Semiconductor Self-assembled Quantum Dot And Quantum Source

Quantum information is a rising interdisciplinary field which combines quantum theory, computer and information sciences, starting from the second half of20th Cen-tury. It contains two major fields, quantum communication and quantum computation. With the developing of the technology of producing integrated chips, the size of chips becomes as small as a nanometer. If it kept going on, the quantum effect should be more and more significant. Moreover, when one wants to increase the number of the chips in a unit area, it is even more difficult to deal with the thermal effects. Landauer pointed out that the classic logical operation is irreversible, which is the origin of the thermal effects, and prevents increasing the integration level. Therefore, it is necessary to study a kind of new frame of computation. Further, thanks to Peter Shor and Bennett, who proposed the quantum factorization algorithm and the protocol of quantum key distri-bution respectively, the great potential of reverible quantum parallel computation and quantum secure communication has been shown, and it makes people place great ex-pectation on quantum information. As a result, it is feasible to develop the quantum information science.Similar to the classic information, the quantum information also needs physical carrier. The photon is such a pure system to carry the quantum information and obtains universal approval. So, it is the oringin of quantum information process that producing single photon and entangled photon pairs. People hope to transmit information between nodes in quantum net by photons. The semiconductor quantum dot seems one of the most promising quantum light source, which is grown by molecular beam epitaxy tech-nology. They are a kind of quasi-zero-dimensional nanometer material, which play a role of a potential well for electrons and holes, so that they form a series of discrete energy level structures. The transition of two levels can emit a single photon. They are compatible with processing technique of semiconductors, and are easy to integrated, and their properies are stable. Moreover, the wavelength of the emitted photons could be controlled to some extent, so they received much concern. In this work, we studied the optical properties of self-assembled quantum dots, and use them to produce single photon and entangled photon pairs. The primary works are as follow:We obtained a single photon source using quantum dots. Quantum dots are formed randomly in the grown process, due to the stress releasing of the crystal lattice. Their size, density and optical properties are all differenct from each other. However, there are some methods to control the distribute, such as to control the growth temper-ature, the ratio of different components and the deposition rate. We made a kind of quantum dot sample of which the density has a gradient distribution, and at the sparse part of the sample, we found and isolated a single quantum dot. As a single photon source, it emitts at least100thousands photons every second, and its second order au-tocorrelation is less than0.27.We studied the four-energy-level model and the correlation functions of quan-tum dots. A quantum dot can confine two pairs of electrons and holes, which is a biex-citon, and it can emitts a pair of photons by the cascade process. The whole process can be abstracted as a four level system model. We studied this model in theory and introduced in the temperature as a parameter. By calculating the correlation functions of the transitions, we can get the polarization density matrix and the optical properties of the photon pair. We made numerical simulation of the model, and explained some experimental phenomenon of the photon pair’s fidelity evolution.We explored the dynamic evolution of the entanglement and quantum corre-lation of photon pairs emitted from a quantum dot. The polarizations of the photon pair emitted from the cascade process are orthogonal. So there are two kind of path of emission. If one cannot distinguish them by other aspects, the two photons are entan-gled. We studied the dynamic evolution of the entanglement of the photon pair by the four-level model, and found the entanglement sudden death phenomenon as the temper-ature increasing. In addition, the quantum correlation is a more fundamental concept than that of entanglement, which contains all the non-classical correlations between two particles. We also discussed the dynamic of quantum correlation of the photon pair us-ing the four-level model, and pointed out that its evolution isn’t affected by the White Noise in the environment, but only related to the assisted process by the phonons in the system.We realized Franson interference using photon pairs from the cascade radia-tion of quantum dots, and carried out investigations on the inhomogeneous broad-ening of biexcitons. The inhomogeneous broadening of the levels in a quantum dot, caused by the randomly trapped charges around it, is close related with the coherent con-trol of the system and the quality of the entangled photon pairs. We firstly present the genuine Franson-type interference to directly measurement the inhomogeneous broad-ening of the biexciton state. The results show that this broadening of the biexciton state is much smaller than that of the exciton state, which suggests that the entangled photon pairs can be well generated by the cascade process..