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July 21, 2003
Volume 81, Number 29
CENEAR 81 29 p. 6
ISSN 0009-2347


Nanoscale sensor couples transistor with crystal to approach quantum limit


A new sensor can measure femtometer-sized movements of a single-crystal oscillator, increasing sensitivity about 10-fold over previous nanoscale devices [Nature, 424, 291 (2003)].


FEMTOSENSOR Oscillations of the GaAs bar (front) affect the current through the single-electron transistor (middle). NATURE © 2003

The apparatus, developed by scientists at the University of California, Santa Barbara, couples a single-electron transistor to a vibrating, 250-nm-wide beam of GaAs. As the beam oscillates back and forth, it causes measurable changes in the current of the transistor. The current variations can then be related back to beam displacement. For a beam oscillating at 116 MHz, postdoctoral researcher Robert G. Knobel and physics professor Andrew N. Cleland report measuring displacements of 2.3 X 10–14 meters at 30 millikelvins.

Femtometer sensitivity puts the researchers within striking distance of observing the effects of the Heisenberg uncertainty principle on a macroscopic object. For a crystal beam about 2,000 atoms across, "we have sensitivity of motion on the order of the size of an atomic nucleus," Knobel says. Another two orders of magnitude more sensitive and the researchers should be able to observe quantum "zero-point" fluctuations of the beam--motions arising directly from the uncertainty in position and velocity.

The device may also have practical applications in sensor technology, especially in areas such as atomic force microscopy, where small, local intra-atomic forces are measured. "If you could do it in three dimensions with chemical selectivity, that would be a powerful tool," says Dan Rugar, manager of nanoscale studies at IBM's Almaden Research Center.

Both Rugar and Knobel note that such applications will take a while to develop. In the meantime, the race is on to eliminate those last few orders of magnitude between the macroscopic and quantum worlds.


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Copyright © 2003 American Chemical Society

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University of California, Santa Barbara

Robert G. Knobel

Andrew N. Cleland

IBM's Almaden Research Center

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