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We investigate the localization of two incoherent point sources witharbitrary angular and axial separations in the paraxial approximation. By usingquantum metrology techniques, we show that a simultaneous estimation of the twoseparations is achievable by a single quantum measurement, with a precisionsaturating the ultimate limit stemming from the quantum Cram\'er-Rao bound.Such a precision is not degraded in the sub-wavelength regime, thus overcomingthe traditional limitations of classical direct imaging derived from Rayleigh'scriterion. Our results are qualitatively independent of the point spreadfunction of the imaging system, and quantitatively illustrated in detail forthe Gaussian instance. This analysis may have relevant applications inthree-dimensional surface measurements.

Towards superresolution surface metrology: Quantum estimation of angular and axial separations