[BugFix] - PennyLane raises an informative error if running circuit(s…

…) with dynamic tracers as wires (#6405)

**Context:** Pennylane provides incorrect results or non-informative
errors (depending on the circumstances) when working with `@jax.jit` and
dynamic wires (that is, JAX tracers) at the same time. For example:

```
dev = qml.device("default.qubit")

@jax.jit
@qml.qnode(dev)
def circuit(input_wires):
    qml.Hadamard(input_wires[1])
    return qml.probs(wires=[0, 1])

circuit([0, 1])

```

**Description of the Change:** Pennylane raises an informative error in
the `validate_device_wires` transform to inform the user that abstract
wires are not currently supported. We cannot raise this error in the
`Wires` class since this would be a problem for Catalyst. This solution
should not (hopefully) cause issues in other repositories.

**Benefits:** This prevents non-informative errors and, most
importantly, wrong results.

**Possible Drawbacks:** This change could potentially break existing
code(s) that somehow used abstract wires and for some reason they worked
fine.

**Related GitHub Issues:** #6380 

**Related ShortCut Stories:** [sc-75756]

doc/releases/changelog-dev.md

@@ -277,6 +277,9 @@
 
 <h3>Bug fixes 🐛</h3>
 
+* The `validate_device_wires` transform now raises an error if abstract wires are provided.
+  [(#6405)](https://github.com/PennyLaneAI/pennylane/pull/6405)
+
 * Fixes `qml.math.expand_matrix` for qutrit and arbitrary qudit operators.
   [(#6398)](https://github.com/PennyLaneAI/pennylane/pull/6398/)
 

pennylane/devices/preprocess.py

@@ -127,9 +127,23 @@ def validate_device_wires(
         The unaltered input circuit. The output type is explained in :func:`qml.transform <pennylane.transform>`.
 
     Raises:
-        WireError: if the tape has a wire not present in the provided wires.
+        WireError: if the tape has a wire not present in the provided wires, or if abstract wires are present.
     """
+
+    if any(qml.math.is_abstract(w) for w in tape.wires):
+        raise WireError(
+            f"Cannot run circuit(s) on {name} as abstract wires are present in the tape: {tape.wires}. "
+            f"Abstract wires are not yet supported."
+        )
+
     if wires:
+
+        if any(qml.math.is_abstract(w) for w in wires):
+            raise WireError(
+                f"Cannot run circuit(s) on {name} as abstract wires are present in the device: {wires}. "
+                f"Abstract wires are not yet supported."
+            )
+
         if extra_wires := set(tape.wires) - set(wires):
             raise WireError(
                 f"Cannot run circuit(s) on {name} as they contain wires "

pennylane/wires.py

@@ -71,6 +71,7 @@ def _process(wires):
         # Note, this is not the same as `isinstance(wires, Iterable)` which would
         # pass for 0-dim numpy arrays that cannot be iterated over.
         tuple_of_wires = tuple(wires)
+
     except TypeError:
         # if not iterable, interpret as single wire label
         try:

tests/devices/test_preprocess.py

@@ -190,6 +190,37 @@ def test_fill_in_wires(self):
         assert batch[0].operations == tape1.operations
         assert batch[0].shots == tape1.shots
 
+    @pytest.mark.jax
+    def test_error_abstract_wires_tape(self):
+        """Tests that an error is raised if abstract wires are present in the tape."""
+
+        import jax
+
+        def jit_wires_tape(wires):
+            tape_with_abstract_wires = QuantumScript([qml.CNOT(wires=qml.wires.Wires(wires))])
+            validate_device_wires(tape_with_abstract_wires, name="fictional_device")
+
+        with pytest.raises(
+            qml.wires.WireError,
+            match="on fictional_device as abstract wires are present in the tape",
+        ):
+            jax.jit(jit_wires_tape)([0, 1])
+
+    @pytest.mark.jax
+    def test_error_abstract_wires_dev(self):
+        """Tests that an error is raised if abstract wires are present in the device."""
+
+        import jax
+
+        def jit_wires_dev(wires):
+            validate_device_wires(QuantumScript([]), wires=wires, name="fictional_device")
+
+        with pytest.raises(
+            qml.wires.WireError,
+            match="on fictional_device as abstract wires are present in the device",
+        ):
+            jax.jit(jit_wires_dev)([0, 1])
+
 
 class TestDecomposeValidation:
     """Unit tests for helper functions in qml.devices.qubit.preprocess"""