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## Homework Statement

Hi

I have a technical question regarding the following paper: http://arxiv.org/pdf/quant-ph/0602170v1.pdf

In it they derive an equation for [itex]\left\langle a \right\rangle[/itex] (equation #5) from the master equation (equation #2). My question is how they do this. Here is what I know

[tex]

\left\langle {\dot a} \right\rangle = \text{Tr}[a\dot\rho]

[/tex]

My problem comes when I have to evaluate this expression. The trace is to be taken over the system states, but what are these?

I would be very happy to get a push in the right direction.

Best,

Niles.