LA声子对激子qubit纯退相干的影响

在一个抛物量子点中，以激子的真空态和基态作为量子比特(qubit)，采用求密度矩阵元的方法，计算了由形变势下声学声子引发的激子量子比特纯退相干.找到了激子量子比特纯退相干因子对时间、温度和量子点受限长度的依赖关系.研究发现,激子量子比特的退相干因子在2.5ps的时间范围内随时间的增加而迅速增加，其纯退相干时间在ps量级；在温度即使为绝对温度0K时由LA声子引发的退相干依然存在，在温度大于3K后退相干因子随温度的增大而开始迅速增大；并同时发现量子点受限长度对退相干因子有重要影响，激子越受限退相干越快.研究结果表明,对激子量子比特使用适当大小量子点，且保持环境低温，并采用低能超快光学操作可以有效地抑制声子对激子量子比特纯退相干的影响. 关键词： 量子点 量子信息 量子比特

Effects of LA phonon on pure dephasing for exciton qubit

In a parabolic quantum dot (QD), the exciton vacuum state and the exciton ground state may be employed as a two-level quantum system——a qubit. The pure dephaseing due to deformation potential exciton-bulk longitudinal acoustic phonon (LAP) interaction for the exciton qubit is investigated by using the reduce density matrix. The dependence of dephasing factor on time and the confinement length of the QD and temperature is calculated. It is shown that the pure dephasing factor increases quickly with increasing time when time is smaller than 2.5ps, and the dephasing time is of the order of ps. Even at absolute zero temperature, exciton-LAP interaction still causes excitonic dephasing. The pure dephasing factor increases with increasing temperature quickly when temperature is higher than 3K. At the same time, it is found that the confinement length of the QD has important effects on pure dephasing of qubit, the smaller the quantum size, the faster the pure dephasing is. The results show that using appropriate QD size at low temperature and using ultra-fast laser technology can effectively suppress pure dephasing due to exciton-LAP interaction.