The Applications Of Quantum Dot Single-photon Sources In Quantum Information And Dimension Witness

The on-demand single photon source is an extremely important source for quantum information science and technology. Important progress has been achieved in the understanding of the quantum dot excitation process and its role on the coherence time of the photons, on the limitations of photon indistinguishability and collection efficiency, on cavity effects on single photon emission, and on the influence of the excitonic fine structure on emission efficiency. High efficiency and low error single-photon sources are required for the implementation of a number of quantum information processing applications. The fastest triggered single-photon sources to date have been demonstrated using epitaxially grown semiconductor quantum dots, which can be conveniently integrated with optical microcavities. Recent advances in quantum dot technology, including demonstrations of high temperature and telecommunications wavelength single-photon emission, have made quantum dot single photon sources more practical.Spectacular achievements include the extension of the wavelength range from the UV spectral region up to the near infrared, polarization control and the demonstration of ultrahigh measured single-photon emis-sion rates, room temperature operation with wide bandgap semiconductor quantum dots, electrical pumping with a cavity design for enhanced photon collection efficiency, electrical pumping up to 80K with an improved heterostructure design, high-purity single photon emission, the generation of high degrees of photon indistinguishability (97%), coherent state preparation in the p shell and s shell of an individual quantum dot, single photon emission from positioned quantum dot in cavities, and single-photon emission in the strong coupling regime.1. Single photon source in self-assembled quantum dotQuantum dot consists of a lower band gap semiconductor embedded in a higher band gap semiconductor. This leads to a three-dimensional electronic confinement due to the band offsets. Initially, electrons are present in the valence band and holes in the conduction band. Optical or electrical excitation can cause an electron to be excited to the conduction band, leaving a hole in the valence band. These electron-hole pairs can be trapped by the quantum dot, and quickly decay non-radiatively into the excited state of the quantum dot, forming an exciton state. Radiative decay of this exciton leads to the emission of a photon. In practice, the quantum dot can be excited to a higher excited state leading to a biexciton state. Due to asymmetries in the quantum dot, there is actually a fine structure splitting in the exciton state due to the different electron and hole spin states of the quantum dot, which lifts the degeneracy of the exciton level due to the electron-hole exchange interaction, leading to slightly different transition frequencies for horizontally and vertically polarized light.2. Violation of Leggett-Garg inequalities in single quantum dotLeggett-Garg (LG) inequalities provide a criterion to characterize the boundary between the quantum realm and classical one and the possibility of identifying the macroscopic quantum coherence. We investigate the violation of Leggett-Garg (LG) inequalities in quantum dots with the stationarity assumption. Because of the relationship between photon and the quantum dot system, we can detect the photon states to evaluate the LG inequalities. By comparing two types of LG inequalities, we find a better one which is easier to be tested in experiment. In addition, we show that the fine-structure splitting, background noise and temperature of quantum dots all influence the violation of LG inequalities. When the fine structure splitting of the quantum dot system, background noise and the temperature become small, we achieve the maximal violation of the Leggett-Garg inequalities.3. Measurement of the inhomogeneous broadening of a bi-exciton state in a quantum dot using Franson-type nonlocal interferenceThe inhomogeneous broadening of the bi-exciton state in quantum dots, i.e., the inhomogeneous broadening of the upper level of the cascade process, is not only a fundamental problem in quantum dots, but also closely related with the coherent control of this complex system and the quality of the entangled photon pairs, especially the time-bin entangled photon pairs. This inhomogeneous broadening is inherently a two-photon correlated phenomenon. In this work, we construct a genuine Franson-type nonlocal interference process to measure the inhomogeneous broadening of the bi-exciton state. The results show that the broadening of the bi-exciton state is considerably smaller than that of the exciton state due to the screen effect, that is why the entangled photon pairs can be generated by the cascade process in the quantum dot. Our results may be helpful for understanding the cascade process and improving the quality of deterministic entangled photon pairs.4. Experimental realization of Dimension witnesses Based on Quantum State DiscriminationDimension witness is an important concept in fundamental physics and quantum information processing which can allow one to test the dimension of an unknown physical system in a device independent manner. Here, we report an experimental test of classical and quantum dimensions in a prepare and measure scenario through dimension witnesses based on the quantum state discrimination. In our work, we can not only distinguish between quantum and classical systems of the same dimension (two, three and four dimensions), but also distinguish between real and complex two-level quantum systems. A strong link between dimension witnesses and quantum state discrimination is also established.