RUI: Topological Excitations in Spin-1 and Spin-2 Bose-Einstein Condensates

对称性是自然界的核心组织原则之一。它适用于我们能想到的能量最高的系统--早期宇宙--也适用于一些能量最低的系统，比如一种稀薄气体，冷却到仅比绝对零度高几十亿分之一度。这个桌面实验项目利用对称性的主题，专注于超冷气体的高度可控、低能环境，在那里可以进行直接的实验研究。尽管我们经常从空间模式的角度来考虑对称性，比如晶体的规则结构，但对称性也可以是内在的，隐藏在视线之外。在这种情况下，它们的影响表现在介质中可能存在的类粒子(或“准粒子”)激发的类型和行为上。可以在超冷气体中观察到的准粒子的例子包括单极、纽结、天子和涡旋，它们中的每一个在宇宙中都有一个(尚未观察到的)类似物。该项目将研究这种准粒子在具有比先前实验更宽的内部对称性范围的介质中的产生和时间演化。由此产生的准粒子行为预计会更加奇特：例如，两个涡旋之间的普通碰撞可能会变成连接细丝在它们之间的区域发展的碰撞，留下了相遇的永久物理记录。该项目为本科生提供了前沿科学技术培训的机会，从而为下一代公民科学家的教育做出了贡献。本实验研究项目探索了光学囚禁Rb-87玻色-爱因斯坦凝聚体中拓扑激发的产生和时间演化。这些超流体中的自旋自由度导致了具有不同内部对称性的各种磁相。每个阶段都可以承载特定的拓扑激励。RB-87凝聚态特别有趣，因为它同时具有自旋1和自旋2的基态超精细流形。自旋1系统相对简单，只有两个磁相。它提供了一个方便的跳板来理解自旋-2系统，该系统有五个磁相，其中两个具有完全离散的对称性：环四面体和双轴向列相。自旋-2 RB-87系统还没有完全表征，近期的实验目标包括(I)确定基态磁相，以及(Ii)了解每个磁相的基线时间演化。双轴向列相和环四面体相中的拓扑激发是相当感兴趣的，因为离散的对称性允许具有分数环流的涡旋。此外，在这些阶段的涡旋之间的碰撞预计会产生“横级涡旋”，这是连接离开的涡旋的永久细丝。除了涡旋之外，这些实验还将研究单轴向列相中单极的产生和时间演化，预计单极将衰变成涡环，以及离散对称磁相中奇异的天子。通过将凝析油暴露在精心定制的随时间变化的磁场和光场中，将产生不同的激发，并将使用现有的成像技术进行表征。PI和他的本科生合作者获得的结果，预计将直接有助于我们对物理许多分支的拓扑激发的科学理解。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势和更广泛的影响审查标准进行评估，被认为值得支持。

Symmetry is one of the central organizing principles in the natural world. It applies to the most energetic system we can contemplate - the early universe - as well as to some of the least energetic, such as a dilute gas cooled to only tens of billionths of a degree above absolute zero. This tabletop experimental project uses the theme of symmetry to focus on the highly-controllable, low-energy environment of an ultracold gas where direct experimental investigation is possible. Although we often think of symmetry in terms of spatial patterns, such as the regular structure of a crystal, symmetries can also be internal and hidden from view. Their influence in such cases manifests itself in the type and behavior of the particle-like (or "quasi-particle") excitations that can exist within the medium. Examples of quasi-particles that can be observed in an ultracold gas include monopoles, knots, skyrmions, and vortices, each of which has an (as-yet unobserved) analogue in the cosmos. The project will study the creation and time evolution of such quasi-particles in a medium with a broader range of internal symmetries than in previous experiments. The resulting quasi-particle behavior is expected to be more exotic: an ordinary collision between two vortices, for instance, can become one in which connecting filaments develop in the region between them, leaving behind a permanent physical record of the encounter. The program provides opportunities for cutting-edge scientific and technological training for undergraduate students, thereby contributing to the education of the next generation of citizen-scientists.This experimental research program explores the creation and time evolution of topological excitations in optically trapped rubidium-87 Bose-Einstein condensates. The spin degree of freedom in these superfluids leads to a variety of magnetic phases with different internal symmetries. Each phase can host specific topological excitations. The Rb-87 condensate is especially interesting because it has both spin-1 and spin-2 ground state hyperfine manifolds. The spin-1 system is relatively simple, with only two magnetic phases. It provides a convenient springboard from which to understand the spin-2 system, which has five magnetic phases, two of which have entirely discrete symmetries: cyclic-tetrahedral, and biaxial nematic. The spin-2 Rb-87 system is not fully characterized, and immediate experimental goals include (i) determining the ground state magnetic phase, and (ii) understanding the baseline time-evolution of each magnetic phase. Topological excitations in the biaxial nematic and cyclic-tetrahedral phases are of considerable interest as the discrete symmetries permit vortices with fractional circulation. Moreover, collisions between vortices in these phases are expected to yield "rung vortices," which are permanent filaments that bridge the departing vortices. Beyond vortices, the experiments will examine the creation and time evolution of monopoles in the uniaxial nematic phase, which are expected to decay into vortex rings, as well as of exotic skyrmions in the discrete-symmetry magnetic phases. The different excitations will be generated by exposing the condensate to carefully tailored time-dependent magnetic and optical fields, and will be characterized using established imaging techniques. The results, obtained by the PI and his undergraduate collaborators, are expected to contribute directly to our scientific understanding of topological excitations across many branches of physics.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.