fix recursion error from power simplification

doc/releases/changelog-dev.md

@@ -514,6 +514,9 @@
 
 <h3>Bug fixes 🐛</h3>
 
+* Simplifying operators raised to integer powers no longer causes recursion errors.
+  [(#8044)](https://github.com/PennyLaneAI/pennylane/pull/8044)
+
 * Fixes the GPU selection issue in `qml.math` with PyTorch when multiple GPUs are present.
   [(#8008)](https://github.com/PennyLaneAI/pennylane/pull/8008)
 

pennylane/operation.py

@@ -1714,7 +1714,7 @@ def __neg__(self):
         """The negation operation of an Operator object."""
         return qml.s_prod(scalar=-1, operator=self, lazy=False)
 
-    def __pow__(self, other: TensorLike):
+    def __pow__(self, other: TensorLike) -> "Operator":
         r"""The power operation of an Operator object."""
         if isinstance(other, TensorLike):
             return qml.pow(self, z=other)

pennylane/ops/op_math/pow.py

@@ -395,8 +395,9 @@ def simplify(self) -> Union["Pow", Identity]:
             ops = base.pow(z=self.z)
             if not ops:
                 return qml.Identity(self.wires)
-            op = qml.prod(*ops) if len(ops) > 1 else ops[0]
-            return op if qml.capture.enabled() else op.simplify()
+            if not qml.capture.enabled():
+                ops = [op.simplify() for op in ops]
+            return qml.prod(*ops) if len(ops) > 1 else ops[0]
         except PowUndefinedError:
             return Pow(base=base, z=self.z)
 

tests/ops/op_math/test_pow_op.py

@@ -108,6 +108,15 @@ def test_nonlazy_simplification_queueing(self):
 
         assert original_op not in q.queue
 
+    def test_simplify_squared(self):
+        """Test that an op without a special pow method can still be simplified when raised to an integer power."""
+
+        class DummyOp(qml.operation.Operator):
+            pass
+
+        simplified = (DummyOp(0) ** 2).simplify()
+        qml.assert_equal(simplified, DummyOp(0) @ DummyOp(0))
+
 
 @pytest.mark.parametrize("power_method", [Pow, pow_using_dunder_method, qml.pow])
 class TestInheritanceMixins: