In an article posted here: http://arxiv.org/abs/1608.00263 the authors estimate that a 50-qubit computer would be capable of computations that could not be replicated in real time by a classical computer. Interestingly, according to "New Scientists" (https://www.newscientist.com/articl...-googles-plan-for-quantum-computer-supremacy/), the authors of this article are Google engineers. My first reaction is to quibble with the term "supremacy" as it implies that this new technology will replace current classical computers. In fact, few of the chores assigned to current computers are appropriate for quantum processors. The article is also forecasting that Google will have a useful quantum processor by the end of next year.