MINDS, MACHINES AND THE MULTIVERSE

Few people are fortunate enough to attend and document the birth of a revolution. The advent of the personal computer, beginning in about 1975, was one such revolution. It changed our lives drastically, and mostly for the better. While much has been written about it, I know of no single volume that records its birth throes — much less one written as they were taking place.

Of course, the inventors of the various pieces of technology required for the personal computer were first hobbyists and then workers within corporations. Neither group has both a long-standing tradition of sharing ideas and a well-organized system to implement the sharing. (The Homebrew Computer Club had both, but only in the Santa Clara Valley of California — the region now known as Silicon Valley. And corporations, of course, are notorious for insistence that any "standards" be based on their proprietary technology.)

Things are different in the academic realm. There, ideas are the main currency, exchanged via the medium of peer-reviewed papers. So it is that the concepts underlying the Quantum Computer, or QC (following "PC") are available for any interested writer to collect and document.

It's lucky we are that Julian Brown was the interested writer. He has followed the very abstruse topic since its inception in the early 1970s; he understands it well, and he can impart that understanding in a very readable narrative.

That narrative begins with a version of Arthur C. Clarke's well-known Third Law: "Any technology that is sufficiently advanced is indistinguishable from magic." Brown then describes the work of David Deutsch, the British theoretical physicist who is arguably the driving force behind the quest for the Quantum Computer. The QC, as Deutsch envisions it, promises to be a truly revolutionary device. Not only would it put today's fastest supercomputers to shame, enabling us to solve problems previously held unsolvable, but it would do so by parcelling out its parallel computations among families of parallel universes. Indeed, Deutsch was led to the concept by his search for a means of proving the existence of these parallel universes — the "Multiverse" of the title.

In quantum mechanics, the branch of physics that describes the behavior of matter at atomic and subatomic levels, one interpretation of the key process (called the collapse of superposition) that occurs when a measurement is made is that — since before the measurement, all possible outcomes are equally true — the act of measuring any quantity spawns a new universe for each possible result. Today, there is no way to test for the existence of such alternate universes, and so the idea remains in the realm of science fiction, where it has been used many times to highly entertaining effect.

But Deutsch believes that, if a Quantum Computer can be built, it can be used to test the mind-boggling concept of the Multiverse. That "if" is a mighty big one. However, physicists have taken some important steps toward the actual hardware of such a computer. Those steps are the subject of Brown's book.

Since it is such a young field, most of the steps he describes are theoretical. But the hardware that has been built and demonstrated so far includes the basic logic gates which correspond to NAND, NOR and XOR gates familiar to designers of classical computers. In addition, there are schemes for error control, and several very promising algorithms have been developed. Probably the most impressive is a method of factoring large numbers: It threatens to make current cryptography techniques obsolete. But QC also offers hope that encryption provably unbreakable can be achieved. Thus, Clarke's quotation is apropos; for many of these applications would seem truly magical.

Chapters 5 and 6 are an extensive discussion of the theory and practice of cryptography, including the details of the factorization program. (It's due to Peter Shor of AT&T Bell Labs.)

Chapter 7 describes the progress in actual hardware to date. Researchers have found four approaches so far: laser-exciting electrons in polymers; laser-exciting phonons (vibrational states) within chains of atoms in ion traps; photons in resonant cavities ("cavity QED"); and hydrogen atoms of organic molecules excited using nuclear magnetic resonance (NMR). Each approach has its advantages and its defects; but all suffer from the critical problem of decoherence. Simply put, the conditions needed for QC are all too evanescent. There is also the "quantum dot transistor". It is stable; but the problem here is how to get the essential coupling (entanglement) between individual quanta (or "qubits"). Yet, if a quantum computer can be built at all, it may be possible to accomplish a lot of computing in a short time. Brown presents on page 266 a chart suggesting how this could be:

We are nowhere close to having a Quantum Computer capable of useful work. Still, with the many breakthroughs Brown describes, it is hard to argue that one can never be built. His account of the progress to date is absorbing. He has a gift for illuminating analogies and, while he includes mathematics and circuit diagrams in the text, he is careful to minimize these technicalities. (More details are provided in appendices.) Along the way he is not loathe to include personal details:

or to digress into philosophy (most of the final chapter) or the question of whether human brains have quantum coherence. This last inquiry ties him to the last book I read, Werner Loewenstein's The Touchstone of Life. Loewenstein also ponders this question, which was first (to my knowledge) raised as a theory by Roger Penrose in his two books The Emperor's New Mind and Shadows of the Mind.

A very helpful feature of the book is its cross-referencing, where Brown will refer to an earlier or later discussion of the same topic. The text is also thoroughly end-noted and indexed, and a bibliography is provided.

Brown and his editor have eliminated most typos. Those I found are in the errata page linked below:

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This page was created in 2000. Its contents were last modified on Sunday, 28 July 2024.