Searching for Born Rule Violation via Triple Interference in a Free-Fall Matter-Wave Interferometer

The Born rule implies that interference fringes in a multi-path interferometer can be reconstructed from the two-path interference fringes of all pairwise path combinations, together with the classical probability associated with each path. New physics beyond the Standard Model could manifest as apparent violations of the Born rule and give rise to three-path interference fringes which cannot be expressed as a sum of pairwise contributions. Experimental searches for Born rule violation have already been performed in a triple-slit optical interferometer, where three-path interference fringes were compared with pairwise fringes obtained by blocking individual slits. Recent theoretical proposals involving quantum gravity have motivated extending these tests to matter-wave interferometers.

We perform an analog of the optical triple-slit experiment using a free-fall atom interferometer, where interference occurs between the paths of an atom rather than optical paths. We independently measure both three-path and pair-wise interference fringes by selectively decohering certain atom paths with dephasing laser pulses. The three interferometer arms are formed using single-photon transitions on the Zeeman sublevels associated with the intercombination line of Sr88.  No statistically significant deviation from Born rule predictions are observed, but these measurements can help place bounds on theoretical models which predict Born rule violation in matter-wave systems.