The collision of two ultracold atoms results in a quantum mechanical superposition of the two possible outcomes: each atom continues without scattering, and each atom scatters as an outgoing spherical wave with an s-wave phase shift. The magnitude of the s-wave phase shift depends very sensitively on the interaction between the atoms. Quantum scattering and the underlying phase shifts are vitally important in many areas of contemporary atomic physics, including Bose–Einstein condensates1,2,3,4,5, degenerate Fermi gases6,7,8,9, frequency shifts in atomic clocks10,11,12 and magnetically tuned Feshbach resonances13. Precise experimental measurements of quantum scattering phase shifts have not been possible because the number of scattered atoms depends on the s-wave phase shifts as well as the atomic density, which cannot be measured precisely. Here we demonstrate a scattering experiment in which the quantum scattering phase shifts of individual atoms are detected using a novel atom interferometer. By performing an atomic clock measurement using only the scattered part of each atom’s wavefunction, we precisely measure the difference of the s-wave phase shifts for the two clock states in a density-independent manner. Our method will enable direct and precise measurements of ultracold atom–atom interactions, and may be used to place stringent limits on the time variations of fundamental constants14.
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We acknowledge discussions with B. Verhaar and S. Kokkelmans and contributions from R. Li. This work was supported by NASA, NSF, ONR and Penn State University.
Reprints and permissions information is available at www.nature.com/reprints. The authors declare no competing financial interests.
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Hart, R., Xu, X., Legere, R. et al. A quantum scattering interferometer. Nature 446, 892–895 (2007). https://doi.org/10.1038/nature05680