Rotational Quantum Beat Lasing without Inversion

In standard lasers, light amplification requires population inversion between an upper and a lower state to break the reciprocity between absorption and stimulated emission. However, in a medium prepared in a specific superposition state, quantum interference may fully suppress absorption while leaving stimulated emission intact, opening the possibility of lasing without inversion (LWI) [1] . Conventional LWI schemes generally strive to maintain a specific phase relationship between the lower-lying states that carry most of the population. We present a scheme [2] , [3] , that does not follow this tradition. It uses only the natural dynamics of a multi-level quantum system and requires no coherence between the excited and the lower electronic states; effectively, LWI comes "for free".