The Slow Light Experiment

     One starts with a beam of hot sodium atoms, glowing like a pale orange street lamp. The atoms are collected in a vacuum chamber and subjected to various cooling methods, one of which involves directing laser light at the atoms to slow their motions. In this bath of laser light and radiating atoms, the system shines brilliantly like a miniature Sun. In the chamber, about 10 billion sodium atoms float almost mysteriously, suspended by magnetic fields and lasers. At this point, the temperature of the tiny, millimeter-sized cloud of atoms is a millikelvin.6 Next, the laser beams are turned off, and the dark, cigar-shaped (0.2-millimeter long and 0.02-millimeter wide) shadow of the cloud can be seen. Its temperature is only 50 microkelvins.1 The sample is so small that lenses must be used for focusing, and microscopes are needed for imaging.
     The final cooling process, which takes only about a minute, is through evaporation.7 Only the coldest two million sodium atoms remain. At a temperature below 435 nanokelvins,2 a dramatic change takes place: A Bose-Einstein condensate forms. The individual atoms loose their identity and dissolve into a kind of global quantum macromolecule.
     At temperatures below a microkelvin, the suspended sodium atoms are opaque. Light is unable to penetrate them. One is unable to "see" through them, much like the way that one cannot see through a block of lead. But then the "miracle" of electromagnetically induced transparency is performed. Laser light of a particular color is directed onto the cloud of sodium atoms. Lo and behold, the cloud becomes transparent. It is an alchemistic feat. It is like turning lead into plastic. When a pulse of light of a certain color is sent through the sodium atoms, it is no longer completely absorbed. A fraction (typically 25%) of it is able to pass through.
     How does this miracle work? In general, light is absorbed by a substance when an electron in an atom of the substance absorbs a photon4 of the light. As a consequence, the electron gains the photon's energy and jumps to a higher energy level in the atom. When the sodium atoms are subjected to laser light of a particular color, electrons cannot easily make the transition to the higher energy levels due to a quantum interference effect: A cancellation occurs in the interacting coupled laser-atom system to render the probability of a transition quite low. Since absorption by electrons of light is inhibited, light is able to pass through the collection of sodium atoms. Without this effect, the sodium atoms would be as optically impenetrable as a block of lead.
     A brief pulse of light of a certain color is shot at the system. It travels at 186,000 miles per second in the vacuum chamber. When it strikes the medium of sodium atoms, it comes to a grinding halt. The length of the pulse collapses 20-million fold. The pulse struggles to pass through the unusual medium, moving at only 38 miles per hour when the system is at 50 nanokelvins. But in a small fraction of a second later, it reaches the end of the tiny collection of atoms. Liberated, it speeds off at 186,000 miles per second again.



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To the "Slow Light" report.