Correlated states are at the heart of some of the most intriguing manifestations of quantum mechanics; they have been subject of historical controversial debates among its founding fathers [1]. Nowadays they represent a very attractive and challenging field of forefront research, addressing both, the most fundamental paradigms of quantum mechanics as well as applications. Prominent examples are twin photons pairs, which have allowed the violation of Bell’s inequalities [2,3] and hence an experimental test falsifying local realism. Correlated photons have been used to realize interferometers whose noise are reduced below the standard quantum limit [4] and are today the key tool for quantum cryptography or quantum communications.
