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

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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.