How giant digital adverts could soon be in our cities   Passers-by on a London street were recently amazed to see a fleeting image of a pink tongue protruding from fruitily plump lips, seemingly suspended in mid-air. It was the famous logo for the Rolling Stones and was part of an experiment by tech start-up Lightvert.a Its technology can produce images that appear to be 200m (656ft) high, but which only exist in the eye of the viewer for a fraction of a second. So could we be on the verge of seeing giant digital ads in our cities, similar…