How can I see the circuit that was actually executed on the device in pennylane?

Question

I do a VQE using the pennylane UCCSD ansatz for execution on the 'default.qubit' device and am interested in the actual circuit that is executed on the device (ideally without making changes in any pennylane code directly). Is there a way to retrieve it?

So far, I found out that the QNode constructs a QuantumTape which has operations and measurements. These operations are executed using execute() or new_execute(). Is that what is executed directly on the device or is there a step where something like QuantumTape.expand() is done. If yes, what controls the depth? Is there a further compilation step? How do I access the tape? What if I use a different device (eg. qiskitAer as plug-in)?

(Addition: If anyone could explain the queueing happening in the QNode in this context a bit as well I would be grateful.)

Follow up: Using the expasion_strategy argument of draw() as explained in the answer is very helpful. But is there a way to also show the transpiled circuit including the rotations for the measurement? Using draw() shows the measurement only as a whole (eg. '' in the example by Isaac), but when using eg. the qiskit plugin the full circuit including the pre-measurement rotations is transpiled, isn't it?

Answer 1

The PennyLane circuit drawer (both the text version qml.draw and the matplotlib version qml.draw_mpl) have keyword arguments that control how you want the circuit displayed. By default, it will print the circuit as defined in the QNode, but you can pass expansion_strategy="device" to instead display the circuit as it will be expanded for the device.

For example:

dev = qml.device("default.qubit", wires=4)
hf_state = np.array([1, 1, 0, 0], requires_grad=True)
coordinates = np.array([0.0, 0.0, -0.66140414, 0.0, 0.0, 0.66140414])
H, _ = qml.qchem.molecular_hamiltonian(["H", "H"], coordinates)
@qml.qnode(dev)
def circuit(weights):
    qml.UCCSD(
        weights,
        wires=[0, 1, 2, 3],
        init_state=hf_state,
        s_wires=[[0, 1, 2], [1, 2, 3]],
        d_wires=[[[0, 1], [2, 3]]],
    )
    return qml.expval(H)
weights = np.random.normal(0, np.pi, 3)

Drawing the circuit with no arguments:

>>> print(qml.draw(circuit)(weights))
0: ─╭UCCSD(M0)─┤ ╭<>
1: ─├UCCSD(M0)─┤ ├<>
2: ─├UCCSD(M0)─┤ ├<>
3: ─╰UCCSD(M0)─┤ ╰<>

Drawing the circuit with the expansion strategy set:

>>> print(qml.draw(circuit, expansion_strategy="device")(weights))
0: ─╭BasisState(M0)──H─────────╭●───────────────────────╭●──────────H──────────RX(-1.57)─╭●──────────────────────────────╭●──────────RX(1.57)───H─────────╭●──────────────────────────────╭●──────────H─────────H─╭●──────────────────────────────╭●──H──RX(-1.57)─╭●────────────────────────╭●──RX(1.57)──H─────────╭●────────────────────────╭●──H──────────RX(-1.57)─╭●────────────────────────╭●──────────RX(1.57)──RX(-1.57)─╭●───────────────────────────────╭●──RX(1.57)──RX(-1.57)─╭●─────────────────╭●──RX(1.57)───H─╭●──────────────────╭●─────────H──────────────────────────────────────────────────────────────────────────────┤ ╭<>
1: ─├BasisState(M0)──H─────────╰X─╭●─────────────────╭●─╰X──────────H──────────H─────────╰X─╭●─────────────────╭●────────╰X──────────H──────────RX(-1.57)─╰X─╭●─────────────────╭●────────╰X──────────RX(1.57)──H─╰X─╭●─────────────────╭●────────╰X──H──H─────────╰X─╭●──────────────────╭●─╰X──H─────────RX(-1.57)─╰X─╭●──────────────────╭●─╰X──RX(1.57)───RX(-1.57)─╰X─╭●──────────────────╭●─╰X──────────RX(1.57)──RX(-1.57)─╰X─╭●──────────────────╭●────────╰X──RX(1.57)────────────╰X─╭●───────────╭●─╰X───────────────╰X─╭●────────────╭●─╰X─────────RX(-1.57)─╭●──────────────────╭●──RX(1.57)───H─╭●─────────────────╭●─────────H─┤ ├<>
2: ─├BasisState(M0)──RX(-1.57)────╰X─╭●───────────╭●─╰X──RX(1.57)───RX(-1.57)───────────────╰X─╭●───────────╭●─╰X─────────RX(1.57)───RX(-1.57)───────────────╰X─╭●───────────╭●─╰X─────────RX(1.57)───H──────────────╰X─╭●───────────╭●─╰X─────────H──H───────────────╰X─╭●────────────╭●─╰X──H──H──────────────────────╰X─╭●────────────╭●─╰X──H──RX(-1.57)───────────────╰X─╭●────────────╭●─╰X──RX(1.57)───H──────────────────────╰X─╭●────────────╭●─╰X─────────H──H──────────────────────╰X──RZ(2.69)─╰X──H──RX(-1.57)───────╰X──RZ(-2.69)─╰X──RX(1.57)────────────╰X─╭●────────────╭●─╰X───────────────╰X─╭●───────────╭●─╰X───────────┤ ├<>
3: ─╰BasisState(M0)──H───────────────╰X──RZ(0.42)─╰X──H──RX(-1.57)─────────────────────────────╰X──RZ(0.42)─╰X──RX(1.57)──RX(-1.57)─────────────────────────────╰X──RZ(0.42)─╰X──RX(1.57)──RX(-1.57)────────────────────╰X──RZ(0.42)─╰X──RX(1.57)──H─────────────────────╰X──RZ(-0.42)─╰X──H──H────────────────────────────╰X──RZ(-0.42)─╰X──H──H─────────────────────────────╰X──RZ(-0.42)─╰X──H──RX(-1.57)────────────────────────────╰X──RZ(-0.42)─╰X──RX(1.57)──H──────────────────────────────────────────────────────────────────────────────────────────────────────╰X──RZ(-2.58)─╰X──H──RX(-1.57)───────╰X──RZ(2.58)─╰X──RX(1.57)────┤ ╰<>