Spin-Dephasing On Quantum Dot System With A Nearby Quantum Point Contact

Semiconductor quantum dot is one of the most promising candidates for quantum computation, electron spins in quantum dot represent a potential qubit. This dissertation introduces the spin-dephasing induced by quantum point contact (QPC) in quantum dot system. The main content of this dissertation are listed as follows:First of all, we introduce the background and motivation of our research, describe the basic knowledge of quantum dot and qubit.Then, this thesis introduces the experiment about dephasing in GaAs semiconductor quantum dot, reviews the mechanism of charge-dephasing induced by QPC in an ideal double quantum system, and we also introduce the quantum Zeno effect.In this dissertation, we study the evolution behaviors of an election spin on a quantum dot due to coupling to a nearby QPC as a measurement. Chapter 3 is devoted to the derivation of the master equation for the system. We find that the depasing time T2Q PCis about 100 ns when the spin decoherence is only induced by the QPC, and the repeated QPC measurement will slow down the transition rate between spin states which can be interpreted in terms of quantum Zeno effect.Meanwhile, we also study the effect of QPC measurement on two-electron spin state in double quantum dot system. The thesis gives an effective Hamiltonian, derives the master equations of the whole system and calculates the time evolution of spin states. We also find the QPC measurement induced dephasing time T2≈1μs, and provide a simple and transparent description of the enhanced QPC measurement which could trap the system for small t and be interpreted in terms of quantum Zeno effect. During the discussion we propose some methods to extend the dephasing time.Furthermore, we take a microwave field into account and discuss evolution of electrons in this situation. We find that the two electrons in double-dot system could perform all the operation of single qubit, and microwave field would extend the dephasing time.Finally, we introduce the relevant knowledge of environment-induced decoherence in quantum dot.