Parameter conjugate (#4619)

* add reno

* add test

qiskit/circuit/parameterexpression.py

@@ -45,6 +45,10 @@ def parameters(self):
         """Returns a set of the unbound Parameters in the expression."""
         return set(self._parameter_symbols.keys())
 
+    def conjugate(self):
+        """Return the conjugate, which is the ParameterExpression itself, since it is real."""
+        return self
+
     def bind(self, parameter_values):
         """Binds the provided set of parameters to their corresponding values.
 

releasenotes/notes/parameter-conjugate-a16fd7ae0dc18ede.yaml

@@ -0,0 +1,7 @@
+---
+fixes:
+  - |
+    Add a ``conjugate`` method to the ``ParameterExpression`` to allow
+    calling ``numpy.conj()`` without raising an error. Since the 
+    ``ParameterExpression`` is real, it returns itself.
+    This behaviour is analogous to Python floats/ints.

test/python/circuit/test_parameters.py

@@ -1189,6 +1189,11 @@ def test_substituting_compound_expression(self):
 
         self.assertEqual(updated_expr, expected)
 
+    def test_conjugate(self):
+        """Test calling conjugate on a ParameterExpression."""
+        x = Parameter('x')
+        self.assertEqual(x, x.conjugate())  # Parameters are real, therefore conjugate returns self
+
 
 class TestParameterEquality(QiskitTestCase):
     """Test equality of Parameters and ParameterExpressions."""