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

爱因斯坦广义相对论最令人惊讶的预测之一是，时间在引力的影响下演化得更慢。这种效应被称为引力红移，它预测距离地球较远的时钟会比距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，它使用了一些最好的光学钟，这是先进的量子时间传感器。具体地说，这个项目将使用光学钟和时间传递系统来推动大地测量超越最先进的水平。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位势差异，展示在1厘米水平上测量地球形状的精确计时的价值。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。这项拟议的工作独特地探索了如何通过最终影响水文、矿产勘探和地震学领域的高时间和空间分辨率测量，更好地设计和应用量子、原子和激光物理的研究和技术，以造福于引力和地球物理。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运行两个光学钟，以及观察它们之间引力红移的测量链路。对于这里提出的大地测量，这个团队将使用目前在NIST运行的Yb光学晶格钟作为参考时钟。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度、频率稳定度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运输的光学时钟，也是基于光学晶格中的超冷Yb原子。在这个项目中，可移动时钟的健壮性将得到提高，其系统不确定度将得到充分评估，以与实验室时钟的系统不确定度相匹配。然后，将通过光学时间传递来比较固定和可运输的时钟，光学时间传递依赖于NIST和山顶之间空气中频率梳的激光双向交换。最终，可移动的时钟将被移到山顶(Mt.埃文斯，14,264英尺)进行相对论大地测量。从山上直达视线。埃文斯到博尔德不存在，因此将使用结合自由空间激光链路的双臂链路，然后是从布卢姆菲尔德到NIST博尔德的基于光纤的链路。将这些部件连接起来，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，两个山脉之间巨大的高程差异。埃文斯和博尔德(2600米)相当于一个大的引力红移，相当于十分之三到十分之十三(10^13)。由于光学钟可以在1018年的2个部分的水平上进行测量，因此红移的分辨率将达到10ppm或更高。与经典的地球势能测定一起，这项拟议的测量将产生有史以来最精确的一般相对论红移测试，无论是陆地测量还是天基测量。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.