Silicon is Slow

Quantum Leaps

Quantum computing combines the nanoscale world of molecular circuits with the speed of DNA's parallel processing-then adds a weirdness all its own.


In a quantum computer, the nuclei of atoms function as so-called qubits-the 1's and 0's of binary code. When the spin of the nucleus points "up," it's a 0; when it points down, it's a 1. But in quantum computing there's yet a third possibility: A nucleus can be in a special kind of quantum state that enables it to occupy both positions at once. This phenomenon is called a superposition, and it underlies the enormous potential power of the quantum computer. For if the nucleus can represent 0, 1, or both simultaneously, one qubit can do the work of two ordinary bits; 2 can do the work of 4; 4 can do the work of 16, and so on. Keep ascending the exponential scale and soon a relatively small quantum computer (say, 40 qubits) attains the capacity of a supercomputer. Indeed, conventional computers have difficulty modeling the quantum behavior of even a small number of atoms precisely, because nuclei are so slippery; a quantum computer might be much better at studying quantum behavior.


Here's the problem: According to the peculiar rules of quantum mechanics, once you observe the state of a nucleus it ceases to be in a superposition and freezes into either a 0 or a 1. The pressure cooker-like machine Chuang worked on at IBM is designed to keep the atoms in a superposition long enough to perform a calculation. "The trick is to create a molecule that can stay in a quantum state for an incredibly long time-in this case, 1.5 seconds," he says. "That's eons for something quantum. In life you don't normally get to see one thing sitting in two places at the same time."


Chuang has helped design four quantum computers, each more sophisticated than its predecessor. Last fall, his 7-qubit machine was the first to implement an important algorithm for factoring prime numbers. Factoring is critical to encryption; Fred Chong, an associate professor of computer science at UC Davis, estimates that today's fastest computer would require billions of years to factor a 300-digit encryption key as it laboriously tried one possibility after another; a quantum computer could crack the code in about 30 hours.