**The conditionally prepared photon and quantum imaging**

I will discuss a classical model allowing one to visualize and characterize the optical mode of the single photon generated by means of a conditional measurement on a biphoton produced in parametric down-conversion. The model is based on Klyshko’s advanced wave interpretation [2], but extends beyond it, providing a precise mathematical description of the advanced wave. The optical mode of the conditional photon can then be calculated as the result of nonlinear interaction of the partially coherent advanced wave with the pump pulse. With this “nonlinear advanced wave model” most coherence properties of the conditional photon become manifest, which permits one to intuitively understand many recent results in quantum imaging.
[1] D.N. Klyshko, Phys. Lett. A 132, 299 (1988); D.N. Klyshko, Phys. Lett. A 128, 133 (1988); D.N. Klyshko, Sov. Phys. Usp. 31, 74 (1988).