I'm not particularly well read on quantum computing, but it seems like we have reduced it to an engineering challenge at this point. We have already constructed and tested quantom computers with several quibits and (I am aware) of no theoretical limit to how many quibits we can have, so the problem is simply in engineering a device that can operate on all of them succesfully.

This is in contrast to most of the second type of breakthrough, where you typically have a theoretical revelation that opens previously closed doors.