Why Ads? 

The Quest for the Quantum Computer *****
Previous US title: Minds, Machines and the Multiverse

Julian Brown, Touchstone, 2001

ISBN 0 684 81481 1

It was after seeing the excellent film Copenhagen that I began to search for a book on quantum theory. Many are very technical and heavy with mathematics. So, although I was not particularly seeking a book on quantum computers, this book by Julian Brown had excellent reviews on Amazon. I was not to be disappointed.

Quantum computers don't exist yet. But as this book explains, if they can be built they will offer a future of huge computing power. Quantum computers will be built using quantum mechanics. So the basic building block is not a transistor gate on a silicon chip (made up of millions (billions?) of atoms), but individual electrons or photons within the atom (i.e. the sub-atomic level). If the building of a quantum computer can be achieved then we will have overcome Moore's Law (the limits of we are approaching fast).

But quantum computing provides more benefits than just miniaturisation (and hence computing speed). Classic computing is based on binary notation. An electronic switch can be in either one of two states: it is either off (0) or on (1). We can thus use multiple switches to hold and manipulate numbers and words etc.. In quantum computing an electron can be in an infinite number of states: from 0 to 1 with an infinite number of values in between all at the same time (called superposition). Therefore, a quantum computer would process infinite amounts of data at the same time (i.e. in parallel)! Naturally, the guys in the Pentagon are very interested, as a quantum computer would be very handy for breaking into encrypted data. There is one major problem though: when you go to examine the quantum state of an electron, you destroy it!

Julian Brown's book begins by explaining the limits of classical computing, the principles behind quantum computing  and the tantilising possibilities if it can be realised in a working machine. In tracing the history of quantum science and computing, he briefly explains the theoretical work of Ralph Hartley at Bell Labs, James Clerk Maxwell, Rolf Landauer at IBM, Ed Frenkin and others. This introduces such concepts as reversibility, time, energy, entrophy - all practical constraints that must be overcome.

Chapter 3 moves on to Richard Fenman's in 1981 and his ideas that a quantum computer would mimic nature rather than simply simulating (as in weather forecasting computer models, for example). The chapter describes the earlier works of Born and Heisenberg (their wartime meeting and discussions on the atomic bomb being the subject of the film Copenhagen). This chapter explains the problem that in measuring a sub-atomic particle (e.g. a photon)  it is destroyed. Chapter 3 goes on to talk about the possibility of many universes (multiverse) all working in parallel and how events in one universe can effect the course of events (history) in another universe. So, if you decide or not to keep reading this review your decision will effect the outcome of other people in other universes!!

Chapter 4 is about parallelism and how superposition may be harnessed to build a parallel processing computer and the sort of practical problems that could then be solved, for example, solving network problems like the travelling salesmen puzzle. Chapter 5 moves this forward in the realm of cryptography and there is an explanation of public key encryption (used, for example, when you you shop with your credit card over the internet). Here we begin to get some mathematics covering factorisation. But understanding of this is not essential as Julian Brown gives an excellent verbal description. Chapter 6 continues with cryptography. It looks at how keys and other information can be transmitted without compromise by using the concept of entanglement.

Chapter 7 addresses the book's title: How to Build a Quantum Computer. It looks at the physical materials that could be used. Chapter 8 looks at the problems of any computer: catching errors and correcting them. Remember, with a quantum computer, looking at any interim stage in the computing process destroys the data! Personally, I found this chapter tedious and so skipped many pages. Chapter 9 sets out a speculative road map for for 'Visions of the Quantum Age'.

All in all an excellent introduction to Quantum theory and computing for the layman. Beware, though, this is a demanding book that is best taken in smallish chunks.