The adiabatic control is a powerful technique for many practical applications in quantum state engineering,light-driven chemical reactions and geometrical quantum computations.This paper reveals a speed limit of nonad...The adiabatic control is a powerful technique for many practical applications in quantum state engineering,light-driven chemical reactions and geometrical quantum computations.This paper reveals a speed limit of nonadiabatic transition in a general time-dependent parametric quantum system that leads to an upper bound function which lays down an optimal criteria for the adiabatic controls.The upper bound function of transition rate between instantaneous eigenstates of a time-dependent system is determined by the power fluctuations of the system relative to the minimum gap between the instantaneous levels.In a parametric Hilbert space,the driving power corresponds to the quantum work done by the parametric force multiplying the parametric velocity along the parametric driving path.The general two-state time-dependent models are investigated as examples to calculate the bound functions in some general driving schemes with one and two driving parameters.The calculations show that the upper bound function provides a tighter real-time estimation of nonadiabatic transition and is closely dependent on the driving frequencies and the energy gap of the system.The deviations of the real phase from Berry phase on different closed paths are induced by the nonadiabatic transitions and can be efficiently controlled by the upper bound functions.When the upper bound is adiabatically controlled,the Berry phases of the electronic spin exhibit nonlinear step-like behaviors and it is closely related to topological structures of the complicated parametric paths on Bloch sphere.展开更多
基金Project supported by the National Natural Science Foundation of China(Emergency Project,Grant Nos.11447025 and 11847308)
文摘The adiabatic control is a powerful technique for many practical applications in quantum state engineering,light-driven chemical reactions and geometrical quantum computations.This paper reveals a speed limit of nonadiabatic transition in a general time-dependent parametric quantum system that leads to an upper bound function which lays down an optimal criteria for the adiabatic controls.The upper bound function of transition rate between instantaneous eigenstates of a time-dependent system is determined by the power fluctuations of the system relative to the minimum gap between the instantaneous levels.In a parametric Hilbert space,the driving power corresponds to the quantum work done by the parametric force multiplying the parametric velocity along the parametric driving path.The general two-state time-dependent models are investigated as examples to calculate the bound functions in some general driving schemes with one and two driving parameters.The calculations show that the upper bound function provides a tighter real-time estimation of nonadiabatic transition and is closely dependent on the driving frequencies and the energy gap of the system.The deviations of the real phase from Berry phase on different closed paths are induced by the nonadiabatic transitions and can be efficiently controlled by the upper bound functions.When the upper bound is adiabatically controlled,the Berry phases of the electronic spin exhibit nonlinear step-like behaviors and it is closely related to topological structures of the complicated parametric paths on Bloch sphere.
文摘针对绳驱动存在的弹性易滑动、误差易累积等问题,提出了一种"钢丝绳+齿形带"的广义驱动方式,将电动机加齿轮、减速器代表的传统刚性动力传递转化为柔性传递.计算得出了钢丝绳拉力大小;运用摩擦力补偿法分析钢丝绳与绳外软管之间的摩擦因数,得出钢丝绳摩擦力与负载的关系.将同一根钢丝绳与绳套的组合进行不同弯曲度的试验;改变钢丝绳在软管中的长度,并于末端施加负载;在固定板上安装钢丝绳,测量拉力及摩擦力的大小进而确定钢丝绳型号.应用ADAMS对绳驱动关节的钢丝绳进行仿真分析.结果表明:弯曲程度对钢丝绳摩擦力影响可忽略;钢丝绳摩擦力与其长度、负载大小都成正比;仿真验证了理论分析正确性.试验得到绳最大紧边压力为8 N,实际摩擦力7.6 N;钢丝绳选用304不锈钢,直径定为1.5 mm,最大承重25 kg.
