Most people’s first exposure to the ideas of modern atomic physics comes through the Bohr model of hydrogen, which treats the atom as something like a little solar system, with the positively charged nucleus as the sun, and negatively charged electrons orbiting in well-defined circular orbits. It’s a very compelling picture, and works well for hydrogen (provided you make a couple of really odd assumptions), but it’s completely wrong on the details. Electrons do not move in well-defined orbits, but rather exists as fuzzy wavefunctions spread over the space near the nucleus.
It turns out, though, that you can make atomic states in which an electron does move in a nice, circular orbit, like the electrons in the Bohr model of hydrogen. the most recent issue of Physics has a nice article by Carlos Stroud explaining how this works in a new experiment by Tom Gallagher’s group at the University of Virginia. (If you go to the Physics story, you can download the paper for free, even if you’re not a subscriber).
They have to work at it a bit– they use laser pulses to excite the outermost electron of a lithium atom into a high-energy state, then a microwave pulse to put the electron into a superposition of multiple states that turns out to correspond to an electron moving back and forth in a linear orbit. Then they add a second microwave pulse to convert the linear orbit into a circle. As long as the circularly polarized microwave field is on, the electron will continue to move in a nice, regular circular orbit, just like the electrons in the Bohr model. They detect the motion by hitting the atoms with very precisely timed laser pulses that will ionize it at particular points in its oscillation, and in this way they can stroboscopically follow the orbit.
Why is this interesting? Well, for one thing, it’s just plain cool to be able to control the states of atoms in this way. As Stroud points out, though, this is also interesting in a fundamental quantum way, as it touches on questions of how to make quantum objects look like classical ones. And the authors suggest that such electron states might be useful as phase-sensitive detectors of light pulses.
But mostly, it’s just cool to be able to manipulate atoms in this way.