Strong field physics and quantum optics

The subject of quantum optics is concerned with the quantum properties of the radiation field, i.e. the properties of photons. Since the word multiphoton has been used, it might seem that strong laser fields are in some way relevant to quantum optics. However, the word multiphoton is something of a misnomer in the strong field regime. In fact, if very many photons are involved, quantisation of the radiation field is more or less irrelevant the intense, coherent laser pulse tends to a quasiclassical beam of light. Indeed, it has been pointed out by several authors [483] that the use of the word photon in the context of laser physics is of questionable validity. 

The condition for a beam to be classical is that the number of photons per cubic wavelength should be much larger than 1. For modern strong field lasers, this number is of the order of 1014 or more. 

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