# I Expectation value in terms of density matrix

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1. May 22, 2016

### entropy1

It says in Susskind's TM:

$\langle L \rangle = Tr \; \rho L = \sum_{a,a'}L_{a',a} \rho_{a,a'}$

with $a$ the index of a basisvector, $L$ an observable and $\rho$ a density matrix. Is this correct? What about the trace in the third part of this equation?

2. May 22, 2016

### entropy1

Never mind, I see it now. Thanks.