MINDS, MACHINES, AND THE MULTIVERSE: THE QUEST FOR THE QUANTUM COMPUTER

by Julian Brown

The traditional and ubiquitous digital computer has changed the world by processing series of binary ones and zeroes...very fast. Like the sideshow juggler spinning plates on billiard cues, the classical computer moves fast enough to keep the plates from falling off. As computers become faster and faster, more and more plates are being added to more and more cues. Imagine, then, a computer in which speed is increased not because it runs faster, but because it has a limitless army of different jugglers, one for each billiard cue. Imagine the quantum computer. Julian Brown's record of the quest for the Holy Grail of computing -- a computer that could, in theory, take seconds to perform calculations that would take today's fastest supercomputers longer than the age of the universe -- is an extraordinary tale, populated by a remarkable cast of characters, including David Deutsch of Oxford University, who first announced the possibility of computation in the Alice-in-Wonderland world of quantum mechanics; Ed Fredkin, who developed a new kind of logic gate as a true step toward universal computation; and the legendary Richard Feynman, who reasoned from the inability to model quantum mechanics on a classical computer the logical inevitability of quantum computing. For, in the fuzzily indeterminate world of the quantum, new computing power is born. Minds, Machines, and the Multiverse details the remarkable uses for quantum computing in code breaking, for quantum computers will be able to crack many of the leading methods of protecting secret information, while offering new unbreakable codes. Quantum computers will also be able to model nuclear and subatomic reactions; offer insights into nanotechnology, teleportation, and time travel; and perhaps change the way chemists and biotechnologists design drugs and study the molecules of life. Farthest along the trail blazed by these pioneers is the ability to visualize the multiple realities of the quantum world not as a mathematical abstraction, but as a real map to a world of multiple universes...a multiverse where every possible event -- from a particular chess move to a comet striking the Earth -- not only can happen, but does. Incorporating lively explanations of ion trap gates, nuclear magnetic resonance computers, quantum dots, quantum algorithms, Fourier transforms, and puzzles of quantum physics, and illustrated with dozens of vivid diagrams, Minds, Machines, and the Multiverse is a mind-stretching look at the still-unbuilt but fascinating machines that, in the words of physicist Stanley Williams, "will reshape the face of science" and offer a new window into the secrets of an infinite number of potential universes.

Genres: Science, Physics, Nonfiction, Popular Science, Quantum Mechanics, Technology, Computer Science

400 pages, Hardcover

First published January 1, 1999

Reviews

[Charles Daney · Rating 4 out of 5]

Considering that this book was published in 2000, one might think that the technology of quantum computing has progressed significantly in the 15 years since then. However, that's not quite clear. Much of the theory of quantum computing, the algorithms in particular, was already in place in 2000. There are newer algorithms, of course, but the most important ones – such as Shor's algorithm for factoring and Grover's algorithm for database searching – were already known.

Yet even now, the technology for actually building a quantum computer is still in its infancy. (That's excluding the so-called "adiabatic quantum computers" of D-Wave Systems, whose status as genuine quantum computers is still controversial, and which are not covered in Brown's book.) The book discusses four possible technologies that were known at the time: ion traps, cavity QED, NMR (nuclear magnetic resonance), and quantum dots). All of these have troublesome limits (to a greater or lesser extent) with switching time (the basic speed of operations), "decoherence" time, and scalability. (Decoherence is the problem that quantum qubits cannot be kept in a state suitable for computation for very long times, due to disruption by external influences.)

At the present time, there are somewhere around 16 different technologies that have been seriously considered, yet the problems mostly remain – especially the problem of decoherence. A useful quantum computer needs to be able to process 1000 or more qubits at a time, perhaps a lot more. For example, just to represent a large number with, say, 500 digits (as would be needed for highly secure encryption), takes 1661 qubits (as a binary number). Several times as many qubits, at least, would be needed for any operations on such numbers.

An additional problem is that reliable error detection and correction must be done, because unavoidable decoherence must be assumed to introduce errors. Detection and correction algorithms exist, but they can multiply the number of qubits required by an order of magnitude, at least. It's rather sobering to realize that to date (2015) the largest number factored by a quantum computer (excluding adiabatic systems) is just 21.

All these issues explain why many people are still skeptical that useful quantum computing will be possible in foreseeable future, if ever. Brown's book does not spend much time discussing the skepticism, since 15 years ago it wasn't clear just how difficult the technical problems are.

The book does, however, go into considerable technical detail about the theoretical aspects. These are fairly well understood. Even so, the architecture of a quantum computer is nothing like a traditional von Neumann computer. Because of this, there are still relatively few general quantum algorithms, since programming a quantum computer is rather tricky. This brings into question whether important applications will actually run a lot faster on a quantum computer. Shor's algorithm offers an exponential speed-up, but many other algorithms, such as Grover's, don't run that fast. (Grover's algorithm can search a database in time proportional to √N, where N is the number of database entries. A non-quantum search runs in time proportional to N, unless the data is sorted (which requires an order of N times log(N) operations).)

Other topics treated in the book include reversible computing (which drastically reduces theoretical amounts of energy required to run an algorithm), the history of quantum computing (which really got started with ideas presented by David Deutsch as early as 1977 and by Richard Feynman in 1981), computability theory (e. g. "P vs. NP"), quantum communication, and quantum cryptography.

You may be wondering how "minds" and the "multiverse" are relevant to this topic. The issue of minds arises, since some people (notably Roger Penrose) have suggested that the human brain might be a quantum computer. (Most neurobiologists doubt that idea.) The "multiverse" refers to Hugh Everett's "many-worlds" interpretation of quantum mechanics. David Deutsch is an especially ardent supporter of this interpretation. He thinks the reason a quantum computer may be very much faster than a conventional one is that it can do computations in a highly parallel way in multiple "parallel" universes. It's an interesting idea, but not essential for believing quantum computers can be much faster. A more prosaic explanation is that when large numbers of qubits are "entangled", operations on all of them can be carried out simultaneously.

If you've found that some of the technical terms in this review are unfamiliar, take that as a sign this book is not an easy-going introduction to the subject. All of these concepts are clearly explained in the book – but some effort by the reader is still required. In addition, a good understanding requires some mathematical prerequisites, such as familiarity with complex numbers and a little elementary number theory. (But at least one doesn't need to know how to solve Schrödinger's equation.) It also helps a lot if a reader can follow logical diagrams and knows a bit about quantum concepts like entanglement and superposition.

So this book isn't a painless introduction to the subject. But it is still one of the few books accessible to non-specialists that actually gives the reader a good idea of what quantum computing is all about.

[Dillon · Rating 4 out of 5]

A beautiful, comprehensive, complete (almost) introduction to the field of quantum computing. This book gives a glimpse into the history and motivations of the field, a pretty thorough look at some of the defining algorithms, an overview of the leading hardware approaches, and lastly the philosophical implications of quantum mechanics. It was also particularly illuminating to peer into the minds of some of the founding fathers of the field, David Deutsch, Richard Jozsa, CQT's very own Artur Ekert, and Peter Shor. Names I've seen in my textbooks, but never really understood as people. The middle chapters were a little dry, but I owe it to the author being thorough in providing some mathematical background for the ideas discussed. Finally, I applaud the author for daring to dip his toes into the philosophical side of physics. Many scientists dare not, or don't care enough to venture into philosophy. While some wacky theories were discussed such as multiverses, the entire universe is a quantum computer? We are all quantum computers? Some of these ideas, as ridiculous as they might seem, at least provoke some thought of the philosophical consequences of quantum theory. All in all, a very thorough introduction to quantum computing. I would highly recommend to anyone who is interested enough in the field to get their hands dirty in the details, and go beyond a 20 minute Veritasium video on YouTube. My only gripe is that the information presented is current as of 2002, which is not very current by today's standards.

So if the visible universe were the extent of physical reality, physical reality would not even remotely contain the resources required to factorise such a large number. Who did factorise it, then? How, and where, was the computation performed?

Things I noted down:
- Quantum computers harness the power of parallel worlds instead of parallel processors.
- Shannon theory: linking information to thermodynamics. Information is the negative form of entropy.
- Interesting thoughts on the relationship between physics and computation. Is computation a tool that helps us to understand physics, or is the universe fundamentally a giant computer, continually computing the next state?
- Hamiltonian mechanics gives a God’s eye view of a system’s dynamics rather than the on-the-spot view offered by Newton’s laws
- How do we turn a qubit from 0/1 state into a superposition state? Depends on physical implementation, of course. For photons, just polarise at 45 degrees. For atoms, play a half pulse (pi/2 pulse).
- if control qubit is in a 0/1 superposition, and target qubits is in 0 state, and we apply a CNOT gate, we obtain entangled state 00 + 11.

[Aleph · Rating 5 out of 5]

Julian Brown si è posto l'obiettivo certamente non facile di raccontare ai piú l’affascinante e sterminato mondo della fisica quantistica in uno dei suoi aspetti piú peculiari e mal interpretati: il calcolo quantistico.
Attraverso nove avvincenti capitoli, Brown ci porta nel cuore della materia, lí dove tutto accade, dove forse anche la coscienza si origina, e sviscera passo passo la fisica necessaria a manipolare gli atomi, con tecniche stravaganti e avveniristiche, per produrre calcoli. Incontreremo teorie «esotiche» o a cui si fatica a credere, come l’interpretazione a molti dell’universo, o disquisizioni sull’origine della coscienza, ma anche cose piú concrete e familiari, come la storia e l’evoluzione dei microprocessori e una spiegazione su come purificare lo stato quantistico di una tazzina di caffè. Tutto questo mentre ci conduce per mano nella vera storia della computazione quantistica, con i suoi protagonisti talvolta poco o per niente conosciuti: abbiamo cosí il piacere di scoprire quanti siano gl’italiani —e gli europei in generale— che hanno dato contributi significativi, talvolta fondamentali e rivoluzionarî, a quest’ambito, dove la dominazione è perlopiú a carattere britannico e statunitense.
Brown ci narra di tecnologiche al limite del possibile, come il teletrasporto quantistico, o come fotografare singoli elettroni, o come si possa rompere la crittografia a chiave pubblica e come riuscire a prevedere l’andamento dei mercati azionarî, oltre a come costruire calcolatori reversibili (sí, dal punto di vista termodinamico!); e fa tutto ciò citando gli articoli scientifici scritti dai luminari di questa scienza, e spiegando per filo e per segno a noi comuni mortali gli esperimenti reali realizzati addirittura nel secolo scorso che dimostrano come costruire veramente tutto ciò! Roba da capogiro.
La lettura è certamente impegnativa, e non adatta per una vacanza rilassante sotto l’ombrellone. Ma è appagante, e se volete davvero imparare qualcosa di piú su questo affascinante mondo, purtroppo troppo spesso avvolto da un alone di baggianate quantistiche, potete stare sicuri che l’autore fa il possibile per guidarvi al meglio lungo questo viaggio e per farvi comprendere davvero la materia.
D’altronde è un saggio scientifico Einaudi, e chi li legge sa che raramente deludono le aspettative, e anzi spesso le superano.
Lettura caldamente consigliata a tutti, ma specialmente a chi non è a digiuno di logica.

[Sami Eerola · Rating 4 out of 5]

Too hard to understand. Only read some parts of this book, because most of it is too technical for me.

[Mangoo · Rating 4 out of 5]

Qual e' il fondamento della realta': la fisica o l'informazione?
Questo tema e' alla base del libro divulgativo di Brown dedicato alla nuova frontiera del calcolo: quello basato sulla meccanica quantistica. In realta' i transistori hanno sempre utilizzato fenomeni quantomeccanici per il loro funzionamento - e sempre di piu' con il ridursi delle dimensioni. Ma il calcolo quantistico mira a utilizzare singoli atomi o cose simili per i suoi scopi. Da qui l'ipotesi che cio' che la materia fa, in fondo, e' calcolare.
L'idea del computer quantistico si deve a David Deutsch, sebbene le sue idee siano state conosciute pubblicamente solo dopo le esternazioni del solito Feynman e Benioff. Ma Deutsch e' stato il primo a estrarne teoricamente le conseguenze e a predire alcune delle caratteristiche che un simile sistema avrebbe a differenza di un analogo classico. E il suo scopo non era informatico, bensi' gnoseologico o ontologico: dimostrare sperimentalmente che l'unica interpretazione vera e coerente della meccanica quantistica e' quella a molti universi - introdotta negli anni '50 da Everett in reazione all'interpretazione standard (detta di Copenhagen).
Il libro e' stato scritto nel 2000, e dato il rapido sviluppo della materia negli ultimi anni rischia gia' di essere vecchio. Ma la sostanza dei ragionamenti e' valida, come il contesto e le motivazioni. Brown divulga argomenti interessantissimi cui finora mancava una adeguata presentazione ad un vasto pubblico.
Si parte con il diavolo di Maxwell e le riflessioni nate dalla soluzione del suo paradosso, che espose le connessioni tra informazione e fisica. Poi si introduce un po'di meccanica quantistica e si fa capire che il computer quantistico, nello sfruttare entanglement e parallelismo quantici, in specifici compiti come la simulazione di sistemi fisici - fisica che simula se stessa - nella fattorizzazione di primi e nella ricerca entro liste ha la capicita' di surclassare i computer classici. Si passano in rassegna le applicazioni alla criptografia e l'importantissimo sviluppo di algoritmi per la correzione di errori quantici, che placarono parzialmente il criticismo di influenti personaggi come Landauer. E si discutono i primi tentativi di implementare i calcolatori quantistici in sistemi fisici (trappole ioniche, cavita' QED, NMR, quantum dots). Si conclude con discussioni sulla coscienza, la singolarita' e i transumanisti, e le applicazioni cosmologiche della meccanica quantistica.
Il libro e' consigliabile perche' chiaro e completo (almeno fino all'epoca di scrittura; servirebbe in aggiornamento o seguito), e colma la lacuna di cui dicevo sopra. E la bibliografia e' ampia e curata.
Cosi' come la teoria della informazione classica si sviluppo' decenni prima che avesse piena applicazione tecnologica, lo stesso sta accadendo con la teoria dell'informazione quantistica, che racchiude la prima come limite classico: la teoria ha visto uno sviluppo rapidissimoe sorprendente nonostante la sua completa applicazione tecnologica sia ancora lontana (un decennio, secondo recenti parole di Deutsch). Ciononostante, le prospettive sono molto promettenti. Senza considerare il grande approfondimento della conoscenza e delle connessioni tra fisica, informazione, matematica e filosofia che la disciplina sta permettendo.

[Joel Lehikoinen · Rating 3 out of 5]

I found the Finnish version of this book in the bookswap phone booth at the office. I think the author said in the introduction that he wanted to make a book that is as easily approachable as Hawking's A Brief History of Time. I really doubt the book can give much to someone who doesn't have prior knowledge of quantum mechanics and computing. I didn't mind it at all though, it was nice to have something a bit more challenging (but still not a text book).

The book covers the basics of quantum computing, its connection to entropy and (possible) multiverse. It also discusses practical implementations of quantum computing, algorithms and impact on e.g. privacy. Despite its age, judging from the other reviews here, it doesn't seem to be so obsolete. All in all, it was an ok enough science book. Like many other science books meant for public, it does suffer from the occasional pointless analog, but all in all, it's an interesting read. I didn't know pretty much anything about information theory before reading this book, so I found the connection between quantum computing and information theory very interesting.

[David Kemp · Rating 4 out of 5]

Great introduction to the relatively new field of "quantum computing" and the quantum weirdness that it relies on. Brown is excellent at explaining the key concepts. From what I have been reading, this book is not too out of date yet despite some of the recent advances in actually physically building these things!

[Brannon · Rating 3 out of 5]

This book discusses the importance of reversability in computer history and how that plays into the idea of quantum computers. It's more enlightening than interesting, if that makes sense.

[Brittany]

Shelving for a while (probably forever). While interesting, it is too physics-oriented for me to grasp the contents. I think a brief skimming won't do it justice.

[Joonas · Rating 2 out of 5]

I read just half. it was too heavy for now.