When I made a study on lasers, I found a webpage saying that the quantum mechanical principles enable us to create laser action without population inversion, which Einstein suggested as one of the essential condition to achieve laser action.
Usually, the gain medium of a laser works on the basis of a population inversion. In the 1990s, however, it was shown that optical amplification and consequently lasing without inversion are possible by using an additional optical or microwave field which induces a quantum coherence in the atoms of the gain medium. The basic idea is to provide two different pathways for atoms to get from the ground state to the excited state – a direct one and another one via a third energy level –, and to induce a quantum coherence, so that the quantum-mechanical probability amplitudes for both processes cancel. In effect, this suppresses the re-absorption and thus makes it possible to obtain gain even with a small population in the upper state.
We show that if two upper levels of a four-level laser system are purely lifetime broadened, and decay to an identical continuum, then there will be an interference in the absorption profile of lower-level atoms, and that this interference is absent from the stimulated emission profile of the upper-level atoms. Laser amplification may then be obtained without inversion. Examples include interfering autoionizing levels, and tunneling systems.
But, Without population inversion, the energy density within the laser material damps down. Then, How does laser action is acheived?!