Lasing without inversion

Lasing without inversion (LWI) is a phenomenon in quantum optics that refers to a mechanism of producing coherent light (laser light) in a system where the usual requirement for population inversion is not met. Normally, in traditional laser operation, a population inversion is necessary, where more atoms or molecules are in an excited state than in a lower energy state. This population inversion is essential for achieving stimulated emission, the fundamental process that amplifies light in a laser.