Spatial Resolution Enhancement in Quantum Imaging beyond the Diffraction   Limit Using Entangled Photon-Number State
  In this paper we study the resolution of images illuminated by sources composed of $N+1$ photons in which one non-degenerate photon is entangled with $N$ degenerate photons. The $N$ degenerate photons illuminate an object and are collected by an $N$ photon detector. The signal from the $N$ photon detector is measured in coincidence with the non-degenerate photon giving rise to a ghost image. We discuss the case of three photons in various configurations and generalize to $N+1$. Using the Rayleigh criterion, we find that the system may give an improvement in resolution by a factor of $N$ compared to using a classical source. For the case that the $N$-photon number detector is a point detector, a coherent image is obtained. If the $N$-photon detector is a bucket detector, the image is incoherent. The visibility of the image in both cases is 1. In the opposite case in which the non-degenerate photon is scattered by the object, then, using an $N$-photon point detector may reduce the Airy disk by a factor of $N$. 
Jianming Wen, Morton H. Rubin, and Yanhua Shih