Hello, I'm trying to understand how to calculate de probability of finding a system in a specific eigenstate using the density operator. In the book of Balian, Haar, Gregg I've found a good definition of it being the expectation value of the projector(adsbygoogle = window.adsbygoogle || []).push({}); Pin the orientation of the eingenstate._{r}

P(a)= tr(D.P_{r}a)

The problem is, since I have then a product between de density matrix tr(D.Pwould have to be a matrix of the same rank of D, write ? To calculate then the_{r}a), P_{r}atr.What is the right way to construct the projection ?

Transform the state, a vector, into a matrix ? Beeing a matrix with just one element in the main diagonal for each of theadirections relative to each eingenstate ?

I'm a new member, so I'm not used to ask questions online. Thank you.

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# How to Calculate Probability using Density Operator?

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