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{{Short description|Time of electromagnetic wave coherence}}
{{for|the similar concept in communication systems|Coherence time (communications systems)}}

For an [[electromagnetic wave]], the '''coherence time''' is the [[time]] over which a propagating wave (especially a [[laser]] or [[maser]] beam) may be considered [[coherence (physics)|coherent]], meaning that its [[Phase (waves)|phase]] is, on average, predictable.

In [[telecommunications|long-distance transmission systems]], the coherence time may be reduced by [[wave propagation|propagation]] factors such as [[dispersion (optics)|dispersion]], [[scattering]], and [[diffraction]].

The coherence time, usually designated {{mvar|τ}}, is calculated by dividing the [[coherence length]] by the [[phase velocity]] of [[light]] in a medium; approximately given by
<math display="block">\tau = \frac{1}{\Delta \nu} \approx \frac{\lambda^2}{c\, \Delta \lambda}</math>
where {{mvar|λ}} is the central [[wavelength]] of the source, {{math|Δ''ν''}} and {{math|Δ''λ''}} is the [[spectral width]] of the source in units of frequency and wavelength respectively, and {{math|''c''}} is the [[speed of light]] 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&nbsp;Hz, corresponding to a coherence time of about one second.<ref>{{cite thesis |title=Precision measurements with atomic hydrogen masers |date=2003-05-12 |type=PhD thesis |url=http://www.physics.harvard.edu/Thesespdfs/humphrey.pdf |access-date=2011-10-19 |url-status=dead |archive-url=https://web.archive.org/web/20120403002427/http://www.physics.harvard.edu/Thesespdfs/humphrey.pdf |archive-date=2012-04-03 }}</ref> Their coherence length approximately corresponds to the distance from the Earth to the Moon.

As of 2022, research groups worldwide have demonstrated [[Superconductivity|superconducting]] [[qubit|qubits]] with coherence times up to several 100 [[microsecond|μs]].<ref name="phys.org">{{cite web|url=https://phys.org/news/2022-08-high-quality-superconducting-qubits-fabricated-cmos-compatible.amp|title=High-quality superconducting qubits fabricated with CMOS-compatible technologies|date=August 19, 2022}}</ref>

==See also==
{{Portal|Physics}}
* [[Atomic coherence]]
* [[Temporal coherence]]
* [[Degree of coherence]]

==References==
<references/>
*{{FS1037C MS188}}

{{DEFAULTSORT:Coherence Time}}
[[Category:Electromagnetic radiation]]
[[Category:Physical optics]]
[[Category:Radio frequency propagation]]
[[Category:Optical quantities]]