Johannes Gutenberg University, Mainz, Germany

A cold-matter physicist is amazed by atoms’ ability to divide themselves up equally.

Imagine having a box containing an even number of objects, N. You want to divide them into two boxes, each of which contains exactly N/2 objects. Sounds easy, right?

But let’s complicate things a bit. Let’s suppose you can’t count the objects, nor look at them. Will you still be able to make the split fairly?

A collaboration of researchers from the Massachusetts Institute of Technology and Harvard University, both in Cambridge, recently showed that it’s possible to do so for atoms. They divided into two equal halves a Bose–Einstein condensate of 1 million sodium atoms (G.-B. Jo et al. Phys. Rev. Lett., in the press; preprint here).

Bose–Einstein condensates are a novel state of matter that forms at a temperature close to absolute zero. They behave like quantum entities with pronounced wave-like properties. These are properties that I exploit in my own work with condensates, and they also underpin the atom division.

The Cambridge team stored their matter waves in microfabricated magnetic traps, made out of thin wires. The researchers changed the currents in the wires to split slowly the one potential well that was holding the atoms into two.

In a non-interacting gas, this splitting process would probably give a skewed distribution of atoms, and the distribution would be different every time. In this case, the quantum interactions favour a system in which each well contains exactly N/2 atoms.

In fact, the evidence suggests that the splitting is accurate to within 50 atoms. I find that truly remarkable from a fundamental point of view. More practically, this dividing of atoms could also be useful in building novel atom interferometers and atomic clocks.