Investigations Probe Ultraslow and Superluminal Light
Daniel S. Burgess
In the few short years following the demonstrations of the slowing and stopping of light in ultracold clouds of atoms and of the superluminal propagation of radiation, researchers have entered a golden age of exploration into such phenomena.
Two recent studies, independently reported by a team at Harvard University in Cambridge, Mass., and at P.N. Lebedev Institute of Physics in Moscow and by another at the University of Rochester in New York, advance the understanding of these effects and pave the way for their eventual application. The former reports the storage and recovery of particular nonclassical properties of photonic states in a rubidium atomic vapor, and the latter describes the observation of ultraslow and superluminal light propagation in an alexandrite crystal at room temperature.
Caspar H. van der Wal and Matthew D. Eisaman of Harvard's department of physics explained that the group's work demonstrates the basis of quantum repeaters for use in quantum key distribution systems, which exploit fundamental properties of physics to produce unbreakable codes. Quantum repeaters would overcome the natural decay of entangled states in communications channels and thereby extend the distances over which the communication fidelity could be preserved by "entanglement swapping" along shorter segments of the optical path. In June, a team at California Institute of Technology in Pasadena independently reported a similar feat.
The Harvard researchers and their colleagues used the 0.7-mW output of an extended-cavity diode laser, tuned about 1-GHz off-resonance from an absorption line in 87Rb, to create a nonclassical atomic state in an atomic vapor sample. They demonstrated that they could map this state onto a second photonic state, an "anti-Stokes field" in the output of a second, 3.2-mW extended-cavity diode laser tuned near resonance that subsequently passed through the atomic vapor cell. Measurements of the intensity correlations between the Stokes field emitted during the initial exposure and the retrieved radiation field verified that the process preserved the nonclassical intensity variance of the initial state.
Van der Wal said that although the setup demonstrates the viability of the basis of the quantum repeater scheme, technical limitations enabled it to display a retrieval efficiency of only 10 to 30 percent in the experiments. The scientists hope to improve this to 90 percent in future studies. Moreover, atomic vapor systems themselves are reliable, he said, but they are relatively slow, so the researchers are interested in investigating other, faster material systems, with which they may apply the techniques that they learn with the setup.
The Rochester team's work represents a continuation of its exploration of solid-state materials for use in slow- and fast-light studies. Recently, the scientists reported group velocities that were as low as 58 meters per second in a ruby crystal at room temperature using coherent population oscillations. A transition to room-temperature solid-state materials would ease the development of devices such as controllable optical delay lines and of others with applications in quantum information and optical data storage.
The researchers again used the output of an argon-ion laser to modify the optical properties of a crystal by manipulating coherent population inversions. In this case, however, alexandrite replaced the ruby sample, and the laser operated at 476 or 488 nm to excite chromium ions at either mirror or inversion sites in the crystal lattice and to induce superluminal or slow-light phenomena, respectively. Specifically, exposing the alexandrite to modulated light at 476 nm yielded group velocities as fast as –800 meters per second, and the 488-nm light yielded group velocities as slow as 91 meters per second.
Matthew S. Bigelow, a doctoral candidate at the university's Institute of Optics, said that this is the first demonstration of both ultrafast and ultraslow light in a solid at room temperature. As with the Harvard team, the researchers hope to investigate alternate material systems. "Our next step is to look at other materials that share the same properties as ruby and alexandrite," he said. "To delay shorter pulses, we need to find a material that has a faster response."
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