Internally used.

def qubit_pairs(args, length):
    """Internally used."""
    if not isinstance(args, tuple):
        raise ValueError("Control and target qubits pair(s) are required.")
    if len(args) != 2:
        raise ValueError("Control and target qubits pair(s) are required.")
    controls = list(slicing(args[0], length))
    targets = list(slicing(args[1], length))
    if len(controls) != len(targets):
        raise ValueError("The number of control qubits and target qubits are must be same.")
    for c, z in zip(controls, targets):
        if c == z:
            raise ValueError("Control qubit and target qubit are must be different.")
    return zip(controls, targets)