Light makes for a terrific carrier of information—witness the prevalence of fiber optics in telecommunications—and the realm of quantum communication is no different. Photons are key quantum objects that can carry information over large distances and that can be entangled in relatively large numbers.







But photons are a hyperactive lot, zipping around at light speed, making the stationary storage of the information they carry a challenge. One cannot simply encode the photonic information in a standard electronic memory—any measurement of a quantum object destroys some of the information contained therein, thereby casting aside the benefits of quantum communication and computation.







Researchers have devised several methods to park the information stored in photons, by encoding the quantum state of the photons into the quantum state of atoms, which can be more easily held in place. But those systems tend to be woefully spotty in their ability to store and recall information. Now a study in the June 24 issue of Nature presents a quantum memory that can store photonic information in a crystal with efficiencies of up to 69 percent. (Scientific American is part of Nature Publishing Group.)







The study's authors used a yttrium orthosilicate (Y 2 SiO 5 ) crystal doped with the rare earth element praseodymium, carefully tailored to have very specific absorption properties, as a quantum memory. Cooled to just three degrees Kelvin and held in an electric field, the crystal memory absorbs a pulse of laser light. But when the external electric field is switched, the memory produces an "echo" of the original photons, their quantum state intact.







"Light entering the crystal is slowed all the way to a stop, where it remains until we let it go again," lead study author Morgan Hedges, a student at the Australian National University's Laser Physics Center, said in a prepared statement. "When we do let it go, we get out essentially everything that went in as a three-dimensional hologram, accurate right down to the last photon."







The storage times demonstrated in the new research are rather short—measured in millionths of a second—but the researchers predict that even more efficient memories should be feasible, with storage times measured in seconds.







Photo credit: ANU