first round. in the format of phone or zoom. I was asked some math questions/basic physics questions. Some of them are like middle school math contest questions. Some of the focus on signal processing. I don't remember all of them. But if you are good in high school. you will be fine.
I applied through college or university. The process took 3 months. I interviewed at Viridien in Dec 2013
Interview
phone interview half hours three around of the interviews: the first two rounds were phone interviews. the hr emailed first to schedule the time for the first around of interview, which included 4 to 5 simple math questions. very simple questions indeed, you don't even need to prepare beforehand. If you pass that, hr will email you to schedule a 1-1 onsite interview, which involve brain teaser problems.
I was interviewed by only one person. They asked me why I wanted to work for them and my skills. They asked me about my thesis and if I was avaiable to move out of my country.
Two rounds of phone interviews. First round to describe yourself about your research and so on and asked some simple math question. Second round asked some brain teaser and more math questions
Interview questions [1]
Question 1
some brain teasers related to geometry algebra questions as well as classical probability