Brewing a quantum computer in a coffee cup.

In Douglas Adams' The Hitchhiker's Guide to the Galaxy, a hungover physicist creates the "infinite improbability drive," which uses unlikely events to propel starships across the cosmos at impossible speeds. He accomplishes this marvel by linking a quantum computer to a "fresh cup of really hot tea." Now, in a case of life imitating art, scientists are proposing to build a quantum computer inside a mug of hot coffee.

"Quantum computing could solve problems faster than ever thought possible," says Isaac L. Chuang of the University of California, Santa Barbara. He explains that, instead of using semiconductor elements as logic switches, these computers of the future would employ single atoms.

Quantum computers will achieve their unparalleled speed by taking advantage of the bizarre behavior of elementary particles. According to the theory of quantum mechanics, the properties of any atom, such as spin, location, or momentum, exist in all possible states at once-as long as they remain unmeasured (SN: 5/14/94, p. 308). In a conventional computer, a bit stores information by assuming one of two possible states. Quantum bits, or qubits, store more information by using all the potential states of an atom. Qubits would allow a quantum computer to perform myriad steps at once. In contrast, today's computers calculate only one step at a time. Past attempts at creating qubits have run into a fundamental problem. "It requires heroic efforts to control a single atom," says Chuang. As an easier alternative, he and Neil A. Gershenfeld of the Massachusetts Institute of Technology propose using nuclear magnetic resonance devices to corral millions of atoms within an evenly heated volume of material. By coordinating the nuclear spin of many particles, the physicists could make these blocks of atoms act collectively as qubits. A liquid with the right thermal properties could hold up to 10 qubits, Chuang and Gershenfeld argue in the Jan. 17 Science.

"We'd probably use a cup of coffee as the medium," quips Gershenfeld, noting the unusually even heating characteristic of java.

"The big challenge is isolating your quantum computer from the environment," notes Gilles Brassard of the University of Montreal. In the new method, the sheer number of atoms in each qubit buffers the computer against unwanted external interactions that disrupt the calculations.

Within a few years, Chuang expects to build such a device and use it to factor numbers, one of the few programs that scientists have already adapted for quantum computers. The hypothetical 10-qubit design, however, would only handle numbers up to 15. To surpass this unimpressive milestone, the scientists will have to cool and control their liquid to eliminate disturbances in the calculations. In theory, by smoothing the temperature variations, they can create a computer with more qubits.

David P. DiVincenzo of the IBM Research Division in Yorktown Heights, N.Y., calls the new approach valuable but adds that "it is not a cure-all." Significant technical problems accompany cooling a material evenly enough to control more qubits. A true quantum computer would need at least 40 qubits, in DiVincenzo's view. "There's lots of physics between here and a working quantum computer," he says.

If the liquid computer design succeeds, Chuang believes that engineers may eventually cook up a useful quantum device out of "a really expensive cup of structured coffee."