Hello everybody. Nice to meet so many people interested in quantum threat to a blockchain in one place. We've been working on one post-quantum project for 2 years and of course we're talking with physicists, pq-cryptographers and other academic minds. And I just want to add a couple things and links you guys might like.
 
First — probably you will be interested in reading about Neven's law (https://www.quantamagazine.org/does-nevens-law-describe-quantum-computings-rise-20190618/). It is not a "law" of course but an interesting thing to keep in mind.
Second — the most powerful quantum chip for today is 128-qubit chip produced by Rigetti (https://medium.com/rigetti/the-rigetti-128-qubit-chip-and-what-it-means-for-quantum-df757d1b71ea).
Third — how many quits you need to crack a blockchain. It depends on a type of encryption, but the point is, it is 2300+ (https://en.wikipedia.org/wiki/Elliptic-curve_cryptography#Quantum_computing_attacks)
Fourth — Intel thinks we'll get 1000 quits by 2024 (https://spectrum.ieee.org/nanoclast/computing/hardware/intels-new-path-to-quantum-computing) and ECDSA will be at risk by 2027 (https://arxiv.org/pdf/1710.10377.pdf).
Fifth — implement post-quantum encryption to existing blockchains? This is a close to impossible task.
Sixth — That's what NIST says about PQC in their project (probably all of you aware of it but https://csrc.nist.gov/Projects/Post-Quantum-Cryptography):

So, yes, quantum threat is a thing to be aware (and probably afraid of) and yes, we gotta start working on it now. Plus, thanks to smart people from NIST we're in good hands. Btw we're working on a utility to secure all of the blockchains from it. I hope this week we'll publish an article about quantum thief where we will explain why the only thing that will save us from it is game theory (and PQC, of course).

I hope I didn't miss anything. Will be happy to answer your questions (but I can get here only a couple times per week max so don't wait for fast replies, sorry)