Advances in Casimir-Polder Interactions between Atoms and Substrates

量子真空是一个繁忙的地方，粒子在短时间内突然出现和消失。这种活动产生了令人惊奇的现象，例如卡西米尔在70多年前发现的间隔很近的不带电导电板之间或中性原子之间的吸引力。如何在实际设备和纳米机械中利用这些卡西米尔力是一个开始。通过改变相互作用的物体的性质，以及它们所处的环境，这些力可以是排斥力而不是吸引力。当物体和原子运动时，原子和表面之间即使不接触也会产生量子摩擦力。这个项目促进了科学的进步，促进了不同群体的学生和研究人员的教育。这项跨学科的工作对生物学、化学、原子和核物理学都有重大影响，甚至在行星科学中也有应用。需要考虑的具体主题包括：(1)负卡西米尔熵。众所周知，原子之间的相互作用熵，或者原子与金属表面之间的相互作用熵，通常是负的。虽然有人认为原子本身的正自熵会抵消这种效应，但我们现在发现，通常情况并非如此，而且自相互作用通常会导致一个负熵区域。(2)非均匀介质中原子与表面之间的量子真空力。几乎所有关于物体间卡西米尔力的研究都假定它们被真空或均匀的介电介质隔开。但是，如果它们被空间变化的介质隔开，就会出现我们无法很好理解的发散。该小组已经就如何提取有意义的相互作用能量提出了一些初步建议，但总体情况将需要更多的工作。(3)排斥性卡西米尔力和非单调转矩。卡西米尔力可以变成排斥力，而不仅仅是用奇异的材料组合。这与随处可见的负熵不无关系，反映了自由能的非单调性。在各向异性材料中，卡西米尔力矩会随着原子和表面之间的距离发生符号变化。正在探索将其应用于诸如冰的冻结和温室气体与基质的相互作用等长期存在的问题。(4)卡西米尔摩擦。当一个原子或一个介电板与附近的板平行运动时，由于与电磁真空的波动相互作用而产生摩擦力。这需要理解非平衡耗散效应。处理这种现象的系统方法正在开发中，目的是提出可访问的实验签名。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The quantum vacuum is a busy place, with particles popping into and out of existence for short periods of time. This activity gives rise to amazing phenomena, such as the attractive force between closely-spaced uncharged conducting plates, or between neutral atoms, discovered by Casimir more than 70 years ago. A beginning is being made to see how to exploit these Casimir forces in practical devices and nanomachinery. By changing the properties of the objects that interact, and the environment in which they are immersed, the forces can be repulsive rather than attractive. When the objects and atoms are in motion, quantum frictional forces arise between atoms and surfaces even when they are not in contact. This project advances the progress of science and promotes the education of a diverse group of students and researchers. This interdisciplinary work has significant impacts in biology, chemistry, atomic and nuclear physics, and is even finding applications in planetary science.Specific topics to be considered include: (1) Negative Casimir entropies. It is now well known that the interaction entropy between atoms, or between atoms and metallic surfaces, for example, are often negative. Although it was suggested that the positive self-entropies of the atoms themselves would cancel this effect, we have now discovered that this is in general not the case, and that self interactions typically lead to a region of negative entropy. (2) Quantum vacuum forces between atoms and surfaces in inhomogeneous media. Nearly all work on Casimir forces between bodies assumes they are separated by vacuum, or by a homogeneous dielectric medium. But if they they are separated by a spatially varying medium, divergences appear which are not well understood. The group has made some preliminary proposals as to how to extract meaningful interaction energies, but the general situation will require much more work. (3) Repulsive Casimir forces and nonmonotonic torques. Casimir forces can turn repulsive, and not merely with exotic combinations of materials. This is not unrelated to the negative entropies seen ubiquitously, and reflects the nonmonotonicity of the free energy. With anisotropic materials, Casimir torques can undergo sign changes with the distances between atoms and surfaces. Real-world applications to such long-standing problems such as the freezing of ice, and the interaction of greenhouse gases with substrates are being explored. (4) Casimir friction. When an atom or a dielectric plate is moved parallel to nearby plate, a frictional force is experienced due to the interaction with the fluctuations in the electromagnetic vacuum. This requires understanding nonequilibrium dissipative effects. Systematic ways of treating such phenomena are being developed, with the intent of proposing accessible experimental signatures.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.