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1/ A thread on the timeline for the arrival of Quantum Computers powerful enough to threaten the 256-bit ECDSA signatures that underpin #Bitcoin. We’ll hear a whole range of predictions from optimists and pessimists alike. Let's rock 💪
2/ Why care about this? Long story short, a powerful enough quantum computer could derive Bitcoin private keys from their respective public keys. A significant % of all bitcoin, namely Satoshi's wallet and others whose private keys are lost, are sitting ducks.
3/ Bitcoin’s devs have a solid plan to migrate to quantum-resistant signatures when necessary, but unlike centralized entities (e.g., banks), they cannot unilaterally upgrade the entire network. This will give rise to some serious challenges when powerful QCs do eventually arrive
4/ It therefore behooves us to figure out how long we have to prepare. Early, decisive action will be key to preventing mayhem down the road. My goal here is to provide a compact, well-documented resource on the development timeline of ECDSA-threatening quantum computers.
5/ Aggarwal et al. (2017, arxiv.org/abs/1710.10377) fit an exponential growth function to historical QC progress to estimate ‘pessimistic’ & ‘optimistic’ timelines for qubit increase and overhead (error) reduction. Note the logarithmic graph axes. 
7/ Their model would put the birth of Bitcoin's ECDSA-cracker at somewhere between 2025 - 2035 (6 - 16 years from now). A major criticism of that projection, however, is that they only used a few datapoints (1-49 qubits) to fit exponential growth. That may not be justified.
8/ A recent commentary in the journal Nature predicts that “within a decade, quantum computers will be able to break a blockchain’s cryptographic codes.” nature.com/articles/d4158…
9/ Harmut Neven, leader of Google’s Quantum AI lab, has said that a MILLION-physical-qubit system is conceivable “on the inside of 10 years.” technologyreview.com/s/603495/10-br…
10/ The aforementioned Dr. Aggarwal himself, in a recent interview, recommends upgrading to quantum-resistant cryptography “if you would like that what you encrypt today to still remain encrypted 5-10 years from now.” thehindubusinessline.com/info-tech/expe…
11/ On the more pessimistic side of the spectrum, Intel’s director of Quantum Hardware, Jim Clarke, expresses skepticism that multi-thousand-qubit QCs are less than a decade away in this interview: scientificamerican.com/article/how-cl…
12/ Chad Rigetti, founder of the leading QC company Rigetti Computing, said on the a16z podcast in mid-2017 that quantum computers that can run Shor’s algorithm are about “20-30 years away.” a16z.com/2017/05/13/qua…
13/ The Economist predicts that, other than tech firms such as Google, IBM, Microsoft, etc., “only government agencies are likely to have their own quantum computers within the next few decades” economist.com/business/2018/…
14/ That same article points out that it’s possible that we are entering a quantum computing “winter”, where there is a lull in hype and development before the really large, useful QCs get built. (Sound familiar?!)
15/ So to wrap up… After several decades of slow and steady progress, a global quantum arms race has begun. The injection of tens of billions of $$ by the USA, China, and EU, as well as tech giants like Google and Intel, will draw more elite talent to QC than ever before.
16/ That talent influx may bring sharply nonlinear progress – or it may not! What is almost certain is that QC will be one of the most influential technologies of the 21st century. It will be fascinating to watch this saga unfold.
17/17 My best guess for when Bitcoin and other cryptocurrencies will be practically threatened by quantum computers? 10-20 years. [END]
