Theoretically, a quantum processor could solve the cryptography and make cryptocurrencies useless. Google reported that its 54-qubit Sycamore processor was able to perform a calculation in 200 seconds that would have taken 10,000 years for the world’s most powerful supercomputer.
Google’s Sycamore processor is more than one and half a billion faster than the conventional bitcoin mining processor. So technically Sycamore processor would be able to mine the rest of 3 million bitcoins in just 2 seconds. No doubt that quantum supremacy is a step forward for human species in the technology field, but this processor possesses a genuine threat to the cryptocurrency world.
My one-sentence impression of recent quantum supremacy stuff so far is that it is to real quantum computing what hydrogen bombs are to nuclear fusion. Proof that a phenomenon and the capability to extract power from it exist, but still far from directed use toward useful things.
— Vitalik Non-giver of Ether (@VitalikButerin) October 24, 2019
However, several experts have reassured that people in the crypto industry have been working on the post-quantum solution for cryptography. IBM, who operates the current fastest supercomputer that Google claimed to have beaten, responded to Google’s claim saying that the same task that the Sycamore processor performed in 200 seconds could be completed in 2.5 days on their supercomputer and not in 10,000 years.
and then