In the PML four-wave mixing experiment (see Figure 1), researchers send “seed” pulses of laser light into a heated cell containing the gain medium, atomic rubidium vapor, along with a separate “pump” beam at a different frequency from the seed pulses. In the medium, the seed pulse is amplified and its peak is shifted so that it becomes superluminal. At the same time, photons from the inserted beams interact with the medium to generate a second pulse, called the “conjugate” because of its mathematical relationship to the seed. Its peak too, the scientists found, can travel faster than an unaltered reference pulse would in a vacuum. Or it can be tuned to travel slower.
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Communications researchers have proposed that slow light could act as a sort of controllable delay line, or storage medium, for light carrying quantum information. Alternatively, the fast conjugate pulse that Lett’s group has observed might act as a way to advance the detection of quantum information, should the fast light retain quantum coherence. (NIST scientists emphasized that, while the information detection could be advanced, no information could actually travel faster than the speed of light and that, consequently, principles like causality in special relativity were always respected in these experiments.)