>>> infidelity = graph.density_matrix_infidelity(
... np.array([[0.5, 0], [0, 0.5]]),
... np.array([[1, 0], [0, 0]]),
... name="infidelity",
... )
>>> result = qctrl.functions.calculate_graph(graph=graph, output_node_names=["infidelity"])
>>> result.output["infidelity"]["value"]
0.5