What is meant by quantum coherence in the context of a two-level atomic system?
Answer
When you prepare a pure quantum state of a two-level system, |ψ⟩=a|0⟩+b|1⟩, the associated density matrix will be ρ=(|a|2ab∗a∗b|b|2).
The non-diagonal terms are usually called the 'coherent' terms, which come from having a pure state instead of a statistical distribution of |0⟩ with probability |a|2 and |1⟩ with probability |b|2.
Interactions with the environment wash away these terms and make you lose coherence (they make the non-diagonal terms →0). The time scale associated is what is called the coherence time.
Personally, I think a better characterization of this idea is purity, which is given by Tr(ρ2) and is basis indepdendent.
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