BOX 4–1

Slow Light

Coherent manipulations of vapors of three-level atoms have led to the observation of remarkable phenomena, such as electromagnetically induced transparency, slow light, and nonlinear optics, at very low (approaching single-photon) light levels. These phenomena originate in the subtle interplay and exchange of optical and atomic coherence. For example, in slow light, optical excitations, which propagate at the speed of light in free space, are reversibly transferred into atomic excitations in a vapor. However, these atomic excitations propagate at a fraction of the speed of light. Once in the vapor, these excitations can be monitored and manipulated. They can be subsequently restored (if desired) to optical excitations. Crucially, the transfer process preserves excitation amplitude and phase information. Envisioned applications include signal processing elements such as delay lines, taps, and bandwidth compressors. As these processes can function at the single-photon level, they also enable new approaches for manipulating and storing quantum information.

Recent work has demonstrated the ability to delay light in optical fibers for applications in fiber-optic communication networks. Slow light would be very useful in all-optical routers, which are used in communication systems to direct information from one point to another. Current routers convert optical information into an electronic form (a so-called communication bottleneck), while an all-optical router would eliminate the optical to electrical to optical conversion and greatly speed up the process. An all-optical router would require an optical buffer—a device that would function as temporary optical storage—to synchronize data packets effectively. A slow light device would accomplish this function. Researchers have recently demonstrated a more than 300-fold reduction in the group velocity of an optical pulse propagating on a silicon chip by using an ultracompact photonic integrated circuit with a silicon photonic crystal waveguide. Many view slow light as an important aspect of our future capability to process and transport information optically: Photonic crystals may be the key to that future.

continue to use short-wavelength light for an astonishing array of applications in basic physics and chemistry, in biology, in materials science and engineering, and in medicine.

New advances in atomic and optical physics are creating brilliant bursts of x-ray beams with laserlike properties. These bright, directed x-ray beams can be focused to the size of a virus and are fast and bright enough to capture the complex dance of atoms within molecules or—even faster—the fleeting motion of electrons within atoms and molecules. These extreme strobe lights, with x-ray vision, will provide a direct view of the electronic and structural changes that govern biology and nanoscience at the molecular level. Scientists have never had such a window through which to explore the nanoworld.

Advanced x-ray sources will be developed through the combined efforts of scientists in universities, national laboratories, and industry. Their scale will range from tabletop systems designed for very short pulses of soft x rays to large national



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