Combine the yoghurt, cream, sugar and lime juice and whisk until smooth. Pour the mixture into a syphon charged with gas and put in the fridge for at least 30 minutes to 1 hour.
Caramelise the inside of the passionfruit by heating them whole in a pan. When the skin begins to crack, take off the heat. Cut the passionfruit in half a scoop into the base of your serving glasses. Top with the chilled mousse from the syphon.