计及厚度下量子点量子比特的电磁场依赖性

在电子-体纵光学(Longitudinal optical,LO)声子强耦合条件下,采用LLP-Pekar型变分法推导出计及厚度下量子点中极化子的基态和第一激发态能量本征值和本征函数以及平均声子数的电磁场依赖性。在此基础上,以极化子的二能级结构为载体构造了量子点量子比特。数值计算结果表明:量子比特的振荡周期T_0随量子点厚度L的增加而增大,随磁场的回旋频率ωc、电场强度F和电声子耦合强度α的增加而减小。量子比特的概率密度︱Ψ(ρ,z,t)~2︱随电子横向坐标ρ的变化呈现"正态分布"并受到量子盘厚度L和有效半径R_0的强烈影响,随电子纵向坐标z、角坐标φ和时间t作周期性振荡变化。消相干时间τ随磁场的回旋频率ω_c、色散系数η和电子-声子耦合常数α的增加而增大,随电场强度F、量子点厚度L和有效半径R_0的增加而减小。量子点的厚度是量子点量子比特的一个重要参数,理论上可以通过设计不同的量子盘厚度并结合调节外加电磁场的强度,达到调控量子比特振荡周期、消相干时间大小的目的。

Electromagnetic Field Dependence of Quantum Dot Qubit with The Thickness of Quantum Dot

Based on the Lee-Low-Pines unitary transformation, the electromagnetic-field dependence of the eigenenergy, the eigenfunctions, and the mean number of phonons of both the ground-state and the first excited-state of the strong-coupling polaron in the quantum dot with the thickness were studied by using the Pekar variational method.On this basis, the quantum dot qubit was formed by means of the two-level structure of the polaron as the carrier.The results of numerical calculation indicate that the oscillation period T0 of the qubit increases with the increasing of the thickness L of the quantum disk, but decreases with the increasing of the cyclotron frequency ωc of the magnetic field, electric-field strength F and electron-phonon coupling strength α.The probability density (Ψ) (ρ,z,t)2 of the qubit presents the normal distribution with the variation of the electronic transverse coordinate ρ.It is significantly influenced by the thickness L and effective radius R0 of the quantum disk and shows the periodic oscillation with the variation of the electronic longitudinal coordinate z, polar angle φ and time t.The decoherence time (τ) increases with the increasing of the cyclotron frequency ωc of the magnetic field, dispersion coefficient η and electron-phonon coupling strength α, but decreases with the increasing of the electric-field strength F, thickness L and effective radius R0 of the quantum disk.The thickness of the quantum dot is an important parameter of the qubit.Theoretically, the target of regulating the oscillation period, decoherence time and quality factor of the free rotation of the qubit can be achieved by designing the different thickness of the quantum disk and regulating the strength of the electromagnetic field.