Qiskit / qiskit / 24895068213

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Committed by web-flow

Commit Message

Fix QPY loading delay integers durations incorrectly (#16076)

* Fix QPY loading delay integers durations incorrectly

This commit fixes an issue with parsing QPY payloads which have circuits
that contain delays with duration of units dt. Durations of dt are
integers and that is preserved in the QPY data. However our rust data
model doesn't have support for an integer in a Rust PackedInstruction's
parameters (this is an inconsistency which we should arguably fix but
that is separate from this bugfix). To workaround this the QPY reader in
rust was sticking the integer into a ParameterExpression as a constant
without any symbols. This resulted in storing the integer in Rust but
treating the object as a ParameterExpression and not an int, which in
Qiskit's rust data model is mapped to a Param::Obj (indicating a Python
object parameter). This mismatch in types was not really noticeable to
Python because the ParameterExpression with the constant integer was
coerced to an integer when it's passed to Python. However, this would
break underlying assumptions for Rust code that is interacting with the
delay. For example, the experimental rust qasm3 exporter would encounter
the ParameterExpression on the delay and error because it can't handle
parameter expressions yet. However fundamentally it could because this
is just an integer. The reproducer for this failure is:

```python
import io
from qiskit import qpy, qasm3, QuantumCircuit

qc = QuantumCircuit(1)
qc.delay(1, 0)

qasm3.dumps_experimental(qc)

with io.BytesIO() as fptr:
    qpy.dump(qc, fptr)
    fptr.seek(0)
    qc2 = qpy.load(fptr)[0]

qasm3.dumps_experimental(qc2)
```

The OQ3 experimental exporter is wrong not to handle
`ParameterExpression::try_as_value` returning an `int`, but it's _more_
wrong that QPY is producing a `ParameterExpression` on deserialisation
in the first place.

To fix this issue this commit removes the conversion of the `int` in the
qpy payload into a ParameterExpression and just... (continued)

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