Two photons everywhere.

We discuss two-photon physics, taking for illustration the particular but topical case of resonance fluorescence. We show that the basic concepts of interferences and correlations provide at the two-photon level an independent and drastically different picture than at the one-photon level, with landscapes of correlations that reveal various processes by spanning over all the possible frequencies at which the system can emit. Such landscapes typically present lines of photon bunching and circles of antibunching. The theoretical edifice to account for these features rests on two pillars: (i) a theory of frequency-resolved photon correlations and (ii) admixing classical and quantum fields. While experimental efforts have been to date concentrated on correlations between spectral peaks, strong correlations exist between photons emitted away from the peaks, which are accessible only through multi-photon observables. These could be exploited for both fundamental understanding of quantum-optical processes as well as applications by harnessing these unsuspected resources. This article is part of the theme issue 'Celebrating the 15th anniversary of the Royal Society Newton International Fellowship'.