Tales of 1001 Atoms

Imagine trying to count grains of rice in a cup that are shaking about, and then assume the grains are less than a nanometer long. This is akin to what David Hume and colleagues from the University of Heidelberg in Germany have recently done [1]. As reported in Physical Review Letters, by observing the light emitted from a collection of laser-cooled and trapped atoms, they can distinguish two ensembles differing by just one atom—one grain—and directly observe a small step in signal when one atom is lost from the trap (Fig. 1). You might also wonder why anybody would want to count atoms to one thousand instead of being satisfied with distinguishing, say, one atom from two. The answer is that one day such particle counting techniques could extend precision quantum measurements well beyond current limits, helping to improve the accuracy of atomic clocks [2, 3], or allowing an increase in the capability of atomic magnetometers.