Add the CONTROLLED modifier to the gate with the given control qubit.

def controlled(self, control_qubit):
        """
        Add the CONTROLLED modifier to the gate with the given control qubit.
        """
        control_qubit = unpack_qubit(control_qubit)

        self.modifiers.insert(0, "CONTROLLED")
        self.qubits.insert(0, control_qubit)

        return self