I always hear how many problems in QC are open and we're working on them.
But what are the problems that we've definitely solved already?
Especially from the algorithms/software/design side, but hardware comments are also welcome :)

I think that we have figured out how to connect to a QC via web interface – not an extremely hard thing to do, but hey – we didn't really have that 10 years ago, right?

Another thing that we're taking for granted and seems like a solved problem to me are gates – probably the most basic layer of abstraction, but an important one. Thinking in terms of unitary matrices is not very intuitive and thinking in terms of microwave pulses even less ;)