The coherence time, usually designated τ, is calculated by dividing the coherence length by the phase velocity of light in a medium; approximately given by τ = 1 Δ ν ≈ λ 2 c Δ λ {\displaystyle \tau ={\frac {1}{\Delta \nu }}\approx {\frac {\lambda ^{2}}{c\,\Delta \lambda }}} where λ is the central wavelength of the source, Δν and Δλ is the spectral width of the source in units of frequency and wavelength respectively, and c is the speed of light in vacuum.

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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.[1] Their coherence length approximately corresponds to the distance from the Earth to the Moon.

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As of 2022, research groups worldwide have demonstrated superconducting qubits with coherence times up to several 100 μs.[2]

For an electromagnetic wave, the coherence time is the time over which a propagating wave (especially a laser or maser 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 factors such as dispersion, scattering, and diffraction.

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