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Optical memory

The secret of the stone

While casually sipping his Martini, a secret agent scans the glamorous guests filling the room. His eyes rest on an innocent-looking woman in a simple dress adorned with a single pale stone. Has he recognized that the stone is the key to all the information that is holding his adversary's evil organization together?

Yuko Ishii et al. would have recognized this hidden device. They have developed a holographic information storage system in which a piece of the mineral ulexite is a key component (Y. Ishii et al. Opt. Express 15, 7218–7223; 2007). As each stone is unique and irreproducible, only the stone that is used to store the information can retrieve it again.

In holographic memory, information is stored optically in a light-sensitive crystal. An 'object' laser beam (containing information) produces an interference pattern with a reference beam, and the pattern is inscribed on the crystal. Information is retrieved again by using a third beam, in combination with the reference laser.

The beauty of the technique is that multiple images can be stored in the same volume, by slightly changing the settings of the reference beam at each recording step.

Although it cannot yet beat magnetic disks in terms of price and performance, holographic data storage has its own advantages, and could become more competitive if its storage capacity could be improved. One way to achieve this is to use random reference beams, which can be generated by randomly rotating a diffuser in front of the reference laser. But this requires a computer-controlled rotator and an algorithm to impose the randomness.

Ishii and colleagues' approach is much simpler and more compact. They use a piece of ulexite as the diffuser and rotate it in regular steps. Because each piece of mineral has a unique composition, the random pattern of reference beams produced cannot be replicated by any other stone. Furthermore, only that stone can be used to retrieve the data. Ishii et al. demonstrate the effectiveness of the scheme by holographically storing images of a running figure and retrieving them as an animation with six frames per second (see figure).

The authors note that the stone could be worn as part of an accessory and so be carried without attracting suspicion — except, that is, from a secret agent who has kept up with the scientific literature.

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Venema, L. The secret of the stone. Nature 447, 787 (2007). https://doi.org/10.1038/447787a

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