source: Big Think 
2013-08-26
Lawrence Krauss describes quantum computing and the technical obstacles we need to overcome to realize this Holy Grail of processing.
Lawrence Krauss: Let me briefly describe the difference between a quantum computer and a regular computer, at some level. In a regular computer, you've got ones and zeros, which you store in binary form and you manipulate them and they do calculations. You can store them, for example, in a way that at least I can argue simply.
Let's say you have an elementary particle that's spinning. If it's spinning, and we say it's spinning, it's pointing up or down depending upon whether it's spinning this way or this way, pointing up or down. And so, I could store the information by having lots of particles and some of them spinning up and some of them spinning down. Right? One's and zero's.
But in the quantum world, it turns out that particles like electrons are actually spinning in all directions at the same time, one of the weird aspects of quantum mechanics. We may measure, by doing a measurement of an electron, find it's spinning this way. But before we did the measurement, it was spinning this way and this way and that way and that way all at the same time. Sounds crazy, but true.
Now that means, if the electron's spinning in many different directions at the same time, if we don't actually measure it, it can be doing many computations at the same time. And so a quantum computer is based on manipulating the state of particles like electrons so that during the calculation, many different calculations are being performed at the same time, and only making a measurement at the end of the computation.
So we exploit that fact of quantum mechanics that particles could do many things at the same time to do many computations at same time. And that's what would make a quantum computer so powerful.
One of the reasons it's so difficult to make a quantum computer, and one of the reasons I'm a little skeptical at the moment, is that - the reason the quantum world seems so strange to us is that we don't behave quantum mechanically. I don't -- you know, you can - not me, but you could run towards the wall behind us from now 'til the end of the universe and bang your head in to it and you'd just get a tremendous headache. But if you're an electron, there's a probability if I throw it towards the wall that it will disappear and appear on the other side due to something called quantum tunneling, okay.
Those weird quantum behaviors are manifest on small scales. We don't obey them - have those behaviors 'cause we're large classical objects and the laws of quantum mechanics tell us, in some sense, that when you have many particles interacting at some level those weird quantum mechanical correlations that produce all the strange phenomena wash away. And so in order to have a quantum mechanical state where you can distinctly utilize and exploit those weird quantum properties, in some sense you have to isolate that system from all of its environment because, if it interacts with the environment, the quantum mechanical weirdness sort of washes away.
And that's the problem with a quantum computer. You want to make this macroscopic object, you want to keep it behaving quantum mechanically which means isolating it very carefully from, within itself, all the interactions and the outside world. And that's the hard part, Is isolating things enough to maintain this what's called quantum coherence. And that's the challenge and it's a huge challenge.
But the potential is unbelievably great. Once you can engineer materials on a scale where quantum mechanical properties are important, a whole new world of phenomenon open up to you. And you might be able to say - as we say, if we created a quantum computer, and I'm not - I must admit I'm skeptical that we'll be able to do that in the near-term, but if we could, we'd be able to do computations in a finite time that would take longer than the age of the universe right now. We'd be able to do strange and wonderful things. And of course, if you ask me what's the next big breakthrough, I'll tell you what I always tell people, which is if I knew, I'd be doing it right now.
Directed / Produced by Jonathan Fowler and Elizabeth Rodd
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