Granted, it will be far too small for her to show off to her friends, but if your potential fiancée has a love of science, she just might accept this bauble over something flashier. It's the world's smallest diamond ring, created by a group of Australian physicists. The ring measures just 5 microns wide and 300 nanometers thick. And no, it's not really for advertising your engagement. The ring is actually part of a device used to produce and detect single photons.