BOOKS in 0—for this purpose. Zero and one are just convenient labels for states of light or matter—for example, a spin-up or -down state of a nucleus or electron, horizontal or vertical polarization of a photon, or left or right chiral states of a molecule. tion and spy books. Less well known, but Once one accepts the notion that in no less important, is that they also excel at formation also can enter into a superposi simulating quantum systems. tion state, the dramatic implications follow w h e n one p o n d e r s the states of many Words such as "efficiently" and "excel" qubits. Two qubits can represent the num have a precise quantitative meaning: In the bers 00, 01,10, and 11 (in bina case of code cracking and sim ulations of quantum mechanics, A S H O R T C U T ry notation) in superposition, and TV qubits can represent all 2N the speedup offered by quan integers from 0 to 2 N -1. T h e tum computers is believed to be shortcut through time refers to exponential in the size of the the parallel processing of all problem. (Equally efficient clas these exponentially many num sical algorithms may still be bers at once, made possible by found, but this seems exceed virtue of their existence in su ingly unlikely.) In the case of perposition. This feat has no database search, the speedup is classical analog: A classical com provably quadratic and cannot p u t e r is d o o m e d to process be improved upon. numbers serially (Of course, a classical par T h e implications are mind-boggling. allel computer is just a bunch of serial ma Consider code-cracking: An eavesdropper chines working in parallel.) in possession of a quantum computer could conceivably gain access to society's bestTo be fair, the situation is in fact more kept secrets in a matter of a few hours or complicated, and concepts such as quan days. And the efficient simulation of quan tum entanglement and measurements play tum mechanical systems could conceivably a crucial role. Johnson admirably explains be exploited for the ab initio design of these subtleties in layman's terms, closely drugs or high-temperature superconduc obeying Stephen Hawking's famous law of tors. As Johnson puts it, "Quantum com popular-science writing, that each equa puting would be to ordinary computing tion halves the number of readers. (I count what nuclear energy is to fire." ed three, such as y = 2x, and these were all illustrated by graphs.) Johnson takes the T h e book's title, Ά Shortcut Through reader on a tour of the state of the art of Time," suggests that quantum computers the field all the way from the beginning perform their computational feats via a (Richard Feynman's first observation trick. From the perspective of "classical" that quantum computers can solve the computers, this is indeed the case. A clas problem of simulating quantum mechan sical computer can be defined as a device ics), to mid-2002, necessarily focusing on that does not use the laws of quantum meonly a few aspects he deems central. The account often reads storylike and is pep pered with occasional anecdotes. W h a t role is there for chemistry in quantum computing research? Interest ingly, the first demonstration of rudimen tary quantum algorithms was performed using a method familiar to every chemist: chanics to represent information. This in liquid-state nuclear magnetic resonance cludes what we currently call desktops, lap spectroscopy In fact, to date, N M R i s still tops, supercomputers, optical computers, the front-runner in terms of the number of and even D N A computing. qubits that have been harnessed for the W h a t does it mean to represent infor purpose of running quantum computing mation quantum mechanically? At the algorithms, setting t h e current world most basic level, it means that the unit of record at seven. information is no longer the classical "0 or 1" bit, but rather the "0 and 1" quantum Unfortunately, this number is not like bit (qubit). "And" refers here to the super ly to grow beyond 10 or so, for noise quick position principle of quantum mechanics, ly overwhelms the N M R signal as the size whereby an object can be at once in two (or of the molecule increases. Indeed, the more) mutually exclusive states. Johnson qubits in N M R are the nuclear spins of introduces the suggestive symbol "Θ"—1 (often specially synthesized) molecules.
QUANTUM COMPUTERS MADE LUCID A SHORTCUT THROUGH TIME: The Path to the Quantum Computer, by George Johnson, Knopf, 2003,224 pages, $24 (ISBN: 0-375-41193-3) REVIEWED BY DANIEL LIDAR
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H E QUEST T O GET S O M E T H I N G
out of nothing dates back at least to the alchemists. In Ά Shortcut Through Time: T h e Path to the Q u a n t u m C o m p u t e r , " his re markably lucid new book about quantum computers, George J o h n s o n describes w h a t may be t h e u l t i m a t e free lunch, spawned by our harnessing of the laws of nature at the quantum level. Potentially bigger than the transition from the abacus to integrated semiconductor circuits, quan tum computers may one day revolution ize computation. The key to their success, as aptly put by Johnson, is that "performing a quantum computation is a matter of jumping on the wagon and going along for the ride." T h e wagon is the mysterious and wonderful world of quantum mechanics, at a level of sophistication no higher than as taught in an advanced undergraduate course. T h e ride is a rather wild one, through the rugged and vast expanses of abstract Hilbert space, traversing superpositions and quantum entanglement. ButJohnson, a science writer for the New York Times and author of several other suc
"What if, in a kind of mathematical jujitsu, quantum mechanics could be pitted against quantum mechanics, fighting fire with fire?" cessful popular-science books, has a re markable ability to tame the abstract using metaphors and pictures, such as his gem of an explanation of modular arithmetic us ing five-houred clocks. His presentation is suitable for and aimed at the nonspecialist. It is the most readable account of quantum computing I have encountered so far in this category Quantum computers are by now rather famous for their (theoretically demon strated!) ability to crack cryptographic codes and efficiently search large databases. They have even trickled into some science fic36
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