Following Tuesday’s post, here is the second piece I wrote for the latest issue of the Quantum Times. It is a review of two recent popular science books on quantum computing by John Gribbin and Jonathan Dowling. Jonathan Dowling has the now obligatory book author’s blog, which you should also check out.

### Book Review

- Title: Computing With Quantum Cats: From Colossus To Qubits
- Author: John Gribbin
- Publisher: Bantam, 2013

- Title: Schrödinger’s Killer App: Race To Build The World’s First Quantum Computer
- Author: Jonathan Dowling
- Publisher: CRC Press, 2013

The task of writing a popular book on quantum computing is a daunting

one. In order to get it right, you need to explain the subtleties of

theoretical computer science, at least to the point of understanding

what makes some problems hard and some easy to tackle on a classical

computer. You then need to explain the subtle distinctions between

classical and quantum physics. Both of these topics could, and indeed

have, filled entire popular books on their own. Gribbin’s strategy is

to divide his book into three sections of roughly equal length, one on

the history of classical computing, one on quantum theory, and one on

quantum computing. The advantage of this is that it makes the book

well paced, as the reader is not introduced to too many new ideas at

the same time. The disadvantage is that there is relatively little

space dedicated to the main topic of the book.

In order to weave the book together into a narrative, Gribbin

dedicates each chapter except the last to an individual prominent

scientist, specifically: Turing, von Neumann, Feynman, Bell and

Deutsch. This works well as it allows him to interleave the science

with biography, making the book more accessible. The first two

sections on classical computing and quantum theory display Gribbin’s

usual adeptness at popular writing. In the quantum section, my usual

pet peeves about things being described as “in two states at the same

time” and undue prominence being given to the many-worlds

interpretation apply, but no more than to any other popular treatment

of quantum theory. The explanations are otherwise very good. I

would, however, quibble with some of the choice of material for the

classical computing section. It seems to me that the story of how we

got from abstract Turing machines to modern day classical computers,

which is the main topic of the von Neumann chapter, is tangential to

the main topic of the book, and Gribbin fails to discuss more relevant

topics such as the circuit model and computational complexity in this

section. Instead these topics are squeezed in very briefly into the

quantum computing section, and Gribbin flubs the description of

computational complexity. For example, see if you can spot the

problems with the following three quotes:

“…problems that can be solved by efficient algorithms belong to a

category that mathematicians call `complexity class P’…”

“Another class of problem, known as NP, are very difficult to

solve…”

“All problems in P are, of course, also in NP.”

The last chapter of Gribbin’s book is an tour of the proposed

experimental implementations of quantum computing and the success

achieved so far. This chapter tries to cover too much material too

quickly and is rather credulous about the prospects of each

technology. Gribbin also persists with the device of including potted

biographies of the main scientists involved. The total effect is like

running at high speed through an unfamiliar woods, while someone slaps

you in the face rapidly with CVs and scientific papers. I think the

inclusion of such a detailed chapter was a mistake, especially since

it will seem badly out of date in just a year or two. Finally,

Gribbin includes an epilogue about the controversial issue of discord

in non-universal models of quantum computing. This is a bold

inclusion, which will either seem prescient or silly after the debate

has died down. My own preference would have been to focus on

well-established theory.

In summary, Gribbin’s has written a good popular book on quantum

computing, perhaps the best so far, but it is not yet a great one. It

is not quite the book you should give to your grandmother to explain

what you do. I fear she will unjustly come out of it thinking she is

not smart enough to understand, whereas in fact the failure is one of

unclear explanation in a few areas on the author’s part.

Dowling’s book is a different kettle of fish from Gribbin’s. He

claims to be aiming for the same audience of scientifically curious

lay readers, but I am afraid they will struggle. Dowling covers more

or less everything he is interested in and I think the rapid fire

topic changes would leave the lay reader confused. However, we all

know that popular science books written by physicists are really meant

to be read by other physicists rather than by the lay reader. From

this perspective, there is much valuable material in Dowling’s book.

Dowling is really on form when he is discussing his personal

experience. This mainly occurs in chapters 4 and 5, which are about

the experimental implementation of quantum computing and other quantum

technologies. There is also a lot of material about the internal

machinations of military and intelligence funding agencies, which

Dowling has copious experience of on both sides of the fence. Much of

this material is amusing and will be of value to those interested in

applying for such funding. As you might expect, Dowling’s assessment

of the prospects of the various proposed technologies is much more

accurate and conservative than Gribbin’s. In particular his treatment

of the cautionary tale of NMR quantum computing is masterful and his

assessment of non fully universal quantum computers, such as the D-Wave

One, is insightful. Dowling also gives an excellent account of quantum

technologies beyond quantum computing and cryptography, such as

quantum metrology, which are often neglected in popular treatments.

Chapter 6 is also interesting, although it is a bit of a hodge-podge

of different topics. It starts with a debunking of David Kaiser’s

thesis that the “hippies” of the Fundamental Fysiks group in Berkeley

were instrumental in the development of quantum information via their

involvement in the no-cloning theorem. Dowling rightly points out

that the origins of quantum cryptography are independent of this,

going back to Wiesner in the 1970’s, and that the no-cloning theorem

would probably have been discovered as a result of this. This section

is only missing a discussion of the role of Wheeler, since he was

really the person who made it OK for mainstream physicists to think

about the foundations of quantum theory again, and who encouraged his

students and postdocs to do so in information theoretic terms. Later

in the chapter, Dowling moves into extremely speculative territory,

arguing for “the reality of Hilbert space” and discussing what quantum

artificial intelligence might be like. I disagree with about as much

as I agree with in this section, but it is stimulating and

entertaining nonetheless.

You may notice that I have avoided talking about the first few

chapters of the book so far. Unfortunately, I do not have many

positive things to say about them.

The first couple of chapters cover the EPR experiment, Bell’s theorem,

and entanglement. Here, Dowling employs the all too common device of

psychoanalysing Einstein. As usual in such treatments, there is a

thin caricature of Einstein’s actual views followed by a lot of

comments along the lines of “Einstein wouldn’t have liked this” and

“tough luck Einstein”. I personally hate this sort of narrative with

a passion, particularly since Einstein’s response to quantum theory

was perfectly rational at the time he made it and who knows what he

would have made of Bell’s theorem? Worse than this, Dowling’s

treatment perpetuates the common myth that determinism is one of the

assumptions of both the EPR argument and Bell’s theorem. Of course,

CHSH does not assume this, but even EPR and Bell’s original argument

only use it when it can be derived from the quantum predictions.

Thus, there is not the option of “uncertainty” for evading the

consequences of these theorems, as Dowling maintains throughout the

book.

However, the worst feature of these chapters is the poor choice of

analogy. Dowling insists on using a single analogy to cover

everything, that of an analog clock or wristwatch. This analogy is

quite good for explaining classical common cause correlations,

e.g. Alice and Bob’s watches will always be anti-correlated if they

are located in timezones with a six hour time difference, and for

explaining the use of modular arithmetic in Shor’s algorithm.

However, since Dowling has earlier placed such great emphasis on the

interpretation of the watch readings in terms of actual time, it falls

flat when describing entanglement in which we have to imagine that the

hour hand randomly points to an hour that has nothing to do with time.

I think this is confusing and that a more abstract analogy,

e.g. colored balls in boxes, would have been better.

There are also a few places where Dowling makes flatly incorrect

statements. For example, he says that the OR gate does mod 2 addition

and he says that the state |00> + |01> + |10> + |11> is entangled. I

also found Dowling’s criterion for when something should be called an

ENT gate (his terminology for the CNOT gate) confusing. He says that

something is not an ENT gate unless it outputs an entangled state, but

of course this depends on what the input state is. For example, he

says that NMR quantum computers have no ENT gates, whereas I think

they do have them, but they just cannot produce the pure input states

needed to generate entanglement from them.

The most annoying thing about this book is that it is in dire need of

a good editor. There are many typos and basic fact-checking errors.

For example, John Bell is apparently Scottish and at one point a D-Wave

computer costs a mere $10,000. There is also far too much repetition.

For example, the tale of how funding for classical optical computing

dried up after Conway and Mead instigated VLSI design for silicon

chips, but then the optical technology was reused used to build the

internet, is told in reasonable detail at least three different times.

The first time it is an insightful comment, but by the third it is

like listening to an older relative with a limited stock of stories.

There are also whole sections that are so tangentially related to the

main topic that they should have been omitted, such as the long anti

string-theory rant in chapter six.

Dowling has a cute and geeky sense of humor, which comes through well

most of the time, but on occasion the humor gets in the way of clear

exposition. For example, in a rather silly analogy between Shor’s

algorithm and a fruitcake, the following occurs:

“We dive into the molassified rum extract of the classical core of the

Shor algorithm fruitcake and emerge (all sticky) with a theorem proved

in the 1760s…”

If he were a writing student, Dowling would surely get kicked out of

class for that. Finally, unless your name is David Foster Wallace, it

is not a good idea to put things that are essential to following the

plot in the footnotes. If you are not a quantum scientist then it is

unlikely that you know who Charlie Bennett and Dave Wineland are or

what NIST is, but then the quirky names chosen in the first few

chapters will be utterly confusing. They are explained in the main

text, but only much later. Otherwise, you have to hope that the

reader is not the sort of person who ignores footnotes. Overall,

having a sense of humor is a good thing, but there is such a thing as

being too cute.

Despite these criticisms, I would still recommend Dowling’s book to

physicists and other academics with a professional interest in quantum

technology. I think it is a valuable resource on the history of the

subject. I would steer the genuine lay reader more in the direction

of Gribbin’s book, at least until a better option becomes available.