## 4 Quantum Integrated Photonic Devices
Thus, ion-exchanged glass devices can provide optimal solutions in quantum optical communications, in particular integrated quantum devices for quantum cryptography [49; 51; 198] as for example generators of quantum states, quantum projectors and Bell states measurement devices. Likewise, experimental results of these devices fabricated by ion exchange in glass provide proofs of concept about the use of ion-exchanged glass integrated optics in quantum information; but as it will be shown these devices would use well-known ion-exchanged glass integrated optical elements as directional couplers (see, for instance, References [199; 200; 201]), integrated optical gratings (see, for instance, references [17; 18; 19]) and so on. Therefore, we have many of the basic ingredients to develop IQP in a IExG platform and we only need to design such quantum devices (quantum circuits), fabricate them and optimize their performance. For example, preliminary results of a quantum projector fabricated by Na\({}^{+}\)/K\({}^{+}\) ion exchange in soda-lime glass has been recently proposed [194]. In all cases, the quantum circuits consist mainly of \(N\) channel waveguides or paths (\(N\)-dimensional device) formed by a set of concatenated integrated optical elements as directional couplers and phase shifters [49] performing quantum operations. As mentioned passive integrated optical elements allow the implementation of devices to be mainly used in quantum communications through different kinds of optical fibers as multicore optical fibers [50] and few-mode optical fibers [52]. On the other hand, it may be useful to recall that space multiplexing is one of the most promising method for increasing the transmission capacity of optical fiber systems [202; 203] and that spatial multiplexing can be used for implementing high-dimensional quantum cryptography. At this regard, ion-exchanged glass modal converters have been already successfully used for spatial multiplexing in few-mode optical fibers [52; 204; 205; 206; 207; 208; 209]. These components are located at the emitter and receiver, where one can already place the ion-exchanged glass IQP devices to generate or measure quantum light states excited in the optical modes of a few-mode optical fiber. Regarding the manufacturing of these devices, we must stress, as commented, that generation and measurement of quantum states in quantum communications can be made with devices formed by the same integrated optical elements fabricated on IExG platform and used in classical optical communications (directional couplers, \(1\times\) N splitters, optical gratings and so on). Therefore, from an industrial point of view, the existing manufacturers of photonic devices on IExG platform can easily enlarge their production to devices designed for quantum optical communications, or even other applications as optical quantum sensors and optical quantum simulators. These applications are well considered by many companies, and governments all over the world invest in quantum technologies that have a disruptive potential. For example, the European Commission has launched in 2018 an ambitious flagship initiative for quantum technologies [210] where, for example, quantum communications play a central role [211; 212] for both increasing commercialization of research results and the interest of large industrial players. On the market side, a recent report by Allied Market Research estimated that the market size of integrated quantum optical circuits could grow from a value of 426.0 million USD in 2017 up to 1460.2 million USD by 2025 [213]. In order to make clear the above considerations, and after describing, for the sake of completeness, the fundamental aspects of quantum integrated optical elements in Section 4.1, we present, in Section 4.2, an integrated generator of quantum light states \(|L\rangle\) based on the use of integrated optical lasers and directional couplers and mainly intended for quantum cryptography.