Fix evolved operator ansatz

[sesajad]

Summary

This resolves #11680, which pertains to a bug encountered when creating an EvolvedOperatorAnsatz with an empty list of operators.

Details and comments

The changes implemented ensure that EvolvedOperatorAnsatz.num_qubits returns a sensible result (specifically zero) when operators is an empty list, rather than throwing an error. This adjustment alone enables the following code to function correctly:

e = EvolvedOperatorAnsatz(operators=[])

By solely doing this, in the above code, e.qregs contains a QuantumRegister with zero qubits. While its base class, QuantumCircuit, handles the case with no qubits by setting qregs to an empty list.

qiskit/qiskit/circuit/quantumcircuit.py

Line 1454 in d033e8a

if regs[0] == 0:

We have mirrored this behavior in EvolvedOperatorAnsatz to ensure that EvolvedOperatorAnsatz(operators=[]) behaves the same as QuantumCircuit(). This equivalence is also asserted in a test.

Please note that a brief release note has been added to the branch.

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Pull Request Test Coverage Report for Build 7942899973

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Details

• 0 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.01%) to 89.298%

---

Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/lex.rs	4	91.94%

Totals
Change from base Build 7938039454:	0.01%
Covered Lines:	58916
Relevant Lines:	65977

---

💛 - Coveralls

[Cryoris]

Thanks for the fix, this looks very good! I left some minor comments below.

[sesajad]

Hi @Cryoris , I know you must've been busy, I just wanted to make sure you didn't miss this.

[kevinhartman]

Looks good to me, thanks for the contribution!

Typically, I would request that we avoid accessing property method getters/setters from within the class's own implementation in most cases (e.g. prefer self._operators over self.operators). Chains of such accesses get confusing pretty quickly and can easily lead to initialization ordering bugs. This class is already guilty of this, and this PR doesn't add any additional use of this, so no changes requested.

[1ucian0]

@Mergifyio backport stable/0.46

[mergify]

backport stable/0.46

✅ Backports have been created

• #12479 Fix evolved operator ansatz (backport #11682) has been created for branch stable/0.46