nonclassical

Nonclassical is an adjective used in physics to describe phenomena that cannot be fully explained by classical theories. In quantum physics, nonclassical refers to states or effects that lack a classical counterpart and cannot be described by a purely classical probability model.

In quantum optics, nonclassical light refers to states of the electromagnetic field that cannot be described as a fluctuating classical field. Indicators include sub-Poissonian photon statistics, quadrature squeezing, and negativity of the Wigner function or other quasi-probability distributions. Common examples are Fock states, squeezed states, and entangled photon states.

Generation and applications: Nonclassical states can be produced through nonlinear optical processes such as parametric down-conversion and four-wave mixing, as well as in cavity quantum electrodynamics and superconducting circuits. They are key resources in quantum information processing, precision metrology, quantum communication, and sensing.

Context and limitations: The classification of a state as nonclassical depends on the measurement scheme and the classical models being used for comparison. The term is thus relative and sometimes used informally. Nevertheless, nonclassicality is a central concept in areas of quantum technologies as a resource beyond classical capabilities.

See also: nonclassical states, quantum optics, coherent state, quasi-probability distribution.