KeyError: 'cr' when loading a circuit containing c_if using qpy

[jiannanWang]

Environment

• Qiskit version: 1.0.2
• Python version: 3.11.8
• Operating system: Ubuntu

What is happening?

Running the below circuit resulted in a key error. I would expect it to run without error.

How can we reproduce the issue?

from qiskit import QuantumCircuit, ClassicalRegister, QuantumRegister
from qiskit.circuit.library.standard_gates import RZXGate

qr = QuantumRegister(2, name='qr')
cr = ClassicalRegister(2, name='cr')
qc = QuantumCircuit(qr, cr, name='qc')
qc.append(RZXGate(0).c_if(cr, 0).repeat(1), qargs=qr, cargs=qc)

from qiskit import qpy
with open('circuit.qpy', 'wb') as fd:
    qpy.dump(qc, fd)
with open('circuit.qpy', 'rb') as fd:
    qc = qpy.load(fd)[0]

What should happen?

Traceback (most recent call last):
  File "/mnt/tests/reproduction/reproduce_cr_1003.py", line 13, in <module>
    qc = qpy.load(fd)[0]
         ^^^^^^^^^^^^
  File "/root/anaconda3/envs/qiskit1.0.2/lib/python3.11/site-packages/qiskit/qpy/interface.py", line 333, in load
    loader(
  File "/root/anaconda3/envs/qiskit1.0.2/lib/python3.11/site-packages/qiskit/qpy/binary_io/circuits.py", line 1263, in read_circuit
    custom_operations = _read_custom_operations(file_obj, version, vectors)
                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/anaconda3/envs/qiskit1.0.2/lib/python3.11/site-packages/qiskit/qpy/binary_io/circuits.py", line 516, in _read_custom_operations
    definition_circuit = common.data_from_binary(
                         ^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/anaconda3/envs/qiskit1.0.2/lib/python3.11/site-packages/qiskit/qpy/common.py", line 262, in data_from_binary
    obj = deserializer(container, **kwargs)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/anaconda3/envs/qiskit1.0.2/lib/python3.11/site-packages/qiskit/qpy/binary_io/circuits.py", line 1265, in read_circuit
    _read_instruction(
  File "/root/anaconda3/envs/qiskit1.0.2/lib/python3.11/site-packages/qiskit/qpy/binary_io/circuits.py", line 216, in _read_instruction
    _loads_register_param(condition_register, circuit, registers),
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/root/anaconda3/envs/qiskit1.0.2/lib/python3.11/site-packages/qiskit/qpy/binary_io/circuits.py", line 185, in _loads_register_param
    return registers["c"][data_bytes]
           ~~~~~~~~~~~~~~^^^^^^^^^^^^
KeyError: 'cr'

Any suggestions?

No response

[jakelishman]

Fixed by #11940.

[jakelishman]

Sorry, that last comment was a bit short. The problem you're seeing should have been fixed by #11940, due to be released in Qiskit 1.1.0 in May - I didn't want to backport it, because it's a bit too much of a change to an existing method for me to feel safe backporting it to stable patch release - but you can install from the main branch if you need it sooner.

Thanks for the report, and please feel free to re-open this if there's more problems I missed.

[jiannanWang]

Thank you for your quick response! Yeah I will try Qiskit 1.1.0 in May and see if it got fixed.