Physics behind the annihilator-creator operator formula

[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!

 

[naima]

A link would help!

 

[naima]

Maybe an idea:
http://en.wikipedia.org/wiki/POVM#Post-measurement_state

It looks like the result of the density matrix after a POVM measurement on your steady state. You get it as a post measurement result at t = 0. If it is not an eigen value of the hamiltonian, it evolves in time.