Is there any way to extract keywords and keyterms of an academic field? I'm trying to do a personal research project on a field, e.g. "quantum physics" and wanted to analyse which keywords are the most important or have the most papers in a field. Is there any way to achieve this?
If you are looking for the last 5 years a good chance would be 


a current textbook/PhD thesis (many are published on university servers and open access) on that field that has a glossary
browsing/downloading the most cited papers on journal websites (there are often tags, keywords for every paper added by the authors) and noting them down
doing text analytics/statistics on your own with such tools after converting PDF to text:


Matlab Text Analytics Toolbox
https://github.com/HamidMosalla/WordFrequencyCounter
http://www.writewords.org.uk/word_count.asp
http://neon.niederlandistik.fu-berlin.de/textstat/