There are already some crazy light sources with line width of less than a single Hz (not a typo) https://www.news.ucsb.edu/2019/019332/quiet-light https://www.n

There are already some crazy light sources with line width of less than a single Hz (not a typo) https://www.news.ucsb.edu/2019/019332/quiet-light https://www.news.ucsb.edu/2019/019332/quiet-light Still other work to be done, and worrying about what happens in the fibre optics, but it is becoming possible, especially if the coherence does not need to be long lived.