PF4.4 Differentiate and repear

Codebook
1

Having trouble with this and got stuck. Any help is appreciated.

dev = qml.device("default.qubit", wires = 2)

@qml.qnode(dev, diff_method = "parameter-shift", max_diff = 2)
def circuit_for_hessian(params):
"""
Implements the circuit shown in the codercise statement
Args:
- params (np.ndarray): [theta_0, theta_1, theta_2, theta_3]
Returns:
- np.tensor: 
"""
qml.RY(params[0], wires = 0)
qml.IsingXX(params[1], wires = (0,1))
qml.RX(params[2], wires = 0)
qml.RX(params[3], wires = 1)


return qml.expval(qml.PauliZ(0))


test_params = np.array([0.1,0.2,0.3,0.4], requires_grad = True)

# Don’t change test_params!

hessian = qml.jacobian(qml.jacobian(circuit_for_hessian))(test_params)
print("The hessian of the circuit is: \n", hessian)
2

Hi @simpsoe , welcome to the Forum!

I can see that you’ve deleted the description of what the circuit should return. Look at the image from the challenge. Is your circuit returning the same thing as the image?

I hope this helps!