In long-distance transmission systems, the coherence time may be reduced by propagation factors such as dispersion, scattering, and diffraction.
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.
As of 2022, research groups worldwide have demonstrated superconducting qubits with coherence times up to several 100 μs.