In the context of Quantum Information, photons are a very promising resource for experimental realizations of quantum communication and quantum computation protocols. Many quantum information experiments have been performed with bulk optics, however, complex quantum optical schemes, realized in bulk optics, suffer from severe drawbacks, as far as stability, precision, and physical size are concerned. The present approach to beat these limitations is to adopt miniaturized optical waveguide devices. All the experiments performed so far with integrated quantum circuits are based only on path-encoded qubits with a given polarization state of the photons. On the other hand, many QI processes and sources of entangled photon states are based on the polarization degree of freedom.
Our approach is to take advantage of the ultrafast laser writing (ULW) technique in order to obtain quantum devices able to support and manipulate polarization encoded qubits. For the realization of ULW circuits a femtosecond laser is foused on a glass substrate and light-guiding structures are produced by translating the substrate with respect to the laser beam: the waveguides are directly written into the glass. A unique feature of this technique is the ability to realize three-dimensional geometries by varying the depth of the beam focus inside the substrate.
Integrated Quantum Photonics
One of the main tasks of this research is to realize optical devices for quantum information. Indeed polarization insensisitive and polarization dependent circuits need to be developed: thanks to the very low glass birefringence such kind of devices can be obtained by simply varying the waveguide geometry. Exploiting this particular feature of the ULW technique integrated beam splitters [1] ad integrated partial polarizing beam splitters and quantum logical gates, like the Controlled-NOT gate, for polarization encoded qubits [2] have been realized.
In addition, integrated optics can been adopted for quantum simulation: a discrete-time quantum walk, which represent one of the most promising resources for the simulation of physical quantum systems, has been realized and the polarization entanglement has been exploited to simulate the bunching-antibunching feature of non interacting bosons and fermions [3].