Overcoming classical measurement limits through entanglement in photon number: an introductionM. Genovese, G. Adenier, D. Calonico, I. P. Degiovanni, S. Micalizio, I. Ruo Berchera and P. TrainaEPJ Web of Conferences, 95 (2015) 03011DOI: https://doi.org/10.1051/epjconf/20149503011