# Synopsis: Nanofiber Optical Memory

Light signals propagating down an ultrathin fiber can be temporarily stored in a cloud of cold atoms surrounding the fiber.

An atomic gas can serve as a memory for the quantum state of a photon. Such a memory would make it easier to use the photon’s quantum properties for quantum communications or computing. But controlling a photon beam is easiest if it is trapped in an optical fiber. A group led by Julien Laurat from the Kastler Brossel Laboratory at the Pierre and Marie Curie University (UPMC), France, showed that a cloud of cold atoms surrounding an ultrathin fiber can store light propagating along the fiber.

Shining a bright laser on atoms can cancel their absorption at a different frequency. This electromagnetically induced transparency (EIT) can alter the propagation of a weaker “signal” beam at that frequency. But exploiting these effects in information-processing schemes requires sending the signal light to specific locations, which is clumsy for beams traveling in free space, as in most current schemes.

For easier routing, the researchers manipulated a beam of single photons without extracting it from an optical fiber. Their fiber included a short section whose $400$-nanometer diameter squeezed about $40%$ of the traveling light energy into an evanescent field just outside the fiber. The team released a trapped low-temperature cloud of cesium atoms in this region and shone a second laser on it to create EIT. The atoms dramatically slowed the light in the fiber. By briefly dimming the control light, the team stored the light signal for microseconds as a collective state of the atoms. Although the current setup recovered only $10%$ of the stored signal, that signal is already 20 times larger than the background noise.

This research is published in Physical Review Letters.

–Don Monroe

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