All-Fibered Memory to Revolutionize Future Quantum Networks

Optical storage devices are not a new technology and hence development of new type of optical storage devices may not get enough attention of technical communities. But the news from Kastler Brossel Laboratory in Paris is really appealing and has the potential to get attention of technical geeks. The researchers B. Gouraud, D. Maxein, A. Nicolas, O. Morin, led by J. Laurat at the Kastler Brossel Laboratory in Pierre and Marie Curie University in Paris have demonstrated to store information in light that propagates in an optical fiber and to release it later when required. The research team found that controlling a photon in an optical fiber is easier. The significance of the research findings lies in the fact that the optical signals propagating along the length of an optical fiber can be temporarily stored in a cloud of cold atoms surrounding the fiber.

Utilizing the quantum properties of a photon, an atomic gas can store and save memory. The research finding points that controlling a photon traveling in an optical fiber is much easier. In an optical fiber, photons are trapped in a mesh of constituting molecules of the medium. Such a memory would make it easier to use the photon’s quantum properties for quantum communications or computing. Scientists found that controlling a photon beam can easily be done if it is trapped in an optical fiber. Focusing 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. Taking advantage of EIT technique 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. The device they used for experiment includes an optical fiber, a section of which is elongated to reduce the diameter to about 0.4 micrometers where 40 percent of the traveling light is squeezed into an evanescent field just outside the fiber. The team released a trapped low-temperature cloud of cesium atoms in this region and focused 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. Thus, by making interaction between the traveling light and a few thousand atoms in the vicinity, the scientists could demonstrate an all-fibered memory. The team used EIT with an optical fiber and demonstrated slow-down of light pulse by 3,000 fold and then halted it completely.

The information conveyed by the laser light is transferred to the atoms in the form of a collective excitation, a large quantum superposition. Around 2,000 cesium atoms very close to the fiber were involved in the process. Later, after a programmable period, the light was released into the fiber, reconstituting the initial encoded information that can once again travel. With the current test setup, the team could stop the light or in other terms Store the information up to 5 micro-seconds. Light travels at a speed of around 300,000 kilometers in one second in vacuum. The speed of light in a silica glass optical fiber is reduced to around 200,000 kilometers due to the refractive index of glass. Refractive index of glass is roughly 1.5 means the light travels 1.5 times slower in glass compared to the vacuum. One seconds is equal to 1000,000 microseconds. Hence 1 microseconds corresponds to 0.2 kilometers (200 meters) of optical length in silica glass fiber. A delay or stop of 5 microseconds corresponds to 1 kilometer in a silica glass optical fiber.

The Kastler–Brossel Laboratory, located in Paris, France, is a research laboratory specializing in fundamental physics of quantum systems. Founded in 1951 by Alfred Kastler and Jean Brossel, it is a joint research unit operated by the French National Centre for Scientific Research (CNRS), the École normale supérieure and Pierre-and-Marie-Curie University. Pierre and Marie Curie University (French: Université Pierre-et-Marie-Curie; abbreviated UPMC), also known as University of Paris VI (Paris 6), is a public research university located on the Jussieu Campus in the Latin Quarter of the 5th arrondissement of Paris, France. It was established in 1971 following the division of the University of Paris (Sorbonne), and is a principal heir to Faculty of Sciences of the University of Paris(French: Faculté des sciences de Paris), although it can trace its roots back to 1109 and the Abbey of St Victor. The French cultural revolution of 1968, commonly known as “the French May”, resulted in the division of the world’s second oldest academic institution, the University of Paris, into thirteen autonomous universities.

The findings of the research team is an important advance in optical communications. We are already using optical fibers for building telecommunication networks. The experiment findings will find its application in future quantum Internet, where quantum information can be transported and synchronized between interconnected nodes. It also demonstrates an all-fibered memory for light. The research team could store the light and release it later into the fiber. The experiments showed that even light pulses containing only one photon can be stored, with a very large signal-to-noise ratio. This feature will enable the use of this device as a quantum memory, an essential ingredient for the creation of future quantum networks.

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Author: James

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