The term "Coherence" is overloaded! I have troubles with modern terminology in the field of quantum information technologies. There are a lot of new terms that everyone is using and no-one takes time to explain, even though modern textbooks did not catch up to these yet. Two of them are optical coherence lifetime vs spin coherence lifetime in the context of atomic clocks and quantum information storage/processing: definitions are needed.



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*Please, notice that I am not asking for a classical definition of coherence. I am rather asking about two very specific terms, listed above in the given context.

*The title was given this way to attract attention and because in the given context the term "coherence" is overloaded and it is a known in the community fact (quantum, optical, quantum information coherence, to name a few).

Thanks to Harry Levine: helpful link
 A: For a quantum system such as an atom, or rare earth ion in a bulk crystal, the coherence lifetime is defined relative to two states of the system. 'Spin coherence' typically would refer to the situation when the two states are the same up to different nuclear or electronic spin orientations. For example, in a typical atom or ion there are many degenerate ground state levels that are split only by coupling from the nuclear spin to the electron spin (different $F$ levels). The splitting between these states is typically on the scale of < 10 GHz, all within the microwave or radio frequency domain.
However, other types of levels can also be considered: for example, one might consider the coherence between a ground state level and an excited state where an electron is in a higher orbital angular momentum or principal quantum number state. In this case, the energy difference between these states would often correspond to very high frequencies of > 100 THz, which are in the optical domain. Coherence between these types of states would be called optical coherence.
These two different settings are practically very different, in the sense that they are probed either with microwave fields or with laser fields. Their coherence can also be affected by different mechanisms (ie., radiative decay may limit the optical coherence but magnetic field noise might limit the spin coherence).
