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In an atom interferometer, an atomic wavepacket is split into two wavepackets that follow different paths before recombining. The phase change of each wavepacket is proportional to the proper time along its path, resulting in an interference pattern when the wavepackets recombine. The detection of the decoherence by observing the fuzziness of this pattern due to space–time fluctuations on the Planck scale would provide experimental access to quantum-gravity effects analogous to the access to atomic scales provided by Brownian motion.