Printing the matrix of a circuit during optimization

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Hi @andrew1, welcome to the forum!

Good question! This isn’t something that’s done by default in PennyLane due to the potential overhead of calculating and storing the unitary as the number of qubits increases. However, it’s possible to put together some code to recover the unitary, for example:

import pennylane as qml
from pennylane import numpy as np

qml.enable_tape()

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


def template(x):
    qml.RX(x, wires=0)
    qml.RY(0.3, wires=1)
    qml.CNOT(wires=[0, 1])


def find_unitary(template, *args, **kwargs):

    @qml.qnode(dev)
    def find_output_state(input_state):
        qml.QubitStateVector(input_state, wires=range(wires))
        template(*args, **kwargs)
        return qml.state()
    
    input_states = np.eye(2 ** wires)
    
    return np.stack([find_output_state(state) for state in input_states]).T


U = find_unitary(template, 0.4)

print(U)

In the example above, you can update your circuit by changing the contents of template() (and the value of wires accordingly).

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