Phase Locked Atomic Interferometers for Gravity Gradiometry

Accurate measurements of gravity can reveal intriguing and important details of the hidden structure of
our surroundings. Since gravity cannot be shielded a tool which can passively make such measurements
has many applications; from measuring the geoid of the earth to surveying subterranean man-made
structures. Developments in quantum technology have enabled the use of atoms as inertial sensors which
greatly increases the accuracy and precision of these measurements. The key process of how these
devices 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 the
absolute value at a single position. This has the benefit of rejecting common-mode noise, a problem
in absolute and relative measurements and a serious issue for applications outside of the laboratory. It
is important that the two locations are connected by a rigid baseline and are addressed by the same
light field. Current atomic interferometers use a single Raman beam passing through both atom clouds
to 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 many
practical applications, as the two atom clouds must have line of sight to each other and ultra-high vacuum
systems are bulky and power hungry.
This project aims to address these problems by separating the atom clouds into separate vacuum
chambers and connecting them with a stabilised optical fibre interferometer. The active optical fibre
interferometer will behave as a phase-locking mechanism to maintain the correlation of the Raman beams
and hence the correlation of the two atom clouds. Thus allowing the rejection of common-mode noise
without line-of-sight between the two atom clouds. This follows a technique used in atomic clocks for
distributing optical phase and demonstrates the required stability levels for this endeavour. Separating
the atom clouds in this manner enables us a great deal of flexibility and portability; making such a device
ideal for practical applications.