This article is part of the theme issue ‘Celebrating the 15th anniversary of the Royal Society Newton International Fellowship’ to which I was invited to contribute as a Newton Fellow 2009 (the first round ever). 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. [Full article]
