Quantum mixtures

Ultracold atoms have become extremely well controlled systems, whose parameters can be settled precisely and independently. Such high level of control and the detection possibilities have opened the way to a large variety of applications. 

The ability to prepare well-controlled atomic mixtures of different internal states of the same species or different elements in a quantum regime greatly enriches the experimental possibilities. Several physical problems associated with mixtures have been studied in recent years. The most well-known example is the mixture of fermionic gases, which permits exploring the famous BEC-BCS crossover between a Bardeen-Cooper-Schrieffer (BCS) type of superfluid - a phenomenon of superconductivity in metals- to a Bose-Einstein condensate of weakly bound dimers. In the crossover, the gas interacts strongly, and the experiments are quantum simulators for complex quantum systems where precise theoretical predictions are scarce. 

Bose and Fermi mixtures have gained renewed interest in recent years. Bose mixtures lead to very interesting new phenomena such as phase separation for repulsive interactions, or quantum droplets that originate from a competition between vanishing mean-field interactions and beyond mean-field corrections. On the other hand, spinor condensates, where spin exchange interaction can take place, leads to a challenging phase diagram and to novel cooling mechanisms when dipolar interactions are at play. 

These mixtures also potentiate the study of polaron physics, i.e., the physics of an impurity immersed in a quantum bath, which is of broad interest in condensed matter physics to understand the electron mobility in semiconductors or organic-semiconductors. Furthermore, they also give access to the rich topic of Efimov-physics first introduced in nuclear physics. Finally, Fermi-mixtures are also promising candidates to explore novel phases of matter such as the Fulde-Ferrell-Larkin-Ovchinnikov, characterizing s-wave Cooper pairs in superconductors with nonzero total momentum and spatially non-uniform order parameter.

Spin mixtures using alkaline earth atoms or lanthanides also offer new possibilities to study topological systems. In this case, the vast internal structure offers the possibility to use the spin degree of freedom as an additional synthetic dimension and emulate gauge fields. 

The wide range of topics covered shows how ultracold atomic quantum mixtures are promising and versatile systems to probe very interesting and challenging topics ranging from condensed-matter theory to nuclear physics in experiments with a high degree of control.