1. S. Stenholm circa (1986) private communication. His statement, if I remember correctly, was “Does it bother you that your Hamiltonian has a spectrum that is unbounded from below?” The very same question was posed soon afterwards, also, by K. Rzążewski after the presentation of a conference paper.
2. W. H. Louisell, Radiation and Noise in Quantum Electronics, McGraw-Hill, New York (1964).
3. J. Peřina, Quantum Statistics of Linear and Nonlinear Optical Phenomena, Kluwer, Dordrecht (1991).
4. P. Meystre and D. F. Walls (Eds.), Nonclassical Effects in Quantum Optics, American Institute of Physics, New York (1991). This is a reprint collection of important early papers in quantum optics, many of them dealing with quantum nonlinear optics.
5. D. F. Walls and G. J. Milburn, Quantum Optics, Springer-Verlag, Berlin (1994).