Leave it as it is.
(Or swap it. But I don't see a benefit in that, so I'd say it is better to leave it.)
First, having duplicates serves exactly the purpose of directing people searching for the same problem in different formulations (which includes different titles) to the same set of answers. This purpose is served here.
Second, the purpose of duplicates is primarily to redirect people searching for sth. to the right answer. It is not clear to me whether people are more likely to use "quantum mechanics" rather than "QM". (Just look around: People are lazy to spell out things and use abbreviations all around. I agree that using less abbreviations would be better (in particular those which are not easily understood to non-native speaker like "SHM", I still stuble across that one, and it took me long to figure it out!). But it might well be that people are more likely to use "QM" - would be interesting to find out, maybe SEDE can be used for that.)
Finally, I think the new duplicate question is more valuable than the original question: With this, I don't mean the quality of the question as such (the two questions are aligned rather differently), but what one learns from reading the question. From reading the first question, I don't learn much, except that this might be an interesting question. From the new question, I learn how to express complex numbers as 2x2 matrices, which is valuable, and as far as I can see not explained in any detail in any of the answers. Thus, I think it would in fact be beneficial if some people end up at the duplicate question first.
Finally, looking at the side bar it seems that this question has lots of duplicates, some using QM and some quantum mechanics. So the very fact that the title has QM or quantum mechanics will likely not be enough to redirect people to any of those questions - there's enough different ways to phrase the very same question in the title.
