Nonclassical Light

Nonclassical light is light with nonclassical quantum noise properties, which can be understood only on the basis of quantum optics. The most common forms of nonclassical light are the following:

• Fock states* (also called photon number states) have a well-defined number of photons, whereas the optical phase is totally undefined. A special case is that of a single-photon state, as generated on demand or probabilistically with single-photon sources. Higher-order Fock states are rather difficult to produce.
• Squeezed states of light* exhibit reduced noise in one quadrature component. The most familiar kinds of squeezed light have either reduced intensity noise (amplitude-squeezed light) or reduced phase noise (phase-squeezed light), with increased noise of the other kind.
• Wigner-function negativity is another sign that a quantum state has no classical phase-space description, although not all states of nonclassical light have that property. The Wigner function is a real, normalized quasi-probability distribution whose marginals reproduce quadrature statistics. Unlike a true probability density, the Wigner function can take negative values, e.g. for Fock states; those negative regions reflect quantum interference between phase-space amplitudes and cannot arise from any classical random mixture of fields.
• Quantum entanglement is often observed, e.g. in photon pairs. This implies nonclassical correlations between photons.

Coherent states most closely resemble classical states of light and exhibit Poissonian photon statistics. They are not considered as nonclassical states.

Nonclassical light is relevant in fundamental quantum physics and also in the context of some high-precision measurements, such as for gravitational wave detection.

The following properties often distinguish nonclassical light from classical light:

• The quantum noise properties are generally modified in some way. For example, there can be reduced noise in a quadrature component, allowing measurements below the standard quantum limit.
• There are often special correlations. For example, nonclassical light sometimes exhibits photon antibunching, i.e. a reduced probability of two photons being detected within a short time interval, or sub-Poissonian photon statistics.
• Quantum entanglement is another profound quantum property.

Nonclassical light is often generated either in nonlinear devices such as in sub-threshold optical parametric oscillators or frequency doublers, or in systems with only a single atom or ion (or just a few such emitters), such as a single-atom laser.

For more details, see the articles on specific sources of nonclassical light, e.g. single-photon sources and photon pair sources.