The rotating wave approximation (RWA) of quantum optics: serious defect

The rotating wave approximation (RWA) is an integral part of the foundations of quantum optics and it is also used extensively in atomic and condensed-matter physics. Here we prove that the model has a serious defect, viz. the spectrum has no lower bound, for all models of physical interest. As a result, the reservoir is not passive (since energy can be extracted from it without limit) and hence the second law of thermodynamics is not satisfied. An alternative to the RWA is discussed.