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. Here we show that lasing without inversion arises naturally during propagation of intense femtosecond laser pulses in air. It is triggered by the combination of molecular ionization and molecular alignment, both unavoidable in intense light fields. The effect could enable inversionless amplification of broadband radiation in many molecular gases, opening unusual opportunities for remote sensing.

中文翻译：

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旋转量子拍激光无反转

在标准激光器中，光放大需要在上下状态之间反转种群，以打破吸收和受激发射之间的互易性。但是，在以特定的叠加状态制备的介质中，量子干涉可以完全抑制吸收，同时完整保留受激发射，从而打开了发射激光而不会倒转的可能性。在这里，我们表明，在空中飞秒级飞秒激光脉冲传播过程中，自然会产生无反转的激光。它是由分子电离和分子排列的结合触发的，这在强光场中都是不可避免的。这种效应可以使许多分子气体中的宽带辐射无反转放大，从而为遥感开辟了难得的机会。