Computing with Quantum Cats by John Gribbin

Reviewed by Peter Hobson

The name John Gribbin will be familiar to many readers with an interest in understanding the mysterious quantum world as he is well known for books on this subject; indeed I have had his book In Search of Schrödinger’s Cat on my own shelves for many years. This book has a subtitle, “From Colossus to Qubits”, which for those with a prior knowledge of the early days of computing, or an interest in the amazing and only recently revealed work that took place at “Station X” at Bletchley Park on code breaking in WW2, will reveal the real subject. The subtitle indicates that this is a book ultimately about the history of the development of computing and where it is (or might be) headed with the development of computers that directly make use of the mysterious property that some quantum systems can be “entangled”. An entangled quantum state, the qubit of the subtitle, can in some sense exist in more than one state simultaneously. This is analogous, except that to make this classical analogy is to point up a very non-classical aspect quantum world, to a light switch that is in both the on and off states simultaneously.

This is a book of three distinct sections; firstly Gribbin takes the reader through the story of the development of conventional computers, the sort of machines that are now often hidden to us in our everyday use, such is the universal embedding of them into smartphones, tablets etc. Huge advances, leading to the first real computers, were made in the dark years of the late 1930s and 1940s, especially by the seminal figures in the UK of Alan Turing and Tommy Flowers and by counterparts such as John von Neumann in the USA. Gribbin covers this area well and in a very readable manner, including many vignettes of the characters involved which add to the interest. After one hundred pages Gribbin takes the reader through a discussion of quantum physics and here we meet another group of famous players, such as Richard Feynman and John Bell (who once tried to teach me at a summer school for PhD students in what was then Czechoslovakia). The final third of the book brings everything back together with a discussion of how quantum computers differ from classical computers, such as the laptop I am writing this review on, and whether they can be built, whether they have been built and if so what sort of problems will they excel in.

Gribbin knows how to tell a complex story. He understands quantum mechanics at a fundamental level and he has a great deal of experience in bringing that world to light and explaining it as best one can to an audience which has not had several courses in the subject at university. He quite appropriately eschews mathematical formalism and concentrates very much on the more philosophical aspects of how one might interpret the entanglement of states, and on what happens when an entangled state is resolved by measurement. He makes a good stab at what is going to be a very hard challenge since it is probably an area where many professional physicists still argue, and yet for most of us it doesn’t actually affect us in any real practical sense at all. If the reader leaves this section still baffled and unconvinced then at least she has been exposed to one of the greatest intellectual debates science has ever engaged in. As an experimental physicist I was pleased to see some good descriptions of the technological approaches to making, storing and “reading” qubits, though I do wonder how much the average non-physicist reader will gain from this.

I liked the overall style of the book and having read some of the author’s other work I am not surprised that I am recommending this book to anyone who would like to see where computers might be headed. Gribbin is, I think, fairly convinced that practical quantum computers will be with us fairly soon, and no doubt GCHQ and NSA and similar state organisations will be keenly awaiting them. About the time that this book was being written, a commercial device that has caused some considerable controversy was released. The D-Wave Two is a 512 qubit machine that has been bought and evaluated by a number of organisations and still perhaps the jury is out as to whether it really is a quantum computer. It is unfortunate that Gribbin was not writing the book in 2014 as I think he would have done a really good job of bringing the complexities of the argument to a wider readership.

I commend the book but have some minor criticisms. The overuse of the word phrase “go-to” as in “go-to man for quantum physics” (on the dust jacket as well!) irritated me; less trivially the section of photographs is less than useful with some pictures of bits of “kit” which even if you are an experimental physicist will mean little, and Gribbin provides no explanation for these at all. What he does provide, however, is a set of useful footnotes for each chapter, four pages of sources and suggested further reading, and (oh joy!) an excellent index.