I had a course last semester which was an introduction to quantum mechanics. The course ended in Perturbation Theory , and Diffusion. So I wonder what should be next to learn in quantum mechanics, i want to continue studying it myself , So what are the next topics to learn and what kind of mathematical formalism do i need to learn them? Any books recommended? This is kind important because there is Course about quantum mechanics will deal with second quantification green function Propagators Coherence and quantum interference. so my other question would be should are these the next level or there is some stuff in between ?