Nobody really knows. It has not been proved that useful QC is possible. (All existing QC is practically useless, and it is not clear that it is even possible to scale it up. This is not snark; if it ever will be useful, we must of course walk before we run.) It is possible that there is no amount of money that will make it useful. It has also not been proved to be impossible, and interesting and tantalizing breakthroughs have been made over time such that it's hard to look at them and firmly promise it's impossible, either. (I fall on the skeptical side myself, but it's what I call "true" skepticism; produce it and I won't try to argue out it out existence, I'll just update my beliefs and move on. And celebrate whoever produces it for doing what I believed to be impossible.)
This is rather like commercial fusion research; it has not yet been shown to even be possible to economically use fusion for power generation, yet it has not been proved impossible either. Contrast this to nuclear fission research, which at this point is more like engineering; modulo lawsuits and administrative compliance, we could probably take a pretty good guess at what it would take to, say, bring a commercial thorium reactor online.
Well anyone with photovoltaics can demonstrate working economical fusion power at a certain large scale. At least there is an existence proof at the extreme there, which QC does not have.
It's a long-term concern. One way to gauge it is that many researchers working seriously on post-quantum cryptography are far more seriously engaged in pre-quantum work like secure elliptic curve.
Ok thanks, last question and I'll leave you alone.
Could you recommend some relatively accessible reading for the layman on how the security world would react to a sudden shift into the post-quantum world?
And, in your opinion, how concerned should average Joes and Janes, like me, be concerned about a sudden shift to a post-quantum world?
Edit: For example: Would my bank be safe? Would my health records be safe?
(On this hyperbolic fear legitimacy scale: Y2K <----> Cryptopocalypse)
Does the current state of quantum computing excite or keep security researchers awake at night?