This grant supports efforts by Gurudev Dutt, Professor of Physics at the University of Pittsburgh, to improve experimental capabilities in physics to demonstrate quantum phenomena in increasingly large, heavy systems. Quantum phenomena, such as an object existing simultaneously in two places, are counterintuitive and have never been directly visualized because quantum states like superpositions have never been realized using macroscopic systems directly accessible to human senses. Why this is the case is a deep question in fundamental physics and falls under a field of research known as ‘quantum foundations.’ It’s thought that environmental interactions—bumping into a gas molecule or absorbing a photon of light—play an important role by destroying typically-fragile quantum states. Even assuming environmental isolation, however, a macroscopic quantum state may not be achievable, as gravitational interactions may destroy (decohere) the quantum state of a sufficiently heavy object. Experiments that work to create ever larger, heavier quantum states that last for increasingly long periods of time are needed to understand the practical and possibly fundamental limits to realizing macroscopic quantum states. Professor Dutt and collaborators are launching a project focused on creating and maintaining large, heavy quantum superpositions. While the work is of interest from a quantum foundations perspective, it’s also critical to enabling a future experiment capable of answering one of the most important open questions in physics: whether gravity is a quantum force.  Achieving large, heavy quantum superpositions stands as a major challenge that must be overcome to enable that experiment. Here the Dutt team intends to magnetically levitate a nanogram-mass, few-micron-sized diamond crystal and put it into a superposition state that lasts for about a second. Levitation under ultra-high vacuum conditions will provide a reasonable degree of environmental isolation for the diamond, and the proposed nanogram-scale superposition would be the heaviest quantum superposition achieved. To date, the heaviest object put into a quantum superposition suitable for matter-wave interferometry is about one million times less massive than the diamond Dutt and colleagues will target.  The Dutt team will first demonstrate a protocol for placing a diamond in a superposition of being in two places at once. In brief, a laser puts a microdiamond crystal into a spin superposition state (diamond simultaneously in two different spin states) and a magnetic field is then used to ‘transfer’ the spin superposition to a spatial superposition (diamond simultaneously in two different locations). Achieving a superposition lifetime of about a second will be challenging, in part due to the lack of good tools for measuring motional decoherence as this makes it difficult to assess the effectiveness of various measures aimed at extending motional coherence lifetimes. Dutt will test a new approach that leverages spin-based measurements to measure motional decoherence.  The team will also develop light-scattering-based and spectroscopy-based diamond-position monitors to directly measure motional noise. As a separate line of research, the research team will explore whether it’s possible to significantly increase superposition size by driving more than one spin excitation in a microdiamond.