Quantum fluids are able to flow past obstacles without creating drag and through channels without any resistance. This superfluidity is a consequence of Bose-Einstein condensation, a process in which all the particles gather in the same, lowest-energy, quantum state. At low flow velocities, energy exchanges between the obstacle and the fluid are forbidden because of the twin requirements of energy and momentum conservation. As the flow speed is increased, however, the superfluidity breaks down at some "critical velocity" above which excitations in the fluid can occur. These excitations can be phonons (i.e. quantized vibrations), vortices or, in the case of superfluid liquid helium, rotons.
Two research groups have recently made significant steps in studying superfluidity. Eric Cornell, Carl Wieman and co-workers at the JILA laboratory in Boulder, Colorado, have made the first direct observation of a vortex in a dilute Bose-Einstein condensate (M Matthews et al.1999 Phys. Rev. Lett. 83 2498). Meanwhile, Wolfgang Ketterle's group at the Massachusetts Institute of Technology has found evidence linking vortex formation with the critical velocity for superfluidity (C Raman et al.1999 Phys. Rev. Lett. 83 2502).
In the January issue of Physics World magazine, Charles Adams from Durham University and Jim McCann from Queen's University, Belfast, UK explain why these experiments may improve our understanding of the fundamental nature of superfluidity.