Theoretical physicist Brian Greene gives a two-minute crash course on quantum computing, which is pretty amazing considering how elaborate such a computer would be. The tl;dr version is pretty much this: If we posit that there are multiple universes in which various probable situations play themselves out, a quantum computer would be able to run calculations within all these different universes at once. Greene, who is chairman of the World Science Festival, explains that the implications of such a machine would be tremendous.
Brian Greene: A quantum computer is a device, a technological device that, in principle, would harness the full capacity of quantum mechanics to undertake calculations that a standard computer would be absolutely unable to achieve. One way of thinking about it is this. There’s an approach to quantum mechanics where one imagines that there are many, in some sense, parallel realities moving along in some larger environment if you will. Where, for instance, if I want to measure an electron, quantum theory says, "Well there’s a 50 percent chance it’s there and a 50 percent chance it’s over there." Now what does that mean? Well one interpretation says well there’s actually two universes and in one universe the electron is here and in another universe it is over there. That’s kind of a crazy-sounding idea but a quantum computer perhaps can harness that by doing some calculations over here and other calculations over there in parallel. Now it’s doing, in some sense, twice as many calculations as a classical computer existing in one world would be able to do. Now imagine taking that idea and spreading it over all of the possible realities allowed by quantum mechanics. Now you’re harnessing all of these different worlds, if you will, to do all of these calculations in parallel much faster, much more powerful. Doing calculations that in a single universe would be impossible. So this is one way of thinking about it and it offers amazing possibilities and we just need to see how successful we are at harnessing these ideas to actually put it into practice. And we don’t know yet. We’ll see going forward, but it’s an exciting possibility.