High-speed feedback trap based on optical tweezers

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My group and I propose to build a high-speed, three-dimensional feedback trap based on optical tweezers, to study fundamental problems in statistical physics and biophysics. Over the past ten years, we have developed a way to use feedback traps to quantitatively study motion of particles trapped in two-dimensional "virtual potentials" that may be arbitrarily manipulated. We have used feedback traps to carry out fundamental tests of nonequilibrium statistical physics, focusing on questions connecting thermodynamics with information theory. Two highlights include a test of Landauer's principle, which states that the erasure of one bit of information inevitably requires work of at least kT ln 2 per bit and the first measurement of the functional form of nonequilibrium system entropy.******Using an instrument developed with funding from a 2014 RTI award, we showed that optical tweezers can provide the force needed to make a feedback trap. The latest version of our trap has demonstrated a speed-up of 1000 times relative to our original design and a decrease in spatial scale by a factor of 100. We have also demonstrated a two-state colloidal system based on a virtual double-well potential that had nearly 1 kHz transition rates at a spatial scale of 10 nm. Finally, for simple trapping, feedback can increase the effective stiffness of the trap by a factor of 40, with no increase in beam intensity. This instrumentation work has just appeared in Applied Physics Letters, where it was highlighted on the Scilights website of the American Institute of Physics.******To date, active feedback manipulation has been confined to lateral (xy) motions of particles in the trap. Trapping in the axial (z) direction was limited to the passive trap provided by feedback tweezers. Unfortunately, the beam geometry (and radiation pressure) leads to axial stiffnesses that are significantly lower than the lateral ones. In the proposed setup, we will improve sensor and actuator design to add active control of vertical motions, as well. Further improvements will increase the overall feedback bandwidth by 10 to 1 MHz.******Adding three-dimensional control and further speeding up the trap bandwidth will lead to qualitatively new types of applications. A first goal is to extend stochastic thermodynamics to trajectory-based quantities, a level of detail that is unexplored in other experiments for lack of statistics. A second goal is the manipulation of unstable objects such as air bubbles in a feedback trap. A third goal is to re-optimize the parameters of optical tweezers to take full advantage of the possibilities offered by feedback control, a feat that has not been possible so far, as feedback control needs to be for all three axes. The instrument that we propose to build will thus make possible new fundamental tests of statistical physics and also new measurement techniques in biophysics.*****