`Clifford.to_circuit()` returns a circuit with emtpy `qregs`

[matteoacrossi]

Environment

• Qiskit version: 1.2
• Python version: 3.10
• Operating system: Linux

What is happening?

Clifford.to_circuit() returns a QuantumCircuit with empty qregs. I don't know if this is an expected behaviour, but it has changed since qiskit 1.1.2.

How can we reproduce the issue?

from qiskit import QuantumCircuit
from qiskit.quantum_info import Clifford

qc = QuantumCircuit(2)
qc.h(0)
qc.cx(0, 1)
cli = Clifford(qc)
cli.to_circuit().qregs

What should happen?

I would expect the QuantumCircuit to contain one QuantumRegister with the correct size, as it was the case in qiskit 1.1.2

>>> cli.to_circuit().qregs
[QuantumRegister(2, 'q')]

Any suggestions?

No response

[matteoacrossi]

With git bisect I found out that the regression was introduced in #12714

[jakelishman]

This change was unintentional and we will backport to fix it, but as a follow up: what are you doing that requires the QuantumRegister? The quantum register type is very close to meaningless in Qiskit, and I'm not sure how other libraries might be using it.

(Fwiw, registers are neither necessary for a circuit nor required to contain only unique bits, so the output of the function is still a valid circuit object, it's only a bug that we deviated from the prior behaviour without meaning to.)