The present work is motivated from the recent paper arxiv.org/pdf/1412.6213v2.pdf, which is also presented in the most famous journal "Nature". The main result is derived much simpler way.
Dear Dr. Martila, this "most famous journal" here is research gate and if you write a question you have to add a decent abstract otherwise I could (should) present you a list of papers (all in most famous journals) which exclude any photon - photon interaction.
I can't see paper, but I don't think wave-function and nonlinear interaction have any problem with each-other, e.g. BEC or Skyrmion ... . What do you mean by question?