Physics Colloquium
April 27, 2006 3:40 pm, Engineering Bldg, Unit 2, Room 138
There will be tea at 3:20 pm in the Barkas Lounge

Can One Weigh a Single Proton?
Professor Andrew Cleland
UC Santa Barbara

Precision mass spectroscopy relies on mass sensors that can distinguish molecules that differ by a minimum of one proton, yet have large enough dynamic range to weigh molecules with hundreds or thousands of carbon atoms. Nanoscale mechanical resonators provide an interesting possible method for achieving these goals, but are limited by the noise intrinsic to their mechanical properties, which at present limits performance to hundreds of proton masses. One possibility of achieving better performance is to use the intrinsic nonlinear properties of these devices. Noise limits the detection limits of linear mechanical sensors, in a manner that is well understood, but also limits nonlinear systems in a less trivial way. Here I describe noise effects on nonlinear resonators operated in a regime where they have one or two stable attractors. We have made quantitative measurements of the nonlinear response of a radiofrequency mechanical resonator with very high quality factor, measuring the noise-induced transitions between the two basins of attraction that appear in the nonlinear regime, and find good agreement with theory. We measure the transition rate response to controlled levels with theory. We measure the transition rate response to controlled levels of white noise, and extract the basin activation energy. This allows us to obtain precise values for the relevant frequencies and the cubic nonlinearity in the Duffing oscillator. The precision extraction of the resonator properties provides one potential means of achieving single proton resolution, with very large dynamic range.