Computers and the Internet have led to an explosion of easily available information. Somewhere, somehow, all of this information must be stored and processed, quickly and economically. Indeed, the decreasing cost of storing data has been a crucial impetus for the information revolution. But magnetic and conventional optical data storage, in which individual bits are stored on the surface of a recording medium, are approaching physical limits beyond which individual bits may be too small to easily write, store or read. Storing information throughout the volume of a medium, rather than just on its surface, is an intriguing high-capacity alternative. One such volumetric approach, holographic data storage, was conceived decades ago. But it tends to be fragile — reading out information tends to destroy what is written. On page 665of this issue1, Buse et al. describe a way to make holographic storage much more durable.
In holographic data storage2, an entire page of information — a million or so data bits — is stored at once as an interference pattern within a thick, photosensitive optical material. This is done by crossing a so-called object wave (typically a laser beam, on which the information has been imprinted by a suitable light modulator, such as a small liquid-crystal TV screen) with a reference beam carrying no added information. A large number of these interference gratings or patterns can be stored in the same piece of material, as long as they are distinguishable by their direction or spacing. That can be accomplished by changing the angle between the object and reference wave or by changing the laser wavelength. A page is read out by illuminating the material with the reference wave that was used to store that page: the wave is scattered in such a fashion that the object beam is reconstructed. The theoretical limit for the storage density of this technique is around tens of terabits per cubic centimetre, although the densities that have been demonstrated so far are three orders of magnitude smaller, comparable to a standard disk drive.
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