A Shortcut Through Time : The Path to the Quantum Computer

Trying to say more about the concepts discussed in this book
in a short review is impossible -- explaining how a particle
will simultaneously exist in all its possible states at once
until its measured will give either a blank stare or a bland
"yeah right I know", meaning it didnt really register -- but
it isnt necessary, because Mr. Johnson does a right nice job
of explaining such matters in this neat brief book.

His writing is extremely clear and concise, at least relative
to the difficult matters he is discussing, and the book is
tidily illustrated. This is, to be sure, a book for beginners,
and in fact it spends some time up-front explaining basics
of computers before it moves on to quantum effects. A
specialist will likely have NO use for it, and might even be
a bit scornful at Mr. Johnsons occasional excursions into
arm-waving.

In fact, I did have some real problems with his discussions
of Shors and Groves quantum-computing algorithms, when
Mr. Johnson did seem to be getting into some real arm-waving.
Well, given the difficulty of the material, he was likely to
fall into that trap in places, and maybe I should just give
him the benefit of doubt, read that material again a few times,
and sleep on it.

However, I was basically familiar with quantum computing and
cryptography (at a layman level) before I read this book, and
at the outset thought it might not tell me anything I didnt
already know. I was wrong since I got a tidy explanation of
the application of quantum teleportation to cryptography
(blank stare out there?), and some other nice tidbits.

Besides, I was thoroughly impressed by Mr. Johnsons
sensibility in his comments about technical writing and his
degree of cautious skepticism in dealing with physicists, some
of whom seem to be slightly around the bend. The late Dick
Feynman, who plays a part of sorts in this book, could
actually understand what the such sorts were saying and
nail them, but the rest of us will just have to sympathize
with Mr. Johnson when he makes comments such as:

"Gilles Brassard tells me that each dim flash, on the average,
contains perhaps one-tenth of a photon, an idea I find rather
difficult to grasp."

You just gotta like this guy.

Johnson acknowledges as much when he quotes French physicists Serge Haroche and Jean-Michel Raimond as saying that the small scale "hands-on experiments" with a few qubits that are currently being done "are more likely to teach us about the processes that would ultimately make the undertaking fail" than to teach "us how to build a large quantum computer." (p. 169)

As I understand it, basically the idea behind quantum compters is that (somehow) individual quanta (atoms, photons, electrons) are able to be in a particular state or not to be in a particular state; that is, either the equivalent of yes or no, but also in an indeterminate state; that is, a state that would signal yes and no at the same time! Somehow (and I hope I am forgiven for not fully appreciating this)--somehow because of this fabled indeterminancy, quanta can be used to compute at a speed that is more than exponentially faster than digital computers.

Johnson spends some series ink in trying to show how the atoms can hold and crunch numbers as long as they are not disturbed; that is, not measured in any way (which would bring about the famous "collapse of the wave function"). In this manner a problem that would take a digital computer weeks or months to solve could be solved in a fraction of a second. Problems now actually impossible to solve in any reasonable length of time might become tractable after all. The traveling salesman problem which grows exponentially more complex with the addition of each city, might very well yield to a quantum computer since the computational ability of a quantum computer itself grows exponentially with the addition of more quanta.

Wow. One of the reasons there is real money going into trying to develop these seemingly magical machines is that at present all the cryptography used by the military and big corporations relies on the fact that digital machines, no matter how fast, are not able to factor the codes. However, a quantum computer could. Furthermore, as Johnson explains, a quantum computer could also develop cryptography that could not be decoded. So, whoever gets there first--assuming somebody can--will at the very least make a whole lot of money.

What I found more interesting than the hope for a quantum computer are some of the insights into the quantum word that Johnson provides incidentally. The biggest stunner for me was his assertion that quantum events can be used to generate random numbers. It may come as a surprise to many people but in the world of classical mechanics there is literally no such thing as a truly random number generator. But because radioactive nuclei decay on a random basis, they can, according to Johnson, be used to generate random numbers. He writes that numbers generated in such a manner are "undeniable random." (p. 91)

Apparently this conclusion is a consequence of quantum indeterminacy. In a way, it is a circular conclusion since if we could somehow predict the rate of radioactive decay we would violate indeterminacy. I say "circular" when perhaps I should say "as a matter of faith" because there is no way a stream of numbers derived from radioactive nuclei decay can be proven to be random. Indeed, no string of numbers can, by examination, be proven to be random. If QM is true--and it is massively established--then the numbers are random.

Perhaps this idea of randomness is similar to the notion of "nothing" in that it is only defined in a negative way, by which I mean random is the absence of order, and order is in the eye of the beholder. What seems random to human beings may be quite orderly from another point of view.

Some of the book is pure fantasy. His discussion of quantum banknotes in Chapter 9 is an example of something that is useful to think about because of the light it sheds on the nature of the quantum world, but any chance that we would actually use quantum banknotes (requiring temperatures near absolute zero!) approaches the null set. (p. 146)

Other parts of the book are largely tangential (but interesting nonetheless). For example Johnsons exploration in Chapter10 of "nondeterministic polynomial-time" problems, such as the above mentioned traveling salesman problem, the protein-folding problem and the software verification problem, is very interesting. I was not aware that such problems were linked, but according to Johnson if one is solved, the others would yield as well. The current thinking is that the only hope of solving such intractable problems is a large-scale quantum computer. (p. 164)

Johnson is hopeful that such a computer can be developed and bases his hope in part on recalling just how intractable the problems toward the development of the sort of computers we have today seemed in the 1940s in the days of the vacuum-tubed Eniac computer which filled an entire room and had only a small fraction of the computational ability of my desktop. (p. 140) However, whether history will repeat itself and the impediments be overcome remains to be seen. Its exciting to think that they will.