Cooled device unveils a quantum limit

Crafting tiny circuits and machines from fewer and fewer atoms, researchers are pushing technology deep into the realm of quantum mechanics. More than a decade ago, physicists discovered unexpected quantum behavior in electronic devices. Now, California experimenters have observed a quantum effect in mechanical characteristics of tiny structures as well.

“It’s the first experiment in which people clearly see quantum mechanics in mechanical structures,” comments Leo P. Kouwenhoven of Delft University of Technology in the Netherlands. Back in 1988, he and his coworkers made the surprising discovery that electrical resistance in certain devices would appear only in multiples of a basic amount, or quantum.

The new experiment by Keith Schwab and his colleagues at the California Institute of Technology in Pasadena demonstrates a similar type of quantum for thermal conductance, or the ease with which heat can flow.

Reported in the April 27 Nature, the results give a first peek into mechanical quantum effects that arise because wavelike particles called phonons—collective, mechanical vibrations of atoms—transmit heat, scientists say. The new findings also impose a fundamental limit on the transmission of information, says team leader Michael L. Roukes of Caltech.

The group suspended a square plate of silicon nitride some 200 atoms thick above a hole in a silicon chip. Four 200-nanometer-wide wires of the same glassy nitride held the plate. The team cooled the device to nearly absolute zero.

Theorists had predicted that at 100 millikelvins only four of countless possible vibrations in each wire would persist. One is a compression, another a twist, and the final two are types of flexing waves. Each of these vibrations would provide one quantum of thermal conductance per wire, according to the theory.

Using thin, gold resistors deposited on the silicon nitride plate to both delicately warm the plate and measure its temperature, the experimenters found that the four wires’ conductance was exactly 16 times the predicted quantum, as expected. The results confirm the quantum’s existence and calculated value, says Schwab, now of the National Security Agency in College Park, Md.

Refining its techniques may soon enable the team to detect lone phonons, Roukes predicts. “We’re on the road to watching heat flow phonon by phonon,” he says. Quantum computing (SN: 11/20/99, p. 334: may also benefit from the methods, Schwab adds.

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