Magneto-optical trapping in a near-surface borehole

TL;DR

This paper demonstrates a compact, borehole-deployable magneto-optical trap for cold atom sensors, enabling in-borehole gravity surveys with potential for improved robustness and deployment in challenging environments.

Contribution

It introduces the first magneto-optical trap designed specifically for borehole deployment, with detailed specifications and successful atom cloud generation in a 50 m deep borehole.

Findings

Generated atom clouds at 1 m intervals in a 50 m borehole

Achieved stable atom numbers with low variability

Demonstrated feasibility of in-borehole cold atom sensing

Abstract

Borehole gravity sensing can be used in a number of applications to measure features around a well including rock-type change mapping and determination of reservoir porosity. Quantum technology gravity sensors based on atom interferometry have the ability to offer increased survey speeds and reduced need for calibration. While surface sensors have been demonstrated in real world environments, significant improvements in robustness and reductions to radial size, weight, and power consumption are required for such devices to be deployed in boreholes. To realise the first step towards the deployment of cold atom-based sensors down boreholes, we demonstrate a borehole-deployable magneto-optical trap, the core package of many cold atom-based systems. The enclosure containing the magneto-optical trap itself had an outer radius of ($60\pm0.1$) mm at its widest point and a length of ($890\pm5$) mm. This system was used to generate atom clouds at 1 m intervals in a 14 cm wide, 50 m deep borehole, to simulate an in-borehole gravity surveys are performed. During the survey the system generated on average clouds of (3.0 $\pm 0.1) \times 10^{5}$ $^{87}$Rb atoms with the standard deviation in atom number across the survey observed to be as low as $9 \times 10^{4}$.