Physical examples of quantum dephasing

Question

Lowering and raising operators are intuitive to understand. But I do not understand quantum dephasing. I know that in quantum dephasing the off-diagonal matrix elements decay. For qubit, quantum dephasing is caused by sigma Z Pauli matrix to a density matrix. But what are some physical examples of it? I know that it is responsible for T2 in NMR. It is somewhat related to spontaneous emission: "From an optoelectronic device point of view, dephasing destroys the coherence ... This is why so much effort has gone into the study of spontaneous emission"

n computational basis, can sigma X pauli matrix cause quantum dephasing also? If not, what is the specific name of the effect by sigma X pauli matrix. 

Ref: https://en.wikipedia.org/wiki/Relaxation_(NMR)#T1_and_T2

Comments

please add context by giving a relevant reference