Back in 2009, Satoshi Nakamoto probably wasn’t overly concerned about advancements in quantum computing when he developed Bitcoin’s key encryption. Even now, quantum computing is still quite far off adoption levels that could be classed as mainstream, with only a handful of companies possessing the technology. But developments do mean that the threat that quantum computing poses to blockchains is starting to become real.

The scary thing is that the pace of development now means that it’s likely that quantum computers could soon break the 309 digit encryption that’s used across many modern systems today - including many blockchains.

https://cryptodaily.co.uk/2019/09/how-will-blockchains-battle-quantum-computing