multi-control gate attributes not preserved by QPY

[richrines1]

Environment

• Qiskit Terra version: 0.45.1
• Python version: 3.11
• Operating system: ubuntu

What is happening?

after QPY serialization + deserialization

• the ctrl_state of multi-controlled gates (e.g. MCXGate, MCPhaseGate, ...) is reverted to its default value.
• MCXVChain._dirty_ancillas is always False

How can we reproduce the issue?

circuit = qiskit.QuantumCircuit(6)
for gate in [
    qiskit.circuit.library.MCU1Gate(1.23, 5, ctrl_state=0),
    qiskit.circuit.library.MCXGate(5, ctrl_state=0),
    qiskit.circuit.library.MCXGrayCode(5, ctrl_state=0),
    qiskit.circuit.library.MCXRecursive(4, ctrl_state=0),
    qiskit.circuit.library.MCXVChain(3, ctrl_state=0, dirty_ancillas=False),
    qiskit.circuit.library.MCXVChain(3, ctrl_state=0, dirty_ancillas=True),
]:
    circuit.append(gate, range(gate.num_qubits))

print(circuit)
print(circuit[-1].operation._dirty_ancillas)

buf = io.BytesIO()
qiskit.qpy.dump(circuit, buf)
circuit = qiskit.qpy.load(io.BytesIO(buf.getvalue()))[0]

print(circuit)
print(circuit[-1].operation._dirty_ancillas)

prints:

q_0: ─o───────────o────o────o────o─────o───
      │           │    │    │    │     │   
q_1: ─o───────────o────o────o────o─────o───
      │           │    │    │    │     │   
q_2: ─o───────────o────o────o────o─────o───
      │           │    │    │  ┌─┴──┐┌─┴──┐
q_3: ─o───────────o────o────o──┤0   ├┤0   ├
      │           │    │  ┌─┴─┐│  X ││  X │
q_4: ─o───────────o────o──┤ X ├┤1   ├┤1   ├
      │U1(1.23) ┌─┴─┐┌─┴─┐└───┘└────┘└────┘
q_5: ─■─────────┤ X ├┤ X ├─────────────────
                └───┘└───┘                 
True
                                           
q_0: ─■───────────■────■────■────■─────■───
      │           │    │    │    │     │   
q_1: ─■───────────■────■────■────■─────■───
      │           │    │    │    │     │   
q_2: ─■───────────■────■────■────■─────■───
      │           │    │    │  ┌─┴──┐┌─┴──┐
q_3: ─■───────────■────■────■──┤0   ├┤0   ├
      │           │    │  ┌─┴─┐│  X ││  X │
q_4: ─■───────────■────■──┤ X ├┤1   ├┤1   ├
      │U1(1.23) ┌─┴─┐┌─┴─┐└───┘└────┘└────┘
q_5: ─■─────────┤ X ├┤ X ├─────────────────
                └───┘└───┘                 
False

What should happen?

both outputs should be the same, i.e. with open-controls, and _dirty_ancillas=True for the final operation

Any suggestions?

No response

[jakelishman]

Thanks for this report. I agree, this just looks like an oversight in QPY - these multi-controlled gate objects aren't heavily used in most cases (they're more properly compiler syntheses, but for historical reasons exposed through the circuit library), which is probably how they've got overlooked. You're right that they need their additional attributes handling specially in QPY.

[fbolton]

@jakelishman I'd be interested in tackling this issue as a first contribution to the Qiskit codebase. Could you assign this issue to me?