Sum-of-states (SOS) for multilevel system
Pros: computationally cheap and intuitive
Note
In the SOS method, decay is introduced phenomelogically.
correlation function approach -- Direct computing the many-point correlation functions with the quantum regression theorem
Valid for open quantum systems where environment effects can be rigirously described with quantum master equations.
Visulize with double-sided Feynman diagrams instead of the time-loop diagrams.
non-perturbative approah -- valid for ALL systems provided a quantum dynamics solver is provided.
Simulating the laser-driven dynamics including explicitly all laser pulses