  Super-resolutive 3D tracking, such as PSF engineering or evanescent field
imaging has long been used to track microparticles and to enhance the
throughput of single molecules force spectroscopic measurements. However,
current methods present two drawbacks. First, they lack precision compared with
optical tweezers or AFM. Second, the dependence of their signal upon the
position is complex creating the need for a time-consuming calibration step.
  Here, we introduce a new optical technique that circumvents both issues and
allows for a simple, versatile and efficient 3D tracking of diluted particles
while offering a sub-nanometer frame-to-frame precision in all three spatial
directions. The principle is to combine stereoscopy and interferometry, such
that the z (axial) position is measured through the distance between two
interferometric fringe patterns. The linearity of this stereoscopy technique
alleviates the need for lookup tables while the structured interferometric
pattern enhances precision. On the other hand, the extended spatial footprint
of this PSF maximizes the number of photons detected per frame without the need
of fancy cameras, nor the need for complex hardware. Hence, thanks to its
simplicity and versatility, we believe that SDI (Stereo Darkfield
Interferometry) technology has the potential to significantly enhance the
spreading of 3D tracking.
  We demonstrate the efficiency of this technique on various single-molecule
measurements thanks to magnetic tweezers. In particular we demonstrate the
precise quantification of two-state dynamics involving axial steps as short as
1 nm. We then show that SDI can be directly embedded in a commercial objective
providing a means to track multiple single cells in 3D .
