Coherent states are defined as eigenstates of the annihilation (lowering) operator for the quantum harmonic oscillator. So why are these called "coherent"? What does "coherence" generally mean in physics? And what does it have to do with what is known as "quantum de-coherence?" Edit: I just realized there is an article on Coherence in physics generally. http://en.wikipedia.org/wiki/Coherence_(physics [Broken]) But I don't see the connection with the eigenstates of the annihilation operator. Also, the article on plain coherence states in its section on quantum coherence: "The quantum description of perfectly coherent paths is called a pure state," so if you can elaborate on the relation between pure states and coherent states as well, that would be appreciated.