def test_nonzero_infidelity_gradient_with_degenerate_hamiltonian():
    """
    Tests that the infidelity gradient is non-zero for a degenerate non-trivial noisy system.
    """
    with tf.Graph().as_default() as g:
        pulse = tf.compat.v1.placeholder(shape=(2,), dtype=tf.float64)
        hamiltonian_pwc = TensorPwc([1, 1],
                                    (tf.cast(pulse[:, None, None], dtype=tf.complex128)
                                     * tf.constant(np.array([np.diag([1, 1, 2, 2]),
                                                             np.diag([1, 1, 2, 2])]),
                                                   dtype=tf.complex128)))
        target = Target(np.eye(4))

        infidelity = create_infidelity(hamiltonian_pwc=hamiltonian_pwc,
                                       noise_operator_pwcs=[],
                                       target=target)

        noise_operator_pwc = TensorPwc([2], tf.constant(np.diag([0, 0, 1, 1])[None],
                                                        dtype=tf.complex128))
        infidelity_noise = create_infidelity(hamiltonian_pwc=hamiltonian_pwc,
                                             noise_operator_pwcs=[noise_operator_pwc],
                                             target=target)

        grad = tf.gradients(infidelity, pulse)[0]
        grad_noise = tf.gradients(infidelity_noise, pulse)[0]

        with tf.compat.v1.Session() as session:
            grad_value = session.run(grad, feed_dict={pulse: np.array([2, 1])})
            grad_noise_value = session.run(grad_noise, feed_dict={pulse: np.array([2, 1])})
    # Changing the pulse should affect the infidelities, so the gradients should be non-zero.
    assert not np.any(np.isnan(grad_value))
    assert not np.allclose(grad_value, 0.)
    assert not np.any(np.isnan(grad_noise_value))
 NORMAL  ᚠ deg_fix!  tests/test_cost.py