Rewritable Holographic Memory

By using lasers to etch data onto microbial proteins, researchers at the University of Connecticut may have demonstrated a way to produce rewritable holographic memory. Holographic memory stores data in three dimensions instead of two and could make data retrieval hundreds of times faster. The first holographic-memory systems have recently come to market, but they do not yet feature discs rewritable in real time.

Researchers at the University of Connecticut, Storrs, led by Jeffrey Stuart, head of the Nanobionics Research Center at the university’s Institute of Materials Science, based their holographic storage system on reengineered versions of proteins produced by bacteria-like organisms commonly found in salt marshes. Simply shining blue light on the proteins erases any data stored in them.

The technology exploits an evolutionary adaptation of the microbe Halobacterium salinarum, which produces a light-sensitive membrane protein when concentrations of oxygen get too low. The protein, known as bacteriorhodopsin, helps the organism convert sunlight into energy. After the protein absorbs light, it cycles through a series of chemical states, releases a proton, and finally resets itself.

When the protein is in some of these states, its ability to absorb light allows it to form holograms. In the natural environment, each of the states lasts only briefly: the whole cycle takes just 10 to 20 milliseconds. But prior research has shown that shining red light on the protein as it nears the end of its chemical cycle can force it into a useful state–known as the “Q state”–that can last for years.

The problem is that the Q state is difficult to produce in the naturally occurring protein. So molecular biologists at UConn, led by Robert Birge of the chemistry department, are genetically manipulating Halobacterium salinarum so that it can produce a protein that enters the Q state more easily.

To serve as part of a holographic system, the protein is suspended in a polymer gel. A green laser beam is split in two, and one beam is encoded with data. The beams are then recombined in the gel, imprinting the proteins with an interference pattern that stores the data. To read the data, the system sends a single, lower-power, red laser beam back through the interference pattern. A blue laser erases the data.

Tim Harvey, CEO of Starzent, a Fairfax, VA, company that is funded by the U.S. Defense Advanced Research Projects Agency and is developing a miniature holographic data storage drive, says “Protein-based holographic media has the potential for low-cost removable media rewritable up to 10 million times.” The protein is extremely robust, he adds, and if the researchers find the right genetic variant, current advances in biotechnology could help quickly produce large amounts of the protein at a low cost.

Among the people interested in the new development is Liz Murphy, vice president of marketing at InPhase Technologies in Longmont, CO, which has demonstrated a holographic device with a storage density of 500 gigabytes per square inch and has several products in the pipeline. “At least one potential advantage is that it is erasable and rewritable, which is rare among currently available media,” Murphy says of the UConn researchers’ device. “However, a drawback is that recording is in the red, and blue light is used to erase the recordings.”

That’s a limitation because “storage density typically increases with shorter wavelengths,” she notes, pointing to the progression from CD to Blue-ray/HD-DVD technology. “So limiting use of the bacterial media to red wavelengths will make it less attractive to use for high-density data-storage applications.”