Originally I was going to put "laser" in the title, but didn't want to restrict wavelength (e.g. masers). I did put "beam" in the title, since there may be highly stable oscillators that are also quite isolated, and I'd like to exclude those for this question.
In this Wikipedia article on Coherence Length it mentions a fiber laser with a coherence length of the order of 100km (yes 100,000 meters, not a typo). The article mentions a bandwidth of a few kHz and that number does get you to that length, and fiber lasers certainly can be collimated to form beautiful beams.
But there is no citation for this measurement in the article as of this moment, and I'm surprised because most laser-related articles used to be curated fairly carefully.
My question "What is the longest coherence length beam demonstrated?" also has the word "demonstrated". Of course it could be a coherence time, or a frequency spread converted to length. They are all roughly the same thing. No not exactly, and please don't open up a long discussion here. If the spread in frequencies has some characteristic width that can be defined in some way, then so will the wavelengths, in this context.
I added "stimulated emission" to the title to be sure to exclude something like a radio transmitter with directional antenna, or it's (in the not so distant future) optical nanoantenna equivalent. I'm looking for beams based on quantum transitions within naturally occurring systems. Thanks!
I thought I remembered seeing HeNe coherence lengths of 1km in catalogues, but now all I see are the 100m values as mentioned in the article. What got me thinking about this is the mention of coherence in this answer.