Three-dimensional storage, a phenomena which has eluded systems, uses electrical interconnects to address the storage locations, such as conventional electronic and magnetic memories. Due to the uncharged nature of photons, light beams can pass through a 3-D transparent storage media to address any site in the storage volume. Although individual sites can be addressed in some of the 3-D systems, the tremendous capacity (thousands of petabits or more) leads to page addressing. Although the access time for locating a page may be relatively slow (microseconds or more), the readout data rates have the potential to be exceptionally fast (megabits per nanosecond).
Numerous 3-D storage systems have been proposed, but those appearing to receive the most attention are: holostore based on photorefractive rods (potential capacity of a terabit and data transfer rate of 10 terabits per second) [], two-photon 3-D memory (potential capacity of 0.1 petabit and data transfer rate of a terabit per second) [], and spectral hole burning. Larger capacities than a terabit have been proposed for holographic storage by combining it with spectral hole burning (4-D memory) to achieve capacities of a petabit, but the high risk of such a storage concept at the present time resulted in the working group not considering to propose this for a PetaFLOPS program.