  The resolution of optical imaging devices is ultimately limited by the
diffraction of light. To circumvent this limit, modern super-resolution
microscopy techniques employ active interaction with the object by exploiting
its optical nonlinearities, nonclassical properties of the illumination beam,
or near-field probing. Thus, they are not applicable whenever such interaction
is not possible, for example, in astronomy or non-invasive biological imaging.
Far-field, linear-optical super-resolution techniques based on passive analysis
of light coming from the object would cover these gaps. In this paper, we
present the first proof-of-principle demonstration of such a technique. It
works by accessing information about spatial correlations of the image optical
field and, hence, about the object itself via measuring projections onto
Hermite-Gaussian transverse spatial modes. With a basis of 21 spatial modes in
both transverse dimensions, we perform two-dimensional imaging with twofold
resolution enhancement beyond the diffraction limit.
