I do not work in quantum information theory, but I think my insights can be generalised with some care. For whatever it’s worth, I have published two single-author papers arising from my PhD in computational physics.
First note that in most subfields of physics I am aware of, single-author papers are rare. This is simply due to the nature of academia where most work is done by a student or postdoc and supervised by a professor. This doesn’t mean however that single-author papers are not valued. Showing tons of examples of multi-author works (as your PhD advisor apparently did) doesn’t prove anything. That doubly applies if those papers were all experimental, as it’s far easier to produce single-author papers in theory.
Since your PhD advisor’s statement was about physics in general, it can also be falsified by just looking at how well single-author papers tend to do. It took me a few minutes to find some recently published single-author papers in Physical Review Letters, and I could not even filter for theoretical papers, let alone single-author papers. If you want to be sure about your particular subfield, look for single-author papers in it and check whether those that you find are regularly cited. Maybe you already cite some yourself. Just beware that since those papers are inherently rare, you really have to search till you have a statistically relevant population.
If you are a postdoc advisor would you encourage your postdocs to write single-author papers? As a PhD advisor what would you think of your ex-students when they write their own papers?
I think that this may be a false proxy for what you actually want to know. Papers do not (or at least should not) become single-author papers because you design your research like that from the beginning. Rather, you set out on a project you conceived and it turns out that you complete it without any external input qualifying anybody else for authorship. Mind that this includes the writing process for which previous publication experience is extremely helpful, if not essential.
That being said, suppose I supervise somebody and the above happened: They conceived a project and completed it without any relevant input by me or anybody else (but with me being aware of it, assuming I am their boss). If I do not see any flaws (which of course would be relevant input), and they have the skills to handle the writing and publication process on their own, I would advise them to do exactly that. I would ensure that they have access to constructive criticism along the process, e.g., I would offer to read the manuscript (internal peer review), unless they have somebody else for that, hoping for nothing but an acknowledgement. One reason to encourage this is that they can demonstrate that they can perform and publish research on their own (which is what a PhD should qualify for), which can be crucial for their career.
Of course, you should be wary that this does not translate to all advisors and situations. Some people think they need to be the author of everything published by somebody in their group (violating authorship ethics). Some people might also get legitimately angry that you pursued a project without telling them anything about it, displaying a lack of trust or transparency towards their boss (in the work sense).
Further reading on this: