RUI: Experiments with Topological Excitations in Bose-Einstein Condensates

现代物理学常常是对极端环境的探索。例如，欧洲核子研究中心的大型强子对撞机是一个粒子加速器，研究质子以极高能量碰撞时会发生什么，短暂地产生模拟早期宇宙的条件。在能量尺度的另一端，极低能量的碰撞（温度仅比绝对零度高几百亿分之一度）会导致超流动性现象，在这种现象中，流体（如非常稀的气体）流动时没有任何粘度。值得注意的是，超流体气体本身就是一个原始的“宇宙”，拥有自己的“粒子”，这些粒子可以与宇宙中存在（或预期存在）的粒子类似。一个例子是一种被称为磁单极子的粒子，它还没有被观察到，但最近在超流体中进行了模拟。创造这种类似粒子的东西使科学家们能够研究宇宙中预期粒子的一些特性，否则这些特性是完全无法达到的，因为它们的创造甚至超过了最强大的粒子加速器的能力。该项目的研究致力于创造和研究几种类粒子结构，包括单极子的直接类似物，以及被称为“结”和“介子”的更奇特的结构。作为模拟，它们有望进一步深入了解宇宙的基本物理过程；但它们本身也很有趣，因为它们是新物理学的例子，在许多情况下，是完全未被探索的物理学。该研究项目还为积极进取的本科生、一名博士后研究员和一名中学教师提供培训机会，我们将与他们接触并激励下一代科学家和公民。拓扑结构是许多不同能量和长度尺度的物理学分支的核心，包括宇宙学、粒子物理学和凝聚态物理学。超流体，如玻色-爱因斯坦凝聚体，为在高度控制的环境中研究这些结构提供了新的和令人兴奋的机会。值得注意的是，描述超流体的序参量的复杂性决定了它能支持什么样的拓扑激励。在这里，研究人员将通过实验研究这些拓扑激发的几个方面，包括非零温度下标量凝聚体（总自旋F=0）中的量子化涡旋动力学，伪自旋凝聚体（有效自旋F=1/2）中涡旋分子（或“介子对”）的产生，以及自旋凝聚体（自旋F=1）中量子场（或“结”）中的连接场构型。通过将凝聚物暴露于随时间变化的磁场和光场，将对激发进行印记，随后将使用已建立的成像技术通过检查原子密度剖面进行分析。后续研究将研究它们的动力学和相互作用，并根据需要开发新的成像技术。

Modern physics is often an exploration of the extreme environments. For example, the Large Hadron Collider at CERN is a particle accelerator that studies what happens when protons collide at extremely high energy, briefly generating conditions that mimic those of the early universe. At the other end of the energy scale, extremely low-energy collisions (at temperatures only tens of billionths of a degree above absolute zero) contribute to the phenomenon of superfluidity, in which a fluid (such as a very dilute gas) flows without any viscosity. Remarkably, the superfluid gas is itself a pristine "universe," hosting its own "particles" that can be analogues of those existing (or expected to exist) in the cosmos. One example is a particle known as a magnetic monopole, which has not yet been observed but has recently been simulated in a superfluid. Creating such particle-like analogues permits scientists to study some of the properties of anticipated particles in the universe that would otherwise be completely inaccessible, as their creation would exceed the capabilities of even the most powerful particle accelerators. The research in this project is devoted to creation and study of several particle-like structures, including direct analogues of monopoles and more exotic structures known as "knots" and "merons." As simulations, they promise additional insight into the fundamental physical processes of our universe; but they are also of interest in their own right as examples of new and, in many cases, completely unexplored physics. The research program also provides training opportunities for highly motivated undergraduates, a postdoctoral researcher, and a secondary school teacher, with whom we will engage and motivate the next generation of scientists and citizens. Topological structures are central to diverse branches of physics at many different energy and length scales, including cosmology, particle physics, and condensed-matter physics. Superfluids, such as Bose-Einstein condensates, provide new and exciting opportunities to examine these structures in highly-controlled environments. Notably, the complexity of the order parameter describing the superfluid determines what kind of topological excitations it can support. Here, researchers will examine experimentally several aspects of these topological excitations, including quantized vortex dynamics in scalar condensates (total spin F=0) at nonzero temperature, creation of vortex molecules (or "meron pairs") in pseudospinor condensates (effective spin F=1/2), and linked field configurations in quantum fields (or "knots") in spinor condensates (spin F=1). The excitations will be imprinted by exposing the condensate to time-dependent magnetic and optical fields, and will subsequently be analyzed through examination of atomic density profiles using established imaging techniques. Subsequent studies will study their dynamics and interactions, developing new imaging techniques as required.