Evidence for Quantized Displacement

Evidence for quantized displacement in nanomechanical oscillators. Physicists at Boston University have performed an experiment in which tiny silicon paddles, sprouting from a central stick of silicon like the vanes from a heat sink, seem to oscillate together in a peculiar manner: the paddles can travel out to certain displacements but not to others. The setup for this experiment consists of a lithographically prepared structure looking like a double-sided comb (see picture at http://nano.bu.edu/antenna-large.jpg ).

Next, a gold-film electrode is deposited on top of the spine. Then a current is sent through the film and an external magnetic field is applied. This sets the structure to vibrating at frequencies as high as one gigahertz. This makes the structure the fastest man-made oscillator. (Atoms and molecules can vibrate faster than this, but not any chunk of matter, until now.) At relatively warm temperatures, this rig, small as it is, behaves according to the dictates of classical physics. The larger the driving force (set up by the magnetic field and the current moving through the gold electrode) the greater the excursion of the paddles. This is no more than Hooke’s law.

At millikelvin temperatures, however, quantum mechanics takes over from classical mechanics. In principle, the energies of the oscillating paddles are quantized, and this in turn should show up as a propensity of the paddles (500 nm long and 200 nm wide) to displace only by discrete amounts. The Boston University experiment sees signs of exactly this sort of behavior. (Gaidarzhy et al., Physical Review Letters, 28 January 2005; contact Pritiraj Mohanty, 617-353-9297, mohanty@buphy.bu.edu; lab website, http://nano.bu.edu/quantum-motion.html )