Two Skype interviews and they flew me in for on-campus interview. The apple travel services was very convenient. Two of the interviewers had to leave on a last minute trip so I only had 6 people to interview. Almost everyone were nice and friendly, interview is very casual and comforting. People are super busy there and you can totally fell that they want to get out of this interview room and back to work. The interview goes to very basic physics questions. Don't waste your time preparing for a professional question.