A description is given of a device that periodically transforms a field from the vacuum state to a pure excited state that corresponds to the propagation of two correlated photons. It employs the phenomenon of parametric scattering, i.e., the emission of pairs of photons by a nonlinear crystal excited by a pulsed coherent pump under phase-matched conditions. In accordance with the well-known Einstein--Podolsky--Rosen gedanken experiment, the device can be used to observe the correlation of either the transverse momenta of the photons (when the two detectors are located in the far-field zone) or their transverse coordinates (when the detectors are in the near-field zone). The device may be of interest in photometry, and also from the methodological point of view as a clear demonstration of the EPR paradox and the complementarity of the transverse coordinate and momentum of a photon.