May 31, 2012

Atomic Vapor Stores Short Movies

The storage of light-encoded messages on film and compact disks and as holograms is ubiquitous---grocery scanners, Netflix disks, credit-card images are just a few examples. And now light signals can be stored as patterns in a room-temperature vapor of atoms. Scientists at the Joint Quantum Institute have stored not one but two letters of the alphabet in a tiny cell filled with rubidium (Rb) atoms which are tailored to absorb and later re-emit messages on demand. This is the first time two images have simultaneously been reliably stored in a non-solid medium and then played back.

In effect, this is the first stored and replayed atomic movie. Because the JQI researchers are able to store and replay two separate images, or "frames," a few micro-seconds apart, the whole sequence can qualify as a feat of cinematography. The new storage process was developed by Paul Lett and his colleagues, who publish their results in the latest issue of the journal Optics Express.

One young man was inspired by the lingo of the JQI paper, especially the storage of images in the atomic memory, and contrived a song which he performs on a YouTube video clip:

We don't yet need to store grocery barcodes in tiny vials of rubidium. The atomic method, however, will come into its own for storing and processing quantum information, where subtle issues of coherence and isolation from the outside world need to be addressed.

The atomic storage medium is a narrow cell some 20 centimeters long, which seems pretty large for a quantum device. That's how much room is needed to accommodate a quantum process called gradient echo memory (GEM). This useful protocol for storage was pioneered at the Australian National University just in the past few years. While many storage media try to cram as much information into as small a place as possible---whether on a magnetized strip or on a compact disk---in GEM an image is stored over the whole range of that 20-cm-long cell.

The image is stored in this extended way, by being absorbed in atoms at any one particular place in the cell, depending on whether those atoms are exposed to three carefully tailored fields: the electric field of the signal light, the electric field of another "control" laser pulse, and a magnetic field (adjusted to be different along the length of the cell) which makes the Rb atoms (each behaving like a magnet itself) precess about. When the image is absorbed into the atoms in the cell, the control beam is turned off. Because this process requires the simultaneous action of two particular photons---one putting the atom in an excited state, the other sending it back down to a slightly different ground state---it cannot easily be undone by atoms subsequently randomly emitting light and returning to the original ground state.

That's how the image is stored. Image readout occurs in a sort of reverse process. The magnetic field is flipped to a contrary orientation, the control beam turned back on, and the atoms start to precess in the opposite direction. Eventually those atoms reemit light, thus reconstituting the image pulse, which proceeds on its way out of the cell.

Having stored one image (the letter N), the JQI physicists then stored a second image, the letter T, before reading both letters back in quick succession. The two "frames" of this movie, about a microsecond apart, were played back successfully every time, although typically only about 8 percent of the original light was redeemed, a percentage that will improve with practice. According to Paul Lett, one of the great challenges in storing images this way is to keep the atoms embodying the image from diffusing away. The longer the storage time (measured so far to be about 20 microseconds) the more diffusion occurs. The result is a fuzzy image.

Paul Lett plans to link up these new developments in storing images with his previous work on squeezed light. "Squeezing" light is one way to partially circumvent the Heisenberg uncertainty principle governing the ultimate measurement limitations. By allowing a poorer knowledge of a stream of light---say the timing of the light, its phase---one gain a sharper knowledge of a separate variable---in this case the light's amplitude. This increased capability, at le ast for the one variable, allows higher precision in certain quantum measurements.

"The big thing here," said Lett, "is that this allows us to do images and do pulses (instead of individual photons) and it can be matched (hopefully) to our squeezed light source, so that we can soon try to store "quantum images" and make essentially a random access memory for continuous variable quantum information. The thing that really attracted us to this method---aside from its being pretty well-matched to our source of squeezed light---is that the ANU group was able to get 87% recovery efficiency from it - which is, I think, the best anyone has seen in any optical system, so it holds great promise for a quantum memory."

The lead author of the new Optics Express article, Quentin Glorieux, feels that the JQI image storage method represents a potentially important addition to the establishment of quantum networks, equipment which exploits quantum effects for computing, communications, and metrology. "It is very exciting because images and movies are familiar to everyone. We want to go to the quantum level. If we manage to store quantum information embedded in an image or maybe in multiple images, that could really hasten the advent of a quantum network/internet."


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