Fix evolved operator ansatz (backport #11682)

[mergify]

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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This is an automatic backport of pull request #11682 done by [Mergify](https://mergify.com).

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[coveralls]

Pull Request Test Coverage Report for Build 9365761074

Details

• 6 of 6 (100.0%) changed or added relevant lines in 1 file are covered.
• 2 unchanged lines in 2 files lost coverage.
• Overall coverage increased (+0.009%) to 86.958%

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Files with Coverage Reduction	New Missed Lines	%
crates/qasm2/src/expr.rs	1	94.02%
crates/qasm2/src/lex.rs	1	93.13%

Totals
Change from base Build 9314544782:	0.009%
Covered Lines:	75020
Relevant Lines:	86272

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💛 - Coveralls