def tile_to_quadkey(tile_x, tile_y, level):
    """This is a function that converts tile coordinates at a certain
    level of detail of a Bing Map to a unique string identifier (QuadKey).
    
    :param tile_x: The x axis coordinate of the tile at `level` level of detail
    :param tile_y: The y axis coordinate of the tile at `level` level of detail
    :param level: The level of detail of the Bing Map
    :return: A `quadkey` string of length given by the `level` level of detail
    """
    ql = []
    for i in range(level, 0, -1):
        digit = ord('0')
        mask = 1 << (i-1)
        if ((tile_x & mask) != 0):
            digit+=1
        if ((tile_y & mask) != 0):
            digit+=2
        ql.append(chr(digit))
    quadkey = ''.join(ql)
    return quadkey