I've encountered some interesting paper on 2D materials where authors use circularly polarized light to break time-reversal symmetry to split energy levels.
Here you can find the paper:
My first question is, why is the degeneracy of levels mentioned in the paper locked by time-reversal symmetry?
The second question is, why does circularly polarized light break time-reversal symmetry?
I would like to have some brief physical explanation, maybe, with some brief mathematics involved.