居量反转（英语：Population inversion），又译为群数反转、密数反转、粒子数反转、反转分布，物理学名词，在统计力学中经常被使用。在一个系统（例如一群原子、或一群分子）中，处在激发状态的成员数量，比起处于较低能阶状态的成员更多时，这就被称为居量反转。让标准雷射进入能够运作的状态的过程中，产生居量反转是一个必要的步骤，因此在雷射科学中，居量反转是很重要的研究主题之一。值得注意的是居能反转不可能是热平衡的稳态解，如二能阶系统中，温度极高或外场极大时的平衡态也只允许激发态与基态粒子数目相等．

In science, specifically statistical mechanics, a population inversion occurs while a system (such as a group of atoms or molecules) exists in a state in which more members of the system are in higher, excited states than in lower, unexcited energy states. It is called an "inversion" because in many familiar and commonly encountered physical systems, this is not possible. The concept is of fundamental importance in laser science because the production of a population inversion is a necessary step in the workings of a standard laser.