Nonclassical light
Encyclopedia
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:
The quantum mechanical version of a classical light wave is a coherent state
of the quantum electromagnetic field
.
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:
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
that cannot be described using classical electromagnetism
Classical electromagnetism
Classical electromagnetism is a branch of theoretical physics that studies consequences of the electromagnetic forces between electric charges and currents...
; its characteristics are described by the quantised electromagnetic field
Electromagnetic field
An electromagnetic field is a physical field produced by moving electrically charged objects. It affects the behavior of charged objects in the vicinity of the field. The electromagnetic field extends indefinitely throughout space and describes the electromagnetic interaction...
and quantum mechanics
Quantum mechanics
Quantum mechanics, also known as quantum physics or quantum theory, is a branch of physics providing a mathematical description of much of the dual particle-like and wave-like behavior and interactions of energy and matter. It departs from classical mechanics primarily at the atomic and subatomic...
. Nonclassical light has nonclassical noise properties called quantum noise
Quantum noise
Quantum noise is uncertainty of some physical quantity due to its quantum origin.In the case of number of particles , the quantum noise is also called shot noise. Most optical communications use amplitude modulation...
, which can be understood on the basis of quantum optics
Quantum optics
Quantum optics is a field of research in physics, dealing with the application of quantum mechanics to phenomena involving light and its interactions with matter.- History 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 stateFock stateA Fock state , in quantum mechanics, is any element of a Fock space with a well-defined number of particles . These states are named after the Soviet physicist, V. A. Fock.-Definition:...
s (also called photonPhotonIn physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
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
Coherent state
In quantum mechanics a coherent state is a specific kind of quantum state of the quantum harmonic oscillator whose dynamics most closely resembles the oscillating behaviour of a classical harmonic oscillator...
of the quantum electromagnetic field
Quantum electrodynamics
Quantum electrodynamics is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved...
.
Glauber-Sudarshan P-representation
It has been shown that the density matrixDensity matrix
In quantum mechanics, a density matrix is a self-adjoint positive-semidefinite matrix of trace one, that describes the statistical state of a quantum system...
for any state of light can be written as:
where
is a coherent stateCoherent state
In quantum mechanics a coherent state is a specific kind of quantum state of the quantum harmonic oscillator whose dynamics most closely resembles the oscillating behaviour of a classical harmonic oscillator...
. A classical state of light is one in which
is a probability distributionProbability distribution
In probability theory, a probability mass, probability density, or probability distribution is a function that describes the probability of a random variable taking certain values....
. 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 functionDirac delta functionThe Dirac delta function, or δ function, is a generalized function depending on a real parameter such that it is zero for all values of the parameter except when the parameter is zero, and its integral over the parameter from −∞ to ∞ is equal to one. It was introduced by theoretical...
.