基于方波脉冲外场的超冷原子-分子绝热转化

Adiabatic conversion of ultracold atoms into molecules via square-shaped pulse field

基于受激拉曼绝热通道技术,研究了方波脉冲外场下的超冷原子-双原子分子转化.运用绝热保真度的方法,详细分析了该原子-分子转化系统相干布居俘获态的动力学演化过程.研究发现,相干布居俘获态的最终绝热保真度随脉冲激光强度的变化呈现出大幅度的周期振荡.这表明本文所设计的方波脉冲方案与高斯脉冲方案相比具有明显的优势,可以在较小的脉冲激光强度下达到较高的绝热保真度并实现较高效率的超冷原子-分子转化.

On the basis of the stimulated Raman adiabatic passage technology, we study the conversion of ultracold atoms into diatomic molecules by using a square-shaped pulse field. By the method of adiabatic fidelity, we analyze the dynamical evolution process of the coherent population trapping state for the atom-molecule conversion system. We introduce two adiabatic fidelities to describe the efficiency of ultracold atom-molecule conversion, i.e.： 1） the final adiabatic fidelity,which gives the value of the adiabatic fidelity at the end of the evolution： the closer to 1 it is, the higher the conversion efficiency is; 2） the final maximum adiabatic fidelity, which denotes the maximum value that can be achieved at the end of evolution, indicating the highest conversion efficiency. With these two quantities, we discuss how to achieve higher adiabatic fidelity for the coherent population trapping state through optimizing the pulse-delay time and the pulse-laser intensity of the stimulated Raman adiabatic passage. In addition, we also discuss the effects of the width of pulses on the ultracold atom-molecule conversion efficiency and the feasibility of continuous light. It is shown that the final adiabatic fidelity of the coherent population trapping state demonstrates a large periodic oscillation with the pulse-laser intensity. By calculating and analyzing the final adiabatic fidelity and the final maximum adiabatic fidelity, we obtain the conditions for higher efficiency conversion, which gives the best choice of the pulse-laser intensity, the pulse-delay time,and the width of pulses. The results show that the scheme of square-shaped pulses we discussed has obvious advantages compared with that of Gaussian-shaped pulses, which can achieve high adiabatic fidelity and realize higher ultracold atom-molecule conversion efficiency via employing the pulse-laser field with low intensity. Further detailed comparison between the square-shaped pulses and the Gaussian-shaped pulses is also given. Particularly, we find that the final adiabatic fidelity shows a periodic oscillation with the pulse width, which means that the high efficiency atom-molecule conversion can be achieved by using a pulse field with small width. Moreover, we find that the high efficiency conversion can also be achieved by using special continuous light under certain conditions.