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In 1963 Glauber /Phys. Rev. 130, 2529 and 131, 2766/ published a
basic theoretical study that revealed the broad variety of possible
photon states. Later on a quantum-statistical analysis showed that
states with classical analog (this light Is termed classical light)
as well as states without classical analog (so-called nonclassical
light) can occur. Following the definition given by Teich, Saleh, and
Perina /JOSAB2 (1985) 275) nonclassical light exhibits one or more
of the attributes squeezed, antibunched, sub-Poissonian, what is
connected with a negative value of the field-fluctuation excess, the
bunching excess, or the Poisson excess. These negative values mean
quantum fluctuations below the classical standard quantum limits of
ideal laser light; thus, nonclassical light may yield an essential
improvement of the optical measurement accuracy, in particular of the
signal-to-noise ratio. There exists a common property of the three
attributes concerning the Glauber-Sudarshan representation P(c ),
which is the real weight function belonging to the incoherent mixture
(density operators) of Glauber states with different complex amplitudes. In the case of classical light P(oç ) is a positive definite
function, whereas nonclassical light requires a nonpositive definite
function P(o.), which cannot be interpreted as a proper probability
distribution - as it is possible in the case of light with a classical analog /M. Schubert, Ann. d. Phys. 4k (1987) 53/. In addition to
its possible important applications nonclassical light yields a deep
insight into the quantum nature of the radiation field.
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