# Quantum Incoherence

Achieving and sustaining “coherence” is one of the toughest problems in quantum computing. What makes quantum computers powerful is that they store information not in the form of classical bits-1 or 0-but as “qubits.” Thanks to the uncertainty principle, qubits act as if they possess an infinite range of values between 0 and 1, enabling a system with only a few qubits to carry out huge calculations in a single stroke. But this state of “superposition” exists only if the physical system representing the qubits is “coherent,” that is, isolated from the outside environment. The slightest interference causes superposition to collapse.

Apparently, coherence is also a challenge for books about quantum computing. When I reviewed Gerard Milburn’s The Feynman Processor, I found it a titillating but insufficient introduction to the topic. Sad to say, Julian Brown’s Minds, Machines, and the Multiverse errs on the opposite extreme, providing more history, circuit diagrams, mathematics and philosophical speculation han I was able to keep straight.

Souls with more patience than I have will profit from Brown’s explanations of different ways to encode quantum states, the new logic gates and error-correction methods required to carry out useful quantum computations and the like, all of which seem thorough. (Though my confidence in the author decreased by a qubit when he identified Creon Levit, a researcher at NASA Ames Research Center, as “Leon Crevitt,” and when he placed Ames itself in Palo Alto; it’s at Moffett Field.) In an excellent chapter on encryption, Brown gives the clearest account I’ve seen of public-key cryptography, how unexpected advances in factoring large

numbers jeopardize the security of even The longest (128-bit) encryption keys now in common use, and why quantum computers could blow factorizationbased security algorithms to smithereens.

But Brown’s true interests lie in more ethereal questions. If the behaviors of particles in coherent quantum states can be hijacked to carry out certain kinds of computations, is it possible that physics itself is computational-or, to put it another way, that the universe is a vast computer, calculating the arc of every fly ball literally “on the fly”? If so, who programmed the universe-God? What are we to make of the many-universes hypothesis favored by quantum-computing pioneer David Deutsch, in which a qubit’s intermediate states are interpreted as discrete realities, each inhabiting a different universe? All of this, while intriguing, is hard to relate back to the very real question of whether silicon-chip manufacturers can keep Moore’s Law going for another decade. I’m still looking for a take on quantum computing that doesn’t decohere.