A quantum computer does not work by trying every solution in superposition and then magically picking out the right one (like a non-deterministic Turing machine would do). This is why quantum computers are not thought to be able to solve NP-complete problems in polynomial time (though, since the P vs NP question is still open, nobody can even prove that a classical
computer can't solve NP-complete problems in polynomial time, it's just strongly suspected that neither computer can.)
The problem of inverting a cryptographically-secure hash function, which is what you're trying to do, can be reduced (converted) to the problem of searching an unordered database. Grover's quantum algorithm (http://en.wikipedia.org/wiki/Grover%27s_algorithm
) can do this in O(√N) time, while a classical computer would take O(N) time (if this doesn't make sense, see http://en.wikipedia.org/wiki/Big_O_notation
). As you can see by looking at the Wikipedia page for Grover's algorithm, though, it is far from trivial to implement and is based around much more subtle effects than picking out the right answer from a multiverse of possibilities. Also, because the improvement is only quadratic rather than exponential, the problem of inverting a hash function with a sufficiently large range remains intractable even on a quantum computer.
Quantum computers can
give an exponential speedup for a limited, known set of problems. The best example of this is Shor's algorithm (http://en.wikipedia.org/wiki/Shor%27s_algorithm
) for factoring numbers into primes (per the fundamental theorem of arithmetic). This algorithm is what made everyone want a quantum computer, since it could be used to break RSA. Again, though, the application here is limited: it can only be used to break public key cyrptosystems based on the difficulty of factoring into primes (and related problems like discrete logarithms). It can't be used to make an otherwise intractable inversion of a hash function suddenly tractable.
The biggest elephant in the room, of course, is that no one has successfully built a quantum computer (the dubious claims of a company called D-Wave notwithstanding), and when a quantum computer is
created, it won't resemble an Arduino in any way, at least not at first. For one thing, all the current quantum computing models are circuit models, not quantum Turing machines, and certainly not random access machines. This is all my speculation, but quantum computers will probably follow the evolution of classical computers. The first will be hard-wired to solve particular problems, and only later will we get programmable, general purpose versions. Maybe in time we'll have personal quantum computers, but we're very far from that right now.