Some choice quotes from a good article: https://thenextweb.com/artificial-intelligence/2018/02/06/heres-why-100-qubit-quantum-computers-could-change-everything/
Quantum brute-force would, theoretically, make classical encryption obsolete.
In May of 2016 a physicist and associate writer for Ars Technica, Chris Lee, reported on IBMs then newly announced 5 qubit system.
Then just 12 months later IBM unveiled a 17 qubit system.
If quantum computing technology (in general) followed Moore’s Law – an observation that CPUs for classical computers tend to double in power every 18-24 months – we should expect a 30 or 40 qubit system next year.
Only, IBM created a working 50 qubit system last year too. In fact, the company rolled it out at CES last month and showed it off to the world.
Realistically, a functional 100 qubit system could be announced before you finish reading this sentence. At 100 qubits a single quantum computer processor would, theoretically, be more powerful than all the supercomputers on the planet combined.
So in 20 months we went from a 5 Qubit to a 50 Qubit super computer. And that is just the ones that are made public. QRL team has estimated that blockchain signatures could be cracked with somewhere in the range of 1,000 - 5,000 Qubits. While that seems "far off" what if the range is actually down to 200 Qubits instead of 1,000, and what if we get there sooner than we think.
MainNet launch can't come soon enough. Expected for Q1 2018. Let's get ready for the party.