NATURE|Vol 441|22 June 2006 BOOKS & ARTS

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goes under the name of decoherence, whichcan be thought of as synonymous with theentanglement between a physical system andits environment.

In fact, quantum computation and decoher-ence are two different sides of the same coin.Entanglement that can be controlled is used in quantum computers; entanglement with the environment that is uncontrollable causesdecoherence, destroying quantum computersand facilitating the emergence of the classicalworld. The distinction between controllable or uncontrollable entanglement makes no dif-ference to the Universe, and is only importantto the tool-building human mind.

Entangled World is a nice collection of essaysand reviews addressing all three of the aboveaspects of entanglement: the philosophical(dead-and-alive cats), the physical (decoher-ence) and the technological (quantum com-puters). This makes it a unique collection, andis its strongest selling point.

The book contains several basic yet eruditeexpositions, many written by people who have made significant contributions to thefield, such as Reinhard Werner, Rainer Blatt, Gerhard Rempe, Harald Weinfurter and ErichJoos. The first few essays are on the basic prin-ciples of quantum mechanics, and then thebook moves on to entanglement and its use in quantum computation, teleportation andother quantum wizardry. The second half ofthe book contains reviews on the emergence of the classical world through decoherence,concluding with an essay by Michael Esfeld onwhat all this really means. Physicists like me,although always wary and suspicious of philo-sophy and the soundness of its reasoning, stilllike to read such musings about the meaning

of quantum physics. This is, after all, why we doscience: to understand and explain the worldwe live in.

My only concern about the book is that itperhaps sits somewhat uneasily between beinga ‘popular science’ book and a third-yearundergraduate introduction to the field. Someof the early essays could easily be part of a popular book, containing as they do very fewequations, whereas the reviews of quantumcomputation and decoherence are more technical and do require some undergraduatephysics training, although nothing terriblyexcessive. More consistency of style and levelthroughout the book would have been desir-able, so it would totally captivate at least one ofits target audiences.

But what about some of the really funda-mental questions in quantum physics? Canlarge-scale quantum computers be made thatwould operate at room temperature? Is theworld really universally quantum mechanical,or do we also need the notion of a classicalworld? If we do, where exactly is the divisionbetween the two worlds? Can cats (or indeedhumans) really be dead and alive at the sametime? Are macroscopic superpositions just aquestion of money and experimental ingenu-ity, or will they one day collapse together withquantum physics? Unfortunately, the reader is unlikely to find answers to these questions in Entangled World, or indeed in any otherphysics book.

What you will find in Entangled World is acomprehensible, friendly and insightful intro-duction into modern ideas in quantum physics,including some fascinating, albeit brief, excur-sions into quantum gravity. These are thesalient points to further enthuse you about the subject and perhaps motivate you to do a little thinking about the big questions. ■

Vlatko Vedral is in the Quantum InformationGroup, School of Physics and Astronomy,University of Leeds, Leeds LS2 9JT, UK.

A weird, wired worldEntangled World: The Fascination ofQuantum Information and Computationedited by Jürgen AudretschJohn Wiley: 2006. 312 pp. $35, £22.50, €33.80

Vlatko VedralQuantum entanglement seems to be at thecore of all the weirdness in quantum mechan-ics. Erwin Schrödinger called it the character-istic trait of quantum mechanics. RichardFeynman said that there was only one mysteryin quantum mechanics and everything elsecan be seen as just its (frequently complex)consequences.

Entanglement — a bewildering set of corre-lations between objects in the quantum world— has historically been thought to lead to var-ious physical, and possibly logical, paradoxesinvolving notions such as non-locality and catsthat exist simultaneously in dead and alivestates. Interestingly for engineers and com-puter scientists, all this quantum weirdness, farfrom being a hindrance, can be used to con-struct quantum computers that can run muchfaster than any current computer. Far from justbeing a theoretical quirk, small-scale quantumcomputers have been experimentally madethat can perform simple calculations.

At least as interesting to physicists, althoughsomewhat more controversial, is the issue that the very existence of the classical (non-quantum) world also crucially depends onquantum weirdness, or more specifically quan-tum entanglement. To put it more provoca-tively, the classical world behaves predictably,deterministically and in a localized way onlybecause of the entanglement that exists on thelarger scale in the Universe. This hypothesis

Virtually wildDazzling the millions who troop past, thehabitat dioramas of New York's AmericanMuseum of Natural History, including thisgorilla showcase, have fused science and artinto hyper-real three-dimensional displays ofanimals in their native savannah, swamp orplain. Windows on Nature: The Great HabitatDioramas of the American Museum of NaturalHistory by Stephen Christopher Quinn (Harry N.Abrams, $40) takes a behind-the-scenes lookat more than 40 of them, each a testament tothe explorers, naturalists, taxidermists andartists who painstakingly brought them to lifemore than half a century ago. This heroic task —it took 18 years to complete the 29 dioramas ofthe North American mammal section alone —inspired a number of conservation measures, as well as considerable awe. B.K.

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