Quantum Amplifiers and Generators
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Under conditions of heat equilibrium, the number of particles with higher energy is always smaller than the number with lower energy. Therefore, n12 > 0, σ > 0, Pabs > 0 and a wave propagating in a medium is damped. Nevertheless, the equilibrium condition can be disturbed by use of an additional energy source (a pumping source). Under definite conditions the number of particles on the upper level becomes greater than in the power level. As a result, n12 > 0, σ > 0, Pabs < 0 and instead of absorption we may observe electromagnetic wave amplification. Such a state is called population inversion or a state with a negative temperature. A medium with inversion is often called an active medium.