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

爱因斯坦广义相对论的一个更令人惊讶的预言是，在引力的影响下，时间的演变更慢。这种效应被称为引力红移，它预示着离地球远的时钟相对于离地球近的时钟走得更快。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，以及这个项目将展示的测量重力和地球形状有重要影响。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。具体来说，该项目将使用光学时钟和时间传输系统，以超越最先进的水平推进大地测量学。这将包括将一个冷原子光学时钟运送到科罗拉多州的山区，使用相对论大地测量学测量地球的位势差异，以显示精确计时在1厘米水平上测量地球形状的价值。该团队还希望对爱因斯坦理论所预测的引力红移进行最好的测试。提出的努力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、原子和激光物理的研究和技术，从而造福于引力和地球物理学，最终将影响水文学、矿产勘探和地震学领域。相对论大地测量学的最低要求是能够在不同的感兴趣的位置操作两个光学时钟，以及观测它们之间的引力红移的测量链路。对于这里提出的测地线，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、频率稳定性和十亿分之一（10^18）或更好的再现性。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。在这个项目中，可移动时钟的鲁棒性将得到提高，其系统不确定性将被充分评估，以匹配实验室时钟。固定时钟和可移动时钟将通过光学时间传输进行比较，该传输依赖于NIST和山顶之间空气中频率梳发出的激光的双向交换。最终，可移动的时钟将被转移到山顶（埃文斯山，14264英尺）进行相对论大地测量。从埃文斯山到博尔德的直接视线不存在，因此将采用双臂连接，结合自由空间激光连接，然后使用基于光纤的连接，从布鲁姆菲尔德到NIST博尔德。将这些碎片连接起来，这种基于量子传感器的大地测量测量的精度将达到或低于2厘米。更重要的是，埃文斯山和博尔德山之间的巨大海拔差（2600米）对应于一个巨大的引力红移，接近10的13次方（10^13）的3分之一。由于光学时钟可以在10^18的2分之一的水平上测量，因此红移将在10 ppm或更好的水平上解决。结合经典的地球势测定，这一提议的测量将产生迄今为止最精确的广义相对论红移测试，无论是在地面还是在太空测量。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.