Dreams of rich leaps in time

Quantum Technology
November 7, 1997

This book is the first volume in a new series aimed at explaining current topics at the "frontiers of science" to "the non-scientific public". The series editor is the noted scientist and populariser of science, Paul Davies: in his foreword to Quantum Technology, Davies characterises the 19th century as the "machine age", the 20th century as the "information age", and the 21st century as the "quantum age". In his opening chapter, Gerard Milburn claims he will describe "the foundations of a nascent technology and billion-dollar industry which promises to enrich our lives". To a sceptical old quantum mechanic like myself, these are bold claims. Nevertheless, by the end of Milburn's little book, I found myself stimulated and excited by the dramatic progress in "applied quantum mechanics" that has been made over the past 15 years. Thus, in spite of some mildly irritating shortcomings, Milburn succeeds magnificently in conveying the message that quantum physics is not only alive and well but close to delivering some startling new technology.

Quantum mechanics is our most fundamental theory of nature - everything, from the hardness of a table to the red sky at night, is ultimately governed by its laws. The surprise is that reality at the quantum level appears very different from the comfortable picture of the world envisaged by Isaac Newton. Even something so ordinary as the trajectory of a tennis ball is reduced, at the quantum level, to a murky haze of probability with no definite trajectory in sight. The puzzle for quantum mechanics is to understand how the certainties of Newton's laws emerge from the uncertainties of Schrodinger's equation for the quantum probability amplitudes. Notwithstanding such problems, Milburn shows that physicists have made great progress in isolating and manipulating quantum objects. Using devices such as the scanning tunnelling microscope, individual atoms can be isolated and their intrinsic quantum behaviour observed. Clever arrangements of laser beams allow atoms to be cooled to near absolute zero and have led to the new field of atom optics with an array of new devices such as atom traps, atom pipes and optical tweezers. Not content to play only with atoms, physicists have made "quantum nanocircuits" using integrated-circuit technology to confine electrons to zero-dimensional structures - "quantum dots" - which can be made to act as "electronic turnstiles" that control the flow of individual electrons.

The final two chapters look to the far future and describe fascinating developments in quantum cryptography and quantum computers. Quantum cryptography uses the subtle effects of quantum probability amplitudes to construct a secure communication system. The system is secure in the sense that any eavesdropper listening in to a private conversation leaves behind a "quantum footprint" revealing that security has been compromised. Unlike the ones and zeros of present-day classical computers, quantum computers deal with quantum bits or "qubits". Remarkably, the idea of a quantum computer can be traced back to a lecture by Richard Feynman in 1981 entitled "Simulating physics with computers". Feynman argued that classical computers are unable to simulate quantum systems and speculated about the possibility of a quantum computer. It was left to David Deutsch to show that quantum computers could calculate much faster than conventional ones because they would be able to explore all possible computational paths simultaneously! Surprisingly, the thermodynamics of computing plays a central role in the discussion, with such things as Landauer's principle linking energy and information loss, reversible logic gates and the delightfully named "Square-Root-of-NOT" gate. In 1994 these ideas were catapulted into prominence by the discovery that a quantum computer could factorise large numbers much faster than a classical computer - and thus, at a stroke, invalidate all present-day "public key" cryptography schemes! Fascinating stuff, but as Milburn shows, we are some way from realising such dreams and it will probably be decades before one will be able to go out and buy a quantum computer, if ever.

Given its exciting content, it is unfortunate that the book has the feel of a cut-price offering with diagrams that are hard to follow and sections that read like a scholarly review article. Nevertheless, Milburn's subject transcends these problems and he leaves us with the intriguing thought that his grandchildren may one day graduate from a department of quantum engineering.

Tony Hey is professor of computation, University of Southampton.

26Jbooks engineeringTHE TIMES 7JNOVEMBER 7J1997 science photo library THE TIMES 7JNOVEMBER 7J1997ENGINEERING BOOKSJ

Quantum Technology

Author - Gerard Milburn
ISBN - 1 86448 146 3
Publisher - Allen and Unwin
Price - £7.99
Pages - 188

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