Opposites Attract: Quantum Computing's Strange WorldOpposites Attract: Quantum Computing's Strange World

Quantum computing exists in a looking-glass world in which things are both up and down, black and white, clockwise and counterclockwise--at the same time. Instead of storing binary bits of information with electrical currents that represent either 0 or 1, quantum computers use the spin of particles or nuclei in atoms or charged ions, the polarization of photons of light, or other methods to represent 0 and 1.

These quantum bits, or qubits, possess the strange properties of the subatomic realm, where electrons and photons appear to occupy more than one place at once and exist in indeterminate states. Quantum computing is difficult in part because those states exist at speed-of-light velocities, within unimaginably short distances, and at extremely low energy levels. Experiments at MIT and the National Institute of Standards and Technology are conducted at fractions of a degree above absolute zero, at which all molecular activity stops.

Qubits used in the experiments can spin clockwise and counterclockwise simultaneously, embodying both 0 and 1, in a phenomenon called superposition. Measuring the system causes the superposition to collapse, yielding answers to computations. Two qubits physically separate in space can be entangled, so the fate of one affects the other--even over great distances. Scientists can coax those quantum bits into performing simple computations using magnetic fields or laser pulses. Each atom is like a tiny switch capable of performing two calculations at once.

That means exponentially higher performance than an electronic computer: Two atoms can perform four computations at the same time, three atoms eight. A quantum computer of 10 qubits could perform 1,024 simultaneous calculations. Twenty qubits would be able to execute a million simultaneous computations; 40, 10 trillion. Mathematicians have proven that a quantum computer with thousands of atoms could find quickly the factors of numbers hundreds of digits long, a feat that would take conventional supercomputers billions of years.

Quantum computers wouldn't perform all tasks better, but for problems where algorithms can be designed--factoring and database searching, for example--the promise is mind-boggling.

Illustration by Steve Keller

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