The Briefist of Introductions to Quantum Computing

Sorry posting has been a bit sparse this semester. I blame a newly developing social life, a schedule that won’t quite settle into a routine, and a course on science writing which is taking up more time than expected.

The latter of these (can you use “latter” when there are more than two items in the list?) is really the biggest problem, as it means I get my writing fix every week before I even get around to thinking about my blog. But it only lasts a few more weeks, at which point we transition into talking about public speaking (yay!) so perhaps you’ll see the blogging pick up again at that point.

Until then, here’s a draft of a piece I’ve been working on for class to satisfy your appetite for accessible science. Because of the nature of the assignment, it’s very, very light on technical details. Perhaps it could be the foreward to a book on quantum computing for public consumption. But it’s the best thing I’ve got to share with you this week. Please feel particularly free to comment on what you do and don’t like about it, because I’m still work-shopping it before turning it in.

Okay: here goes:

There is a problem with physics these days—it’s very hard. And not just for freshmen struggling to stay awake in lecture halls. It’s hard for scientists too.

This was essentially the first point made by physicist Mike Freedman in a recent talk on quantum computing: that ever since the arrival of modern physics, it has simply been a fact that human brains are not wired to understand the most fundamental workings of the universe in an elegant and direct way. And by extension, the same is true of any conventional computer we design to aid us in that pursuit.

Hence, Freedman’s second point: in order to truly expand our understanding of the universe, we need to design a new kind of computer.

The problem is quantum mechanics. This theory, written to explain strange and unexpected behavior of exceptionally small things like individual atoms, describes a world whose individual members do not need to play by the same rules as we humans do. A person cannot—as any housewife will tell you in frustration—be in two places at once. But an individual electron can, at least in a certain sense. Alternatively, if I am driving down the highway, I can do so at any speed that I like, limited only by my engine’s power and the generosity of the local 5-O. Under certain conditions, however, an individual electron does not have this freedom, but must pick only from a handful of specific velocities; 35 and 50 mph might be legal, for example, but nothing in between. That this is counterintuitive even to scientists is a clear understatement.

And yet, unfamiliar though it may seem, quantum mechanics has been tested in experiments time and time again and found to be more consistent with the real behavior of the universe than any competing theory. No serious physicist alive doubts its validity as the underlying law of the land; instead, all have come to terms with the fact that the intuition we as human beings have developed about the universe, which has evolved in us through thousands of years of experiences with objects much much larger than individual electrons, is simply not suited to anticipating the behavior of things that are sub-microscopically small.

As a result, not only is our intuition maladapted to studying the universe at its most fundamental level, but so is our internal mathematics. Our brains, to the extent that they work like computers, like to process “real” numbers: numbers like twelve, negative six, or the slightly more intangible “pi.”  Even this broad family, however, does not seem to be the natural language of the universe, which prefers to use an even larger category of numbers called “complex numbers” when describing the properties of its fundamental particles.

In response to this, we have dutifully developed a system that allows us to describe complex numbers in terms of the more familiar “real” numbers. But this means that we are never really working in a language that is comfortable to us when we study the behavior of subatomic particles. We are like students in a high school French class, clumsily discussing a piece of poetry in a language still unfamiliar to us. Naturally, we are still able to make some basic observations (“it was a very sad poem.”  “I did not understand the word “pamplemousse”), but of course the discussion is nothing compared to what we would be able to say in comfortable old English.

It was the great physicist Richard Feynman who first realized that this apparent inaccessibility of quantum mechanics might be made to work in our favor instead. What Feynman realized was that while conventional computers, designed to think they way human beings do, would be just as unaccustomed as us to the language of quantum mechanics, there was no particular reason why a machine couldn’t be designed which operated through the laws of quantum mechanics, and would therefore be fully at home calculating in that language. Once such a machine was built, with a suitable human interface, it could act as a translator between mankind and the quantum world. It would allow us to ask questions in a natural language about the way systems of small particles behaved, but also have the answers computed rapidly and efficiently by a machine to whom the concepts of quantum mechanics were second-nature. Or perhaps more accurately, for whom these concepts were it’s true nature.

For the trick to work, a quantum computer would itself have to be a system of subatomic particles, so that it could play by the quantum rules. Traditional computers, for example, encode information in strings of tiny switches, each one set to either “on” or “off.” But in the same way an electron can seemingly be in two places at once, so can a “switch” in a quantum computer choose between on, off, or a simultaneous combination of the two. Unsurprisingly, this kind of power allows a quantum computer to effectively work with several numbers at once, skipping steps and significantly reducing computation times.

But this built-in parallel-processing power is just the beginning. By capitalizing on all of the “new rules” which come into play for quantum systems, a computer could theoretically be built which was so superior that no traditional computer could match it, at least not using any technique currently imaginable. It would be capable of storing enormous amounts of data in a microscopic space, of searching such data several times more rapidly. And it would be able to perform many traditional algorithms at an exponentially faster pace, meaning encryption systems that currently can only be broken after centuries of computer work could be cracked in a matter of seconds.

But perhaps most excitingly, particularly to a physicist like Mike Freedman, would be a quantum computer’s ability to predict the behavior of other systems of small particles in a relatively short amount of time, something which is only barely possible on a traditional computer and only for the simplest of cases. Such a computer would have a tremendous impact for physicists, chemists, and nano-engineers everywhere. It would suddenly make trivial entire families of problems currently believed to be nearly unsolvable in practice, merely because it would be doing the work directly in the language of quantum mechanics itself.

A practical quantum computer is still many years off, possibly decades away. Naturally, the original act of the designing and building must still be done with the clumsy tools and computers presently at our disposal, a daunting task to say the least. But because it holds the key to such enormous potential, the field is still one of the most active in physics. As Freedman and others continue to argue, what’s at stake is not simply the ability to search our hard-drives slightly faster. Rather, it is our very ability to continue to unlock the secrets of the universe. Of course, humankind may yet prove resourceful enough to solve such problems on our own. But without a quantum computer, we will always be working in a language we find painfully foreign.

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About Colin West
Colin West is a graduate student in quantum information theory, working at the Yang Institute for Theoretical Physics at Stony Brook University. Originally from Colorado (where he attended college), his interests outside of physics include politics, paper-folding, puzzles, playing-cards, and apparently, plosives.

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