Codercise PF.2.4 - Gates under control

Codebook
1

Why this returns Error: Incorrect: The output of ctrl_circuit doesn’t look quite right!.

dev = qml.device("default.qubit", wires=3)

@qml.qnode(dev)
def ctrl_circuit(theta, phi):
    # Left column
    qml.RY(phi, wires=0)
    qml.H(wires=1)
    qml.RX(theta, wires=2)

    # Middle column
    qml.ctrl(qml.S, control=0)(wires=1)  # black dot control

    qml.X(wires=1)                       # white dot on control-0
    qml.ctrl(qml.T, control=1)(wires=2)
    qml.X(wires=1)

    # Right column
    qml.H(wires=0)

    return qml.state()
2

Hi @l.a.moncayo-martinez, welcome to the Forum!
You are very close to the correct answer. However, notice the wire coming out the Hadamard in what you called “right column”. It is a controlled Hadamard. Fixing that should give you the right answer.

I hope this helps.

3

Absolutely. Thanks,

dev = qml.device("default.qubit", wires = 3)

@qml.qnode(dev)
def ctrl_circuit(theta,phi):
    """Implements the circuit shown in the Codercise statement
    Args:
        theta (float): Rotation angle for RX
        phi (float): Rotation angle for RY
    Returns:
        (numpy.array): The output state of the QNode
    """

    ####################
    ###YOUR CODE HERE###
    ####################
    qml.RY(phi, wires=0)
    qml.Hadamard(wires=1)
    qml.RX(theta, wires=2)

    # Controlled-S (control=0, target=1) — standard control (●)
    qml.ctrl(qml.S, control=0)(wires=1)

    # Controlled-T with inverted control (○ control=1, target=2)
    qml.X(wires=1)                                # Flip qubit 1 to simulate control-|0⟩
    qml.ctrl(qml.T, control=1)(wires=2)
    qml.X(wires=1)                                # Flip back

    # Controlled-H (control=0, target=2) — standard control (●)
    qml.ctrl(qml.Hadamard, control=2)(wires=0)
    
    return qml.state()
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