Returns the bitstring in lexical order that corresponds to the given index in 0 to 2^(qubit_num)
    :param int index:
    :param int qubit_num:
    :return: the bitstring
    :rtype: str

def get_bitstring_from_index(index, qubit_num):
    """
    Returns the bitstring in lexical order that corresponds to the given index in 0 to 2^(qubit_num)
    :param int index:
    :param int qubit_num:
    :return: the bitstring
    :rtype: str
    """
    if index > (2**qubit_num - 1):
        raise IndexError("Index {} too large for {} qubits.".format(index, qubit_num))
    return bin(index)[2:].rjust(qubit_num, '0')