ABSTRACT
Real atoms are typically complex, having ground and excited states with spin structure.Themolecules formed from the atoms typically have a rich spectrumof nearthreshold bound and quasi-bound molecular states when the molecular spin, rotational, and vibrational structure is taken into account.When an ultracold gas of atoms is produced, the atoms are prepared in specific quantum states, and collisions between the atoms occur with an extremely precisely defined energy close to the E = 0 collision threshold of the interacting atoms, where E denotes energy. The collision then makes the near-threshold spectrum of the molecular complex of the two atoms accessible to electromagnetic probing. An external magnetic or electromagnetic field can be precisely tuned to couple the colliding atoms to a specific molecular state, which can be viewed as a scattering resonance. This permits both extraordinary spectral accuracy in probing near-threshold level positions (order of E/h = 10 kHz accuracy for 1μK atoms) and precise resonant control of the collisions that determine both static and dynamical macroscopic properties of quantum gases. Consequently, understanding the near-threshold bound and scattering states is essential for understanding the collisions and interactions of ultracold atoms. This is also true for interactions of ultracold molecules.
