String theorists attempt to explain all the fundamental particles as vibrations on tiny strings on length scales of about 10-33 centimetres. The theory naturally includes "supersymmetry" - a symmetry that connects particles with integer spin, known as bosons, to particles with half-integer spin, which are known as fermions. The particles that carry the fundamental forces of nature, such as the photon and the gluon, are bosons, while the quarks and leptons that make up matter are fermions. Although superstring theory is the leading candidate for a theory of everything, there is no experimental evidence to date for strings or supersymmetry.


Now Michiel Snoek, Masudul Haque, Stefan Vandoren and Henk Stoof of Utrecht University have proposed making a "non-relativistic Green-Schwarz superstring" by trapping an ultracold cloud of fermionic atoms along the core of a quantized vortex in a Bose-Einstein condensate (BEC). A BEC is a special state of matter in which all the particles are in the same quantum ground state. Bosonic atoms such as rubidium-87 can enter such as state because, unlike fermions, they do not obey the Pauli exclusion principle.

The bosonic part of the superstring would consist of a vortex line created by rapidly rotating a one-dimensional BEC in an optical lattice (see figure). Next, a gas of fermion atoms, such as potassium-40, would be trapped within this vortex, which is possible under certain conditions. Snoek and colleagues say that it should be possible to observe the supersymmetry between the fermions and bosons by carefully tuning the interactions between the two types of atom with a laser.

Quantized vortices were first seen in superfluid helium. They are formed inside a rotating superfluid when it begins to spin faster than a certain critical speed. In the mid-1990s it was suggested that these vortices could simulate the formation of cosmic strings in the early universe.