# Why the formula (it is the density operator in its initial state) below has this form?

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1. May 26, 2015

### Lizzie Mann

a
In the formula above, on the left hand side, ρ(0) is a system's density operator in its initial state. a is the annihilator operator of the system, and a+ is the create operator of the system. ρss is the system's density operator in its steady state.

But I don't understand why this formula writes in this way. Could anyone tell me what is the physics behind this formula?

Many thanks!

2. May 27, 2015