Monday 19 August 2019

quantum mechanics - What is the entropy of a pure state?


Well, zero of course. Because


$S = -\text{tr}(\rho \ln \rho)$ and $\rho$ for a pure state gives zero entropy.


But... all quantum states are really pure states right? A mixed state just describes our ignorance about a particular system. So how can properties like entropy and temperature come out of ignorance of information? That doesn't make sense to me.




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