Single-particle tracking experiments have provided the opportunity to monitor dynamical processes on length scales from millimeter to subnanometer. Especially in biology, where relevant processes on the molecular level demand high spatial and temporal resolution, numerous optical methods have emerged. In this thesis, a new method is introduced, the differential interferometric particle tracking. It combines a relatively simple experimental setup with high spatial (0.9 nm) and temporal (50 µs) resolution and inherent elimination of microscope stage drift. Differential interferometric particle tracking is based on interference between the scattered light of a probe particle and a fixed reference particle. An upright microscope has been equipped with laser dark field illumination, and a compact interferometer has been mounted on top of the camera exit of the microscope. Two avalanche photodiodes suffice to detect the interference signal, which provides information about one degree of freedom of the particle movement. Furthermore, this detection scheme is insensitive to laser power fluctuations and pointing fluctuations of the illuminating beam. To demonstrate the performance of this method it has been applied to the protein complex kinesin microtubule. The kinesin is labelled with a polystyrene bead with diameter 0.5 µm. Steps with a size of 8 nm could be clearly resolved without the use of an optical tweezer.
