Thermally activated state transition technique for femto-Newton-level force measurement.

We develop and test a thermally activated state transition technique for ultraweak force measurement. As a force sensor, the technique was demonstrated on a classical Brownian bead immersed in water and restrained by a bistable optical trap. A femto-Newton-level flow force imposed on this sensor was measured by monitoring changes in the transition rates of the bead hopping between two energy states. The treatment of thermal disturbances as a requirement instead of a limiting factor is the major feature of the technique, and provides a new strategy by which to measure other ultraweak forces beyond the thermal noise limit.