# 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.

1. 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.

2. 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).

• The claim that the term is 'overloaded' is, at best, an over-reaction. Coherence is a perfectly well-defined term in wave mechanics, and it translates directly and unambiguously to matter because matter is also a wave. – Emilio Pisanty Aug 16 '18 at 17:42
• If you find the usage confusing, then you should ask a much more detailed question. Saying "I am confused" doesn't provide much of a foothold to provide useful answers: what is it you already understand, what aspects don't you understand, and what specific aspects do you perceive as being in conflict with one another? Similarly, going on the offensive against the literature (where the likely reason for your confusion is that you simply have not been looking for textbooks that are specialized enough) really doesn't help. – Emilio Pisanty Aug 16 '18 at 17:54
• Do you have access to the wikipedia article: en.m.wikipedia.org/wiki/Coherence_(physics)? This is a good starting point. – my2cts Aug 16 '18 at 18:18

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.