  Quantum imaging with undetected photons is a recently introduced technique
that goes significantly beyond what was previously possible. In this technique,
images are formed without detecting the light that interacted with the object
that is imaged. Given this unique advantage over the existing imaging schemes,
it is now of utmost importance to understand its resolution limits, in
particular what governs the maximal achievable spatial resolution. We show both
theoretically and experimentally that the momentum correlation between the
detected and undetected photons governs the spatial resolution - a stronger
correlation results in a higher resolution. In our experiment, the momentum
correlation plays the dominating role in determining the resolution compared to
the effect of diffraction. We find that the resolution is determined by the
wavelength of the undetected light rather than the wavelength of the detected
light. Our results thus show that it is in principle possible to obtain
resolution characterized by a wavelength much shorter than the detected
wavelength.
