# Special Issue on Quantum Optics for Fundamental Quantum Mechanics
###### Abstract
On the other side, in [16], a theoretical investigation of the photon statistics in a coupled cavity system mediated by a two-level atom is shown: the authors find that there exists a photon blockade in the linear cavity for different parameter regimes, where the coupling strength between the atom and the nonlinear cavity is greater (or less) than the dissipation rate of the linear cavity. Moreover, they provide other extensions to this study to show the potential applications of a single photon source in a weakly nonlinear system. The arguments of [17; 18] concern photon emission and statistics. The high-frequency electromagnetic emission from non-local wavefunctions is analyzed in [17], where it is shown that, in systems with non-local potentials or other kinds of non-locality, the Landauer-Buttiker formula of quantum transport leads to replacing the usual gauge-invariant current density \(\mathsf{J}\) with a current \(\mathsf{J}_{ext}\), which has a non-local part and coincides with the current of the extended Aharonov-Bohm electrodynamics. Peculiar properties of the analyzed scenario are analyzed by the authors to provide possible applications concerning the radiation field generated by Josephson tunneling in thick superconductor-normal-superconductor (SNS) junctions and by current flow in molecular nanodevices. The process of second harmonic generation is addressed in a quantum frame in [18]: the analysis here starts with a perturbative approach, showing how it is possible to achieve a number of analytic results, ranging from the up-conversion probability to the statistical properties of the generated light. Moreover, the authors investigate the nonclassicality of some benchmark states by inspecting the corresponding autocorrelation function. Finally, this Special Issue concludes with methods in quantum technologies [19; 20], a very promising and vivid field that nowadays centralizes the great efforts of the quantum optics community. The study reported in [19] articulates the context of multiparameter quantum estimation theory: the authors investigate the construction of linear schemes in order to infer two classical parameters that are encoded in the quadratures of two quantum coherent states. The optimality of the scheme, built on two-phase conjugated coherent states, is proven with the saturation of the quantum Cramer- Rao bound under global energy constraints. The study is then extended on n-mode schemes that can be used to encode n classical parameters into n quantum coherent states, and then estimate all parameters optimally and simultaneously. In conclusion, paper [20] focuses on Quantum Key Distribution (QKD) Networks, that actually are an important support for quantum communication. QKD Networks have achieved a relatively mature level of development and, consequently, are facing greater requirements in terms of their implementation capabilities, thus requiring an effective management plane to control and coordinate the QKD resources. The presented study reports how, as a promising technology, software-defined networking (SDN) can separate the control and management of QKD networks from the actual forwarding of the quantum keys. This research was supported by grant number (FQXi FFF Grant number FQXi-RFP-1812) from the Foundational Questions Institute and Fetzer Franklin Fund, a donor advised fund of Silicon Valley Community Foundation. This work has received funding from PATHOS EU H2020 FET-OPEN grant no. 828946, and from the European Union Horizon 2020 and the EMPIR Participating States in the context of the project EMPIR-17 FUN01 'BeCOMe'. As Guest Editors of this Special Issue, we would like to thank all of the authors and anonymous peer reviewers for their valuable contributions. Undoubtedly, this issue would not be possible without the excellent and tireless work done by the MDPI management and staff, to whom we convey our most sincere thanks, with a special thanks to [PERSON], Managing Editor at the MDPI Branch Office, Beijing. The authors declare no conflict of interest.