I'm not sure if this is the right place for this question but If I have a specific question about a derivation step in a paper published in Physical Review A, for example. Can I ask it here?
It probably depends on how you word the question itself.
If you simply ask, "How is this step done?" you will likely find the question closed under the homework policy.
If, instead, you ask, "I thought that situation X should apply here, but they say it's Y and I don't see why. Can someone fill in why?" or something similar, the question will likely remain open.