First, I must say that I'm not in graduate school for physics, but rather pursuing math/biophysics ... however, the application process had lead me to look at quite a few physics programs over the past year or two. This is what I've observed that may answer your question:
Most MS (or PhDs) in physics require you to pass qualifying exams in all the major areas of physics: thermodynamics/statistical mechanics, electromagnetic field theory, classical mechanics, and quantum mechanics. Often the degree requires you to take at least a single course in each. Many MS programs are 6-8 courses + a thesis. If you end up needing to take the standard sequence to prep for the qualifying exams, you'll usually end up with a full first year looking something like:
fall
math methods for physics
statistical mechanics
classical mechanics
spring
electromagnetic field theory
quantum mechanics
general relativity
You would take your qualifying exams shortly after your first year (during the summer). Your second year could be made up of an advanced QM survey over the two semesters and writing of your thesis in some area of QM I suppose.
I had looked at a program or two that had a very open requirement assuming you could pass the entrance exam based on your undergrad preparation. You were basically allowed to take 8 classes in whatever you wanted, no thesis required. I was looking at these for the freedom of taking biophysics electives but it could be just as easily extended to your case where you want to focus on QM.
In theory, you could go part-time, taking one QM class per semester, extend the degree over the maximum 4 years and each of those courses would be a higher level QM sequence pretty much exhausting the formal courses the departments offer in QM.
I'm not sure how practical any of that is since it relies on being able to pass the general exams in all areas then going strictly part-time so you can take all the QM courses in sequence over 4 years, but at least according to those few university's academic bulletins, it would earn you a MS in Physics after having taken 8 courses, all in QM and nothing else.
p.s. throughout this post, I am referring to universities I've seen in the U.S. I have no clue if any of this is applicable anywhere else.