  Quantum imaging with undetected photons relies on the principle of induced
coherence without induced emission and uses two sources of photon-pairs with a
signal- and an idler photon. Each pair shares strong quantum correlations in
both position and momentum, which allows to image an object illuminated with
idler photons by just measuring signal photons that never interact with the
object. In this work, we theoretically investigate the transverse resolution of
this non-local imaging scheme through a general formalism that treats
propagating photons beyond the commonly used paraxial approximation. We hereby
prove that the resolution of quantum imaging with undetected photons is
fundamentally diffraction limited to the longer wavelength of the signal and
idler pairs. Moreover, we conclude that this result is also valid for other
non-local two-photon imaging schemes.
