Qiskit Aer does not handle isolated Clbits

[taalexander]

Informations

• Qiskit Aer version: 0.11.2
• Python version: 3.9
• Operating system: OSX

What is the current behavior?

Qiskit Aer does not handle isolated qubits (ie., not in a register)

Steps to reproduce the problem

from qiskit import Aer, QuantumCircuit, execute
from qiskit.circuit import Qubit, Clbit

sim = Aer.get_backend("qasm_simulator")

bits = [Qubit(), Clbit(), Clbit()]
qc = QuantumCircuit(bits)
qc.x(bits[0])
qc.measure(bits[0], bits[1])
qc.measure(bits[0], bits[2])


result = execute(
    qc, backend=sim
).result()

Gives

Simulation failed and returned the following error message:
ERROR: Failed to load qobj: Invalid Qobj experiment: not enough memory slots.

What is the expected behavior?

Completes with

{'1 1': 1024}

Suggested solutions

As of https://github.com/Qiskit/qiskit-terra/blob/b4bc559228c3898817978c3d8c3fc97372beecad/qiskit/circuit/bit.py#L65
Qiskit Terra allows clbits to not be in registers (or be in multiple registers!). This seems to break result handling.

[hhorii]

@taalexander Could you test with 0.12.0? This looks similar to #1660 and resolved by #1717.

[taalexander]

I can confirm 0.12.0 resolves this.