QPY fails to serialize MCXGrayCode gate

[ElePT]

Environment

• Qiskit Terra version: 0.22.4
• Python version: 3.9
• Operating system: macOS Monterey 12.6

What is happening?

When you try to serialize a circuit with an MCXGrayCode gate using qpy, you get TypeError: __init__() missing 1 required positional argument: 'num_ctrl_qubits'. As far as I have seen, other mcx gates do not run into this issue.

I discovered this while trying to run iterative amplitude estimation using the runtime Sampler, which relies on qpy for serialization.

How can we reproduce the issue?

  from qiskit import QuantumCircuit, qpy
  circuit = QuantumCircuit(11)
  circuit.mcx(list(range(0,9)), 10)
  print('gate name:', circuit.data[0][0].name)

  with open('test.qpy', 'wb') as fd:
      qpy.dump(circuit, fd)

  with open('test.qpy', 'rb') as fd:
      new_qc = qpy.load(fd)[0]

This snippet will fail with a similar traceback to:

gate name: mcx_gray
  File "/Users/ept/Downloads/ExamplePEO.py", line 131, in <module>
    new_qc = qpy.load(fd)[0]
  File "/Users/ept/Desktop/qgt/qiskit/qpy/interface.py", line 269, in load
    loader(
  File "/Users/ept/Desktop/qgt/qiskit/qpy/binary_io/circuits.py", line 864, in read_circuit
    _read_instruction(file_obj, circ, out_registers, custom_operations, version, vectors)
  File "/Users/ept/Desktop/qgt/qiskit/qpy/binary_io/circuits.py", line 258, in _read_instruction
    gate = gate_class(*params)
TypeError: __init__() missing 1 required positional argument: 'num_ctrl_qubits'

What should happen?

It should not have failed.

Any suggestions?

I would modify the following line of qpy.binary_io.circuits.py to add "MCXGrayCode". There seem to be other gates that have been already accounted for this same issue, and adding "MCXGrayCode" to the list fixes it:

        if gate_name in {"MCPhaseGate", "MCU1Gate", "MCXGrayCode"}:
            gate = gate_class(*params, instruction.num_ctrl_qubits)

I'm happy to open a bugfix PR, but because I'm not that familiar with qpy, I wanted to document the problem in this issue first.