Nonclassical light is light that cannot be described using classical electromagnetism; its characteristics are described by the quantized electromagnetic field and quantum mechanics. Nonclassical light has nonclassical noise properties called quantum noise, which can be understood on the basis of quantum optics.
Common described forms of nonclassical light are the following:
- Squeezed light exhibits reduced noise in one quadrature component. The most familiar kinds of squeezed light have either reduced amplitude noise or reduced phase noise, with increased noise of the other component.
- Fock states (also called photon number states) have a well-defined number of photons (stored e.g. in a cavity), while the phase is totally undefined.
Glauber–Sudarshan P representation 
It has been shown that the density matrix for any state of light can be written as:
Aspects of that would make it nonclassical are:
- a negative value at any point;
- being more singular than a Dirac delta function.
The matter is not quite simple. According to Mandel and Wolf: "The different coherent states are not [mutually] orthogonal, so that even if behaved like a true probability density [function], it would not describe probabilities of mutually exclusive states."
- Mandel & Wolf 1995, p. 541
Citation bibliography 
General references 
- R. J. Glauber, “Coherent and incoherent states of the radiation field”, Phys. Rev. 131 (6), 2766 (1963)
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