Spin Qubits In Quantum Dots

Spin qubits in semiconductor quantum dots,due to its long coherence time and the easy integration features have attracted much attention.In this paper,we focus on the study of spin qubits in GaAs double quantum dots.The main contents are as follows:1.A brief introduction to the background and concepts of quantum computing.Some basic phenomena in quantum dots are explained using the constant interaction model,including Coulomb oscillation,Coulomb blockade,triple point,bias triangle and so on.2.A simple principle analysis of single-electron spin qubits is given.The Hamiltonian of the two-electron singlet-triplet qubit and the experimental details are described in detail,including the two methods of observing spin blockade and the method of characterizing the system.Finally,two classical experiments and our experimental results are introduced.3.The blockade of the spin quadruplet states is observed in the three-electron(1,2)-(2,1)charge transition region by using the asymmetry of the double quantum dots.Using this spin blockade,we achieve the Landau-Zener-Stuckelberg interference and exchange oscillations between the spin quadruplet and the doublet states.And finally we supplement our additional experimental results in two other charge transition regions with odd number of electrons,(1,2)-(0,3)and(1,6)-(0,7).4.A signal amplification mechanism is found in the measurement of the exchange oscillations in(1,3)-(0,4)charge transition region.We have analyzed and experimentally verified this mechanism.A method of controlling the amplification process is designed and the tunneling rates of different spin states are extracted by this method.5.We perform transport measurement on a double quantum dot.Spin blockade is observed in five consecutive charge transition regions.The blocked spin states of each region are determined by analyzing the data.The main innovations of this paper are:1.The spin blockade phenomenon was observed for the first time in(1,2)-(2,1)charge transition region in double quantum dot.Using this spin blockade,the spin states can be read out and the coherent operations can be demonstrated.Using the method developed in this region,we explored other odd-numbered electron regions and got similar results.Our results and experimental methods will benefit future exploration of multi-electron coded spin qubits.2.A new spin-to-charge conversion mechanism was discovered.This mechanism produces a stronger signal than the conventional spin blockade method when it is used to read out the state of spin qubits.In the experiment we achieved a amplification ratio of 2.We illustrate how to use a simple scheme to achieve a significant increase in amplification ratio.3.Spin blockade was observed for the first time in successive charge transition regions.Especially the spin blockade in odd-numbered electron regions,combined with the strong controllability of lateral double quantum dots,provides a new and powerful tool for studying the interaction between electron spins and nuclear spins.