In general, applications are sought biannually, so that students can begin the program either at the start of the winter semester (beginning of October) or at the start of the spring semester (in February). Lists of thesis topics (for Physics and Chemistry separately) are published biannually and the applicants apply for a specific topic (with the option to provide a backup topic). It is very important to choose a topic which you find truly interesting, since strong motivation is critically important for success.
The selection process involves two stages. The first stage is focused on the education and prior research experience relevant for the chosen topic. Graduate students work closely with senior scientists pursuing cutting-edge research, and those scientists play a decisive role in the selection process. The applicants should thus highlight in their research statements any relevant experience (such as involvement in research projects). If they have little relevant experience (e.g. because they wish to pursue something very different in their Ph.D.) they should take care to explain their motivation for wishing to pursue a particular line of research. This may play an important role in whether they will go on to the second stage. Some topics attract many applications.
Those applicants deemed most suitable for their chosen projects will be invited to take part in the second stage, which includes an oral exam designed to ensure that all incoming graduate students have a firm grasp of basic physics concepts, followed by a conversation about their research experience and interests.