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

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.

The focus of this proposal is on expanding the scope of gravity gradient tools by providing an intriguing route toward more compact, and scalable, sensors. We wish to bring gravity gradiometry into civil engineering in which it will have its greatest impact. In order to begin any groundworks one must assess and survey the site for stability as well as avoiding, or integrating, with existing utilities. Unknown voids due to emerging sinkholes or undocumented mineshafts always pose a significant hazard, and existing survey tools may not be able to penetrate the surface deep enough to detect them. Gravity gradiometry can provide this capability.
Roadworks are a significant, and daily annoyance, which not only incur large costs to implement, but also have a knock on effect to the economy via delays in ground transportation of goods and people. It is surprising to learn that only 30% of underground utilities are well documented. Thus, in order to begin ground works several exploratory trenches are dug to identify where the utility cables and pipes are actually situated. Gradiometry could provide the necessary sensitivity to mass density variations caused by some utilities without breaking the ground. This could also offer the possibility of digging laterally underneath a road, thus avoiding traffic delays.
How can we bring about this new surveying tool? Alongside the development of the PLAIN-GG sensor we will engage with civil engineers, primarily through links built up from the EPSRC GG-TOP programme as well as the primarily EPSRC funded 'assessing the underworld' collaboration (www.assessingtheunderworld.org) to understand end user requirements in tool sensitivity, measurement bandwidth, mounting possibilities, and data analysis/interpretation. By the end of the project we aim to operate the cold atom gravity gradiometer outside of the laboratory to demonstrate 'real-world' robustness and sensitivity to massive objects. Our results will not only presented in optics and atomic physics journals, such as (PRL, Optics Letters, PRApplied and New Journal of Physics) but also in civil engineering journals concerned with underground surveying (such as Near Surface Geophysics, Journal of Applied Geophysics, and Tunnelling and Underground Space Technology).
There are also applications in deep sea surveying in which small compact sensors can be mounted on autonomous or remotely operated submersibles for geophysics surveying, resource exploration, and for pipeline inspection. Better surveying can identify new natural resources, or aid the management of existing deposits, thus more efficiently manage and secure the nation's reserves. One can also image submarine defence applications in which sensors are placed across the hull to detect underwater objects entirely passively for covert navigation. We will engage with both these communities to explore opportunities for collaboration and further research.
We highlight that we are not attempting to achieve the high sensitivity levels required for practical applications. PLAIN-GG is a technology testbed to aid in the development of high stability optical fields for cold atom interferometry using state of the art optical fibre control systems. The optical stabilization methods developed during the project will benefit other researchers looking into distributed atomic time, quantum communication and networking, as well other cold atom sensors. Many of component technologies could be commercialized to meet the demands of the UK quantum technology programme as well as the EU flagship programme.
As part of the emerging field of quantum technology, a key element in the take up of new paradigms is the acceptance and understanding of the public. Therefore the PI will provide public lectures on the field as well as take part in local 'meet the scientist' events. We also promote the research during the yearly Science Week be demonstrating the sensitivity of optical interferometers.