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toward the light ‘see’ photons that are Doppler-shifted closer to resonance and thus absorb more momentum in a direction opposite to their motion. The result is that they slow down. The amazing thing is that such cold temperatures (microkelvins or less) can be reached.”

The researchers explored the rather complex energy-level structure of

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Er, identifying five possible laser-cooling transitions (from high to low energy, they are 401, 583, 631, 841, and 1299 nm), only two (401 and 583 nm) of which had previously known rates. They also determined whether or not the transitions had “optical leaks” (a spontaneous decay to metastable states other than the ground state, which can interfere with laser cooling); the lower three transitions had virtually no opti-

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Fluorescence measurements In an experiment, a beam of Er atoms from a heated piece of Er in a vacuum was directed into a crossed-beam laser-induced-fluorescence setup (see figure). Two transitions were studied—the 841-nm line with an 80-m W Ti:sapphire laser, and the 631-nm line with a grating-stabilized 0.8-m W diode laser. Fluorescence measurements produced spectra identifying the characteristics of individual Er isotopes. By chopping the laser light with an acousto-optic modulator, the fluorescent lifetimes were measured; the results agreed well with calculations.

The researchers note that the large capture velocity of the 401-nm line allows capturing of Er atoms from a thermal source. And, because the recoil velocity of the 401-nm line is smaller than the capture velocity of the 841-nm line, thermally trapped Er atoms can be further cooled by the 841-nm line. In fact, they can be cooled to 80 nK—a much lower temperature than that for other narrowband cooling atoms such as calcium, strontium, and ytterbium, making record-low laser-cooling temperatures possible.

“The colder an atom is, the longer it stays in one place, and the longer one can interact with it, the easier it is to trap—for example, with a magnetic or laser field,” says McClelland. “Erbium is particularly good for magnetic trapping because it has a very large magnetic mo-