Now, the paper of Mahboob et al. reports a significant step towards this goal. The authors create a purely mechanical system of three energy levels that is reminiscent of a three-level laser scheme (see Fig. 1, top panel). In a three-level laser, atoms are excited by an optical pump (usually another laser or a flashlamp) from the ground level (1) to a higher energy level (3), and then quickly relax through spontaneous emission and accumulate in the upper laser level (2). If the population of level
2exceeds that of level 1(a condition known as population inversion), photons resonant with the 1→2laser transition get amplified through stimulated emission: the medium acts as an optical amplifier and, when placed within a cavity, lasing occurs if the gain exceeds the losses.
The link given above gives you access to the actual paper, so don't miss it.