Circuit decoding error

[samanthavbarron]

Environment

• Qiskit Terra version: 0.45.1
• Python version: 3.10.12
• Operating system: macOS 13.6 (22G120)

What is happening?

Serialization decoding of a circuit with qpy fails when qargs for a delay are passed in as a set and len(qargs) > 1. The error raised here is

ValueError: b'\x00' is not a valid Value

and when running the circuit on a device, the job will fail with something like:

Failed - UnicodeDecodeError: 'utf-8' codec can't decode byte 0xa0 in position 87: invalid start byte

How can we reproduce the issue?

from qiskit import QuantumCircuit, qpy

# This case fails when the delay is present
num_qubits = 2
qubits = {0, 1}

# This case always runs
#num_qubits = 1
#qubits = {0}

qc = QuantumCircuit(num_qubits)
qc.delay(duration=160, qarg=qubits)
qc.x(qubits)

with open("test.qpy", "wb") as f:
    qpy.dump(qc, f)

with open("test.qpy", "rb") as f:
    qpy.load(f)

What should happen?

This should run regardless of the cases described.

Any suggestions?

No response

[jakelishman]

This is a bug with QuantumCircuit.delay rather than QPY - it's the same thing as #11208 going on, where the initial QuantumCircuit is in a totally invalid state that's somehow not noticed. It's because of our old "friend" broadcasted arguments.

As an immediate work-around, turn the set into a list and the circuit will be valid (and roundtrip through QPY, etc).