Exploring quantum features of large spin and photonic systems

In this thesis, we will present different theoretical tools for demonstrating the quantum features of possibly large photonic and spin systems. Our aim is not only to propose experiments demonstrating entanglement/non-locality in possibly large spin and photonic systems, but to clarify on the requirements that the detection systems need to fulfil to detect the quantum nature of these large systems. We will see that collective projections in which the subsystems are measured along the same direction, does not prevent the detection of entanglement and non-locality. Even threshold detectors with low efficiencies can be used to reveal non-classical features of states involving many photons. In parallel, we will make steps to clarify on the meaning of large/macroscopic when describing quantum states.