Quantum mixtures of ultracold gases of neutral atoms

After decades of improvements in cooling techniques of several atomic species and in finding methods for the achievement of stable quantum mixtures, the field is now ready for an extensive use of such a versatile experimental platform for the investigation of various physical problems. Among them, relevant examples are the dynamics of impurities in a quantum gas, the miscibility condition of different gases, the study of exotic topological structures, the interplay between magnetism and superfluidity, the formation of artificial molecules or new few-body states. We illustrate the differences among possible quantum mixtures — whether homonuclear spin mixtures or heteronuclear ones — and show how they can be exploited to investigate a plethora of topics from the few-body to the many-body regime. In particular, we discuss quantum mixtures of ultracold gases under three different perspectives: systems made of a few atoms of different kinds, single impurities immersed in a host quantum gas and quantum mixtures of two interacting gases. We restrict the discussion to single harmonic or flat traps, predominantly in a 3D configuration. A selection of results on recent experiments and possible interesting future directions are given.