'Slow light' on a chip holds promise for optical communications

'Slow light' on a chip holds promise for optical communications: Several different techniques have been used to slow light to a crawl and even bring it to a complete halt for a few hundredths of a millisecond. Previously, however, systems based on quantum interference required low temperatures or laboratory setups too elaborate 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.

Those quantum effects produce not only slow light but other interactions between light and matter that raise the possibility of radically new optical devices for quantum computing and quantum communication systems, according to Schmidt. In addition, the system makes it easy to turn the effect on and off and tune it to the desired speed of light.

"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.