Mason Inman - science journalist

15 October 2005, for New Scientist

Mix some static over a DJ's tunes and you will surely kill the party.

Do the same to the vibrations of a nanoscale silicon beam, and it will transform the way it responds to feeble signals.

The device, which exploits a phenomenon called stochastic resonance, could act as the basis of a sensitive probe for tiny forces, or as a mechanical alternative to conventional computer memory.

Stochastic resonance is the name given to the enhanced sensitivity of a device to a weak signal that occurs when random noise is added to the mix. The effect shows up throughout nature. Paddlefish use it to detect weak electric fields given off by their plankton prey, and it is at work in vibrating shoes that help people who are unsteady on their feet to better sense the ground and so avoid dangerous falls (New Scientist, 2 November 2002, p 22). Now, for the first time, physicists Pritiraj Mohanty and Robert Badzey of Boston University have put the effect to work in a nanomachine.

To make their nanoscale beam, they etched an 8-micrometre notch most of the way across a thin silicon wafer, leaving a sliver of material a few hundred nanometres wide bridging the gap. They set this beam of silicon vibrating by applying an alternating current to it. At low current, the vibration was steady. But by increasing the current they were able to make the vibration bistable: its amplitude jumped back and forth between two distinct levels.

When the researchers then mixed in noise - either by superimposing a small random electric current on the alternating current, or in the form of heat - stochastic resonance kicked in, causing the beam to shimmy between the two vibrational modes even when the current driving the vibrations was a quarter of what was normally needed (Nature, vol 437, p 995).

By turning the noise up and down, the researchers were able to switch the beam between two discrete states, in which the beam spent different proportions of time in each vibrational mode. They were also able to completely switch the vibration to one mode or the other. Using these two states to encode a digital 1 or 0 would allow such devices to be used for computer memory, and they could be packed more tightly and operate more efficiently than conventional computer memory, Mohanty says. The team's findings suggest that the device could also act as a sensitive probe to measure tiny weights or distances.

"It's very impressive they were able to show stochastic resonance in such a small system," says theoretical physicist Milena Grifoni of the University of Regensburg in Germany. It is extremely difficult to control and make measurements in such small devices, but in this experiment, she says, "they had 100 per cent control".