At a conference in 2014, bioengineer Jeffrey Fredberg of Harvard University presented pictures of asthma cells. To most people, the images would have been indistinguishable – they all showed tightly packed layers of cells from the airways of people with asthma. But as a physicist, Lisa Manning saw something no one else had spotted; she could tell, just by looking, that some of the model tissues were solid and some were fluid.

Animal tissues must be able to rearrange and flow but also switch to a state where they can withstand mechanical stress. However, whereas solid-liquid transitions are generally associated with a density change, many cellular systems, including asthma cells, can change from rigid to fluid-like at a constant packing density.

Many of a tissue’s properties depend on biochemical processes in its constituent cells, but some collective behaviours can be captured by mathematical models, which is the focus of Manning’s research. At the time, she was working with postdoctoral associate Dapeng Bi on a theory that a tissue’s rigidity depends on the shape of the cells, with cells in a rigid state touching more neighbouring cells than those in a fluid-like one. When she saw the pictures of the asthma cells she knew she was right. “That was a very cool moment,” she says.

Manning – now the William R Kenan, Jr Professor of Physics at Syracuse University in the US – began her research career in theoretical condensed-matter physics, completing a PhD at the University of California, Santa Barbara, in 2008. The thesis was on the mechanical properties of amorphous solids – materials that don’t have long-ranged order like a crystal but are nevertheless rigid. Amorphous solids include many plastics, soils and foods, but towards the end of her graduate studies, Manning started thinking about where else she could apply her work.

I was looking for a project where I could use some of the skills that I had been developing as a graduate student in an orthogonal way

“I was looking for a project where I could use some of the skills that I had been developing as a graduate student in an orthogonal way,” Manning recalls. Inspiration came from of a series of talks on tissue dynamics at the Kavli Institute for Theoretical Physics, where she recognized that the theories she had worked on could also apply to biological systems. “I thought it was amazing that you could apply physical principles to those systems,” she says.

The physics of life

Manning has been at Syracuse since completing a postdoc at Princeton University, and although she has many experimental collaborators, she is happy to still be a theorist. Whereas experimentalists in the biological sciences generally specialize in just one or two experimental models, she looks for “commonalities across a wide range of developmental systems”. That principle has led Manning to study everything from cancer to congenital disease and the development of embryos.

“In animal development, pretty universally one of the things that you must do is change from something that’s the shape of a ball of cells into something that is elongated,” says Manning, who working to understand how this happens. With collaborator Karen Kasza at Columbia University, she has demonstrated that rather than stretching as a solid, it’s energy efficient for embryos to change shape by undergoing a phase transition to a fluid, and many of their predictions have been confirmed in fruit fly embryo models.

More recently, Manning has been looking at how ideas from AI and machine learning can be applied to embryogenesis. Unlike most condensed-matter systems, tissues continuously tune individual interactions between cells, and it’s these localized forces that drive complex shape changes during embryonic development. Together with Andrea Liu of the University of Pennsylvania, Manning is now developing a framework that treats cell–cell interactions like weights in a neural network that can be adjusted to produce a desired outcome.

“I think you really need almost a new type of statistical physics that we don’t have yet to describe systems where you have these individually tunable degrees of freedom,” she says, “as opposed to systems where you have maybe one control parameter, like a temperature or a pressure.”

Developing the next generation

Manning’s transition to biophysics was spurred by an unexpected encounter with scientists outside her field. Between 2019 and 2023, she was director of the Bio-inspired Institute at Syracuse University, which supported similar opportunities for other researchers, including PhD students and postdocs. “As a graduate student, it’s a little easy to get focused on the one project that you know about, in the corner of the universe that your PhD is in,” she says.

As well as supporting science, one of the first things Manning spearheaded at the institute was a professional development programme for early-career researchers. “During our graduate schools, we’re typically mostly trained on how to do the academic stuff,” she says, “and then later in our careers, we’re expected to do a lot of other types of things like manage groups and manage funding.” To support their wider careers, participants in the programme build non-technical skills in areas such as project management, intellectual property and graphic design.

What I realized is that I did have implicit expectations that were based on my culture and background, and that they were distinct from those of some of my students

Manning’s senior role has also brought opportunities to build her own skills, with the COVID-19 pandemic in particular making her reflect and reevaluate how she approached mentorship. One of the appeals of academia is the freedom to explore independent research, but Manning began to see that her fear of micromanaging her students was sometimes creating confusion.

“What I realized is that I did have implicit expectations that were based on my culture and background, and that they were distinct from those of some of my students,” she says. “Because I didn’t name them, I was actually doing my students a disservice.” If she could give advice to her younger self, it would be that the best way to support early-career researchers as equals is to set clear expectations as soon as possible.

When Manning started at Syracuse, most of her students wanted to pursue research in academia, and she would often encourage them to think about other career options, such as  working in industry. However, now she thinks academia is perceived as the poorer choice. “Some students have really started to get this idea that academia is too challenging and it’s really hard and not at all great and not rewarding.”

Manning doesn’t want anyone to be put off pursuing their interests, and she feels a responsibility to be outspoken about why she loves her job. For her, the best thing about being a scientist is encapsulated by the moment with the asthma cells: “The thrill of discovering something is a joy,”  she says, “being for just a moment, the only person in the world that understands something new.”

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Waseem completed his DPhil in physics at the University of Oxford in the UK, where he worked on applied process-relational philosophy and employed string diagrams to study interpretations of quantum theory, constructor theory, wave-based logic, quantum computing and natural language processing. At Oxford, Waseem continues to teach mathematics and physics at Magdalen College, the Mathematical Institute, and the Department of Computer Science.

Waseem has played a key role in organizing the Lahore Science Mela, the largest annual science festival in Pakistan. He also co-founded Spectra, an online magazine dedicated to training popular-science writers in Pakistan. For his work popularizing science he received the 2021 Diana Award, was highly commended at the 2021 SEPnet Public Engagement Awards, and won an impact award in 2024 from Oxford’s Mathematical, Physical and Life Sciences (MPLS) division.

What skills do you use every day in your job?

I’m a theoretical physicist, so if you’re thinking about what I do every day, I use chalk and a blackboard, and maybe a pen and paper. However, for theoretical physics, I believe the most important skill is creativity, and the ability to dream and imagine.

What do you like best and least about your job?

That’s a difficult one because I’ve only been in this job for a few weeks. What I like about my job is the academic freedom and the opportunity to work on both education and research. My role is divided 50/50, so 50% of the time I’m thinking about the structure of natural languages like English and Urdu, and how to use quantum computers for natural language processing. The other half is spent using our diagrammatic formalism called “quantum picturalism” to make quantum physics accessible to everyone in the world. So, I think that’s the best part. On the other hand, when you have a lot of smart people together in the same room or building, there can be interpersonal issues. So, the worst part of my job is dealing with those conflicts.

What do you know today, that you wish you knew when you were starting out in your career?

It’s a cynical view, but I think scientists are not always very rational or fair in their dealings with other people and their work. If I could go back and give myself one piece of advice, it would be that sometimes even rational and smart people make naive mistakes. It’s good to recognize that, at the end of the day, we are all human.

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“I could have sworn I put it somewhere safe,” is something we’ve all said when looking for our keys, but the frustration of searching for lost objects is also a common, and very costly, headache for civil engineers. The few metres of earth under our feet are a tangle of pipes and cables that provide water, electricity, broadband and waste disposal. However, once this infrastructure is buried, it’s often difficult to locate it again.

“We damage pipes and cables in the ground roughly 60,000 times a year, which costs the country about 2.4 billion pounds,” explains Nicole Metje, a civil engineer at the University of Birmingham in the UK. “The ground is such a high risk, but also such a significant opportunity.”

The standard procedure for imaging the subsurface is to use electromagnetic waves. This is done either with ground penetrating radar (GPR), where the signal reflects off interfaces between objects in the ground, or with locators that use electromagnetic induction to find objects. Though they are stalwarts of the civil engineering toolbox, the performance of both these techniques is limited by many factors, including the soil type and moisture.

Metje and her team in Birmingham have participated in several research projects improving subsurface mapping. But her career took an unexpected turn in 2009 when one of her colleagues was contacted out of the blue by Kai Bongs – a researcher in the Birmingham school of physics. Bongs, who became the director of the Institute for Quantum Technologies at the German Aerospace Centre (DLR) in 2023, explained that his group was building quantum devices to sense tiny changes in gravity and thought this might be just what the civil engineers needed. However, there was a problem. The device required a high-stability, low-noise environment – rarely compatible with the location of engineering surveys. But as Bongs spoke to more engineers he became more interested. “I understood why tunnels and sewers are very interesting,” he says, and saw an opportunity to “do something really meaningful and impactful”.

What lies beneath

Although most physicists are happy to treat g, the acceleration due to gravity, as 9.81 m/s², it actually varies across the surface of Earth. Changes in g indicate the presence of buried objects and varying soil composition and can even signal the movement of tectonic plates and oceans. The engineers in Birmingham were well aware of this; classical devices that measure changes in gravity using the extension of springs are already used in engineering surveys, though they aren’t as widely adopted as electromagnetic signals. These machines – called gravimeters – don’t require holes to be dug and the measurement isn’t limited by soil conditions, but changes in the properties of the spring over time cause drift, requiring frequent recalibration.

The perfect test mass would be a single atom – it has no moving mechanical parts, can be swapped out for any of the same isotope, and its mass will never change

More sensitive devices have been developed that use a levitating superconducting sphere. These devices have been used for long-term monitoring of geophysical phenomena such as tides, volcanos and seismic activity, but they are less appropriate for engineering surveys where speed and portability are of the essence.

The perfect test mass would be a single atom – it has no moving mechanical parts, can be swapped out for any of the same isotope, and its mass will never change. “Today or tomorrow or in 100 years’ time, it’ll be exactly the same,” says physicist Michael Holynski, the principal investigator of the UK Quantum Technology Hub for Sensors and Timing led by the University of Birmingham.

Falling atoms

The gravity sensing project in Birmingham uses a technique called cold-atom interferometry, first demonstrated in 1991 by Steven Chu and Mark Kasevich at Stanford University in the US (Phys. Rev. Lett. 67 181). In the cold-atom interferometer, two atomic test masses fall from different heights, and g is calculated by comparing their displacement in a given time.

Because it’s a quantum object, a single atom can act as both test masses at once. To do this, the interferometer uses three laser pulses that sends the atom on two trajectories. First, a laser pulse puts the atom in a superposition of two states, where one state gets a momentum “kick” and recoils away from the other. This means that when the atom is allowed to freefall, the state nearest the centre of the Earth accelerates faster. Halfway through the freefall, a second laser pulse then switches the state with the momentum kick. The two states start to catch up with each other, both still falling under gravity.

Finally, another laser pulse, identical to the first, is applied. If the acceleration due to gravity were constant everywhere in space, the two states would fall exactly the same distance and overlap at the end of the sequence. In this case, the final pulse would effectively reverse the first, and the atom would end up back in the ground state. However, because in the real world the atom’s acceleration changes as it falls through the gravity gradient, the two states don’t quite find each other at the end. Since the atom is wavelike, this spatial separation is equivalent to a phase difference. Now, the outcome of the final laser pulse is less certain; sometimes it will return the atom to the ground state, but sometimes it will collapse the wavefunction to the excited state instead.

If a cloud of millions of atoms is dropped at once, the proportion that finishes in each state (which is measured by making the atoms fluoresce) can be used to calculate the phase difference, which is proportional to the atom’s average gravitational acceleration.

To measure these phase shifts, the thermal noise of the atoms must be minimized. This can be achieved using a magneto-optical trap and laser cooling, a technique pioneered by Chu, in which spatially varying magnetic fields and lasers trap atoms and cool them close to absolute zero. Chu, along with William H Phillips and Claude Cohen-Tannoudji, was awarded the 1997 Nobel Prize in Physics for his work on laser cooling.

Bad vibrations

Unlike the spring or the superconducting gravimeter, the cold-atom device produces an absolute rather than a relative measurement of g. In their first demonstration, Chu and Kasevich measured the acceleration due to gravity to three parts in 100 million. This was about a million times better than previous attempts with single atoms, but it trailed behind the best absolute measurements, which were made using a macroscopic object in free fall.

Whether spring or quantum-based, gravimeters share the same major source of noise – vibrations

“It’s always one thing to do the first demonstration of principle, and then it’s a different thing to really get it to a performance level where it actually is useful and competitive,” says Achim Peters, who started a PhD with Chu in 1992 and is now a researcher at the Humboldt University of Berlin.

Whether spring or quantum-based, gravimeters share the same major source of noise – vibrations. Although we don’t feel it, the ground, which is the test mass’s reference frame, is never completely still. According to the Einstein equivalence principle, we can’t differentiate the acceleration due to these vibrations from the acceleration of the test mass due to gravity.

When Peters was at Stanford he built a sophisticated vibration isolation system where the extension of mechanical springs was controlled by electronic feedback. This brought the quantum device in line with other state-of-the-art measurement techniques, but such a complex apparatus would be difficult to operate outside a laboratory.

However, if a cold-atom gravity sensor could operate outside without being hampered by vibrations it would have an instant advantage over spring devices, where vibrations have to be averaged out by taking longer measurements. “If we want to measure several hectares, you’re talking about three weeks or plus [with spring gravimeters],” explains Metje. “That takes a lot of time and therefore also a lot of cost.”

Enter the gravity gradiometer

A few years after Chu and Kasevich published the first cold-atom interferometer result, the US Navy declassified a technology that had been developed by Bell Aerospace (later acquired by Lockheed Martin) for submarines and which transformed the field of geophysics. This device – called a gravity gradiometer – calculated the gravity gradient by measuring the acceleration of several spinning discs. As well as finding objects, gravity can identify a geographical location, meaning that gravity sensors have applications in GPS-free navigation. Compared to gravimeters, a gradiometer is more sensitive to nearby objects and when the gravity gradiometer was declassified it was seized upon for use in oil and gas exploration. The Lockheed Martin device remains the industry standard – it measures gravity gradient in three dimensions and its sophisticated vibration-isolation system means it can be used in the field, including in airborne surveys – but it is prohibitively costly for most researchers.

In 1998 Kasevich’s group demonstrated a gradiometer built from two cold-atom interferometers stacked one above the other, where the difference between the phases on the atom clouds was used to calculate the gravity gradient (Phys. Rev. Lett. 81 971). In this configuration, the interferometry pulses illuminating the two clouds come from the same laser beams, which means that the vibrations that had previously required a complex damping system are cancelled out. In the laboratory, cold-atom gravity gradiometers have many applications in fundamental physics – they have been used to test the Einstein equivalence principle to one part in a trillion, and a 100 m tall interferometer is currently under construction at Fermilab, where it will be used to hunt for gravitational waves.

It was around this time, in 2000, when Bongs first encountered cold-atom interferometry, as a postdoc with Kasevich, then at Yale. He explains that the goal was to “get one of the lab-based systems, which were essentially the standard at the time, out into the field”. Even without the problem of vibrational noise, this was a significant challenge. Temperature fluctuations, external magnetic fields and laser stability will all limit the performance of the gradiometer. The portability of the system must also be balanced against the fact that a taller device will allow longer freefall and more sensitive measurements. What’s more, the interferometers will rarely be perfectly directed towards the centre of the Earth, which means the atoms fall slightly sideways relative to the laser beams.

In the summer of 2008, by which time Bongs was in Birmingham, Kasevich’s group, now back at Stanford, mounted a cold-atom gradiometer in a truck and measured the gravity gradient as they drove in and out of a loading bay on the Stanford campus. They measured a peak that coincided with the building’s outer wall, but this demonstration took place with a levelling platform and temperature control inside the truck. The demonstration of the first truly free-standing, outdoor cold-atom gradiometer was still up for grabs.

Ears to the ground

The portable cold-atom gravity sensor project in Birmingham began in earnest in 2011, as a collaboration between the engineers and the physicists. The team knew that building a device that was robust enough to operate outside would be only half the challenge. They also needed to make something cost-effective and easy to operate. “If you can manage to make the laser system small and compact and cheap and robust, then you more or less own quantum technologies,” says Bongs.

When lasers propagate in free space, small knocks and bumps easily misalign the optical components. To make their device portable, the researchers made an early decision to instead use optical fibres, which direct light to the right place even if the device is jolted during transportation or operation.

However, they quickly realized that this was easier said than done. In a standard magneto-optical trap, atoms are cooled by three orthogonal pairs of laser beams that cool and trap them in three dimensions. In the team’s original configuration, this light came from three fibres that were split from a single laser. Bending and temperature fluctuations exert stresses on the optical fibre that alter the polarization of the light as it propagates. Unstable polarizations in the beams meant that the atom clouds were moving around in the optical traps. “It wasn’t very robust,” says Holynski, “we needed a different approach”.

To solve this problem, they adopted a new solution in which light enters the chamber from the top and bottom, where it bounces off a configuration of mirrors to create the two atom traps. Because the beams can’t be individually adjusted, this sacrifices some efficiency, but if it fixed the laser polarization problem, the team decided it was worth a try.

In the world of quantum technologies, 1550 is something of a magic number. This is the most common wavelength of telecoms lasers because light of this wavelength propagates furthest in optical fibres. The telecoms industry has therefore invested significant time and money into developing robust lasers operating close to 1550 nm.

By lucky chance, 1550 nm is also almost twice the main resonant frequency of rubidium-87 (780 nm), an alkali metal that is well-suited to atom interferometry. Conveniently close to rubidium-87’s resonant frequency are hyperfine transitions that can be used to cool the atoms, measure their final state and put them into a superposition for interferometry. Frequency doubling using nonlinear crystals is a well-established optical technique, so combining a rubidium interferometer with a telecoms laser was an ideal solution.

By 2018, as part of the hub and under contract with the UK Ministry of Defence, had assembled a freestanding gradiometer – a 2 m tall tube containing the two interferometers, attached to a box of electronics and the lasers, both mounted on wheels. The researchers performed outdoor trials in 2018 and 2019, including a trip to an underground cave in the Peak District, but they still weren’t getting the performance they wanted. “People get their hopes up,” says Holynski. “This was quite a big journey.”

The researchers worked out that another gamble they had made, this time to reduce the cost of the magnetic shield, wasn’t performing as well as hoped. External magnetic fields shift the atom’s energy levels, but unlike the phase shift due to gravity, this source of error is the same whether the momentum kick is directed up or down. By taking two successive measurements with a downwards and upwards kick, they thought they could remove magnetic noise, enabling them to reduce the cost of the expensive alloy they were using to shield the interferometers.

This worked as expected, but because they were operating outside a controlled laboratory environment, the large variation of the magnetic fields in space and time introduced other errors. It was back to the lab, where the team disassembled the sensor and rebuilt it again with full magnetic shielding.

By 2020 the researchers were ready to take the new device outside. However, the COVID-19 pandemic ground work to a halt and they had to wait until the following year.

Quantum tunnelling

“One of the things that changes about you when you work on gravity gradiometers is you start looking around for potential targets everywhere you go,” says Holynski. In March 2021 a team of physicists and engineers that included Bongs, Metje and Holynski took the newly rebuilt gradiometer for its first outside trial, where they trundled it repeatedly over a road on the University of Birmingham campus. They knew that running under the road was a two-by-two-metre hollow tunnel, built to carry utility lines. They also knew approximately where it was, but wanted to see if the gradiometer could find it.

The first time they did this, they noticed a dip in the gravity gradient that seemed to have the right dimensions for the tunnel, and when they repeated the measurements, they saw it again. Because of their previous unsuccessful attempts, Holynski remained trepidatious. “People get quite excited. And then you have to say to them, ‘Sorry, I don’t think that’s quite conclusive enough yet’.”

Elsewhere on campus, another team was busy analysing the data. The results, when they were done, were consistent with a hollow object, about two-by-two metres across, and about a metre below the surface. Millions of people will have walked over that road without thinking once about what’s beneath it, but to the researchers, this was the culmination of a decade of work, and proof that cold-atom gradiometers can operate outside the lab (Nature 602 590).

The valley of death

“It’s one more step in the direction of making quantum sensors available for real-world everyday use,” says Holger Müller, a physicist at the University of California, Berkeley. In 2019 Müller’s group published the results of a gravity survey it had taken with a cold-atom interferometer during a drive through the California hills (Sci. Adv. 5 10.1126/sciadv.aax0800). He is also involved in a NASA project that aims to perform atom interferometry on the International Space Station (Nature Communications 15 6414). Müller thinks that for researchers especially, cold-atom gradiometers could make gravity gradient surveys more accessible than with the Lockheed Martin device.

By now, the Birmingham gravity gradiometer is well travelled. As well as land-based trials, it has been on two ship voyages, one lasting several weeks, to test its performance in different environments and its potential for use in navigation. The project has also become a flagship of the UK’s national quantum technologies programme, garnering industry partners including Network Rail and RSK and spinning out into start-up DeltaG (of which Holynski is a co-founder). Another project in France led by the company iXblue has also built a prototype gravity gradiometer that has been demonstrated inside (Phys. Rev. A 105 022801).

However, if cold-atom gravity gradiometers are to become an alternative to electromagnetic surveys or spring gravimeters, they must escape the “Valley of Death” – the critical phase in a technology journey when it has been demonstrated but not yet been commercialized.

This won’t be easy. The team has estimated that the gravity gradiometer currently performs about 1.5 times better than the industry-leading spring gravimeter. Spring gravimeters are small, easy to operate and significantly cheaper than the quantum alternative. The cost of the lasers in the quantum gradiometer alone are several hundreds of thousands of pounds, compared to about £100,000 for a spring-based instrument.

The quantum device is also large, requires a team of scientists to operate and maintain it, and consumes much more power than a spring gravimeter. As well as saving time compared to spring gravimeters, a potential advantage of the quantum gravity gradiometer is that because it has no machined moving parts it could be used for passive, long-term environmental monitoring. However, unless the power consumption is reduced it will be tricky to operate it in remote conditions.

In the years since the first test, the team has built another prototype that is about half the size, consumes significantly less power, and delivers the cooling, detection and interferometry using a single laser, which will significantly reduce the total cost. Holynski explains that this system is a “work in progress” that is currently being tested in the laboratory.

A large focus of the group’s efforts has been bringing down the cost of the lasers. “We’ve taken available components from the telecom community and found ways to make them work in our system,” says Holynski. “Now we’re starting to work with the telecom community, the academic and industry community, to think ‘how can we twist their technology and make it cheaper to fit what we need?’”

When Chu and Kasevich demonstrated it for the first time, the idea of atom interferometry was already four decades old, having been proposed by David Bohm and later Eugene Wigner (Am. J. Phys. 31 6). Rather than lasers, this theoretical device was based on the Stern–Gerlach effect, in which an atom is in a superposition of spin states, deflected in opposite directions in a magnetic field. Atoms have a much smaller characteristic wavelength than photons, so a practical interferometer requires exquisite control over the atomic wavefronts. In the decades after it was proposed, several theorists, including Julian Schwinger, investigated the idea but found that a useful interferometer would require an extraordinarily controlled low-noise environment that then seemed inaccessible (Found. Phys. 18 1045).

Decades in the making, the mobile cold-atom interferometer is a triumph of practical problem-solving and even if the commercial applications have yet to be realized, one thing is clear: when it comes to pushing the boundaries of quantum physics, sometimes it pays to think like an engineer.

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