Many-Body Physics with Ultracold Atomic Fermions and Bosons

原子气体冷却到绝对零度以上1000亿分之一度的超低温，已经成为研究集体多体行为的一个高度通用的平台（多体行为是关于物体的集合如何以不同于人们对物体如何单独行为的知识所期望的方式进行行为的物理学）。这种集体现象通常与固体物质中的电子有关，其中晶体结构的细节，电子之间的强相互作用，以及描述系统的物理学基础理论（量子力学）可以结合起来创造出具有高度不寻常且有时非常实用的特性的材料。这种材料最著名的例子是高温超导体，在高温超导体中，电流流动没有任何阻力。令人惊讶的是，在发现高温超导体近30年后，物理学家仍然不了解产生高温超导体的潜在机制；他们也不知道超导温度是否有上限。用超冷原子在光波形成的所谓“光学晶格”上进行实验，可能有助于解开这个谜团，从而促进高温超导体实际应用的发展，包括有效传输电能和更具成本效益的医学成像。在这个实验中，锂原子与电子遵循相同的基本物理定律，将在实际材料中充当电子的替身。原子系统比实际材料干净得多，因为没有杂质、缺陷或晶格错位。此外，原子系统的参数，包括原子-原子相互作用强度、密度和晶格参数，是高度可调的。超冷锂原子将用于创建最有希望的模型，并确定它是否包含超导的本质。该小组将利用锂同位素，费米子Li-6和玻色子Li-7中广泛的费什巴赫共振，在具有基础和实际意义的背景下探索晶格内外的多体物理。由于无法冷却到足够低的温度，在光学晶格中实现超冷原子的全部潜力一直受到阻碍。该小组最近展示了一种在光学晶格中蒸发冷却的方法，该方法在费米-哈伯德模型中观察到反铁磁相关性，费米-哈伯德模型是凝聚态物理的原型模型，也是最著名的高温超导模型。甚至需要更低的温度来探索最新颖的强相关现象。该小组建议改进冷却方法，使用磁相关的布拉格散射作为敏感的原位温度计。有了这个系统，该小组计划研究Hubbard模型的相图，用于排斥和吸引相互作用。理论表明，结合自旋极化（种群不平衡）的吸引相互作用为观察奇异的FFLO配对机制提供了一个令人兴奋的机会。最后，对于玻色子同位素，该小组将对非相互作用和相互作用玻色凝聚体的物质波隧穿进行基础研究，以探索非线性的作用，包括孤子的状态。障壁形成分束器，分束器在一维几何结构中构成马赫-曾德尔干涉仪。它的相干特性，因此，它的适用性作为一个物质波干涉仪，将探讨。

Gases of atoms cooled to the ultra-low temperatures of 100 billionths of a degree above absolute zero have emerged as a highly versatile platform for the study of collective many-body behavior (the physics of how a collection of objects behaves in ways that is different from what one would expect from knowledge of how the objects behave individually). Such collective phenomena are usually associated with electrons in solid matter, where details about the crystal structure, strong interactions between electrons, and the underlying theory in physics which describes the system (quantum mechanics) can combine to create materials with highly unusual and sometimes very practical properties. The best known examples of such materials are high-temperature superconductors, in which electron currents flow without any resistance. Surprisingly, nearly 30 years after their discovery, physicists still do not understand the underlying mechanisms that create high-temperature superconductors; nor do they know if there is an upper bound on the superconducting temperature. Experiments with ultra-cold atoms on a so-called "optical lattice" created with light waves may help solve this mystery, thereby facilitating the development of practical applications of high temperature superconductors, including the efficient transmission of electrical energy and more cost-effective medical imaging. In this experiment, lithium atoms, which obey the same basic laws of physics as electrons, will act as stand-ins for the electrons in real materials. The atomic system is much cleaner than the real material, since there are no impurities, defects, or lattice dislocations. Furthermore, the parameters of the atomic system, including the atom-atom interaction strength, density, and the lattice parameters, are highly tunable. Ultra-cold lithium atoms will be used to create the most promising model and determine whether or not it contains the essence of superconductivity. The group will exploit the broad Feshbach resonances in the lithium isotopes, the fermion Li-6 and the boson Li-7, to explore many-body physics, in and out of lattices, in contexts that have both fundamental and practical implications. Realization of the full potential of ultracold atoms in optical lattices has been impeded by an inability to cool to sufficiently low temperatures. The group has recently demonstrated a method to evaporatively cool in optical lattices which resulted in the observation of antiferromagnetic correlations in the Fermi-Hubbard model, an archetypal model of condensed matter physics and the most prominent model of high-temperature superconductivity. Even lower temperatures are needed to explore the most novel strongly correlated phenomena. The group proposes to refine the cooling method, using Bragg scattering from magnetic correlations as a sensitive in-situ thermometer. With this system, the group plans to study the phase diagram of the Hubbard model for both repulsive and attractive interactions. Theory suggests that an attractive interaction combined with spin polarization (population imbalance) offers an exciting opportunity to observe the exotic FFLO pairing mechanism. Finally, for the bosonic isotope, the group will conduct a fundamental investigation of matter-wave tunneling for non-interacting and interacting Bose condensates to explore the role of the nonlinearity, including the regime of solitons. The barrier forms a beam splitter, which in a one-dimensional geometry constitutes a Mach-Zehnder interferometer. Its coherence properties and hence, its suitability as a matter-wave interferometer, will be explored.