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.