30 years of the quantum cascade laser

It was January 1994, when the first quantum cascade laser (QCL) displayed laser action in Bell Laboratories. During these 30 years the QCL evolved incessantly, from a lab curiosity to the main on-chip source of coherent radiation in the Mid-IR and THz ranges. The journey has seen an impressive development of the QCL in several fields of laser physics and its applications, with a steady growth of research groups and companies worldwide. The 30-years development of quantum cascade lasers has established them as a go-to source of coherent radiation in the Mid-IR and THz ranges. In this comment, the authors guide the reader through the landmark achievements of this technology, from a lab curiosity to a mature technology adopted by research groups and companies.