In recent years, experimental progress has led to the mastery of ultracold atomic gases. This allowed the first observation of Bose-Einstein condensation, a quantum statistical effect resulting in a macroscopic quantum object, the Bose-Einstein condensate (BEC). In the spirit of Feynman's quantum simulator, these systems can be employed to model fundamental problems of condensed-matter physics, with flexible control over the parameters of the system.
In our group, we study in particular the phenomenon of Bloch oscillations (BOs): a quantum particle in a periodic potential exposed to a force cannot accelerate uniformly, but performs BOs, because at the band edge the effective mass is negative. Real crystals are not clean enough for BOs, so we consider a BEC in a tilted optical lattice, where BOs are indeed observable. With ultacold atoms, new control possibilities open up: using Feshbach resonances, one can modulate the atom-atom interaction as function of time. Interestingly, such a modulated interaction leaves the BO intact, if frequency and phase (with respect to the free BO) are chosen correctly.