JAM

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Yes, it is necessary for you to give an entrance either its JAM or JEST because IISER offers integrated Ph.D. programme. Here is the link:
http://www.iisermohali.ac.in/home/front-page-articles/admissions-to-int-phd-program-august-2018
IISER, Pune has the best research faculty among IISER institutes. Meanwhile, there is also an interview to judge your core interest and research ability in the respective field of Science which you want to pursue.

Hello, Online registration for IIT JAM 2019 has not been started yet. The exact dates have not bee declared. But it is expected to begin in the month of September 2018. IIT JAM 2019 Exam will be conducted in the month of February 2019. So, prepare yourself well for it. Best of luck.

Yes, you can get admission to top IIT colleges through JAM. It's an admission test for getting admission to M.Sc.
Regards.

Of course, not. If there would have been any provision of sports quota in IITs then it would have been clearly stated.

You can only give an exam with 51.83% if you belong to the SC or ST category.

Take a look at the eligibility. Eligibility:
For M.Sc. Physics:
IITB, IITD, IITG, IITGN, IITH, IITI, IITJ, IITK, IITM, IITP, IITR, IITRPR and IITISM. Physics for at least two years/four semesters and mathematics for at least one year/two semesters. JAM Eligibility Criteria 2019:
Common Eligibility:
For General/OBC (NCL) category, the minimum marks will be 55% or 5.5 out of 10 aggregate in languages and subsidiaries of all the years combined. Candidates belonging to the SC/ST and PwD categories must have secured 50% or 5.0 out of 10 aggregate. Hope this will solve your query. All the best.

Hii Sriniwas,
If you go with the trend, pursuing Ph.D after M.Sc seems to be the most followed option. However, most of the IITs have great campus recruitment facilities for not only B.Tech students but also for other programmes such as M.Sc, M.Tech, integrated Ph.D, etc. For sure, you'll get a decent paying job.

Hi, Firstly, there is no age limitation to appear in IIT JAM exam. Secondly, I am mentioning some of the government sectors where you can get a job after qualifying JAM exam:
Defense Research and Development organisation (DRDO)
Physical Research Laboratory, Ahmedabad
Nuclear Science Centre, New Delhi
Saha Institute of Nuclear Physics, Kolkata
Bhabha Atomic Research Centre (BARC)
Indian Space Research organisation (ISRO).

People generally take a year drop for JEE but for IIT JAM exam we are little apprehensive. But in my opinion taking one year off can give you enough time to crack IIT JAM or even any other exam. There are JEST TIFR etc. For M.Sc. Admission. You can read about IIT JAM here - https://www.eduncle.com/iit-jam-exam.