Quantum computers are already revolutionizing computers today. Tomorrow they will be more and more generilized and yes, they could theoretically crack modern encryption and threaten to defeat blockchain encryption!
Fortunately, and ironically, modern encryption is benefiting from quantum computers at the same time quantum computers are threatening to crack modern cryptography.
Want to know more?
Check out this animated 15 minute video on exactly this topic. The creator is excellent. Be sure to follow him.
-Frame of Essence, Youtube, April 5, 2017