If the magnetic moment of the molecular state differs from that of the atoms, one can use a magnetic field to vary the energy difference between the atomic and molecular states. When the two atoms thus flirt with entering a molecular state, the scattering is greatly enhanced. As the magnetic field goes through this Feshbach resonance, a grows rapidly stronger, changes sign, and then weakens as the field is taken farther from resonance. The use of Feshbach resonances to tune atomic interactions was suggested in 1992 by Boudewijn Verhaar, of the Eindhoven University of Technology in the Netherlands, and his colleagues. 3 Scattering length versus magnetic field near a Feshbach resonance at 155 G. Data points indicate the scattering length determined from the size of the condensate.
Smooth curve is the theoretical expectation.
The scattering length is given relative to the Bohr radius. Inset shows the full extent of the Feshbach resonance. Although many atoms have Feshbach resonances, near which one can tune the interactions, such resonances have so far been seen only in a few atoms. That's because the resonances are only evident at very low temperatures, and they usually occur at incon veniently large magnetic fields. Two years ago, researchers at MIT were able to observe a Feshbach resonance in a sodium BEC by using optical rather than magnetic fields to trap the atoms.