Quantum encryption: Researchers convert entangled photons to telecom

Sept. 29, 2010
Using optically dense, ultracold clouds of rubidium atoms, researchers have improved three key elements needed to create telecom-style quantum-information networks.

Atlanta, GA--Using optically dense, ultracold clouds of rubidium atoms, researchers have improved three key elements needed to create telecom-style quantum information networks (which securely encode information by entangling photons and atoms). The advances include:

--development of an efficient, low-noise system for converting photons with a 795 nm wavelength carrying quantum information to the 1.3 micron wavelength suitable for transmission on conventional telecom systems; the system maintains the entangled information during conversion to telecom wavelengths

--a significant improvement in the length of time that a quantum repeater (necessary to transmit the information) can maintain the information in memory; the researchers reported memory lasting as long as 0.1 s, 30 times longer than previously reported for systems based on cold neutral atoms, and approaching the quantum-memory goal of at least one second (long enough to transmit the information to the next node in the network)

--an efficient, low-noise system able to convert photons at longer telecom wavelengths back to 795 nm; such a system would be necessary for detecting entangled photons transmitted by a quantum information system

Four-wave mixing

Wavelength conversion takes place in a system based on a cloud of rubidium atoms packed closely together in gaseous form to maximize the likelihood of interaction with photons entering the samples. Two separate laser beams excite the rubidium atoms, which are held in a cigar-shaped magneto-optical trap about six millimeters long. The setup creates a four-wave-mixing process that changes the wavelength of photons entering it.

By changing the shape, size and density of the rubidium cloud, the researchers boosted efficiency to as high as 65%. The four-wave mixing process does not add noise to the signal, allowing the system to maintain the information encoded onto photons by the quantum memory. Once the photons are converted to telecom wavelengths, they move through optical fiber and loop back into the magneto-optical trap. They are then converted back to the shorter wavelength for testing to verify that entanglement has been maintained.

Researchers at the Georgia Institute of Technology reported the findings Sept. 26 in Nature Physics, and in a manuscript submitted for publication in Physical Review Letters.

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About the Author

John Wallace | Senior Technical Editor (1998-2022)

John Wallace was with Laser Focus World for nearly 25 years, retiring in late June 2022. He obtained a bachelor's degree in mechanical engineering and physics at Rutgers University and a master's in optical engineering at the University of Rochester. Before becoming an editor, John worked as an engineer at RCA, Exxon, Eastman Kodak, and GCA Corporation.

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