Book Review: an Introduction to Quantum Computing by Phillip

Royal revolutionaries of Victorian astronomy

systems and from the danger it poses to Clark is writing for a popular- The Sun Kings humans in space. science audience who will enjoy his The Unexpected Tragedy of In setting the stage for his biography lively and eminently readable account Richard Carrington and the of the founders of Victorian solar as- of the lives and scientific careers of Tale of How Modern tronomy, Clark, a writer for the Euro- those whose work furthered the under- Astronomy Began pean Space Agency who has a PhD in standing of the Sun–Earth connection. astrophysics, introduces the phenom- Moreover, all readers, professional Stuart Clark physicists and interested laypersons Princeton U. Press, Princeton, NJ, ena of space weather with a blow-by- 2007. $24.95 (211 pp.). blow account of the ferocious solar alike, will appreciate that The Sun Kings ISBN 978-0-691-12660-9 storms that took place around late is no dry, biographical tome. Clark’s October and early November 2003. He skillful writing weaves into his protag- Reviewed by Richard C. Canfield starts by discussing the radiation effects onists’ research the aspects of their non- Stuart Clark’s The Sun Kings: The Unex- of those storms on astronauts aboard professional lives that so strongly influ- pected Tragedy of Richard Carrington and the International Space Station, passen- enced their work. That fact is captured the Tale of How Modern Astronomy Began gers on airplanes flying polar routes, in the book’s rather embellished subti- is a lively, informative discourse on the and instrumentation aboard scientific tle. Clark successfully presents the way research that led to a discovery that in and weather satellites. This rhetorical a major subfield of modern astronomy Victorian times was revolutionary: a device works well; it gives the reader a began while introducing readers to one cause-and-effect relationship between framework and motivation for under- of its key players. Carrington led a hard events on the Sun and Earth. Although standing the final outcome of the con- life, punctuated by failed, turbulent re- the book is biographical, the science is voluted and intensely personal research lationships, professional and personal. not secondary: The path taken by the pioneers of solar Contemporary physicists will recog- characters and their astronomy. nize that irrational and counterpro- research are skill- The author traces the development ductive rivalries, petty jealousies, and fully interwoven in of the comprehension of the Sun–Earth affairs of the heart were every bit as the narrative. The connection from the 17th through 19th pervasive in the scientific life of the inclusion of the dis- centuries, beginning with the telescopic Victorian era as they are today. coveries and per- observations of sunspots by Galileo in sonas of so many of 1610. Key players in the 19th century in- the pioneers of Vic- cluded mathematician and Astronomer An Introduction torian astrophysics Royal George Biddell Airy and natural will make Clark’s scientist Alexander von Humboldt. to Quantum book an enjoyable and meaningful read During the time that 18th-century as- for anyone, professional physicist and tronomer William Herschel, the discov- Computing layperson alike, who has an interest in erer of the planet Uranus, was making Phillip Kaye, the roots of physics and astronomy. his observations, it was well accepted Raymond Laflamme, Before the age of the “Sun Kings”— that sunspots were indeed located on and Michele Mosca the Victorian astronomers who are the the surface of the Sun. But, interest- Oxford U. Press, New York, 2007. focus of the book—solar astronomy was ingly, notions such as Herschel’s that $150.00, $50.00 paper (274 pp.). strictly an intellectual pursuit. Nowa- living creatures inhabited the Sun were ISBN 978-0-19-857000-4, days, that field’s liveliest branch is ar- not rejected out of hand. ISBN 978-0-19-857049-3 paper guably the study of what contemporary The event that had the most impact geophysicists call space weather, the on the solar astronomy that Clark physics and phenomena of storms in chronicles was Carrington’s telescopic Introduction interplanetary space and in Earth’s observation in 1859 of a white-light magnetosphere, ionosphere, and outer- to Quantum solar flare. The sighting temporally co- most atmosphere that are caused by incided with an impulsive change in solar flares and coronal mass ejections. Information the direction of compass needles at a The practical relevance of space weather nearby magnetic observatory followed Science accrues from its threat to both space- by an auroral display of extraordinary Vlatko Vedral borne and ground-based technological intensity, and with electrical arcs and Oxford U. Press, New York, 2006. Richard Canfield is a research professor the smell of scorched paint at telegraph $70.00 (183 pp.). in the department of physics at Montana stations. Although Carrington was fa- ISBN 978-0-19-9215706 State University in Bozeman. His interests mously conservative, cautioning that There was a time when computer and include solar magnetism and flares, as “one swallow does not make a sum- information science appeared to be a well as solar eruptions and their inter- mer,” Clark rightly notes that the events discipline of pure logical reasoning, planetary consequences. were a tipping point for astronomy. divorced from physics. Questions of

© 2008 American Institute of Physics, S-0031-9228-0802-240-0 February 2008 Physics Today 61 what a computer could do and aid. A series of exercises pre- be coupled with concrete examples of what resources would be sented early in the book, for physical implementations. Although needed to do it seemed to be example, guide the reader to the fundamental results of quantum largely independent of its the existence of a universal set information science depend on the logi- physical construction. The of quantum gates. For better cal structure of quantum mechanics— connection between informa- or worse, in some places im- unitary evolution in Hilbert space and, tion and physics, thermody- portant results are simply de- perhaps, the projection postulate—and namics in particular, has been clared as theorems without not on any specific qubit implementa- developed during the past proof. tion, neglecting experimental realiza- 50 years or so. A famous ex- The authors do an excellent tions is unwise. A novice reading Kaye, ample of that relation was dis- job breaking up Shor’s factor- Laflamme, and Mosca’s book, for ex- covered by Rolf Landauer, who in 1961 ing algorithm into pieces that students ample, might reasonably reach the con- showed that erasing, or otherwise irre- can easily digest. They take the ap- clusion that if there is a universal set of versibly losing, a bit of information in a proach—not original, but deftly pre- gates, clearly delineated quantum algo- computer must dissipate at least kT ln2 sented—of reducing the order-finding rithms, and error-correcting codes on of energy. problem at the heart of factoring to the paper, then a large-scale quantum com- That idea, now called Landauer’s problem of finding the eigenvalues of a puter should be just around the corner. principle, was succinctly summarized unitary operator, delegating much of the The reader should at least be given a in his declaration that “information is work to the reader through exercises. glimpse of the challenges involved in physical,” which has become a mantra The book contains a chapter on quantum making a robust controlled-NOT gate or in the fields of quantum information complexity that is more formal than the performing a complete Bell-state meas- science and quantum computing. (See rest of the text and may not appeal to urement; both are primitives on the Landauer’s article in PHYSICS TODAY, some readers. However, readers who quantum information theorist’s palette May 1991, page 23.) We now know that skip or skim that chapter will have little but are challenging to perform in the a computer that exploits the trouble digesting the remain- lab. The closest these books come to dis- unique features of quantum der of the text. cussing experiments are brief mentions mechanics can solve some Vedral’s Introduction to of idealized Mach–Zehnder interferom- problems more efficiently Quantum Information Science is eters. Remarkably, because of an algebra than any classical computer, a billed as a graduate text but is mistake, Vedral gets that example result that firmly links com- not really—it contains no ex- wrong—the photon exits the wrong puter and information science ercises. Distilled from a series way from the symmetric interferometer. to physics. Quantum informa- of lectures by Vedral, a profes- There is another, deeper reason ex- tion science, broadly defined, sor of quantum information periments in this field deserve more at- has expanded enormously science at the University of tention. If researchers fail to construct over the past two decades, and Leeds in the UK, it is a bit un- a large-scale quantum computer, it numerous books on the subject at all even, both in the selection of topics and may be because of some practical limi- levels have appeared, including Phillip style of presentation. Sometimes more tation, or it may be the sign of some- Kaye, Raymond Laflamme, and Michele space is given to the algebra of a simple thing more interesting. A spectacular, Mosca’s An Introduction to Quantum example than to a fairly abstract proof. albeit speculative, possibility is that we Computing and Vlatko Vedral’s Introduc- Most of the book focuses on quantify- find quantum mechanics breaks down, tion to Quantum Information Science. ing quantum entanglement, the author’s and there emerge some fundamentally An Introduction to Quantum Comput- primary current research field, and new laws of physics and, by extension, ing covers a small subset of the topics in quantum information by using entropic of information. Michael Nielsen and Isaac Chuang’s measures. That approach pervades his Jonathan R. Friedman Quantum Computation and Quantum In- treatment of quantum computing, meas- Amherst College formation (Cambridge U. Press, 2000), urement, and error correction. Formal Amherst, Massachusetts which has become a standard text in the results are sometimes followed by clear field. That tome, still relevant eight discussions of analogies to thermody- years after its publication, is probably namics. Error correction, for example, is The Grid too unwieldy for most undergraduates cast in terms of swapping quantum states A Journey Through the Heart looking for an introduction to the field. and entropy into environmental degrees of Our Electrified World In contrast, Kaye, Laflamme, and Mosca’s of freedom. The book is a good, technical book is very accessible. LaFlamme and read, with many pithy or whimsical foot- Phillip F. Schewe Mosca are on the faculty of the Institute notes sprinkled throughout. Unfortu- Joseph Henry Press, Washington, for Quantum Computing at the Univer- nately, Vedral’s text contains numerous DC, 2007. $27.95 (311 pp.). sity of Waterloo in Canada and are well- typographical errors and notational am- ISBN 978-0-309-10260-5 known researchers in quantum infor- biguities. Most are merely distracting, Electricity uniquely combines three at- mation science; Kaye is a doctoral but some may lead to real confusion. In tributes: It is crucial, even for minimally student at the university. Based on a contrast, the few errors I found in Kaye, developed societies; fragile, in that pro- course taught by the authors, the text is Laflamme, and Mosca’s text are almost hibitive storage costs require produc- tightly focused on quantum computing entirely inconsequential. tion to equal use by the minute; and and carefully leads the reader through Both An Introduction to Quantum interconnected, because failure of one Deutsch’s, Shor’s, and Grover’s algo- Computing and Introduction to Quantum supplier to meet demand can black out rithms, among others. Coupled with Information Science would benefit from a half a continent. In The Grid: A Journey the authors’careful exposition is a cadre stronger adherence to Landauer’s dic- Through the Heart of Our Electrified of exercises, integrated into the text, tum. If information is physical, discus- World, Phillip Schewe, chief science that forms an important pedagogical sions of quantum information need to writer at the American Institute of

62 February 2008 Physics Today www.physicstoday.org