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

PLAIN-GG：用于重力梯度测量的锁相原子干涉仪

• 批准号: EP/R041806/1
• 负责人: Matthew Himsworth
• 金额: $ 31.4万
• 依托单位: University of Southampton
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2018
• 资助国家: 英国
• 起止时间: 2018 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=EP%2FR041806%2F1
• 关键词: PLAIN; GG; Phase; Locked; Atomic

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.

地球上的重力不是均匀的，也不是恒定的。质量密度的任何变化都会轻微地改变当地的重力，这可以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点之间重力加速度差的技术。这项技术已经应用了几十年，用于确定地下石油和天然气储量，监测海洋环流，探测地质断层，以及测量精确导航所必需的地球重力场形状。目前的重力梯度仪是大型、笨重和复杂的设备，通常安装在专门的测量飞机上，甚至是卫星上（GOCE任务），因此仅限于投资非常高的项目。我们设想未来重力梯度仪将成为一种更常见和广泛的传感器。土木工程将是最大的受益者，它可以在没有探索性挖掘的情况下发现公用设施（减少道路工程），有助于确定由于未记录的矿井和天坑而导致的不稳定区域，或者补充一般调查以评估地面稳定性。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，高度紧凑的梯度仪仍然需要获得非常高的灵敏度，所有这些都在一个经济的包装内。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。重力梯度仪基本上由两个测试质量组成，它们在重力作用下落下，它们的路径之间的任何差异都提供了重力方差的测量。提高灵敏度的关键是消除所有其他力（如平台运动），这些力会压倒极小的引力，并确保测试质量完全相同。单原子保持在超高真空中，可以说是理想的测试质量，因为它们总是相同的，并且不受磨损。人们还必须确保用相同的“尺子”测量每个原子的落差。这是通过单个激光束照射两个原子，以及原子干涉测量法来实现的，原子干涉测量法通过原子波函数的干涉来测量原子的路径，原子钟具有惊人的精度。为了达到土木工程应用所需的灵敏度，原子之间必须间隔一米左右的基线。这包括一个大型超高真空室，高功率真空泵，多种光学，昂贵的磁屏蔽以及几个激光系统。这种梯度仪可能与它们更“经典”的前辈一样有笨重的局限性，尽管有提高灵敏度的潜力。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光纤将它们耦合的方法来克服这一障碍。原子重力梯度仪的关键是两个原子都经历相同的激光“标尺”。我们将通过将每个原子测试质量放置在光学干涉仪的臂上来实现这一点，该干涉仪可以控制一个原子的光场在另一个原子上精确地再现。这种方法通常用于将光学相位传输数百公里以分配原子钟时间，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，并提供可变基线来调整分辨率（在感知更深、更大的物体到更浅、更小的特征之间切换），并且还允许多个相关加速度计沿着许多不同的轴或位置提供梯度。我们的目标是为实际应用程序设计一个健壮的、可伸缩的和实用的体系结构。