论文标题
量子在弱电场和BLOCH振荡中行走
Quantum walks in weak electric fields and Bloch oscillations
论文作者
论文摘要
当将电场叠加在带有紧密结合的哈密顿量(TBH)的晶格上的量子粒子上时,Bloch振荡就会出现。这种现象将被称为TBH Bloch振荡。已知类似的现象出现在所谓的电离散时间量子步行(DQWS)中。这种现象将称为DQW Bloch振荡。当DQW的电场较弱时,这种相似性尤其重要。对于宽的(即空间扩展的初始条件),一个数值观察到半古典振荡,即用于电气TBH和电气DQW的局部粒子的振荡。更准确地说:数值模拟强烈表明,半古典的DQW Bloch振荡对应于两个反向传播的半古典TBH Bloch振荡。在这项工作中,表明在某些假设下,电场的电气DQW解决方案和宽的初始条件的溶液通过两个连续时间表达式的叠加很好地近似,这是电TBH的反向传播溶液,其跳高振幅是任意硬币操纵杆混合角的余弦。相反,如果人们希望对空间定位的初始条件保持连续的时间近似,则至少需要DQW是懒惰的,如数值模拟所暗示的,并且在消失的电场中已经证明了这一事实。
Bloch oscillations appear when an electric field is superimposed on a quantum particle that evolves on a lattice with a tight-binding Hamiltonian (TBH), i.e., evolves via what we will call an electric TBH; this phenomenon will be referred to as TBH Bloch oscillations. A similar phenomenon is known to show up in so-called electric discrete-time quantum walks (DQWs); this phenomenon will be referred to as DQW Bloch oscillations. This similarity is particularly salient when the electric field of the DQW is weak. For a wide, i.e., spatially extended initial condition, one numerically observes semi-classical oscillations, i.e., oscillations of a localized particle, both for the electric TBH and the electric DQW. More precisely: The numerical simulations strongly suggest that the semi-classical DQW Bloch oscillations correspond to two counter-propagating semi-classical TBH Bloch oscillations. In this work it is shown that, under certain assumptions, the solution of the electric DQW for a weak electric field and a wide initial condition is well approximated by the superposition of two continuous-time expressions, which are counter-propagating solutions of an electric TBH whose hopping amplitude is the cosine of the arbitrary coin-operator mixing angle. In contrast, if one wishes the continuous-time approximation to hold for spatially localized initial conditions, one needs at least the DQW to be lazy, as suggested by numerical simulations and by the fact that this has been proven in the case of a vanishing electric field.