From construction works, water pipes and electricity cables, to natural resource management and irrigation, ground conditions impact everything from railways and housing to agriculture. Yet current sensing technologies can only penetrate a few centimetres below the ground surface.

Quantum gravity sensors, developed by physicists and civil engineers at the University of Birmingham, promise to illuminate the underworld, with far-reaching economic and social benefits.

Below the ground, a dense network of pipelines and cables delivers our water, sanitation, electricity and communications. New infrastructures, from housing to railways, are also critically dependent on what lies beneath: tunnels, mine-shafts and unrevealed sinkholes could all pose geotechnical risks. Companies spend millions of pounds investigating ground conditions, and those working on upgrading and maintaining underground utilities are often working with utilitiy records and mechanical excavators, analogous to a treasure hunter with a map and spade, according to Chris Rogers, Professor of Geotechnical Engineering at the University of Birmingham. Public works to upgrade utility infrastructures can shut down roads for thousands or millions of days a year. 

Being able to ‘see’ what’s under the surface would make construction safer, cut costs, and avoid some of the overruns and overspend that bedevil infrastructure projects. One Netherlands-based study estimated that 37% of construction project delays are linked to ground condition problems. But currently most available sensor technologies rely on radio frequency electromagnetic waves, which can sometimes only delve a few centimetres deep, as signals are attenuated and distorted by soil type and water content. Now, researchers at the University of Birmingham, a lead participant in the UK National Quantum Technologies Hub initiative, are building quantum gravity sensors to penetrate far deeper - theoretically, at least, all the way to the centre of the earth.  

Quantum gravity sensors work by dropping charged atom clouds that, because of the principle of superposition, can be in two different states at once. This mind-boggling quality enables observers to deduce underground conditions by comparing the different ways that atom clouds fall. Quantum gravity gradiometers promise to reduce measurement times and can detect targets like mine-shafts which may be ten metres or more below ground. “Quantum sensors are promising to be far superior to what we use traditionally for imaging, and can reveal things we are interested in and can’t currently see,” says Professor Nicole Metje from the University of Birmingham’s civil engineering department.

Safe as houses

Infrastructure is a kingpin asset to an economy, yet has been a weakly performing sector in terms of productivity, and remains under-invested in, from rich countries to emerging economies, braking growth. Construction, linked to 13% of global GDP, has enjoyed only a 1% annual productivity increase over the last 20 years, with no gain in labour productivity in infrastructure overall for 20 years in Japan, Germany and the USA. An estimated $1 trillion in savings would be possible, according to McKinsey, if a 60% increase in productivity could be realised. The consultancy also estimates that $57 trillion infrastructure investment will be required between 2013 and 2030 to keep up with projected global GDP growth. The European Investment Bank last year warned that the continent’s spending on infrastructure was at a ‘chronic’ low level, partly as a result of diminished public spending in the austerity era. 

Infrastructure is also one of the biggest brakes on growth, and a political priority in promising emerging economies like Indonesia. Quantum gravity sensors could be impactful in fast-growing emerging economies typified by slums, informal settlements, patchy existing infrastructure, and scarce data records, where geotechnical dangers could be more severe. 

Resource stewardship 

Quantum gravity sensors could also benefit natural resource management, the environment and agriculture. Shale oil extraction, and carbon capture and storage (CCS), require geophysical surveys and continuous monitoring to be regulation-compliant. Freshly captured carbon is only visible in current seismic surveys initially; when it dissolves in underground fluids, only microgravity measurements can spot it. This insight is essential to pinpoint upward movements of CO2 out of reservoirs.  

In agriculture, quantum sensors could also inform decisions like ploughing, to tackle soil compaction, which reduces crop yields. Gravimeters could monitor soil moisture, optimise irrigation, and track surface and subsurface water levels. Groundwater extraction is a major water sustainability challenge, especially parts of Asia like India, China, Pakistan and Bangladesh, with the potential to lower the water table, and increasingly likely to contain harmful pollutants like arsenic as depletion deepens. Soil erosion, meanwhile, is one less-discussed problem facing the global environment; worldwide gross investment requirements in irrigation and water management amount to $1 trillion by 2050, according to a UN study. Being able to map soil conditions would provide necessary insights to optimise soil management. 

Cross-pollination of quantum research 

As quantum sensor research and development progresses, more diverse use cases are emerging across disciplines. Finding and exploiting these opportunities, and forming academic and industry partnerships to explore real-world applications, takes an entrepreneurial, interdisciplinary mindset, and serendipitous interactions between experts. 

Professor Metje’s civil engineering collaboration with the quantum team led by Professor Kai Bongs, director of the UK Quantum Technology Hub for Sensors and Metrology, came about through a college research conference in which Professor Bongs learned more about the ‘Mapping the Underworld’ project, a multi-million pound research programme finding ways to pinpoint accurately and quickly the location of buried infrastructure without the need to dig holes and trenches. “I remember thinking these kinds of projects could benefit from gravity measurements,” recalls Professor Bongs and the collaboration was initiated, providing valuable trans-disciplinary input to advance this world-leading research.