This is a quite interesting article on Quantum Computing and how hard it really is.

It is well known that Quantum Computers are prone to Quantum Errors, and this issue grows with the number of Qubits. The typical estimate is that an useful Quantum Computer would need approx. 1.000 physical Qubits to correct the Quantum Errors of a single “logical” Qubit. Even if there are advancements in this topic (see for example this post), this is still a problem to be solved in practice.

Another potential issue is that Quantum Computers have been proposed to efficiently solve many problems including optimization, fluid dynamics etc. besides those problems for which a Quantum Computer would provide exponential speed-up, such as factoring large numbers and simulating quantum systems. But if a Quantum Computer does not provide an exponential speed-up in solving a problem, there is the possibility that actually it would be slower than a current “classical” computer.

But the big question remains: will a real useful Quantum Computer arrive soon? If yes, how soon?