Single Electron Charge And Spin Measurements In Gaas Quantum Dots By Strong Back-action

Information Theory brought unprecedented great development to human society in the20th century. With the continuous progress of science and technology and the rapid development of quantum information tenchnology, the twenty first century will be the era of quantum information. Electronically controlled semiconductor quantum dots which were considered the most likely to achieve solid-state quantum computing by the scientific community become one of the most important and most popular research field in the present international forefront physics. Because the preparation process of the semiconductor quantum dots have many similarities with traditional chip technology, so it greatly promotesa fast development and application for the variety of micro and nano processing technology, RF application measurement technology and the very low temperature technology. This thesis have done the experimental study of single electron charge and spin in very low temperature for the semiconductor gallium arsenide gate electronically controlled quantum dot devices in the international forefront of research areas. The back-action of quantum point contact can drive electron tunneling out of quantum dot or tunneling from ground state to excited state. By measuring the random telegraph signal (RTS), the back-action can be quantitatively characterrized as a back-action-induced tunneling-out rate. We studied the last six electrons, and found back-action driven spin singlet-triplet excitation only for all the even number of electrons and back-action-induced tunneling-out for all the odd number of electrons. We use back-action as an excitation source to probe the spin excited states spectroscopy for both the odd and even electron numbers under a varying parallel magnetic field. Using the three-step pulses, we measured the relaxation time T1between spin triplet and singlet states for the last few even number of electrons. We also measured the different relaxation time T1with the different back-action strength for two electrons.The main content of this thesis includes:1. A brief introduction to some basic concepts of quantum computing and quantum dot.Described the basic principles of measurement of quantum point contact as a highly sensitive measurement channels, and random telegraph signal and the electron charge counting statistics in gate electronically controlled semiconductor quantum dots. Finally, the the relevant experimental techniques and the basic physical concepts of electronic relaxation time measurements have been introduced.2. Introduced the nanofabrication and the preparation of semiconductor gateelectronically controlled quantum dot devices. Described the variety of precision machining equipments and the sample preparation process. A brief introduction to the instrument of extremely low temperatures and the variety of quantum transport measurement techniques in very low-temperature.3. A quantum point contact (QPC) next to a quantum dot (QD) is widely used to perform electron charge counting and plays an important role in many tasks such as the read-out of electron charge or spin-based qubits. However, QPC measurem-ents have an inevitable side effect known as the back-action. We study real time charge counting statistics measured by a quantum point contact (QPC) coupled to a single quantum dot (QD) subject to different back-action strengths. By tuning the QD-QPC coupling or the QPC bias, we control the QPC back-action, which drives the QD electrons out of thermal equilibrium. The random telegraph signal (RTS) statistics show strong and tunable non-thermal-equilibrium saturation effect, which can be quantitatively characterized as a back-action-induced tunneling-out rate. We find that the QD-QPC coupling and QPC bias voltage play different roles in determining the back-action strength and the cutoff energy.4. In single quantum dot (QD), the electrons were driven out of thermal equilibrium by the back-action from a nearby quantum point contact (QPC). We found the driving to energy excited states can be probed with the random telegraph signal (RTS) statistics, when the excited states relax slowly compared with RTS tunneling rate. We studied the last few electrons, and found back-action driven spin singlet-triplet (S-T) excitation for and only for all the even number of electrons. We developed a phenomenological model to quantitatively characterize the spin S-T excitation rate, which enabled us to evaluate the influence of back-action on spin S-T based qubit operations.5. We use back-action as an excitation source to probe the spin excited states spectroscopy for both the odd and even electron numbers under a varying parallel magnetic field. For a single electron, we observed the Zeeman splitting. For two electrons, we observed the splitting of the spin triplet states.All these informations were revealed through the real-time charge counting statistics.6. In a GaAs single quantum dot, the relaxation time T1between spin triplet and singlet states hasbeen measured for the last few even numbers of electrons. The singlet-triplet energy separation EST is found to drastically decrease with increasing electron number. With EST controlled at the same magnitude, T1shows a steady decrease from2-electrons to6-electrons. We also measured the relaxation time T1between spin triplet and singlet states with different back-action strength.The main innovations of thesis are:1. For the first time, we study real time charge counting statistics measured by a quantum point contact (QPC) coupled to a single quantum dot (QD) subject to different back-action strengths. The random telegraph signal (RTS) statistics show strong and tunable non-thermal-equilibrium saturation effect, which can be quantitatively characterized as a back-action-induced tunneling-out rate.2. For the first time, we found the driving to energy excited states can be probed with the random telegraph signal (RTS) statistics with the strong back-action, when the excited states relaxslowly compared with RTS tunneling rate. We developed a phenomenological model to quantitatively characterize the spin S-T excitation rate, which enabled us to evaluate the influence of back-action on spin S-T based qubit operations.3. We use back-action as an excitation source to probe the spin excited states spectroscopy for both the odd and even electron numbers under a varying parallel magnetic field. For a single electron, we observed the Zeeman splitting. For two electrons, we observed the splitting of the spin triplet states. And we found that back-cation drived the singlet state|S> over whelmingly to|T+> other than|T°>.4. For the first time, the relaxation time T1between spin triplet and singlet states has been measured for the last few even numbers of electrons. The T1shows a steady decrease from2-electrons to6-electrons. We also measured the relaxation timeT1between spin triplet and singlet states with different back-action strength.