AS HARD DRIVES GET MORE and more cramped, digital space is running out. For decades, researchers have hoped that holograms created in the subatomic structure of crystals might hold an answer. Stanford University physicists have demonstrated the first fully digital model of such a device, storing and retrieving the "Mona Lisa." The crystal held only 163 kilobytes of memory comparable to some early home computers but researchers think holographic units could hold a million megabits. Crystals store in three dimensions and could be 10 times faster than today's fastest systems.