## 4 Quantum Integrated Photonic Devices  
As indicated in the Introduction section, different platforms have been proposed for implementing IQP devices. The main platforms are based on III-IV semiconductor, such as gallium arsenide and indium phosphide, lithium niobate and those ones based on silica such as silica-on-silicon, silicon-on-insulator and silicon nitride, as reviewed, for instance, in References [44, 45, 46]. IExG platform is not usually included as a possible quantum integrated photonics platform, despite the fact that it is present in many classical optical communications systems and optical sensors. We must indicate that the closest platform to the IExG one is obviously, silica-on-silicon; however, in our opinion, IExG presents a lower technological complexity and retains all the classical advantages of this platform, that is, low-cost, low propagation losses and high compatibility with optical fibers, together with a slightly better modal transverse coupling due to the graded-index profile unlike the strictly step profile of silica-on-silicon waveguides. We must stress that the main limitation, shared with silica-on-silicon, is the low contrast index (therefore low curvature radii and consequently limited device densities). The main consequence is that IExG platform becomes remarkably efficient when quantum applications require only a few qubits. This requirement is fulfilled in most devices for optical quantum communications, optical quantum sensors and even optical quantum simulators. On the other hand, IExG is a passive platform, that is, it has not electro-optical properties and presents a low nonlinearity, although efforts based on glass poling [188] and/or hybrid integration [189] have been made to achieve significant nonlinear effects. Therefore, in a monolithic version, IExG platform would perform only passive functions, but, as shown in Section 3, doping with rare-earth ions adds active functionality [60] and opens new possibilities for integrated quantum photonics. We must also underline that thermo-optic reconfiguration would also be possible in suitable glasses, so providing an additional capability. Accordingly, while applications to quantum computing, as for example the implementation of complex quantum processors for specific or universal purpose, are reserved for other platforms (as lithium niobate, silicon-on-insulator,...), applications requiring a less amount of qubits, as quantum cryptography, quantum optical sensing and even quantum optical simulating, could be implemented by using the IExG platform. A few examples have already appeared in the scientific literature, and the primary aim of this section is to justify such possible applications. Indeed, recently, theoretical and experimental studies of the IExG platform for implementing different integrated quantum devices has begun to be reported. Thus, numerical modeling of ion exchange in glass for quantum computations tasks has been discussed [190, 191]; nanoemitters (nanodiamond) coupled to ion-exchanged glass waveguides for quantum photonics have been realized [192, 193]; ion-exchanged glass directional couplers for implementing quantum projectors have been proposed [194]. On the other hand, ion-exchanged waveguides in thin glass substrates, suitable for photonic system integration and EOCB (electro-optical circuit board) panel size integration [195], can open new possibilities in IQP. Likewise, other proposed quantum systems could be implemented,as, for example, quantum simulators of geometrical phases [72] and boson sampling [196], quantum random walk devices [197] and so on. As commented, these implementations will be efficient as long as a few qubits are considered, a limit which is in turn related to the present scalability difficulty of quantum integrated photonics.