Antiferromagnetic Spinor Bose-Einstein Condensates

我们的实验项目探索了钠原子的自旋玻色-爱因斯坦凝聚（BECs）。量子流体的独特性质源于它的反铁磁相互作用，这些特性引导我们探索超冷原子和相关电子材料之间丰富的前沿领域。与它的铁磁性表亲铷-87不同，光阱中的钠BEC具有向列序参数。提出了一种新的非破坏性成像方案来揭示这一顺序的空间依赖性，基于检测自旋排列而不是方向。向列性的空间分布将在二次塞曼效应介导的量子相变附近测量。该方法将用于检查空间波动的动力学，包括量子半漩涡的成核，以及平衡基态的结构。反铁磁相互作用激励我们通过实验实现和探索完美混相双组分BEC中运动退相干的极限。最后，在二维反铁磁BEC中，预测了由具有非平凡自旋（反）相关的玻色子原子对介导的Berezinskii-Kosterlitz-Thouless跃迁。磁力与自旋密切相关。在冷却到接近绝对零度的原子气体中，单个原子的自旋相互作用产生有序状态——如果磁矩彼此对齐，则为铁磁状态；如果磁矩彼此相反，则为反铁磁状态。在反铁磁体中，自旋在空间中选择一个轴，并沿着该轴安排自己正或负指向，因此总磁矩为零。反铁磁体和铁磁体之间的能量差，如果用温度来表示，只比绝对零度高几十亿分之一度。值得注意的是，我们可以通过实验区分这两种状态，所以原子确实非常强烈地表达了它们的偏好。我们的研究将研究反铁磁体如何响应外部磁场重新排列其对准轴。这就是所谓的相变，类似于温度升高到接近沸点时液体的变化。然而，在我们的气体中，这种转变不是由热效应介导的，而是由量子力学介导的。众所周知，量子相变发生在许多固态材料中，包括超导体，我们的研究可以揭示它们的性质。除了基本利益之外，它们还可能导致利用量子力学定律的新设备，如拓扑量子计算机。

Our experimental program explores spinor Bose-Einstein condensates (BECs) of sodium atoms. Unique properties of the quantum fluid derive from its antiferromagnetic interactions, features that lead us to explore the rich frontier area between ultracold atoms and correlated electronic materials. Unlike its ferromagnetic cousin, rubidium-87, the sodium BEC in an optical trap possesses a nematic order parameter. A novel non-destructive imaging scheme is proposed to uncover the spatial dependence of this order, based upon detection of spin alignment rather than orientation. The spatial distribution of nematicity will be measured in the vicinity of a quantum phase transition mediated by the quadratic Zeeman Effect. The method will be used to examine both the dynamics of spatial fluctuations, including the nucleation of quantum half-vortices, as well as the structure of the equilibrium ground state. The antiferromagnetic interactions motivate us to experimentally realize and explore the limits to motional decoherence in a perfectly miscible two-component BEC. Finally, in a two dimensional antiferromagnetic BEC a Berezinskii-Kosterlitz-Thouless transition is predicted that is mediated by bosonic pairs of atoms with non-trivial spin (anti)-correlations.Magnetism is intimately connected to spin. In a gas of atoms cooled to near absolute zero the spins of the individual atoms interact with one another to create ordered states - ferromagnetic if the magnetic moments are aligned with each other, or antiferromagnetic if the moments tend to oppose one another. In an antiferromagnet, the spins choose an axis in space and arrange themselves to point both positively and negatively along that axis, so that the total magnetic moment is zero. The energy difference between antiferromagnet and ferromagnet, if it were expressed as a temperature, is only a few billionths of a degree above absolute zero. Remarkably, we can experimentally distinguish between these two states, so the atoms are expressing their preference very strongly indeed. Our research will examine how an antiferromagnet reorders its axis of alignment in response to an external magnetic field. This is known as a phase transition, analogous to what happens to a liquid as the temperature increases to near its boiling point. In our gas, however, the transition is not mediated by thermal effects, but by quantum mechanics. Quantum phase transitions, as these are known, occur in many solid state materials, including superconductors, and our research can shed light on their properties. Apart from fundamental interest, they could lead to new devices that exploit the laws of quantum mechanics such as topological quantum computers.