Hello, I hope my title is clear enough but I will detail my problems below. I am interested in almost everything aspects of physics (theoretical, fundamental, applied, experimental, not against some numerical simulations too...) and almost every field too. I have more background in quantum physics, condensed matter physics and nanotechnologies, so I have no intention to apply for cosmology PhD for example. But even with a restriction to nanotechnology and condensed matter physics, there seem to exist infinite subfields extremely specialized. So I have mainly two problems: 1) I truly do not know what to choose and what I should write in motivation letters because for every PhD offer I must find a different motivation which I am not sure myself about. Thus I cannot answer questions during an interview such as: "Why do you want to apply for this position and not another in related field?" (e.g. cold ions versus cavity quantum electrodynamics for applications in quantum computing...) 2) I feel I don't have the background for anything, that there will be another candidate to fit better the position (I did several little projects on very different topics but no big projects very specialized so far). Should I read a lot on the topics before applying and mention it on my CV even if I will have no proof that I understood the books or scientific papers? And for experimental positions, is it possible to succeed without previous experience? (one cannot trully learn experimental physics with books some months before going on a PhD project) I have a master degree in applied/engineering physics. Thank you in advance for any answer, if people have already met this situation.