# Coherence time

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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](/source/electromagnetic_wave), the '''coherence time''' is the [time](/source/time) over which a propagating wave (especially a [laser](/source/laser) or [maser](/source/maser) beam) may be considered [coherent](/source/coherence_(physics)), meaning that its [phase](/source/Phase_(waves)) is, on average, predictable.

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

The coherence time, usually designated {{mvar|τ}}, is calculated by dividing the [coherence length](/source/coherence_length) by the [phase velocity](/source/phase_velocity) of [light](/source/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](/source/wavelength) of the source, {{math|Δ''ν''}} and {{math|Δ''λ''}} is the [spectral width](/source/spectral_width) of the source in units of frequency and wavelength respectively, and {{math|''c''}} is the [speed of light](/source/speed_of_light) in vacuum.

A single mode [fiber laser](/source/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 [superconducting](/source/Superconductivity) [qubits](/source/qubit) with coherence times up to several 100 [μs](/source/microsecond).<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](/source/Atomic_coherence)
* [Temporal coherence](/source/Temporal_coherence)
* [Degree of coherence](/source/Degree_of_coherence)

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

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

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Adapted from the Wikipedia article [Coherence time](https://en.wikipedia.org/wiki/Coherence_time) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Coherence_time?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.