实现粒子布居高效转移的两种激光脉冲时序方案的理论研究

Theoretical research on an efficient population transfer based on two different laser pulse sequences

基于最近实验工作的结果(2010 Nat.Phys.6 265)即Danzl等在五能级M型级联系统中分别利用连续型和四光子型受激拉曼绝热通道(stimulated Raman adiabatic passage,STIRAP)实现了将Feshbach态上弱束缚的Cs2有效转移到其振转基态,本文理论研究了两种STIRAP方案实施的基本条件,解析推导系统的准暗态、绝热参数的具体形式并分析其存在的必要性,详细讨论布居转移效率对相关参量的依赖关系.通过比较激光脉冲的时序、中间能级的失谐量和自发辐射率、光场脉冲的幅值等诸多参量的不同影响,讨论方案各自的优缺点,找到了参量优化的方法以实现最高效的粒子布居数转移.与前人的实验结果相比,本文研究表明,实验观测值(约0.60)均低于理论预估最佳值(约0.97)的主要原因是受限于激发态能级的自发辐射率过大.该理论方案还可用于制备量子纠缠态,在量子逻辑门操控、量子信息传输等领域都有潜在的应用.

A quantum gas of ultracold molecules, with long-range and anisotropic interactions, will enable a series of fundamental studies in physics and chemistry. In particular, samples of ground-state molecules at ultralow temperatures and high number densities will facilitate the explorations of a large number of many-body physical phenomena and applications in quantum information processing. However, due to the lack of efficiently cooling techniques such as laser cooling for atomic gases, high number densities for ultracold molecular samples are not readily attainable. Associating ultracold atoms to weakly bound dimer molecules via Feshbach resonance and subsequently transferring them to a wanted molecular ro-vibronic ground state by a stimulated Raman adiabatic passages（STIRAP） have proved to be an effective way in producing ideal ultracold molecular samples. As a typical illustration, in a recent study（2010 Nat. Phys. 6 265）Danzl et al. experimentally realized the preparation of Cs2 molecule into its ro-vibronic ground state via two different multi-level STIRAPs： one is based on a single conversion route and the others are based on a cascade-connected route（labeled by 4 p-STIRAP and s-STIRAP, respectively）. In this work, we present a theoretical study for these two STIRAP schemes, focusing on the differences in physical principle and realistic performance between them. On the one hand,according to the theoretical approach of quasi-dark eigenstates, we conclude that a highly efficient population transfer is achievable in both schemes. On the other hand, by systematically studying the influences of the relevant parameters,including the spontaneous decays and the detunings from the intermediate states, and the temporal sequence and the amplitude of the laser pulses, we disclose their respective advantages and weaknesses in the realistic implementation.We theoretically predict that for both schemes their maximal conversion efficiencies each can attain 0.97 as long as the spontaneous decays from the intermediate excited states are sufficiently suppressed. Yet considering the fact that the already implemented efficiency is only around 0.6 for both schemes, there is still room for optimization, e.g. using stable Rydberg energy levels in future experiment. Furthermore, the success of these two schemes can provide a new route to the controllable entanglement preparation, opening more applications in the fields of quantum logic gate and so on.