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.