We present a novel method for mapping in situ the spatial distribution of photon momentum across a laser beam using a Bose-Einstein condensate (BEC) as a moving probe. By displacing the BEC, we measure the photon recoil by atom interferometry at different positions in the laser beam and thus reconstruct a two-dimensional map of the local intensity and effective dispersion of the $\vec{k}$ wave vector. Applied to a beam diffracted by a diaphragm, this method reveals a local extra recoil effect, which exceeds the magnitude $h\nu /c$ of the individual plane waves over which the beam can be decomposed. This method offers a new way to precisely characterize wavefront distortions and to evaluate one of the major systematic bias sources in quantum sensors based on atom interferometry.

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• Angular momentum of light
• Atom interferometry
• Atom optics
• Bose-Einstein condensates
• Cold atoms & matter waves
• Cold gases in optical lattices
• Cooling & trapping
• Lasers
• Metrology
• Spatial profiles of optical beams