Add pow for Parameter

Adds powering for qiskit.circuit.ParameterExpression allowing to use a**n, n**a, a**b, pow(a,b) etc. for ParameterExpressions a, b and numbers n. Minimal change using default support of pow by Sympy/Symengine.
Added pow to list of operators in TestParameterExpressions test case for unit testing.
fixes Qiskit#8959
Changelog: New Feature

qiskit/circuit/parameterexpression.py

@@ -362,6 +362,12 @@ def __truediv__(self, other):
     def __rtruediv__(self, other):
         return self._apply_operation(operator.truediv, other, reflected=True)
 
+    def __pow__(self, other):
+        return self._apply_operation(pow, other)
+
+    def __rpow__(self, other):
+        return self._apply_operation(pow, other, reflected=True)
+
     def _call(self, ufunc):
         return ParameterExpression(self._parameter_symbols, ufunc(self._symbol_expr))
 

releasenotes/notes/add-parameter-pow-ff5f8d10813f5733.yaml

@@ -0,0 +1,7 @@
+---
+features:
+  - |
+    :class:`~qiskit.circuit.ParameterExpression` (and thus also
+    :class:`~qiskit.circuit.Parameter`) now support powering: :code:`x**y`
+    where :code:`x` and :code:`y` can be any combination of
+    :class:`~qiskit.circuit.ParameterExpression` and number types.

test/python/circuit/test_parameters.py

@@ -1332,7 +1332,7 @@ def _paramvec_names(prefix, length):
 class TestParameterExpressions(QiskitTestCase):
     """Test expressions of Parameters."""
 
-    supported_operations = [add, sub, mul, truediv]
+    supported_operations = [add, sub, mul, truediv, pow]
 
     def setUp(self):
         super().setUp()