Patent Number: 
Section: claims

1. Method for reducing interference and crosstalk in a double optical tweezers apparatus for measuring forces applied to beads, comprising a single laser source, the method comprising the sequential steps of:a. splitting the laser beam by polarization into a first laser beam and a second laser beam,a1. shifting the frequency of the first laser beam to a value different from the frequency of the second laser beam by a frequency shift,b. passing the first and second laser beams through a trapping objective and then through a condenser objective,c. reflecting one of the first and second laser beams so as to select a beam to be imaged, andd. imaging using back focal plane interferometry the selected laser beam on a position sensitive detector having a bandwidth smaller than the frequency shift. 2. Method according to claim 1, wherein, before step a, the laser beam is expanded, and, before step b, the first and second laser beams are steered. 3. Double optical tweezers apparatus for measuring forces applied to beads, comprising:a single laser source;a laser beam splitter for sequentially splitting a laser beam from the single laser beam source into a first laser beam and a second laser beam, wherein the laser beam splitter comprises an optic frequency shifter that shifts the first laser beam to a value different from the frequency of the second laser beam by a frequency shift;a trapping means for passing the first and second laser beams through a trapping objective and then through a condenser objective;a polarizer configured to select one of the first and second laser beams; anda position sensitive detector configured to image using back focal plane interferometry the selected laser beam, wherein the position sensitive detector has a bandwidth smaller than the frequency shift. 4. Apparatus according to claim 3, wherein, the splitter further comprises a piezoelectric tilt mirror, the optic frequency shifter is positioned before the piezoelectric mirror. 5. Apparatus according to claim 3 or 4, wherein the apparatus further comprises a beam expander and a beam steering.