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Rubidium Vapor Slow Light Photonics Chip

A minute optical device in a silicon chip has attained the slowest light propagation on a chip to up until now. Developed by a team of researchers at UC Santa Cruz and Brigham Young University, the chip is capable reducing the speed of light by a factor of 1,200. The study was published in Nature Photonics September 5 online edition, and the November print issue. The facility to control light pulses on an integrated chip-based platform is a key step toward the realization of all-optical quantum communication networks. They hold the potential for huge improvements in ultra-low-power performance.

“Slow light and other quantum coherence effects have been known for quite awhile, but in order to use them in practical applications we have to be able to implement them on a platform that can be mass-produced and will work at room temperature or higher, and that’s what our chips accomplish,” professor Holger Schmidt, team leader said.

While today’s optical fibers regularly transmit data at light speed, routing and data processing operations still necessitate converting light signals to electronic signals. All-optical data processing will require compact, reliable devices that can slow, store, and process light pulses.

“The simplest example of how slow light can be used is to provide a data buffer or tunable signal delay in an optical network, but we are looking beyond that with our integrated photonic chip,” Schmidt said.

The device uses quantum interference effects in a rubidium vapor inside a hollow-core optical waveguide. The waveguide is built into a silicon chip using standard manufacturing techniques, and is a progression of earlier work by Schmidt and his collaborators that enabled them to perform atomic spectroscopy on a chip.

Several different methods have been used to slow light to a crawl and even bring it to a complete halt for a few hundredths of a millisecond. Formerly though, systems based on quantum interference needed low temperatures or laboratory setups too complex for practical use. In 2008, researchers at NTT Laboratories in Japan developed a specially structured silicon chip that could slow light pulses by a factor of 170. Called a photonic crystal waveguide, it has advantages for certain applications, but it does not produce the quantum effects of the atomic spectroscopy chip developed by Schmidt’s group.

“By changing the power of a control laser, we can change the speed of light–just by turning the power control knob,” he said.

The control laser modifies the optical properties of the rubidium vapor in the hollow-core waveguide. Under the combined action of two laser fields (control and signal), electrons in the rubidium atoms are transferred into a coherent superposition of two quantum states. In the strange world of quantum physics, they exist in two different states at the same time. One result is an effect known as electromagnetically induced transparency, which is key to producing slow light.

“Normally, the rubidium vapor absorbs the light from the signal laser, so nothing gets through. Then you turn on the control laser and boom, the material becomes transparent and the signal pulse not only makes it through, but it also moves significantly more slowly,” Schmidt said. “This has implications for looking at nonlinear optical effects beyond slow light,” Schmidt said. “We can potentially use this to create all-optical switches, single-photon detectors, quantum memory devices, and other exciting possibilities.”

Reference:

Slow light on a chip via atomic quantum state control
Bin Wu, John F. Hulbert, Evan J. Lunt, Katie Hurd, Aaron R. Hawkins & Holger Schmidt
Nature Photonics
Published online: 5 September 2010 | doi:10.1038/nphoton.2010.211

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