A Shortcut Through Time: The Path To the Quantum Computer by George Johnson. Knopf, $24, 204 pages.
The Quest for the Quantum Computer by Julian Brown. Simon & Schuster, $16, 396 pages.

The Chapel Hill News

February 15, 2004

Is Your Credit Card Safe?

By Phillip Manning

When you buy a bauble over the Internet, you send your credit card number out into the wild blue yonder. Fortunately, most vendors encrypt that number in a way that even the best hacker cannot dig it out. The key to the encryption is hidden in a number. To get the key, you must find the factors of this number. (Factors are numbers other than 1 that divide evenly into a larger number. Thus, the number 15 has two factors, 3 and 5.) However, the number that contains the encryption key can be hundreds of digits long, and the world’s fastest supercomputer could not factor it in a million years. Governments use similar methods to encode their secrets. So, in the early 1980s, when mathematicians and physicists started playing around with an idea for a quantum computer that could quickly factor such huge numbers, people started paying attention.

Quantum computers are built around the weirdness of quantum physics, which is altogether different from the classical physics used to construct conventional digital computers. Conventional computers are circuits of ON and OFF switches, each of which represents one bit of data. An ON switch means 1, an OFF means 0. Put two bits together, say a 1 and a 0, and you get the binary number 10, which is 2 in everyday numbers. Put more bits together, and you can produce a sequence of ONs and OFFs that represent any number and any letter. Combine the 40 million ON-OFF switches in a Pentium chip with sophisticated software, and you get all the functions of a modern digital computer.

At the simplest level, a quantum computer uses a property of atoms or subatomic particles called spin, which can be measured as either UP or DOWN. Like ON and OFF switches, spin can be used to represent binary numbers. An UP spin could be decreed 1 and a DOWN spin 0. But atoms and electrons have a mysterious property that classical computer switches don’t have: they can be in both states at the same time.

This property of small particles arises from Heisenberg’s uncertainty principle and is called quantum indeterminacy. There is no point in trying to picture this surreal state in your head or racking your brain for the perfect analogy. We humans evolved in a world of large objects where quantum effects are negligible, and our minds were built to comprehend that environment. As Nobel laureate Richard Feynman said, “Things on a very small scale behave like nothing that you have any direct experience about.” But quantum indeterminacy is real; it has been proven time and again in the lab.

Two recent books explore the ideas behind quantum computing and assess its potential. Science writer George Johnson’s book, “A Shortcut Through Time: The Path to the Quantum Computer (Knopf, $24), is the more accessible of the two, but Julian Brown’s “The Quest for the Quantum Computer” (Simon & Schuster, $16) has more technical depth. The key concept is developed in both books. A row of three classical ON-OFF switches requires eight sets of these switches to store all of the possible patterns (0,0,0; 0,0,1; 0,1,0; etc.). But only three quantum switches — each of which can, in effect, express a 0 and a 1 at the same time — could represent all eight patterns. This property enables quantum computers to perform some types of calculations — such as factoring large numbers — very rapidly. As Johnson says, “Quantum computing would be to ordinary computing what nuclear energy is to fire.”

However, don’t forgo your Internet buying habit yet. Quantum computing is in its infancy, and your credit card number is still safe. The most quantum bits ever put together in a system is seven, and the largest number ever factored is 15. Furthermore, rooms full of elaborate equipment are required for even the simplest calculation. But don’t count quantum computing out.

My first programming experience was on a cranky, tennis court-sized computer packed with unreliable vacuum tubes attended by somber operators in white lab coats. Data was entered on punched cards and batch processed. The idea that a computer with far more power would one day fit in a briefcase was not even a pipe dream. And the idea that great gobs of data would one day be transferred between computers was inconceivable. Even though quantum computers are still in the early vacuum-tube stage of development, some very smart men and women are working hard to make it a practical reality. As Johnson says, “Everything in the universe is made from quantum particles . . . . There are so many possibilities yet to try.”
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