Much of modern experimental atomic physics relies on a counterintuitive principle: Under the right circumstances, zapping matter with a laser doesn’t inject energy into the system; rather, it sucks energy out. By cooling the system to a fraction of a degree above absolute zero, one can observe quantum effects that are otherwise hidden.
Laser cooling works like a charm, but only when a system’s ladder of quantum states contains a transition that the lasers can repeatedly and reliably cycle to leave the system with a bit less energy each time. Atoms of alkali metals and a few other elements are ideal. Molecules are much more challenging: Their vibrational and rotational degrees of freedom create a multitude of low-lying quantum states that can disrupt a cooling cycle. And fundamental particles such as protons, which lack internal states altogether, can’t be laser cooled at all.
Nevertheless, there’s a lot of interest in...
