Quantum Computing and Other Risks to Blockchain Technology

We have done over 40 episodes on blockchain and talked about the common risks of technology and adoption. With most ICO's and projects, the overwhelming, evident risks are: 1. No product market fit 2. Inability to get adoption, network effects. They come up frequently on the show. But they aren't the only risks. Some are more abstract and less evident. For example, the risk of quantum computing. What is it exactly and could it be a threat to bitcoin and other blockchains?