Excerpt from Attack of the 50 Foot Blockchain by David Gerard
Before founding Ethereum, Buterin put considerable effort in 2013 into trying to convince investors to fund him in constructing a quantum computer. (Note that no quantum computers able to solve practical problems are verified as existing as of early 2017.) His plan was to use this quantum computer to solve computationally infeasible problems that can’t be done practically on an ordinary computer, such as reversing cryptographic hash functions.1
Since he didn’t know how to build a quantum computer, his plan was to simulate one on an ordinary computer – since this apparently wouldn’t count as just running a program to solve the impossible problem. This was an idea that had long been put forward by Jordan Ash, his associate in this endeavour, who had put considerable effort into this startlingly crank mathematical notion.2
Buterin and Ash’s plan was to use this simulated quantum computer not to revolutionise computation – but only to use it to mine bitcoins faster than anyone else and corner the market.
Sadly for their Fields Medal hopes, they failed to secure sufficient funding to break mathematics. Investors may have been put off by the pointed questions from the crowd on how, quite apart from the mathematical implausibility, this would destroy any confidence in Bitcoin and kill the golden goose.
It’s also worth noting that a quantum computer would be able to solve the SHA-256 hash used in Bitcoin somewhat faster than an ordinary computer3 – but it could also quickly break the public-key encryption that protects a user’s Bitcoin balance. So if you secretly had a quantum computer, you could mine a bit faster, or you could just steal everyone else’s bitcoins.
Buterin later said he had “greatly overestimated” the likelihood of the team breaking mathematics, estimating this task at maybe 1% to 5% possible (apparently a purely subjective guess, with no basis given for even this number), and assures us that his skepticism concerning quantum claims has “substantially increased.” He now puts the probability at “<0.1%”, though competent observers would likely consider even that on the high side for a mathematical impossibility.4