For an
electromagnetic wave
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visib ...
, the coherence time is the
time
Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, into the future. It is a component quantity of various measurements used to sequence events, ...
over which a propagating wave (especially a
laser
A laser is a device that emits light through a process of optical amplification based on the stimulated emission of electromagnetic radiation. The word "laser" is an acronym for "light amplification by stimulated emission of radiation". The fi ...
or
maser
A maser (, an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The first maser was built by Charles H. Townes, Ja ...
beam) may be considered
coherent, meaning that its
phase is, on average, predictable.
In long-distance
transmission systems, the coherence time may be reduced by
propagation
Propagation can refer to:
* Chain propagation in a chemical reaction mechanism
*Crack propagation, the growth of a crack during the fracture of materials
* Propaganda, non-objective information used to further an agenda
* Reproduction, and other fo ...
factors such as
dispersion
Dispersion may refer to:
Economics and finance
* Dispersion (finance), a measure for the statistical distribution of portfolio returns
* Price dispersion, a variation in prices across sellers of the same item
*Wage dispersion, the amount of variat ...
,
scattering, and
diffraction.
The coherence time, usually designated , is calculated by dividing the
coherence length
In physics, coherence length is the propagation distance over which a coherent wave (e.g. an electromagnetic wave) maintains a specified degree of coherence. Wave interference is strong when the paths taken by all of the interfering waves dif ...
by the
phase velocity of
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 te ...
in a medium; approximately given by
where is the central
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, t ...
of the source, and is the
spectral width In telecommunications, spectral width is the wavelength interval over which the magnitude of all spectral components is equal to or greater than a specified fraction of the magnitude of the component having the maximum value.
In optical communicat ...
of the source in units of frequency and wavelength respectively, and is the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
in vacuum.
A single mode
fiber laser has a linewidth of a few kHz, corresponding to a coherence time of a few hundred microseconds. Hydrogen masers have linewidth around 1 Hz, corresponding to a coherence time of about one second.
[ - Precision measurements with atomic hydrogen masers] Their coherence length approximately corresponds to the distance from the Earth to the Moon.
As of 2022, research groups worldwide have demonstrated
superconducting
Superconductivity is a set of physical properties observed in certain materials where electrical resistance vanishes and magnetic flux fields are expelled from the material. Any material exhibiting these properties is a superconductor. Unlike ...
qubits
In quantum computing, a qubit () or quantum bit is a basic unit of quantum information—the quantum version of the classic binary bit physically realized with a two-state device. A qubit is a two-state (or two-level) quantum-mechanical system, ...
with coherence times up to several 100
µs
A microsecond is a unit of time in the International System of Units (SI) equal to one millionth (0.000001 or 10−6 or ) of a second. Its symbol is μs, sometimes simplified to us when Unicode is not available.
A microsecond is equal to 1 ...
.
See also
*
Atomic coherence
Atomic may refer to:
* Of or relating to the atom, the smallest particle of a chemical element that retains its chemical properties
* Atomic physics, the study of the atom
* Atomic Age, also known as the "Atomic Era"
* Atomic scale, distances com ...
*
Temporal coherence
*
Degree of coherence
References
*
{{DEFAULTSORT:Coherence Time
Electromagnetic radiation
Physical optics
Radio frequency propagation