  The field of optical metrology with its high precision position, rotation and
wavefront sensors represents the basis for lithography and high resolution
microscopy. However, the on-chip integration - a task highly relevant for
future nanotechnological devices - necessitates the reduction of the spatial
footprint of sensing schemes by the deployment of novel concepts. A promising
route towards this goal is predicated on the controllable directional emission
of the fundamentally smallest emitters of light, i.e. dipoles, as an indicator.
Here we realize an integrated displacement sensor based on the directional
emission of Huygens dipoles excited in an individual dipolar antenna. The
position of the antenna relative to the excitation field determines its
directional coupling into a six-way crossing of photonic crystal waveguides. In
our experimental study supported by theoretical calculations, we demonstrate
the first prototype of an integrated displacement sensor with a standard
deviation of the position accuracy below $\lambda$/300 at room temperature and
ambient conditions.
