In the mid-1990s, researchers cooled atomic vapors to temperatures low enough to form a dilute Bose-Einstein condensate (BEC) where the atoms would all lock together in a single ground state. Another famous superfluid discovered earlier—low-temperature liquid helium—is a Bose condensate with much stronger interactions. Now researchers at JILA and the University of British Columbia have been able to tune the atom-atom scattering length in a rubidium BEC to a strongly interacting regime reminiscent of liquid helium.
To adjust the interactions between rubidium-85 atoms, the team used a mechanism called a Feshbach resonance in which colliding atoms strongly interact if their kinetic energy is equal to the energy of a bound state involving both atoms. This resonance can be tuned with an applied magnetic field, resulting in an adjustable scattering length. To measure the spectrum of excitations in the BEC, the researchers use Bragg spectroscopy: two counter-propagating lasers form an interference pattern that acts essentially as a moving diffraction grating. Rubidium atoms are scattered off the grating with momentum transfer determined by the period of the grating. Images of the BEC yield the momentum transfer as a function of excitation energy and the results showed substantial deviations from the case of a dilute weakly interacting BEC.
The strongly interacting BEC is interesting from a theoretical standpoint, and the Bragg interference technique provides a useful means of monitoring how transferring energy and momentum into such a system determines its excitations. – David Voss