## 4 Quantum Integrated Photonic Devices
### Fabrication and Characterization of a Quantum Project  
We present the fabrication of the basic units \(X_{\pi/4}\) and \(X_{\pi/2}\), that is, 2 \(\times\) 2 directional couplers in glass, by using two consecutive purely thermal \(\mathrm{K}^{+}/\mathrm{Na}^{+}\) ion-exchange processes in glass. The first process is a selective ion exchange, that is, it is produced only in non-masked regions that corresponds to the channel waveguides defining the integrated optical component, while the second one is only a burial process through the same mask [194]. We must underline that this two-step process is different from the one proposed in Reference [199] where two consecutive \(\mathrm{K}^{+}/\mathrm{Na}^{+}\) ion exchanges are made, but the second is a planar ion exchange. Rigorously, the second step (burial process) would not be necessary but presents two important advantages. On the one hand, the modal field is lo  
Figure 14: Integrated Bell projector for entangled quantum states excited in four cores _ao_\(j\), \(j=1,2,3,4\) of two-core optical fibers \(F_{a}\) and \(F_{b}\). Detectors \(D_{aj}\) are used for measuring photon coincidences. cated further from the glass surface, which can present irregularities, and therefore surface losses are reduced, and, on the other hand, it decreases the typical anisotropy in K\({}^{+}\)/Na\({}^{+}\) waveguides produced by mechanical stress due to the size difference of the two ions, one of the main drawback of these integrated guides. Of course there are many other strategies for fabricating directional couplers by K\({}^{+}\)/Na\({}^{+}\) ion exchange (see, for example [16; 200; 201]), which could also be suitable for this kind of applications. Another approach would be to use ion-exchanged MMI couplers based on multimode interference [223; 224], which would have less stringent lithographic requirements. There are also proposals of reconfigurable integrated elements as directional couplers by thermo-optic modulation [225], a strategy that can be oriented to reconfigure quantum operations. In another work, an interesting design of asymmetric Y-branch waveguides for bidirectional transmission on PCB (printed circuit board) was described [226], which would allow us to join IQP and microelectronics. The fabrication strategy presented here has given excellent experimental results for \(X_{\pi/4}\) and \(X_{\pi/2}\) directional couplers with both low losses and anisotropic effects (coupling between TE and TM guided modes, one of the drawbacks of K\({}^{+}\)/Na\({}^{+}\) ion exchange) thanks to the burial process. The starting point has been to use previous results [54; 201; 207; 227], which have allowed us to establish a fixed temperature \(T=400\)\({}^{\circ}\)C for both the first and the second (burial) ion exchange in a soda-lime glass with index \(n_{s}=1.5104\) at \(\lambda_{o}=633\) nm. Several tests of planar thermal ion exchange in the same type of glass substrate were made, and the effective indices of the different guided modes were measured by prism coupling method in an automated system (Metricon, Model 2010/M). From this empiric calibration and taking into account numerical results by using the EIM for channel guides, a diffusion time for the first exchange equal to \(t_{1}=30\) min and a time \(t_{2}=10\) min for the burial process were fixed in order to achieve single-mode channel waveguides. In order to design and fabricate integrated quantum optical projectors, a prior optical characterization of the full fabrication process of the basic elements (the directional couplers as \(X_{\pi/4}\) and \(X_{\pi/2}\)) was needed. A series of masks on the same substrate for different directional couplers have been made by changing the separation \(s\) between channel guides and the coupling length \(l\) of the couplers (a sketch of the directional coupler is shown in Figure 15), while keeping the width of the channel guides constant (\(w=3\) um). The connection between channel guides have been made through curves following the profile used by [PERSON] et al. (MC curve) [62].