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

爱因斯坦的广义相对论的一个更令人惊讶的预测是，在引力的影响下，时间的演化速度会更慢。这种效应被称为引力红移，它预测离地球较远的时钟会比离地球较近的时钟走得更快。从人类的角度来看，影响的幅度很小，但它对GPS导航，基本计时以及本项目将展示的测量重力和地球形状具有重要影响。这个概念被称为相对论大地测量学，它采用了一些最好的光学时钟，这是先进的量子时间传感器。具体而言，该项目将使用光学钟和时间传递系统，使大地测量学超越最先进水平，包括将冷原子光学钟运到科罗拉多山区，利用相对论大地测量学测量地球位差，显示精确计时对测量1厘米高度的地球形状的价值。该团队还希望对爱因斯坦理论预测的引力红移进行最好的测试。这项计划独特地探索了如何通过高时空分辨率测量更好地设计和应用量子、原子和激光物理学的研究和技术，以造福于引力和地球物理学，这些测量最终将影响水文、矿产勘探和地震学领域。以及观测它们之间引力红移的测量链路。对于这里提出的大地测量，该团队将使用目前在NIST运行的镱光学晶格时钟作为参考时钟。这个时钟位于固定的引力参考系中，并且已经证明了系统的不确定性，频率稳定性和重复性为十亿分之一（10^18）或更好。第二个时钟系统将是一个可移动的光学时钟，也是基于光学晶格中的超冷镱原子。在该计划中，将提高可移动时钟的鲁棒性，并对其系统不确定度进行全面评估，以匹配实验室时钟。然后，将通过光学时间传输来比较固定和可移动的时钟，该光学时间传输依赖于NIST和山顶之间空气中频率梳激光的双向交换。最终，可移动的时钟将被移动到一个山顶（山。埃文斯，14，264英尺）进行相对论大地测量。从Mt.埃文斯到博尔德不存在，因此将使用双臂链路，该链路结合了自由空间激光链路，然后是从布鲁姆菲尔德到NIST博尔德的基于光纤的链路。将这些碎片连接起来，这种基于量子传感器的大地测量将产生2厘米或以下的精度。更重要的是，山之间的巨大海拔差异。埃文斯和博尔德（2600米）对应的是一个大的引力红移，接近3/10的10次方（10^13）。由于光学钟可以测量到2/10^18的水平，因此红移将被分辨到10 ppm或更好的水平。与经典的地球势测定一起，这个提议的测量将产生有史以来最精确的广义相对论红移测试，无论是地面还是天基测量。这个奖项反映了NSF的法定使命，并被认为值得通过使用基金会的智力价值和更广泛的影响审查标准进行评估来支持。

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.