A PauliProductMeasurement serialized in 2.4 can't be deserialized in 2.3

[ihincks]

Environment

• Qiskit version: 2.4.1 (writer); 2.3.x (reader)
• Python version:
• Operating system:

I am opening this issue in collaboration with Claude 4.6, who originally found it, and did the technical writing below. I verified that the snippet fails as described.

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What is happening?

A QPY file containing a PauliProductMeasurement gate written by Qiskit 2.4 cannot be loaded by Qiskit 2.3. The Rust writer in 2.4 serializes the three parameters of PauliProductMeasurement as two GenericValue::Tuple<Bool> (the z and x bit arrays) and one GenericValue::Bool (the neg flag). The Bool type uses type key b'b'.

The Python 2.3 _loads_instruction_parameter in circuits.py handles Container.TUPLE = b"t" directly (before reaching loads_value()), so the outer tuple level is read successfully. The failure occurs one level deeper: each inner Bool element has type key b'b', which falls through all explicit checks in _loads_instruction_parameter to the else branch, which calls value.loads_value(b'b', ...). That function tries type_keys.Value(b'b') and raises:

ValueError: b'b' is not a valid Value

because b'b' is not a member of the 2.3 Value enum. The 2.4→2.4 round-trip works because the Rust reader has ValueType::Bool = b'b'. The bug only manifests in the cross-version 2.4→2.3 direction.

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How can we reproduce the issue?

# Step 1: in a Qiskit 2.4 environment — write the file
import io
from qiskit import QuantumCircuit
from qiskit.circuit.library import PauliProductMeasurement
from qiskit.quantum_info import Pauli
from qiskit import qpy

ppm = PauliProductMeasurement(Pauli("ZXY"))
qc = QuantumCircuit(ppm.num_qubits, ppm.num_clbits)
qc.append(ppm, range(ppm.num_qubits), range(ppm.num_clbits))

with open("/tmp/ppm.qpy", "wb") as f:
    qpy.dump(qc, f)
print("Written /tmp/ppm.qpy")

# Step 2: in a Qiskit 2.3 environment — try to load the file
from qiskit import qpy
with open("/tmp/ppm.qpy", "rb") as f:
    qpy.load(f)   # raises ValueError: b'b' is not a valid Value

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What should happen?

A QPY file written by Qiskit 2.4 containing standard library gates such as PauliProductMeasurement should be loadable by Qiskit 2.3. QPY version 17 should be forwards-compatible for gates that existed in 2.3.

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Any suggestions?

[jakelishman]

Sounds like the 2,4 serialisation is wrong - b is not a valid type key for INSTRUCTION_PARAM to the best of my knowledge.

[gadial]

Sounds like the 2,4 serialisation is wrong - b is not a valid type key for INSTRUCTION_PARAM to the best of my knowledge.

It's definitely wrong. It's a mistake I introduced at a stage I did not understand the ways in which forward comptibility will be compromised and basically did QPY18 without raising the QPY version to 18. Thankfully the fix here is trivial.

I am adding a reproducing test to the forwards-compatibility PR #16041, after verifying it reproduces the error I can also make the fix as part of that PR.

[gadial]

Turns out the issue was worse - adding the forwards compatibility test revealed a backwards compatibility issue as well, since 2.3 used numpy arrays to encode the boolean z_values and x_values and 2.4 code cannot (yet) coerce numpy objects into Vec<bool>. I'm trying to find the least messy way to solve this.