Investigations in Gravitational Quantum Physics

由亚原子粒子、原子和小分子组成的微观量子世界的行为方式可能与日常经验的宏观经典世界截然不同。特别是，一个小分子可以奇怪地“同时”沿着两条不同的路径运动，而另一方面，当你把一个球抛向空中时，它只会沿着一条由你投掷它的方式决定的路径运动。普遍接受的解释是，系统越大，它与环境的相互作用就越多；通常，单个任性的光量子（即光子）与宏观物体相互作用，使其坍缩到单一路径上就足够了。然而，原则上我们可以减少与环境的相互作用，这就提出了一个有趣的问题：一个宏观量子物体“同时在两个地方”能有多大？在这个项目中，PI将量化重力作为宏观经典性可能的基本执行者的影响；与其他日常环境相比，重力是无法消除的。该项目旨在提供有助于指导宏观量子实验发展的预测，宏观量子实验目前是一个活跃和发展中的研究领域。这些结果将直接引起国际相对论量子信息社区以及从事量子信息科学的实验家和理论家的兴趣，例如提供了量子计算机可以实现多大的基本限制。该项目将为一名博士后和一名研究生提供为期三年的理论物理主题培训。该项目由量子信息科学计划（物理部）和促进竞争研究的既定计划（EPSCoR）共同资助。迄今为止，局域位置态的量子叠加已经被实验证明是在米级分离的原子、亚微米级分离的大原子序数分子和亚皮米级分离的微米级振动结构上。人们普遍接受的原因是，在宏观的日常情况下，没有观察到类似Schrödinger猫的状态，因为它们与空气分子、光子、物体内部缺陷等的相互作用，会极快地退散。这种与物体环境的相互作用原则上可以通过在超高真空和电磁辐射屏蔽内冷却悬浮物体来抑制。然而，有一种环境无法被屏蔽，那就是重力，即引力波背景辐射。最近的一些努力已经开始解决宏观质量和能量叠加态的退相干率，目标是在原则上提供宏观叠加态寿命的基本界限。拟议的活动包括引力量子物理学领域的两个项目，定义为弱引力存在下的量子动力学研究。一个项目将利用涉及弱重力的量子场论技术来量化由引力诱导退相干设定的质量系统空间叠加态寿命的上限。该方法将考虑一个思想实验，其中通过量子干涉测量获得退相干率。另一个项目虽然与第一个项目不同，但确实通过等效原理与重力联系在一起。特别是，该项目将考虑在微波腔中经历振荡加速的缺陷状光电探测器云，并量化由此产生的真空光子探测/产生。超过一定的临界探测器数后，光子产生率可能发生相变，成探测器数的平方，从而显著提高了超出正常随探测器数成比例的产生率。这种超辐射相的存在可能会增加实验验证加速光电探测器在真空中产生光子的可能性。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 microscopic quantum world of subatomic particles, atoms, and small molecules can behave in a radically different way from the macroscopic classical world of everyday experience. In particular, a small molecule can bizarrely travel along two different paths 'at the same time’, while on the other hand when you throw a ball up in the air for example, it only ever follows a single path determined by how you threw it. The commonly accepted explanation is that the larger the system, the more it interacts with its environment; it is usually sufficient for a single wayward light quantum (i.e. photon) to interact with a macroscopic object to cause it to collapse onto a single path. However, we can in principle reduce the interactions with the environment, which then raises the fascinating question: how large can a macroscopic quantum object be in 'two places at once'? In this project, the PI will quantify the effects of gravity as the possible fundamental enforcer of macroscopic classicality; in contrast to the other everyday environments, gravity cannot be removed. The project will aim to provide predictions that can help guide the development of macroscopic quantum experiments, currently an active and developing area of research. The outcomes will be of direct interest to the international relativistic quantum information community, as well as to experimentalists and theorists working in quantum information science, providing for example fundamental limits on how large a quantum computer can be realized. The projects will provide training over three years for one postdoctoral fellow and one graduate student in a diverse range of theoretical physics topics. This project is jointly funded by the Quantum Information Science Program (Physics Division), and the Established Program to Stimulate Competitive Research (EPSCoR). Quantum superpositions of localized position states have to date been experimentally demonstrated for atoms with meter-scale separations, for large atomic number molecules with sub-micrometer scale separations, and for micrometer-sized vibrating structures with sub-picometer scale separations. The commonly accepted reason for not observing similar Schrödinger cat-like states in macroscopic, everyday situations is that they decohere away extremely rapidly due to interactions with air molecules, photons, defects internal to the objects etc. Such interactions with the object's environment can in principle be suppressed by cooling the suspended object in ultrahigh vacuum and inside an electromagnetic radiation shield. The one environment that cannot be shielded out, however, is gravity, i.e., gravitational wave background radiation. A number of recent efforts have set out to address the decoherence rates of macroscopic mass and energy superposition states, with a goal to provide in principle fundamental bounds on the lifetimes of macroscopic superposition states. The proposed activity comprises two projects within the area of Gravitational Quantum Physics, defined as the study of quantum dynamics in the presence of weak gravity. One project will utilize quantum field theoretic techniques involving weak gravity to quantify the upper limits on the lifetimes of mass system spatial superposition states set by gravitationally induced decoherence. The approach will consider a thought experiment, where the decoherence rate is obtained through a quantum interference measurement. The other project, while distinct from the first one, does connect to gravity through the equivalence principle. In particular, the project will consider a cloud of defect-like photodetectors undergoing oscillatory acceleration in a microwave cavity and quantify the photon detection/production from vacuum that results. Beyond a certain critical detector number, the photon production rate may undergo a phase transition, scaling as the square of the detector number and thus significantly enhancing the production rate beyond the normal scaling with detector number. The existence of this superradiant-like phase may increase the possibility of experimentally verifying photon production from vacuum for accelerating photodetectors.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.