Last year I shared a stop-motion animation made using single trapped molecules by researchers at IBM. I went on to briefly talk about the technique’s application in quantum computing, but in retrospect I don’t feel like I did the technology justice. QC provides an enormous potential to scale down the size of computers, as I mentioned. This, though, is only half the magic (because it is, basically, magic). The reality-bending nature of quantum mechanics allows an object to be in more than one state at the same time (explanation of ‘state’ to come), which can cause some astounding things to be made possible. A company called D-Wave who purport to be the world’s first and only quantum computing company (I say ‘purport’ as there’s a bit of controversy surrounding whether or not the computers they use are actually based on quantum mechanics) have produced their own video which explains the process quite well. For my own explanation, I will invoke the (somewhat tired) analogy of Schrödinger’s Cat:
Schrödinger’s Cat is a thought experiment devised by the Austrian physicist after whom the experiment is named. Our unfortunate feline finds itself trapped inside a sealed box, into which the outside observer has absolutely no way of observing. Next to the cat is a single radioactive atom, a Geiger counter (which detects radiation) and a vial of poison. Radioactivity is very much a statistical phenomenon – given a certain length of time, there is a certain probability the atom will ‘decay’ (and give out radiation) and a certain probability it won’t. If the atom decays, the Geiger counter observes it, causing the vial to break. The cat becomes, to use the wise words of John Cleese, an ex-cat.
Or does it? Quantum mechanics dictates to us that, whilst the box is closed and nobody can see inside, the cat is nether alive, nor dead, but a combination of both alive and dead – in what physicists call a ‘superposition state’. It’s only when one opens the box and observes the cat that it is essentially forced into a state of either life or death. Bizarre? Well, if you’ve ever heard someone talking about the ‘strange nature’ of quantum mechanics, this thought experiment essentially underpins that strangeness. When you get down to a quantum mechanical level, things just don’t behave the way you’d expect. For example, a single electron attached to the nucleus of an atom cannot be located to one single place, instead it has a ‘probability distribution’ – areas where it is likely to be, and areas where it isn’t likely to be. It’s not until you actually try and observe this property (‘opening the box’) that it is forced to take a precise location. The same goes for its speed – as it ‘orbits’ around the nucleus, it’s likely to be at a specific speed, though you can’t be sure, and the further away from that speed, the less likely it is. When you measure the speed, it takes a certain value. This goes for just about every other parameter relating to the particle.
Now, I should state that in reality, were someone to be so ruthless as to actually do this to a cat (in the name of sciece!) then it would be plain old dead or alive, as we’d expect. This non-intuitive behaviour only occurs at the quantum level, i.e. really, really, really small – so when an object is said to behave ‘quantum mechanically’, it’s generally something like a single particle. In the (relatively enormous) biological system which constitutes a cat, the probabilistic nature (‘likely to do this, not likely to do that’) of a single (quantum-mechanical) particle is summed up over the vast number of particles, resulting in a life or death probability which is so enormously high that it’s vitually certain it’ll be either alive or dead. By the same token, though, that means that quantum mechanically, virtually anything is possible (or, at the very least, it isn’t impossible). To use an analogy by John Gribbin, if I’m looking at a granite statue, it’s not impossible that the atoms in its hand could suddenly rearrange themselves such that it appears to wave at me. Mind-boggling stuff.
Fundamental explanation of quantum mechanics aside, what’s so magic (I hope you now agree with me about the magic sentiment) about quantum computers? Think, if you will, about the ‘bits’ (ones and zeroes) in your computer, each stored within a single capacitor. Millions of these combined form data. As I’ve previously mentioned, the QC uses the ‘qubit’ – similar to the bit, but far more useful. This is because not only can the qubit store a zero or one (often, but not exclusively, via its spin-state), but also a superposition state between the two (as in the dead/alive quantum cat). So now, where a classical computer is contrained to searching for individual combinations of zeroes and ones, its quantum equivalent can search through multiple combinations simultaneously. Again, the video by D-Wave explains this quite well but you could maybe think of it in terms of coins. If I have 10,000 coins in a row and am looking for a unique combination of heads and tails, as a classical computer I’d have to move all 10,000 coins into one composition; if that’s not the right one I’d have to move all 10,000 into another composition, and continue this process until I find the right setup. As a shiny quantum computer though, I’d be able to look at multiple combinations at once – so I’d have multiple rows of 10,000 which I could move simultaneously. The possible reductions in computation time are astronomical.
The scaled-down size and enormous speed allowed by this technology have huge potential implications. It names quite a few in the video, though I particularly like the example given where a natural disaster has recently struck and the most efficient distribution of rescue resources has to be computed. Such a huge number of alternative scenarios demands an enormous computing power, which only a QC could provide in a reasonable timeframe. Huge numbers of lives could be saved. One thing which did make me slightly uneasy, though, was the mention of the potential to sift through huge datasets ‘to catch bad guys’. After everything we’ve heard about the misdeeds of various governments over the last couple of years I’m not sure I want GCHQ or the NSA to have that kind of snooping ability at their fingertips. Perhaps if they were catching cat-killers my mind would be more at ease?