Control Unitary Gates

Hi! How can I design a controlled-U gate?

Hi @Leeseok_Kim,

Depending on what type of controlled-U gate you wish to apply, there are several different possible solutions to this.

If you simply want to create an arbitrary controlled-unitary gate you would need to write down the matrix representation of it and then use qml.QubitUnitary to apply it (see here for details). Then you could e.g. define a two qubit arbitrary controlled-unitary like this:

CU  = np.array([[1, 0, 0, 0],
                [0, 1, 0, 0],
                [0, 0, a, b],
                [0, 0, c, d]])

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

@qml.qnode(dev)
def my_cu(U):
    ...
    qml.QubitUnitary(U, wires=[0, 1])
    ...
    return qml.expval(qml.PauliZ(1))

print(my_cu(CU))

where a, b, c and d would be the unitary that you wish to apply to the target qubit and ... could be replaced by any other operations you wish to apply in the circuit. This can be expanded to include several control qubits and/or several target qubits, as well as changing which qubits are control/target. You would need to write down and define these specific unitaries though.

In the above case you could also use the qml.CRot gate (see here) since you only have 1 control qubit and 1 target qubit. If you want to have more complicated unitaries you would need to use the above QubitUnitary approach.

Thank you so much! What if I already set up the U gate through lists of quantum gates not in terms of matrix and try to make it to controlled_U that is controlled by some specific wire index?

Should I do something like decomposing the U gate into matrix form and change that matrix and then use QubitUnitary thing?

Hi @Leeseok_Kim,

Unfortunately, I think that extracting the matrix representation and then using QubitUnitary to define a new gate (adding the conditional qubit to it in-between) might be necessary.

It would definitely be nice to be able to add controls to already existing operations, but as of now it’s possible in PennyLane in a straight-forward manner.

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