You have to make use of our Advanced Search Query:
- On the far right on the top menu bar click on the blue link Adv. Query to obtain the following search form:
http://www.physicsoverflow.org/query
- In the first text field titled "tagged as" type the tag "quantum-field-theory" and start the search by clicking the button "start query" at the bottom to obtain a list of all questions with this tag:
http://www.physicsoverflow.org/query-results?catid=1&otags=308
- Now sort this list by votes by clicking the button "most votes":
http://www.physicsoverflow.org/query-results?sort=votes&catid=1&otags=308
It admittedly might seem that this search works a bit less intuitive than on SE, but in fact our Advanced Search Query offers much more possibilities to search for specific question than what is possible on SE. Enjoy testing and playing around with it ;-)