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We demonstrate a single-photon stored-light interferometer, where a photon is stored in a laser-cooled atomic ensemble in the form of a Rydberg polariton with a spatial extent of $10 times1times1mu m^3$. The photon is subject to a Ramsey sequence, i.e. `split into a superposition of two paths. After a delay of up to 450 ns, the two paths are recombined to give an output dependent on their relative phase. The superposition time of 450 ns is equivalent to a free-space propagation distance of 135 m. We show that the interferometer fringes are sensitive to external fields, and suggest that stored-light interferometry could be useful for localized sensing applications.
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