Update qml.expval to correctly cast value type

doc/releases/changelog-dev.md

@@ -266,6 +266,9 @@
 
 <h3>Bug fixes 🐛</h3>
 
+* `qml.expval` now longer silently casts to a real number when observable coefficients are imaginary.
+  [(#6939)](https://github.com/PennyLaneAI/pennylane/pull/6939)
+
 * `qml.capture.PlxprInterpreter` now correctly handles propagation of constants when interpreting higher-order primitives
   [(#6913)](https://github.com/PennyLaneAI/pennylane/pull/6913)
 

pennylane/measurements/expval.py

@@ -162,5 +162,4 @@ def _calculate_expectation(self, probabilities):
         Args:
             probabilities (array): the probabilities of collapsing to eigen states
         """
-        eigvals = qml.math.cast_like(self.eigvals(), 1.0)
-        return qml.math.dot(probabilities, eigvals)
+        return qml.math.dot(probabilities, self.eigvals())

tests/measurements/test_expval.py

@@ -48,6 +48,17 @@ def qnode():
 class TestExpval:
     """Tests for the expval function"""
 
+    @pytest.mark.parametrize("coeffs", [1, 0.5, 0.5j, 0.5 + 0.5j])
+    def test_expected_dtype(self, coeffs):
+        """Test that the return type of the expval function is correct"""
+
+        @qml.qnode(qml.device("default.qubit"))
+        def circuit(coeffs):
+            return qml.expval(coeffs * qml.PauliZ(0))
+
+        res = circuit(coeffs)
+        assert np.allclose(res, coeffs)
+
     @pytest.mark.parametrize("shots", [None, 1111, [1111, 1111]])
     def test_value(self, tol, shots, seed):
         """Test that the expval interface works"""