Collapse models and their experimental tests

Testing the limits of validity of the superposition principle is of crucial importance in the foundations of quantum mechanics and the development of quantum technologies. A way to quantify possible breakdowns of the superposition principle is given by collapse models. These models modify the Schrödinger equation, by adding non-linear and stochastic terms which describe spontaneous collapse in space of the wavefunction. The effects of the non-linear terms are negligible for microscopic systems however, because of an amplification mechanism built in the models, they become dominant for macroscopic objects, providing in this way a natural solution to the measurement problem. Because in collapse models the Schrödinger equation is modified, they make different predictions compared to Quantum Mechanics, hence they can be tested in experiments. We will introduce the most relevant collapse models, the Continuous Spontaneous Localization (CSL) model, and the Diósi-Penrose (DP) model. Then, we will give a summary of the current bounds set by different experiments on their phenomenological parameters. In particular, we will focus on experiments based on the study of radiation emission from matter.