Making Bosons Act Like Fermions

In what would represent an unprecedented manipulation of matter, physicists in Germany and Austria have proposed ways of making bosons, one of the two major classes of matter, act like fermions, the other kind of matter.

Fermions (such as electrons) obey the Pauli exclusion principle: If you put multiple electrons in a box, they all must differ from each other in some way, for example by being in a different place or having a different value of a quantum property such as spin.

Bosons (such as photons and the hydrogen atom) have no such restrictions: a limitless number of them can be in the identical quantum state.

Physics is replete with examples of making the fussy fermions behave like the more easygoing bosons, thanks to the phenomena of superconductivity and superfluidity. Causing fermions to pair up, as they do in superconductors, gives the pairs the same key properties as bosons, and so they act just like bosons.

But the reverse--making the normally undiscriminating bosons act like picky fermions--has never been done before. Now, researchers (Belen Paredes, Max Planck Institute for Quantum Optics, Belen.Paredes@mpq.mpg.de) have a couple of proposals for accomplishing this.

One way, they suggest, would be to rotate a Bose-Einstein condensate. At rest, the BEC has several different low-energy levels due to different possible values of angular momentum in the atoms.

However, rotating the BEC at just the right rate causes these levels to become equal to one another in energy since the rotation will cancel out the energy gains due to angular momentum. All stuck in the low-energy well, the atoms would be forced to minimize their repulsions with one another, and they'd do this by assuming slightly different values of angular momentum, thereby acting like fermions.

Rotating BECs is now possible with lasers or mechanical devices. But to observe "fermionization" in BECs with currently obtainable rotation speeds, researchers would need to create a BEC with only a handful of atoms, say 5, instead of the typical 10,000 or so.

But in case this turns out to be infeasible, the researchers have another proposal: rotate an optical lattice, a light-based web of atom traps, containing 5 atoms in each trap. Such a situation is experimentally possible and could produce a stronger signal than that from a single BEC. (Paredes, Zoller and Cirac, Physical Review A, September 2002; also Paredes and Cirac, cond-mat/0207040 at arXiv.org)