Studies on quantum dots (QDs) provide great opportunities in single photon detection as well as single circular polarized photon emission, which are the key technology for future quantum information processing. For single photon detection, the quantum-dot-resonant-tunneling-diode (QD-RTD) is evaluated as one of the most promising scheme but still suffering from the ultralow working temperature (～5 K) and lack the capability to discriminate photon numbers. Here we demonstrate a photon-number-resolving detector based on quantum dot coupled resonant tunneling diodes (QD-cRTD). Individual QDs coupled closely with adjacent quantum well (QW) of resonant tunneling diode operate as photon-gated switches which turn on (off) the RTD tunneling current when they trap photon-generated holes (recombine with injected electrons). With proper decision regions defined, 1-photon and 2-photon states are resolved in 4.2 K with excellent propabilities of accuracy of 90% and 98% respectively. Further, by identifying step-like photon responses, the photon-number-resolving capability is sustained to 77 K, making the detector a promising candidate for advanced quantum information applications where photon-number-states should be accurately distinguished. On the other hand, we firstly performed the magneto-optical studies on single InGaAs/GaAs self-assembled QDs. We observed the exciton Zeeman splitting and diamagnetic shift of a single QD under magnetic field, and the exciton g factor and diamagnetic coefficient was extracted by fitting the magnetic field dependent PL energies. By comparing with theories, we discussed on the effect of QD size, shape and composition on these two parameters. Based on these work, we investigated the single QD exciton-cavity mode coupling effect under external magnetic field. By first time we observed the interaction of Zeeman splitted exciton spin states with the cavity mode and realized the selective enhancement of the SE rate of the exciton state with specific spin configuration by means of magnetic manipulation of Purcell effect. In this sense, single QD emission with higher circular polarization degree under non-polarized excitation was realized. Our results have high potential to open up a way to novel quantum light sources and quantum information processing applications based on cavity quantum electrodynamics effects.