Yes, Harvard University is best for pursuing PhD degree. The University provides cutting-edge infrastructure that supports high-quality research in a wide range of academic disciplines. 

The duration of PhD program at Harvard University is 5 years. The first 2 years are reserved for doctoral coursework and the following 3 years are reserved for conducting original research and thesis preparation. 

No, Harvard University does not offer PhD program in Online Mode. Currently, Harvard University offers only certification programs in Online mode. However, students can join Online DBA which is equivalent to the On-Campus PhD in USA. 

Yes, you have to leave your current job to pursue a PhD at Harvard University. However, you can still pursue a doctorate from some of the top universities in the USA through Online DBA without leaving your current job.