimized subroutines.

Correctness validation (n = 2..16)

I compared old vs. new construction for all n in [2, 16]:

1. Build both circuits.
2. Remove final measurements.
3. Compare resulting statevectors using fidelity.

Result:

• All checks passed for n=2..16.
• Minimum fidelity: 0.9999999999999802 (numerical precision tolerance).

Performance validation (same environment, same run, n = 2..16)

n	before (s)	after (s)	speedup
2	0.033425	0.006768	4.9x
3	0.003550	0.003284	1.1x
4	0.005631	0.004448	1.3x
5	0.010618	0.005664	1.9x
6	0.012951	0.006786	1.9x
7	0.021017	0.007985	2.6x
8	0.036682	0.009122	4.0x
9	0.063978	0.010370	6.2x
10	0.118278	0.011781	10.0x
11	0.258973	0.013150	19.7x
12	0.544852	0.015034	36.2x
13	1.212202	0.016025	75.6x
14	2.685636	0.017285	155.4x
15	5.434519	0.018494	293.9x
16	13.701815	0.020821	658.1x

No additional dependencies are required.

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• I have added entries to the changelog for any noteworthy additions, changes, fixes, or removals.
• I have added migration instructions to the upgrade guide (if needed).
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

If PR contains AI-assisted content:

• I have disclosed the use of AI tools in the PR description as per our AI Usage Guidelines.
• AI-assisted commits include an Assisted-by: [Model Name] via [Tool Name] footer.
• I confirm that I have personally reviewed and understood all AI-generated content, and accept full responsibility for it.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: ASSERTIVE

Plan: Pro

Run ID: 0ddad19b-0938-476b-a0a5-86b5be9a9c69

📥 Commits

Reviewing files that changed from the base of the PR and between ca1df6c and 1f0d69a.

📒 Files selected for processing (1)

• src/mqt/bench/benchmarks/ae.py

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📝 Walkthrough

Summary by CodeRabbit

• Refactor
  • Updated phase estimation circuit construction for improved operator representation in benchmark measurements.

Walkthrough

The phase estimation circuit now wraps the Grover operator into a named gate before passing it to the phase estimation function, changing the input type from a QuantumCircuit object to a Gate object while preserving all downstream circuit operations.

Changes

Cohort / File(s)	Summary
Phase Estimation Gate Wrapping src/mqt/bench/benchmarks/ae.py	Modified Grover operator input to phase estimation by wrapping it with to_gate(label="Q"), converting from direct QuantumCircuit to Gate representation.

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~2 minutes

Poem

🐰 A gate wrapped neat in label "Q",
The Grover operator finds its view,
One line changed with careful grace,
Phase estimation finds its place! ✨

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and specifically describes the main change: converting a QuantumCircuit to a Gate in the AE phase_estimation code path.
Description check	✅ Passed	The description is comprehensive with detailed validation, performance data, and reasoning; however, unchecked items show tests, docs, changelog, and CI verification remain incomplete.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!

[burgholzer]

Nice, thanks! 🙏