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We have experimentally and theoretically studied how the interactions affect the interference pattern of two expanding 87Rb condensates. Our analysis shows that the condensate phase is modified by the mutual interaction only in the region where the wave packets overlap. This result proves that the general assumption of phase rigidity has to be abandoned for an accurate description of matter-wave interference. A. Burchianti et al. |
Considering the hurdles of experiments with more than one atomic species, the temptation arises of rephrasing Arthur L. Schawlow: "Double-species Bose-Einstein condensates are condensates with one species too many". We think otherwise. Quantum mixtures allow the investigation of a wealth of genuinely quantum phenomena: mixed phases of superfluids and Mott insulators, impurities and polarons, chemistry at zero-temperature.