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. In other words, it is the time interval within which its phase is, on average, predictable.
A single mode fiber laser has a linewidth of a few kHz. The Schawlow-Townes limit for some cw lasers can be below 1 Hz. Hydrogen masers have linewidth around 1 Hz; their coherence length approximately corresponds to the distance from the Earth to the Moon.
- http://www.physics.harvard.edu/Thesespdfs/humphrey.pdf - Precision measurements with atomic hydrogen masers
- Balasubramanian; et al. (2009). "Direct Ultralong spin coherence time in isotopically engineered diamond". Nature Materials. 8: 383. Bibcode:2009NatMa...8..383B. doi:10.1038/nmat2420.