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