efficient_su2 & co should handle the num_qubits=1 case

[Cryoris]

Environment

• Qiskit version: 1.3.0rc2
• Python version: 3.11.9
• Operating system: macOS

What is happening?

Calling some library functions, like efficient_su2 don't work for num_qubits=1, since the entangling gate requires 2 qubits. This should still work, but simply not add an entangling gate.

We should check this edge case for all other functions, as well.

How can we reproduce the issue?

from qiskit.circuit.library import efficient_su2

efficient_su2(1)

What should happen?

Build a circuit without CX entangling gate. This should be handled in the efficient_su2 function and adjust the internal call to n_local.

Any suggestions?

No response

[trigpolynom]

I'd like to take this one on.

[Cryoris]

Great @trigpolynom! You can verify what should happen if you run the circuit-versions of these functions with a single qubit, e.g.

from qiskit.circuit.library import EfficientSU2, efficient_su2

circuit = EfficientSU2(1, flatten=True)
also_circuit = efficient_su2(1)  # should equal ``circuit``

We also need to add tests in this file and add a release note. You can check out the contributing guidelines on how to do that.

Let me know if you have any questions, I'm happy to discuss!

[trigpolynom]

Something like this @Cryoris ? https://github.com/Qiskit/qiskit/pull/13523/files