The answer irrelevant of how long it might take is yes. One key reasoning is from Forbes who says; “At a major crypto event at University of California Santa Barbara this week, Martinis talked about why it could take a decade or more to build a quantum computer. “This is really, really hard, way harder than building a classical computer,” he said.

He went on to explain to the packed room at Crypto 2017, a four-day conference sponsored by the International Association for Cryptographic Research, that the main reason building a quantum computer is so tough is because qubits (quantum bits), the counterpart of bits in classical computers, are unstable. And that creates extra work for physicists trying to solve the problem.”

Article link: https://www.google.com/amp/s/www.forbes.com/sites/amycastor/2017/08/25/why-quantum-computings-threat-to-bitcoin-and-blockchain-is-a-long-way-off/amp/

# Big picture

No matter what he said on the time needed to build these computers, one thing is clear. After the 10 years or so it will take to build, it can be destructive to a crypto currency. Remember that having 51% of the network hash, means that you are in control on the network. With these computers, the network can be overrun for malicious attempt, and once reaching 51% can be exposed badly. So no matter if it’s in a year, a decade or a century, there will be a time in the future when quantum computing can overrun crypto currencies.