In 10 years from now a quantum computer will have sufficient computation power to give a 51% attack on the bitcoin network.
Please discuss.
Unlikely, given that quantum computing are unlikely to offer any advantage in attacking SHA256:
There may be breakthroughs in generic quantum collision search algorithms, or someone may find specialized cryptanalysis of SHA-256 that works only on quantum computers (e.g., see below about exotic constructions). But in the current state of the public literature, classical collision searches already appear cheaper than any known quantum collision searches would be even in the unbelievable scenario that qubit operations became as cheap as bit operations.
Granted, running the hashing algorithm and searching for collisions are two very different things, but it goes to show that quantum computing is not some kind of magic all-seeing oracle for which the laws of mathematics don't apply.
Additionally modern Bitcoin mining is done via ASICs which offer even more of an advantage over regular CPUs that are usually the benchmark for comparing classical to quantum computing.
Also keep in mind that even if quantum computers where more efficient than ASICs at mining Bitcoins they would more likely be used to simply mine Bitcoin rather than attacking the network.