The application of integrated photonic technologies to quantum optics has recently enabled a wealth of
breakthrough experiments in several quantum information areas. In particular, femtosecond laser written
optical circuits revealed to be the ideal tool for investigating the features of polarization encoded qubits.
However, the difficulty of integrating half and quarter wave plates in such circuits avoids the possibility to
perform arbitrary rotations of the polarization state of photons on chip.
Femtosecond laser written waveguides intrinsically exhibit a certain degree of birefringence and thus they
could be exploited as integrated waveplates. In practice, the direction of the birefringence axes of the
waveguides is the same of the propagation direction of the writing femtosecond laser beam, namely
perpendicular to the substrate surface. Its fine rotation in a controlled fashion, preserving the accuracy of the
positioning of the laser focal spot required by the fabrication process, is extremely challenging. In order to
achieve this goal, we combine a high NA (1.4) focusing objective partially filled with a reduced diameter
writing beam. In this way, the translation of the beam with respect to the objective center produces a rotation
of the focusing direction, without altering the focal spot position. With this method we are able to tilt the
birefringence axes of the waveguides up to 45°, and thus to use them as integrated light polarization rotators.
In order to demonstrate the effectiveness of these components, we developed a fully integrated device capable
to perform the quantum tomography of an arbitrary two-photon polarization state.