Not going to disagree that it’s not an advising failure as well, but if you’re so interested in Quantum computing that you’d like to devote 6-8+ years studying

Not going to disagree that it’s not an advising failure as well, but if you’re so interested in Quantum computing that you’d like to devote 6-8+ years studying the subject under a professor, you might want to check first that the professor knows something about quantum computing. Step 1 of doing a PhD is to find a person who you want to work under for probably 10% of your entire life. If you’re serious about it, it’s something you should not quit at literally the first roadblock. If I may read between the lines here, because I see this often enough: it seems like this student had a passing interest in QC; joined a department without talking to anyone there to find out if the research they do is compatible with their interests; proposed a deep theoretical research topic that neither anyone in the department nor the author knows much about; and then promptly quit when they told him it’s probably not the best idea.