ion of mercury, Hg+. This species shows a strong spectral line with wavelength close to 2800 Å, corresponding to a frequency of some 1.07 × 1015 hertz. The natural line width is only 43 megahertz, but at room temperature Doppler effects broaden the line to 3 gigahertz, blurring out its details. Weinland, by contrast, has cooled his Hg+ ion, which has no name, to 0.006 Kelvin, or degrees above absolute zero. More recently, he claims to have reached 0.00024 K. Working with an adjacent spectral line at 2815 Å, his group has observed line widths as narrow as 30 kilohertz, some 100,000 times finer than the room temperature line. Nor is this the limit; the 2815-Å line has a natural width of as little as 1.6 hertz, which might be approached in practice using a laser with sufficient stability in its frequency.
Atoms are electrically neutral; hence, unlike ions, one cannot trap them by using electric fields. Perhaps the simplest trap involves three sets of laser beams, oriented respectively to define x, y, and z axes and intersecting within a small region of space. These lasers are tuned to just below the frequency of a strong spectral line. An atom in motion, whatever its direction, will absorb photons more effectively and hence will experience a drag force that acts to slow it down. Indeed, provided its velocity is small, the force it feels will increase with its speed. The atom then behaves as if it were held within a viscous fluid that resists its motion. Steven Chu of Stanford University, who first demonstrated this technique while at AT&T Bell Laboratories, calls this approach "optical molasses," noting that the idea of using lasers to cool neutral atoms was first proposed by Ted Hansch and Arthur Schawlow.
A different technique takes advantage of the fact that while atoms carry no electric charge they do have electrical properties. Light, such as comes from a laser, carries a rapidly varying electric field that oscillates at its frequency. With a wavelength of 5000 Å, for instance, which is a reasonably typical value, the frequency is 6 × 1014 hertz. Electrons within an atom tend to oscillate in response to this field. The consequence is a rapidly varying redistribution of charge within the atom that indeed makes it responsive to electric forces.
This effect resembles what happens when you rub a comb with fur and use it to attract small bits of paper. Rubbing the comb gives it a charge of static electricity. The bits of paper, like atoms, carry no such charge; but when the comb approaches, it carries an electric field that redistributes the electrons within the paper. The paper then acquires an