Phase Locked Atomic Interferometers for Gravity Gradiometry

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Accurate measurements of gravity can reveal intriguing and important details of the hidden structure ofour surroundings. Since gravity cannot be shielded a tool which can passively make such measurementshas many applications; from measuring the geoid of the earth to surveying subterranean man-madestructures. Developments in quantum technology have enabled the use of atoms as inertial sensors whichgreatly increases the accuracy and precision of these measurements. The key process of how thesedevices work is by dropping atoms and measuring the change in their Doppler shift to detect acceleration.Gravity gradiometry measures the difference of gravity between two chosen points rather than theabsolute value at a single position. This has the benefit of rejecting common-mode noise, a problemin absolute and relative measurements and a serious issue for applications outside of the laboratory. Itis important that the two locations are connected by a rigid baseline and are addressed by the samelight field. Current atomic interferometers use a single Raman beam passing through both atom cloudsto eliminate this common-mode noise. These clouds are typically in the same vacuum chamber.Unfortunately this limits geometry and portability of such devices, making them unsuitable for manypractical applications, as the two atom clouds must have line of sight to each other and ultra-high vacuumsystems are bulky and power hungry.This project aims to address these problems by separating the atom clouds into separate vacuumchambers and connecting them with a stabilised optical fibre interferometer. The active optical fibreinterferometer will behave as a phase-locking mechanism to maintain the correlation of the Raman beamsand hence the correlation of the two atom clouds. Thus allowing the rejection of common-mode noisewithout line-of-sight between the two atom clouds. This follows a technique used in atomic clocks fordistributing optical phase and demonstrates the required stability levels for this endeavour. Separatingthe atom clouds in this manner enables us a great deal of flexibility and portability; making such a deviceideal for practical applications.