Quantum Computing

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Professor Scott Aaronson
Photo: Jason Dorfman, CSAIL photographer

Quantum computing is one of the most fascinating – if counterintuitive – final frontiers in the computing world today. Saddled with technical limitations and the potential impossibility of their pursuit, experimentalists and theoreticians alike have found themselves beset from all sides by uncertainty. In Professor Scott Aaronson’s view, this is where some of the most fascinating work occurs.

Gleefully described by Aaronson as “what we can’t do with computers we don’t have,” investigating the limitations of the currently theoretical builds on its predecessors in a slightly unusual way. Rather than carrying forward the suppositions of past researchers to create new models, it involves a great deal of poking holes in the theories that have gone before to see if they continue to hold water. This process of irreverent trial and error appeals to the researcher’s fundamental curiosity about the nature of computing, and devotion to pushing its boundaries.

Aaronson himself has made great strides in that arena. This year he became one of just over 100 professors worldwide who were selected for Sloan Foundation Fellowships. On the less formal side, he has become something of an expert commentator on the topic of quantum computing. His blog, Shtetl Optimized, boasts a wide audience, allowing its creator to achieve that uncommon distinction of a high degree of relevance in academic as well technical and pop cultural circles.

But whither the ready-to-use quantum computers themselves? As of this writing, they do not exist – and that’s where things get interesting. At its most basic, a quantum computer would be a device that leveraged the laws of quantum mechanics for enhanced problem solving. Quantum mechanics itself deals in a kind of molecular probability theory.

On the atomic level, there is a great deal of uncertainty: what are all the possibilities for how a molecule will behave? Each of those possibilities contributes a complex number, known as an amplitude. They differ from regular probabilities in that they can be positive or negative; the potential that amplitudes can cancel each other out is the linchpin upon which quantum mechanics turns. In computing, if amplitudes could be manipulated as a computational base, researchers would be able to harness a level of compute power that’s currently unheard of.

More to the point, in Scott Aaronson’s view, is this: if experimentalists find that quantum computers are simply an impossibility, it will be a blow to their field. But such an impossibility holds the potentially profound implication that quantum mechanics – the dominant theory in physics for over a century – is wrong. To Aaronson, this is a stunning possibility.

His entrée into the world of computers began, as it does for so many, with video games: specifically the realization that he might like to create his own some day. He jokingly refers to his discovery of programming as “a revelation on a par with learning where babies come from – why hadn’t anyone told me this?”

Mesmerized by the creative potential, he learned Basic, and then became curious as to whether or not it was possible to create a new programming language which enabled the user to do things not able to be realized with any extant languages. But the Church-Turing hypothesis helped him understand that all programming languages for known computers were fundamentally equivalent to each other – which lead him directly to the search for some new, unknown computer.

When speaking of the things that people in his field hope to achieve in future, Aaronson frequently refers to great figures of the past. In his ruminations on our potential limitations, Charles Babbage comes to his mind. In the 1820s, Babbage realized the possibilities of a mechanism that was able to perform mathematical functions automatically. He dreamed of a machine called the analytical engine, built of gears and pulleys – which, if it had ever come to fruition, would have been the first computer. But in order for his vision to come to fruition, the transistor had to be invented.

As research continues to try and divine what the quantum linchpin will be, Aaronson and others in his field will continue questioning what we know about the world and how we think we know it, trying to reach an agreement of understanding on what’s possible.

May 24, 2009
Adwoa Gyimah-Brempong, CSAIL