Using spontaneous parametric down conversion as a source of entangled photonpairs, correlations are measured between the orbital angular momentum (OAM) ina target beam (which contains an unknown object) and that in an empty referencebeam. Unlike previous studies, the effects of the object on off-diagonalelements of the OAM correlation matrix are examined. Due to the presence of theobject, terms appear in which the signal and idler OAM do not add up to that ofthe pump. Using these off-diagonal correlations, the potential forhigh-efficiency object identification by means of correlated OAM states isexperimentally demonstrated for the first time. The higher-dimensional OAMHilbert space enhances the information capacity of this approach, while thepresence of the off-diagonal correlations allows for recognition of specificspatial signatures present in the object. In particular, this allows thedetection of discrete rotational symmetries and the efficient evaluation ofmultiple azimuthal Fourier coefficients using fewer resources than inconventional pixel-by-pixel imaging. This represents a demonstration of sparsesensing using OAM states, as well as being the first correlated OAM experimentto measure properties of a real, stand-alone object, a necessary first steptoward correlated OAM-based remote sensing.
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