martedì 17 marzo 2009

The race to build a quantum computer

Quantum computation was a highly speculative enterprise facing serious technological obstacles until a shy young physicist came along. Dave Bacon tells the story of Alexei Kitaev’s big idea.

When Russian physicist Alexei Kitaev heard from a colleague that Peter Shor, a researcher from Bell Labs in New Jersey, had discovered an algorithm for factoring numbers on a quantum computer, he got excited. Kitaev had long pondered how the — at the time — obscure field of quantum computation could be useful, and he knew that the Bell Labs result was likely to radically change our understanding of computing. This was a major result that he just had to understand.
Unfortunately for Kitaev, the research facility where he worked, the L D Landau Institute for Theoretical Physics in Chernogolovka, Russia, did not have a copy of the conference proceedings in which Shor’s result was published. Now, most researchers when confronted with a missing article would probably get on the phone to their local librarian or to a colleague at a neighbouring institution and attempt to obtain a copy of the paper. But for Kitaev, by nature rather shy, this normal mode of proceeding did not hold. Famously, when Nobel-prize winner Richard Feynman died in 1988, written on one of his office blackboards was the phrase “What I cannot create, I do not understand”. It was in this vein that Kitaev did what comes naturally to brilliant minds like himself and Feynman: he simply sat down and rederived this major new result.
The ability to reconstruct, with only the hint of a solution, Shor’s algorithm, is no doubt the mark of an amazingly sharp mind. But the standards of genius in fields like theoretical physics and computer science extend even higher: one must not just recreate prior results, but also create something totally new.
In 1997, three years after Shor’s breakthrough, Kitaev wrote a paper that made a radical new suggestion about how one could build a quantum computer (arXiv:quant-ph/9707021). At the time, researchers vehemently believed that building a quantum computer was going to require a physics and engineering tour de force, akin to replaying many decades of the computer revolution. Kitaev, contrary to the opinion of nearly everyone involved in the field of quantum computation at the time, proposed that building a quantum computer might not be more difficult than finding a proper physical substrate, an analogue to the transistor for quantum computers.
The idea was a heresy of the highest order. But like many heretical prophets before him, Kitaev’s proposal soon attracted the attention of a group of dedicated followers that has been quietly pursuing his alternative path. Kitaev’s vision for what a future quantum computer might look like, then, sparked a race between his disciples and those following the traditional path towards constructing a quantum computer: a race for the soul of a future quantum computer.
In the February issue of Physics World, Dave Bacon looks at the world of quantum computing and the key figures in the quest for a working computer. Bacon looks at the physics of Alexei Kitaev who was met with bewilderment and scepticism, when he showed in 2003 that it was possible to build a many-body quantum system with an inbuilt quantum-correcting code. Over the following six years, with the backing of a Field’s medal winner, Kitaev’s work has come to represent a key milestone in the quest for a workable machine.
To read the full version of this article — and the rest of the February issue of Physics World — please subscribe to our print edition. Members of the Institute of Physics can read all articles for free by accessing our digital edition.

About the author:
Dave Bacon is assistant research professor at the Department of Physics and at the Department of Computing Science and Engineering in the University of Washington, US

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