- Squeezed light exhibits reduced noise in one quadrature component. The most familiar kinds of squeezed light have either reduced intensity 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), whereas the phase is totally undefined. A special case is that of a single-photon state, as generated on demand by a photon gun.
- 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.
- Also, there are states with special correlations between two or more different light beams. For example, signal and idler in parametric amplification exhibit strong intensity noise correlations, because signal/idler photons are generated in pairs. Nonclassical correlations of different light beams also occur in the context of quantum nondemolition measurements.
Coherent states most closely resemble classical states of light and exhibit Poissonian photon statistics. They are not considered as nonclassical states.
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. It is relevant in fundamental quantum physics and also in the context of some high-precision measurements, such as for gravitational wave detection.
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