Investigations in Gravitational Quantum Physics

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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.