Atomic Quantum Fluids

Atomic gasses can be trapped in magneto-optic traps and cooled down to a few billionth of a degree above absolute zero. In doing so the quantum mechanical nature of the particles shows its colours: in Bose-Einstein condensates and fermionic superfluids the quantum mechanical properties, usually only visible on very small scales, are now transferred to the entire cloud of atoms. These atomic quantum gasses are produced in experiments around the world and it seems that experimentalists have a large degree of control on these macroscopic quantum systems: not only temperature and the number of atoms are adjustable, but also the interaction strength between the atoms and the geometry in which the gas is trapped (2D/3D or a grid). It is possible to imitate the Hamiltonians that appear in quantum mechanics with quantum gasses, and thus to test the theory with unprecedented precision and to bring the systems in regimes and under conditions that were previously unachievable. This way the atomic quantum gasses give a whole new look and unprecedented possibilities to construct many-particle quantum theories and test them. In this we participate enthusiastically with, among other, the following research themes:

• Fermionic superfluidity: a few bosonic atoms can all occupy the same quantum state. But one could also make fermionic atoms superfluid, under the condition that it is possible to pair them two by two. This can not only be done by forming molecules, but also through weakly bound Cooper pairs. The latter are also key to superconductivity. The atomic gasses allow one to study Cooper pairs under novel conditions. For example:
  • If there isn’t an equal number of pairing partners ('spin imbalance', FFLO states),
  • if they are limited to 2D-movement (Kosterlitz-Thouless transition & pseudogap)
  • if there is spin-orbit coupling
  • en if there is competition with itinerant ferromagnetisme
• An equation for the order parameter: Fermionic superfluids are described by a macroscopic wavefunction serving as an orderparameter. This is also the case for Bose-Einstein condensates and superconductors. For condensates the orderparameter must obey the Gross-Pitaevskii equation, and for superconductors there is the Ginzburg-Landau equation. We’ve succeed in finding a similar equation for fermionic superfluids, generalizing the Ginzburg-Landau equation for superconductors to all temperatures. And now we can focus on minimizing the energy functional for several applications.
• Quantum turbulence: Classic turbulence describes how energy, injected in a fluid on large scales, is redistributed to microscopic scales through large vortices breaking up into ever smaller vortices. Quantum mechanically though, there is a limit: there is a smallest quantized vortex. We study how the story of turbulence continues in the quantum regime.