PLAIN-GG: Phase-Locked Atomic INterferometers for Gravity Gradiometry

地球上的引力既不是均匀的，也不是恒定的。质量密度的任何变化都会略微改变局部引力，并为我们提供一个独特的机会来探测隐藏在视线之外的特征。重力梯度法是一种测量由固定基线隔开的两个点之间由于重力引起的加速度差的技术。这项技术几十年来一直用于识别地下石油和天然气储量，监测海洋环流，探测地质断层，以及测量地球重力场的形状，这是精确导航所必需的。目前的重力梯度仪是大型、笨重和复杂的设备，通常安装在专门的勘测飞机上，甚至安装在卫星上(GOCE任务)，因此仅限于投资非常高的项目。我们展望未来，重力梯度仪将成为一种更常见、更广泛的传感器。土木工程将受益最大，能够在不进行探索性挖掘的情况下发现公用事业(减少道路工程)，帮助识别由于未记录的矿井和天坑而导致的不稳定地区，或补充用于评估地面稳定性的一般调查。人们还可以设想在考古学和深海勘探中的应用。为了实现这一目标，需要高度紧凑的梯度计，而且仍然获得非常高的灵敏度，所有这些都在经济的包装内。量子技术领域的最新发展将为实现这一目标提供重大飞跃。从根本上讲，重力梯度仪由两个测试质量组成，它们可以在重力作用下下落，它们的路径之间的任何差异都提供了对重力变化的测量。提高灵敏度的关键是消除所有可能压倒极小引力的其他力(如平台运动)，并确保测试质量完全相同。保持在超高真空中的单原子可以提供理想的测试质量，因为它们总是相同的，不会受到磨损。人们还必须确保每个原子的水滴都是用相同的“尺子”测量的。这是通过一束激光照射两个原子来实现的，以及原子干涉测量的方法-它提供了原子钟惊人的准确性-通过原子波函数的干涉来测量原子的路径。为了达到土木工程应用所必需的灵敏度，原子必须以一米左右的基线分开。这包括一个大型超高真空室、高功率真空泵、多个光学元件、昂贵的磁屏蔽以及几个激光系统。这样的梯度计可能会和它们更经典的前辈一样，具有同样的笨重局限性，尽管有可能提高灵敏度。我们的目标是通过探索分离两个原子测试质量并通过主动稳定的光纤将它们耦合的方法来克服这一障碍。原子重力梯度计的关键方面是两个原子都经历了一把相同的激光‘尺子’。我们将通过将每个原子测试质量放置在光学干涉仪的臂中来实现这一点，光学干涉仪受到控制，使一个原子上的光场在另一个原子上精确地复制。这种方法通常用于将跨越数百公里的光学相位传递到分布的原子钟时间，并且落后于LIGO重力波探测器的灵敏度。通过采用这种方法，我们可以显著减小传感器的尺寸、重量和功率，并提供可变的基线来调整分辨率(在感测更深、更大的物体和更浅、更小的特征之间切换)，还允许多个连续加速度计沿许多不同的轴或位置提供梯度。我们的目标是为实际应用设计一个健壮、可伸缩且实用的架构。

The force of gravity across the earth is not uniform, nor constant. Any variation in mass density acts to slightly alter the local force of gravity and can provide us with a unique opportunity for detecting features which are hidden from view. Gravity gradiometry is a technique for measuring the difference in the acceleration due to gravity between two points separated by a fixed baseline. This technique has been in use for several decades for identifying underground oil and gas reserves, monitoring ocean circulation, detecting geological faults, as well as measuring the shape of the earth's gravitational field, which is necessary for accurate navigation. Current gravity gradiometers are large, heavy and complex devices typically mounted on specialised survey aeroplanes, or even on satellites (GOCE mission), and so are confined to projects with very high investment. We envision a future in which gravity gradiometers will become a more common and widespread sensor. Civil engineering will benefit the most, enabling the discovery of utilities without exploratory digging (reducing roadworks), help identify unstable areas due to unrecorded mineshafts and sinkholes, or complement general surveys for assessing ground stability. One may also envision applications in archaeology and deep sea exploration. To achieve this goal, highly compact gradiometers which still obtain very high sensitivities are needed, all within an economic package. A recent development in the field of quantum technology will provide a significant jump toward this goal. A gravity gradiometer, fundamentally, consists of two test masses which are allowed to fall under gravity and any differences between their paths provides a measurement of gravitational variance. The key to increasing sensitivity is to remove all other forces (such as platform motion) which can overwhelm the extremely small gravitational forces, and also ensure the test masses are absolutely identical. Single atoms held within ultra-high vacuum provide, arguably, ideal test masses as they are always identical and are not subject to wear and tear. One must also ensure each atom's drop is measured using identical 'rulers'. This is achieved with a single laser beam illuminating both atoms, as well as methods from atom interferometry - which provides atomic clocks with their astonishing accuracy - to measure the atom's path via the interference of atomic wavefunctions. To achieve the necessary sensitivity for civil engineering applications the atoms must be separated by baseline of a metre or so. This involves a large ultra-high vacuum chamber, high power vacuum pumps, multiple optics, expensive magnetic shielding as well as several laser systems. Such gradiometers are likely to have the same bulky limitations as their more 'classical' predecessors, albeit with the potential for improved sensitivity. We aim to overcome this hurdle by exploring methods to separate the two atomic test masses and couple them via actively stabilized optical fibres. The key aspect of atomic gravity gradiometers is that both atoms experience an identical laser 'ruler'. We will achieve this by placing each atomic test mass in the arms of an optical interferometer which is controlled such that the optical field at one atom is reproduced exactly at the other. Such methods are commonly employed to transfer optical phase across hundreds of kilometres to distributing atomic clock time and are behind the sensitivity of the LIGO gravity wave detector. By adopting this method we can significantly reduce the size, weight and power of the sensor, as well as providing a variable baseline to adjust resolution (to switch between sensing deeper, larger, objects to shallower, smaller, features), and also allow multiple corrolated accelerometers to provide gradients along many different axes or position. Our goal is to engineer a robust, scalable, and practical architecture for practical applications.