## 3 Integrated Quantum Photonics Systems  
To enhance the quantum information and computation integrability and scalability, researchers have dealt with the problem of increasing the circuit complexity. _Photonic Processors_--Since 2017, improved technologies and fabrication processes have allowed the realization of much more complex integrated circuits able to perform many operations on the chip. A SOI-quantum-photonic device that integrates the capabilities for the generation, manipulation, and analysis of two-qubit entangled states has been reported [49]. The device can prepare and operate on a variety of separable and entangled states, using a switchable entangling gate. The performances were measured using on-chip quantum-state tomography. The increasing number of elements in a single device brought about the birth of the first quantum photonic processor, which consisted of a programmable single device able to perform many different operations. One of the first reconfigurable integrated quantum photonic processors was developed in 2015 by [PERSON] et al. [39]. Such a universal linear-optic processor heralded both quantum logic gates and entangling gates, by means of 15 Mach-Zehnder interferometers made of 30 silica-on-silicon-waveguides directional couplers and 30 electronically controlled phase-shifters, programmed with four microcontrollers with a total 32-channel 12-bit digital-to-analogue converters. A picture of the photonic processor is shown in Figure 2a. The processor was also programmed to implement 100 different boson-sampling routines. Boson sampling, which is believed to require exponential time by using classical resources, is a computational task that samples from the output of a linear-network interferometer the distribution of identical photons sent into it [50, 51]. Silicon chips started to integrate more and more elements and, in 2017, [PERSON] et al. [52] realized a programmable nanophotonic chip made up of 264 elements [see Figure 2b]: 88 interferometers and 176 individually tunable phase shifters, which could be programmed by means of a 240-channel, 16-bit precision electronic biasing system. The authors, using such a processor, simulated different transport regimes over a set of 64,400 experiments, observing also the signatures of the environment-assisted quantum transport: a particle propagating through a strong disordered system became exponentially localized in space (Anderson localization), inhibiting transport; still, increasing environmental noise over a finite range resulted in enhanced transport. Later on, in 2018, more than 200 photonic components were used to realize a fully programmable quantum circuit with two qubits [53]. CMOS-compatible processing was used for fabrication. The quantum processor was programmed to compute 98 different quantum operations with two qubits, with an average quantum-process fidelity of \((93.2\pm 4.5)\%\). On  
Figure 1: Advancement and milestones reached in the field in the last decade. The achievements until 2018 are partially taken from Ref. [20] (red fonts). Starting from 2013, the following results and milestones have been achieved: demonstrations of boson sampling with multiple photons [34, 35, 36, 37, 38]; the first integration of spontaneous four-wave mixing sources with quantum circuits [33]; demonstration of universal linear-optic circuit [39]; test of [PERSON]’s search algorithm [40]; test of quantum-Hamiltonian-learning algorithm [41]; large-scale quantum circuits in silicon [42]; programmable chip [43]; supercompact quantum gate [44]; on-chip single-photon detector [16]; silicon programmable qudit processor [45]; chip-based entanglement network [46]; universal quantum classifier [47]; and photonic circuit with 2446 components [48]. the same track, [PERSON] et al. [42] realized an integrated multidimensional quantum photonic platform that had more than 550 photonic components [see Figure 2c]. It implemented the generation, manipulation, and analysis of multi-dimension quantum systems directly on a single chip. In 2021, [PERSON] et al.