The prevalence of the Global Navigation Satellite System (GNSS) has made it a ubiquitous tool for navigators. However, the presence of GNSS-denied environments created by high-density media in satellite-receiver lines-of-sight or by deliberate signal interference is a key barrier to robust navigation, particularly in maritime contexts for navigating underwater and autonomous shipping. Whilst an inertial navigation system (INS) can be an important navigation tool in GNSS-denied environments, such systems are prone to drift-induced readout errors, resulting in a loss of positional accuracy over time. In this article, we discuss the potential use of quantum sensors for alternative navigation via gravity gradient map matching to enable stable navigation trajectories. Through their ability to reject specific inertial noise sources, atom interferometers have potential as an interesting sensor technology in such contexts. However, while atom interferometers have demonstrated performance relevant to map matching in static conditions, a key barrier is improving their deployability on dynamic platforms. We here provide initial reports on two maritime demonstrations of gravity gradiometry via atom interferometry in a strap-down configuration, showing the realization of gravity gradiometry on a moving marine vessel without any inertial stabilization. We provide an overview of the trials and the impact of the maritime vessel and inertial environments on the interferometer performance. These trials are a first step towards a full alternative navigation capability, which will require further work on sensor robustness across different inertial conditions, sensor integration into the platform, and integration into the navigation system via filtering. The trials are informing a granular understanding of the challenges of operating in these environments, progressing the roadmap towards further demonstrations and upgrades for the next generation of sensors to bring benefit to maritime navigators.

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 ± 0.1) mm at its widest point and a length of (890 ± 5) mm. This system was used to generate atom clouds at 1 m intervals in a 14 cm wide, 50 m deep borehole, to simulate how in-borehole gravity surveys are performed. During the survey, the system generated, on average, clouds of (3.0 ± 0.1) × 105 87Rb atoms with the standard deviation in atom number across the survey observed to be as low as 8.9 × 104.

Quantum sensors based on cold atoms are being developed which produce measurements of unprecedented accuracy. Due to shifts in atomic energy levels, quantum sensors often have stringent requirements on their internal magnetic field environment. Typically, background magnetic fields are attenuated using high permeability magnetic shielding, with the cancelling of residual and introduction of quantisation fields implemented with coils inside the shield. The high permeability shield, however, distorts all magnetic fields, including those generated inside the sensor. Here, we demonstrate a solution by designing multiple coils overlaid on a 3D-printed former to generate three uniform and three constant linear gradient magnetic fields inside the capped cylindrical magnetic shield of a cold atom interferometer. The fields are characterised in-situ and match their desired forms to high accuracy. For example, the uniform transverse field, $B_x$, deviates by less than $0.2$% over more than $40$% of the length of the shield. We also map the field directly using the cold atoms and investigate the potential of the coil system to reduce bias from the quadratic Zeeman effect. This coil design technology enables targeted field compensation over large spatial volumes and has the potential to reduce systematic shifts and noise in numerous cold atom systems.

The sensing of gravity has emerged as a tool in geophysics applications such as engineering and climate research 1 – 3 , including the monitoring of temporal variations in aquifers 4 and geodesy 5 . However, it is impractical to use gravity cartography to resolve metre-scale underground features because of the long measurement times needed for the removal of vibrational noise 6 . Here we overcome this limitation by realizing a practical quantum gravity gradient sensor. Our design suppresses the effects of micro-seismic and laser noise, thermal and magnetic field variations, and instrument tilt. The instrument achieves a statistical uncertainty of 20 E (1 E = 10 −9 s −2 ) and is used to perform a 0.5-metre-spatial-resolution survey across an 8.5-metre-long line, detecting a 2-metre tunnel with a signal-to-noise ratio of 8. Using a Bayesian inference method, we determine the centre to ±0.19 metres horizontally and the centre depth as (1.89 −0.59/+2.3) metres. The removal of vibrational noise enables improvements in instrument performance to directly translate into reduced measurement time in mapping. The sensor parameters are compatible with applications in mapping aquifers and evaluating impacts on the water table 7 , archaeology 8 – 11 , determination of soil properties 12 and water content 13 , and reducing the risk of unforeseen ground conditions in the construction of critical energy, transport and utilities infrastructure 14 , providing a new window into the underground.

The extraordinary performance offered by cold atom-based clocks and sensors has the opportunity to profoundly affect a range of applications, for example in gravity surveys, enabling long term monitoring applications through low drift measurements. While ground-based devices are already starting to enter the commercial market, significant improvements in robustness and reductions to size, weight, and power are required for such devices to be deployed by Unstaffed Aerial Vehicle systems (UAV). In this article, we realise the first step towards the deployment of cold atom based clocks and sensors on UAV’s by demonstrating an UAV portable magneto-optical trap system, the core package of cold atom based systems. This system is able to generate clouds of 2.1±0.2×107 atoms, in a package of 370 mm × 350 mm × 100 mm, weighing 6.56 kg, consuming 80 W of power.

The sensing of gravity has emerged as an important tool in geophysics for applications such as engineering and climate research 1,2,3 where it provides the capability to probe otherwise inaccessible features under the surface of the Earth. Examples include the monitoring of temporal variations such as those found in aquifers 4 and geodesy 5 . However, resolving metre scale underground features is rendered impractical by the long measurement times needed for the removal of vibrational noise 6 . Here, we overcome this limitation and open up the field of gravity cartography by realising a practical quantum gravity gradient sensor. Our design suppresses the effects of micro-seismic and laser noise, as well as thermal and magnetic field variations, and instrument tilt. The instrument achieves an uncertainty of 20 E (1 E = 10-9 s-2) and is used to perform a 0.5 m spatial resolution survey across a 8.5 m long line, detecting a 2 m tunnel with a signal to noise ratio of 8. The tunnel centre is localised using a Bayesian inference method, determining the centre to within ± 0.19 m in the horizontal direction and finding the centre depth as (1.89 -0.59/+2.3) m. The removal of vibrational noise enables improvements in instrument performance to directly translate into reduced measurement time in mapping. This opens new applications such as mapping the water distribution of aquifers and evaluating impacts on the water table 7 , detecting new features in archaeology 8,9,10,11 , determination of soil properties 12 and water content 13 , and reducing the risk of unforeseen ground conditions in the construction of critical energy, transport and utilities infrastructure 14 , providing a new window into the underground.

“Cold atoms” can be used as ultra-sensitive sensors for measuring accelerations and are capable of mapping changes in the strength of gravity across the surface of the Earth. They could offer significant benefits to existing space based gravity sensing capabilities. Gravity sensors in space are already used for many Earth observation applications including monitoring polar ice mass, ocean currents and sea level. Cold atom sensors could enable higher resolution measurements which would allow monitoring of smaller water sources and discovery of new underground natural resources which are currently undetectable. The adoption of cold atom technology is constrained by low technology readiness level (TRL). Teledyne e2v and its partners are addressing this maturity gap through project Cold Atom Space PAyload (CASPA) which is an Innovate UK and Engineering and Physical Sciences Research Council (EPSRC) funded project, involving the University of Birmingham as science lead, XCAM, Clyde Space, Covesion, Gooch & Housego, and the University of Southampton. Through the CASPA project the consortium have built and vibration tested a 6U (approximate dimensions: 100 × 200 × 300 mm) cube Satellite (CubeSat) that is capable of laser cooling atoms down to 100’s of micro kelvin, as a pre-cursor to gravity sensors for future Earth observation missions.

Recent advances in the understanding and control of cold atom systems have resulted in devices with extraordinary metrological performance. To further improve the performance in these systems, additional methods of noise reduction are needed. Here, we examine the noise reduction possible from vacuum compatible low reflection coatings in cold atom systems by characterizing a black coating and its compatibility in a Magneto-Optical Trap (MOT). We demonstrate that the commercially available PCO35® coating provides low-reflectivity surfaces that are ultra-high vacuum compatible. The reflective properties of the coating are compared to titanium, a common vacuum chamber material, and the reduction to scattered light is characterized over a range of angles and wavelengths. The outgassing properties of the coating are measured to be less than that of the vacuum system used to test the coating, which is limited to 3 × 10−8 mbar L cm−2 s−1. The coating is applied to a vacuum chamber housing a rubidium prism MOT, and its vacuum compatibility is assessed and compared to an identical non-coated system. Finally, the effect of scattered light reduction in a generalized system is explored theoretically. These results show promise for reducing background light in cold atom experiments via the use of low-reflectivity coatings.

The high precision and scalable technology offered by atom interferometry has the opportunity to profoundly affect gravity surveys, enabling the detection of features of either smaller size or greater depth. While such systems are already starting to enter into the commercial market, significant reductions are required in order to reach the size, weight and power of conventional devices. In this article, the potential for atom interferometry based gravimetry is assessed, suggesting that the key opportunity resides within the development of gravity gradiometry sensors to enable drastic improvements in measurement time. To push forward in realizing more compact systems, techniques have been pursued to realize a highly portable magneto-optical trap system, which represents the core package of an atom interferometry system. This can create clouds of 107 atoms within a system package of 20 l and 10 kg, consuming 80 W of power.This article is part of the themed issue 'Quantum technology for the 21st century'.