Handling a case of reviewer reporting a result he read in a paper he had rejected Long ago, as a new researcher to an established field, I submitted a paper to the leading journal, with the first time experimental observation of a weak but theoretically predicted quantum transition. The paper was rejected by the reviewer, a professor and leading specialist in the field, claiming my observation was impossible.  I did not resubmit the paper elsewhere.  I have discovered that this same reviewer and his group published a later paper claiming to have observed the same transition first, which I had submitted.  

What are my options for handling this situation?
Did the supposed reviewer really copy your result/idea?

Double check the paper in question to see if it does match your result exactly. Many papers tend to appear similar but would've obtained with different approach and methods. This is a perfectly acceptable scientific conduct. So make sure your claim can be justified.

If your manuscript was copied, do you have proof?

If you are absolutely sure that the manuscript was copied by the reviewer, you may file a plagiarism report to the publisher of the concerned paper. But, the chances of you winning is slim. You may have your submission report from your mail or submission repository, but it wouldn't help much. From your post, it doesn't seem like you have a preprint in arXiv either. Sadly, you couldn't do much at this point.

To avoid this in future it would be better to make a preprint available in arXiv. Just make sure you confirm if it is allowed by the publisher's guidelines. You may consult with the editor if this is unclear.