QuSeC-TAQS: Improving Geodesy and Gravitational Sensing with Quantum Sensors of Time

One of the more surprising predictions of Einstein’s theory of general relativity is that time evolves more slowly under the influence of gravity. Known as the gravitational redshift, the effect predicts that a clock further from the Earth will tick faster relative to one closer to the Earth. From the human perspective, the magnitude of the effect is small, but it has important impacts for GPS navigation, fundamental timekeeping, and as this project will show, for measuring gravity and the shape of the Earth. This concept is known as relativistic geodesy, and it employs the some of best optical clocks, which are advanced quantum sensors of time. Specifically, this project will use optical clocks and time transfer systems to advance geodesy beyond the state-of-the-art. This will involve transporting a cold-atom optical clock to the mountainous regions of Colorado to measure geopotential differences using relativistic geodesy, showing the value of precision timekeeping for measuring the shape of the Earth at the 1 cm level. This team also expects to make the best test of the gravitational redshift predicted by Einstein’s theory. The proposed effort uniquely explores how research and technologies from quantum, atomic, and laser physics can be better engineered and applied for the benefit of gravitational and geophysics through high temporal and spatial resolution measurements that will ultimately impact the fields of hydrology, mineral exploration, and seismology. A minimum requirement for relativistic geodesy is the ability to operate two optical clocks at distinct locations of interest, as well as a measurement link to observe the gravitational redshift between them. For the geodesy proposed here, this team will use an ytterbium optical lattice clock currently operating at NIST as the reference clock. This clock is located in the fixed gravitational reference frame, and has already demonstrated systematic uncertainty, frequency stability, and reproducibility at 1 part in a billion billion (10^18) or better. The second clock system will be a transportable optical clock, also based on ultracold ytterbium atoms in an optical lattice. In this program, the robustness of the transportable clock will be improved, and its systematic uncertainty will be fully evaluated to match that of the laboratory clock. The fixed and transportable clocks will then be compared via optical time transfer that relies on the two-way exchange of laser light from frequency combs across the air between NIST and the mountaintop. Ultimately, the transportable clock will be moved to a mountain summit (Mt. Evans, 14,264 feet) to perform relativistic geodesy. Direct line of sight from Mt. Evans to Boulder does not exist, so a two-arm link will be utilized that combines free-space laser link, followed by a fiber-optic based link from Broomfield to NIST Boulder. Connecting the pieces, this quantum-sensor-based geodesy measurement will yield accuracy at or below 2 cm. More significantly, the sizeable elevation difference between Mt. Evans and Boulder (>2600 m) corresponds to a large gravitational redshift of nearly 3 parts in ten to the thirteen (10^13). Since the optical clocks can measure at the level of 2 parts in 10^18, the redshift will be resolved at 10 ppm level or better. Together with classical geopotential determination, this proposed measurement will yield the most precise test of the general relativistic redshift ever, either for terrestrial or space-based measurements. 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.