ARISING FROM T. Kovachy et al. Nature 528, 530–533 (2015); doi: 10.1038/nature16155

Although the existence of quantum superpositions of massive particles over microscopic separations has been established since the founding of quantum mechanics, the maintenance of superposition states over macroscopic separations is a subject of modern experimental tests. Kovachy et al.1 report on applying optical pulses to place a freely falling Bose–Einstein condensate into a superposition of two trajectories that separate by an impressive distance of 54 cm before being redirected towards one another. When the trajectories overlap, a final optical pulse produces interference with high contrast, but random phase, between the two wave packets. Contrary to ref. 1, we argue that the observed interference is consistent with, but does not prove, the claim that the spatially separated atomic ensembles were in a quantum superposition state; therefore, the persistence of such superposition states remains experimentally unestablished. There is a Reply to this Brief Communication Arising by Kovachy, T. et al. Nature 537, http://dx.doi.org/10.1038/nature19109 (2016).

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