Nonclassical light is any state of light that cannot be described using classical electromagnetism; its characteristics are described by the quantised 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 kind.
- 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.
The quantum mechanical version of a classical light wave is a coherent state of the quantum electromagnetic field.
Glauber-Sudarshan P-representation
It has been shown that the density matrix for any state of light can be written as:
where 
is a coherent state. A classical state of light is one in which 
is a probability distribution. If it is not, the state is said to be nonclassical.
Aspects of that would make it nonclassical are:
- a negative value at any point;
- being more singular than a Dirac delta function.
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