One of the most iconic representations of the struggle physicists went through to understand quantum mechanics is a series of black-and-white pictures showing Albert Einstein locked in an animated scientific discussion with Niels Bohr — walking, sitting and smoking (pictured). A famous paradox the pair debated is embodied in the recoiling double-slit experiment.
Although arguably less romantic than old pictures, the newest chapter in the history of this thought experiment takes place in the lab, where Xiao-Jing Liu and co-workers have now tested it using resonant X-ray photoemission from molecular oxygen. Each of the two oxygen atoms in the molecule served as a slit, and the Auger-ejected electron played the role of the particle.
By tuning the energy of the exciting X-ray photons, the electron could be emitted with or without dissociating the O2 molecule. In the latter case, the two oxygen atoms recoiled together — yielding no information about the atom from which the electron was emitted — and spectral bands exhibited an interference pattern. In the former case, only one of the atoms recoiled, and the availability of path information destroyed any interference, in full agreement with Bohr's complementarity. So far, it seems, he is still winning the argument.
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Levi, F. Tell-tale oxygen. Nature Phys 11, 92 (2015). https://doi.org/10.1038/nphys3253
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DOI: https://doi.org/10.1038/nphys3253
