  The year 2015 was the International Year of Light. It marked, however, also
the 20th anniversary of the first observation of Bose-Einstein condensation in
atomic vapors by Eric Cornell, Carl Wieman and Wolfgang Ketterle. This
discovery can be considered as one of the greatest achievements of quantum
optics that has triggered an avalanche of further seminal discoveries and
achievements. For this reason we devote this essay for focus issue on "Quantum
Optics in the International Year of Light" to the recent revolutionary
developments in quantum optics at the frontiers of all physics: atomic physics,
molecular physics, condensed matter physics, high energy physics and quantum
information science. We follow here the lines of the introduction to our book
"Ultracold atoms in optical lattices: Simulating quantum many-body systems"
[1]. The book, however, was published in 2012, and many things has happened
since then -- the present essay is therefore upgraded to include the latest
developments.
