Of the four fundamental forces in nature, three are well-described by quantum theory while the fourth — gravity — is not. Obtaining a unified theory that describes all four forces ranks among the most important goals in fundamental physics. Perhaps the most pressing question on the road to unification is whether gravity is a quantum force. Experiments that directly probe quantum gravity have been considered infeasible because the fundamental unit of ‘quantized’ space is exceedingly small. Directly probing this scale would require a particle accelerator the size of our galaxy. Instead, what physicists have been trying to do for many decades is to think of viable experiments that would teach us something about quantum gravity. Two recent papers envision just such an experiment.  That experiment, unfortunately, is not feasible given the limitations of current experimental physics. Yet there’s considerable optimism in some physics communities that these limitations may be overcome in the reasonably near future, perhaps in five to ten years. This grant provides funding to Gavin Morley, Professor of Physics at Warwick University, who is leading a collaboration that aims to tackle the primary challenges to performing the proposed experiment. Summarized broadly, what’s needed is the experimental capacity to create and manipulate large, heavy quantum systems and to arrange for conditions that allow gravity to be the dominant interaction between two laboratory-scale objects. In quantum physics, entanglement refers to a non-intuitive connection between objects that’s evidenced by measuring some property of the objects and showing that the measurements are more highly correlated than is allowed by classical physics. The proposed future experiment aims to determine whether two objects (diamonds) can become entangled via their mutual gravitational attraction. If they can, then gravity must be a quantum force, as asserted by two recently published papers. Morley and his team will address three lines of research critical to enabling that future quantum gravity experiment. First, it’s important that gravity be the dominant interaction between two objects, here micron-scale diamond samples (microdiamonds). Electromagnetism is much stronger than gravity so electromagnetic (EM) interactions must be heavily attenuated. The plan is to first measure the strength of EM interactions between two microdiamonds, and between a microdiamond and nearby experimental components. After measuring and understanding the sources of microdiamond EM interaction, the PIs will implement attenuation measures. Next, the researchers will demonstrate matter-wave interferometry involving objects much heavier than what’s been achieved to date. Heavy objects are required because the strength of gravity scales with mass so heavy objects are more easily entangled. Interferometry is required because the entanglement will be evidenced by a gravity-induced shift in the interference pattern. Morley will release a magnetically-levitated diamond and allow it to fall onto a laser-induced diffraction grating that creates a matter-wave interference pattern. By studying the ‘sharpness’ of the matter-wave interference pattern, Morley will learn about sources of ‘decoherence’ that limit how long the superposition lasts. Finally, the team will follow a protocol they’ve proposed to put a microdiamond into a spatial superposition of being in two places at once by using a laser to create a spin superposition (diamond simultaneously in two spin states) and then using a magnetic field to drive the two different spin components in opposite directions; thereby also achieving a spatial superposition. This will be done under conditions where EM forces are attenuated by rotating the diamond and by using a conductive screen.

General relativity—which explains gravity on large length and mass scales—and quantum mechanics—which explains the behavior of atoms and molecules--are our two most fundamental descriptions of nature. One of the most important unsolved problems in physics is uncovering the correct way to bring these two theories together. While theorists have proposed many possibilities, experiments are needed to guide the way. Relevant experimental data is hard to come by, however, because gravity is weak and extremely sensitive instruments are needed to detect its influence on (typically small) quantum systems. Funds from this grant support Jun Ye, Adjoint Professor of Physics at the University of Colorado at Boulder, to make improvements to the most precise instruments around—atomic clocks—and to then use these improved clocks to perform two types of laboratory-scale experiments: novel laboratory-scale tests of general relativity, and the first experiments where general relativity has measurable effects on the evolution of a quantum system. Starting with the first class of experiments, while general relativity (GR) has been tested previously, foundational GR principles are related to one another, making it challenging to test any one principle in isolation. Ye and his team will conduct atomic-clock-based measurements that will allow tests either of isolated principles or of the principles in different combinations as well as set  limits on parameters that can be used to develop GR-alternative theories. Principles to be tested include the Einstein Equivalence Principle; the Accelerated Clock Hypothesis; the equivalence of energy and mass; and the equivalence of gravitational and inertial mass. As to the second class of experiments, the improvements in atomic clocks will enable Ye and his team to perform measurements that probe the differential flow of time (GR time-dilation) across the wave function of a quantum system. Project theorists will compute the precise dynamics of such systems and help determine how they can best be detected using atomic clock methods. Ye’s efforts promise to improve the precision of atomic clocks by a factor of 10, achieve new standards of precision in measurements that employ quantum measurement protocols, and develop entirely new measurement protocols for detecting novel phenomena that arise at the interface of quantum physics and general relativity.

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.

Advances in AI and machine learning, and specifically large language models such as ChatGPT, Bard and Claude, herald a transformational period in access to knowledge.   This grant provides three years of support to the Wikimedia Foundation, the parent organization of Wikipedia, for a new initiative to harness these advances to improve Wikipedia’s core function—access to reliable knowledge for half a billion people each month—and to protect the largest encyclopedia in human history from vandalism and even potential obsolescence.  Planned activities over the grant period include the iterative improvement and user testing of a Wikipedia plug-in using ChatGPt—now available in an experimental beta version and generating 1000 queries per day; use of AI and machine learning tools to suggest valuable edits that human editors could use to improve articles; an improved open-source neural machine translation model that supports over 200 languages, significantly boosting Wikipedia’s translation capabilities; machine learning tools to help moderators filter bad edits; a machine learning model that will let volunteers build and host their own AI models and tools (versus those built by Wikipedia staff); and a new ML moderator tool called Automoderator that will defend against vandalism with automated prevention or reversion of bad edits.  Wikipedia staff will also experiment and observe the evolving ways that contributors, volunteers, donors, and users interact use AI tools to interact with the site and whether infrastructure changes are needed to accommodate chatbot use or other tools.