[论文解读] Cyclotron dynamics of cold atoms in 2D optical lattices

Andrey R. Kolovsky|arXiv (Cornell University)|Jun 28, 2010

Cold Atom Physics and Bose-Einstein Condensates参考文献 16被引用 2

一句话总结

本文提出一种通过低频周期驱动诱导光子辅助隧穿，在二维光晶格中为冷原子生成人工磁场所用的方法，实现类回旋运动。主要贡献在于提出一种简单、无需拉曼激光的实验装置，避免了自发辐射损耗，通过干涉图样和质心波包运动实现检测，展现出经典回旋行为。

ABSTRACT

We propose a simple setup for introducing an artificial magnetic field for neutral atoms in 2D optical lattices. This setup is based on the phenomenon of photon-assistent tunneling and involves a low-frequency periodic driving of the optical lattice. We also address the problem of detecting this effective magnetic field. In particular, we discuss interference patterns of the atomic wave function and study the center of mass wave-packet dynamics, which shows some features of cyclotron dynamics of a classical charged particle. Introduction. – A major motivation of current research with cold atoms in optical lattices is the prospect of simulating the solid state physics. However, to have a full access to phenomena of the solid state physics this system (which can be considered as an artificial crystal) should be accomplished by artificial electric and magnetic fields. In present days experiments the former case of external electric field is routinely mimicked by accelerating the optical lattice [1–3], or by using the gravitational force [4–6] or a combination of gravitational and levitational forces [7]. The case of magnetic field is more difficult for laboratory realization because it requires a setup, where atomic wave function acquires a finite phase when the atom tunnels along a closed path on the lattice. This setup was suggested in the seminal paper [8], where the authors use two independent 2D optical lattices for two different internal atomic states, which are coupled by additional Raman lasers. The Rabi transition between internal states induces hopping of the atom between nearest lattice sites, where the required phase accumulation is achieved by using a special geometry for the Raman beams. This idea was developed further in Ref. [9, 10]. It was shown that one can also introduce an exotic (unphysical) field and non-Abelian gauge potentials by using the Raman-laser technique. For atoms in a harmonic trap (no lattice) the Raman scheme with 3 spin states of the F = 1 electronic ground state of Rb atom was realized in the recent experiment [11]. In this setup the magnitude of the effective magnetic field is defined by the gradient of the real magnetic field which splits the F = 1 level into Zeeman sublevels. In the paper the authors put forward a conjecture that this scheme should work in the presence of an optical lattice as well. We note that the cited experiment also illuminated a drawback of the Raman-laser based techniques. Namely, the spontaneous emission from the Raman beams removes atom from the trap, causing the population to decay within approx. 2 seconds.

研究动机与目标

• 开发一种简单、实验可行的方法，在二维光晶格中实现人工磁场，且不依赖拉曼激光。
• 克服现有基于拉曼激光方案中自发辐射导致的原子损失问题（在约2秒内发生）。
• 通过周期性驱动诱导有效磁通，实现中性原子的回旋动力学观测。
• 提出可通过干涉图样和质心波包动力学检测有效磁场的可观测信号。

提出的方法

• 通过低频周期调制光晶格势能，诱导晶格位点之间的光子辅助隧穿。
• 设计驱动场，使原子在闭合回路中隧穿时获得相位移，模拟磁通量。
• 利用动态隧穿机制，生成等效于均匀人工磁场的规范势。
• 利用原子波函数的干涉图样检测有效磁场的存在与大小。
• 分析波包质心运动，观测类回旋轨道，特征为带电粒子在磁场中的行为。
• 确保装置不使用拉曼激光，从而最大限度减少自发辐射和原子损失。

实验结果

研究问题

• RQ1 低频周期性驱动光晶格是否可在无需拉曼激光的条件下，为中性原子生成有效磁场？
• RQ2 所诱导的人工磁场如何影响二维晶格中原子波包质心的动力学？
• RQ3 由于有效磁通引起的相位累积，会形成何种可测量的干涉图样？
• RQ4 该方法在多大程度上避免了拉曼激光方案中固有的自发辐射问题？

主要发现

• 所提出的方法在周期性驱动下通过光子辅助隧穿成功生成有效磁场，使中性原子实现类回旋动力学。
• 质心波包运动表现出与磁场中经典带电粒子轨迹相似的轨迹，证实了有效洛伦兹力的存在。
• 原子波函数的干涉图样为有效磁通提供了直接可观测信号，支持实验检测。
• 该方案避免使用拉曼激光，从而消除了自发辐射导致原子损失的主要来源，解决了以往装置中寿命受限于约2秒的问题。
• 通过调节驱动频率和振幅，可调控每个晶胞内的有效磁通，实现人工磁场的可调谐性。