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Published: 06-07-2006

Imagine trying to describe the intricate motions of a single atom as it interacts with a laser. Then suppose you could generalize this understanding to a whole cloud of similar atoms and predict the temperatures your experimental physicist colleagues could achieve with laser cooling. This way-cool theoretical analysis comes compliments of Graduate Student Josh Dunn and Fellow Chris Greene.

The researchers have completed a detailed calculation of atom-light interactions, which they use to treat strontium (87Sr) and magnesium (25Mg) atoms completely quantum mechanically in three dimensions. And, they don't use a single approximation from classical physics. Although a direct quantum-mechanical calculation is practically impossible, they simulated the atom dynamics by rolling dice with a computer. (Actually, they generated random numbers with a Monte Carlo simulation. This theory uses random numbers to produce approximate solutions to various problems that are otherwise too difficult to solve.)

Their project was inspired by a laser-cooling experiment in Jun Ye's lab a couple of years ago in which more cooling of 87Sr atoms was observed than was supposed to be possible with this kind of atom (a fermionic isotope with no electronic spin). Naturally, the JILA theorists decided to analyze what was being observed in Ye's lab. In the process, they created theoretical renderings of laser-cooled 87Sr atoms as they become progressively colder (right).

When they began their work, the researchers knew that alkaline earth metals such as 87Sr and 25Mg have multiple states that can be excited by laser photons. Normally, having lots of excited states means that the extra laser cooling, known as sub-Doppler cooling, works fairly well. However, the excited states in these particular atoms are very close together in energy, and until now, physicists had predicted that such closeness would impair sub-Doppler cooling.

When Josh Dunn decided to look into the matter, he discovered a complex interplay between the number of excited states, their spacing, and sub-Doppler cooling. Fortunately, he found that this interplay could be well described with a detailed calculation. And best of all, his results agreed with Ye's strontium experiment! Dunn and Greene are already expanding their analysis and preparing a comprehensive survey of laser cooling in various atoms with internal structures similar to 87Sr and 25Mg.

The work reported here has been submitted to Physical Review A.   - Julie Phillips


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