Short Movies Stored in Atomic Vapor

Using lasers and a magnetic field, physicists stored and replayed two letters of the alphabet in a tiny cell of rubidium atoms, the first time two images have been reliably stored in a nonsolid medium.

Scientists at the Joint Quantum Institute essentially demonstrated the first atomic movie because they were able to store two separate images, or frames, and replay them a few microseconds apart.

The physicists used lasers and a magnetic field to store the letters “N” and “T” as separate images in a small container of rubidium atoms. The image is stored using a quantum process called gradient echo memory (GEM), which was recently pioneered at the Australian National University. Using GEM, an image is stored over the whole range of a 20-cm-long cell.

The image is stored when it is absorbed in atoms at any one particular place in the cell. The location depends on whether the 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 that makes the rubidium atoms (each behaving like a magnet itself) precess about. The control beam is turned off once the image is absorbed by the atoms in the cell.

A gradient echo memory setup. The image to be stored, the letter N encoded by a signal laser beam and a mask, enters from the left (pink light) and enters the storage cell filled with Rb atoms. The components of this image will be absorbed by the atoms when, at locations all over the body of the cell, a part of the signal beam and parts of a separate “control” laser beam — entering from the side (shaded green) via a polarizing beamsplitter (PBS) — and (last but not least) the strength of a magnetic field (delivered by the brown coil around the cell) are just right. The stored image can later be read out and observed with a CCD camera. (Image: NIST)

The process cannot easily be undone by atoms subsequently randomly emitting light and returning to the original ground state because it requires 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.

For image readout to occur, the scientists must reverse the process, flipping the magnetic field so that the atomic vapor re-emits the images of the letters as light signals. The very short movie played back successfully each time, but only about 8 percent of the original light was redeemed, a percentage that will improve with practice.

One of the greatest challenges in storing images this way is keeping the atoms embodying the image from diffusing away, said Paul Lett of JQI. The longer the storage time (measured so far to be about 20 µs), the more diffusion occurs, resulting in a fuzzy image.

Lett plans to link this new technique for storing images with his previous work on squeezed light. “Squeezing” light can partially circumvent the Heisenberg uncertainty principle that governs the ultimate measurement limitations. One can gain a better understanding of a separate variable — in this case, the light’s amplitude — by allowing a poorer knowledge of a stream of light. This increased capability, at least for this one variable, could enable higher precision in certain quantum measurements.

“The big thing here 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,” Lett said. “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 [Australian National University] group was able to get 87 percent recovery efficiency from it — which is, I think, the best anyone has seen in any optical systems, so it holds great promise for a quantum memory.”

The image storage method offers a great addition to the establishment of quantum networks — equipment that exploits quantum effects for computing, metrology and communications.

“It is very exciting because images and movies are familiar to everyone,” said Quentin Glorieux, a JQI physicist. “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.”

The results appeared in Optics Express.

For more information, visit: www.umd.edu