From basic quantum mechanics to ultracold atoms: dynamics of two Lithium-6 atoms close to a Feshbach resonance

Feshbach resonances provide a remarkable tool to tune interatomic interactions in ultracold Fermi gases using external magnetic fields. Starting from basic quantum mechanics, we investigate collisions between two Lithium-6 atoms, explicitly considering their internal states and the Zeeman effect, which couples these states to the magnetic field. We analyze how the scattering length depends on the external field and highlight the dramatic changes in collision dynamics when it diverges. In particular, we discuss the Wigner time delay, the role of bound states in multichannel scattering, and the emergence of molecules, establishing connections to the BEC–BCS crossover in ultracold gases. Our approach offers a pedagogical yet rigorous framework, providing valuable insights and teaching tools for students, educators, and early-career researchers.