In the introduction to a paper in press in the Journal of Biotechnology, Virgile Adam of the Katholieke Universiteit Leuven in Belgium, along with a long list of collaborators from other institutions, describe the ultimate in holographic (three-dimensional) data storage: a chemically pure crystal composed solely of proteins that can be read and reversibly switched between at least two different states using nothing but light.
Embedded within the proper array of lasers (it would take at least two), such a crystal would represent something approaching the theoretical limit of data density in a storage medium: each bit would be represented by a single molecule.
With their latest work, Adam et al. take us a step closer to this dream, at least in the laboratory. Not only did they manage to encode and read data on chemically pure crystals of mutant descendants of fluorescent proteins originally derived from corals, but they also demonstrated that at least one of these proteins, known as IrisFP, actually has the ability to store data in four different states, versus the two different states (on and off) encoded by a traditional bit. In other words, this protein could store data in base 4 instead of base 2.