Nonclassical light is light that cannot be described using classical electromagnetism; its characteristics are described by the quantized electromagnetic field and quantum mechanics.
The most common described forms of nonclassical light are the following: The density matrix for any state of light can be written as: where
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, L.; Wolf, E. (1995), Optical Coherence and Quantum Optics, Cambridge UK: Cambridge University Press, ISBN 0-521-41711-2