[en] We compare the original formalism in which the travelling-wave setup of the experiment due to Zou et al (1991 Phys. Rev. Lett. 67 318) was analysed with a simple quantum optical approach that was later used for the same purpose. The analyses, both original and subsequent, included the quantum correlation between the signal beams. Whereas the original work spoke of a single photon per beam, the subsequent papers generalized to many photons per mode. We try to show that, in the original theory as well, one may speak of many photons per beam, without changing the relationship with the experiment. We conclude that the original analysis represents multiphoton stationary fields by interval restrictions comprising single photons