While the physics of uncorking a bottle of champagne has been well documented, less is known about the mechanisms at play when opening a swing-top bottle of beer.
Physicist Max Koch from the University of Göttingen in Germany, decided to find out more.
Koch, who is a keen homebrewer, and colleagues used a high-speed camera and a microphone to capture what was going on together with computational fluid-dynamics simulations.
When opening a carbonated bottle under pressure, the difference between the gas pressure in the bottleneck and ambient pressure as it opens results in a rapid escape of gas from the bottle, which can reach the speed of sound.
In a champagne bottle, this results in the creation of a Mach disc as well the classic “pop” sound as it is uncorked.
To investigate the gas dynamics in swing-top bottles, Kock and colleagues examined transparent 0.33 litre bottles, which contained home-brewed ginger beer under 2–5 bars of pressure.
The team found that the sound emitted when opening the bottles, what can be described as an “ah” sound, wasn’t due to a single shockwave, but rather condensation in the bottleneck forming a standing wave.
“The pop’s frequency is much lower than the resonation if you blow on the full bottle like a whistle,” notes Koch. “This is caused by the sudden expansion of the carbon dioxide and air mixture in the bottle, as well as a strong cooling effect to about minus 50 degrees Celsius, which reduces sound speed.”
The team also investigated the sloshing of the beverage as it is opened. First, the dissolved carbon dioxide inside the beer triggers the level of the liquid to rise while the motion of the bottle as it opens also causes the liquid to slosh.
Another effect during opening is the bottle-top hitting the glass with its sharp edge. This triggers further “gushing” in the liquid due to the enhanced formation of bubbles.
There are still some unanswered questions, however, which will require further work. “One thing we didn’t resolve is that our numerical simulations showed an initial strong peak in the acoustic emission before the short ‘ah’ resonance, but this peak was absent in the experimentation,” adds Koch.
Scientists who have been publicly accused of sexual misconduct see a significant and immediate decrease in the rate at which their work is cited, according to a study by behavioural scientists in the US. However, researchers who are publicly accused of scientific misconduct are found not to suffer the same drop in citations (PLOS One20 e0317736). Despite its flaws, citation rates are often seen a marker of impact and quality.
The study was carried by a team led by Giulia Maimone from the University of California, Los Angeles, who collected data from the Web of Science covering 31,941 scientific publications across 18 disciplines. They then analysed the citation rates for 5888 papers authored by 30 researchers accused of either sexual or scientific misconduct, the latter including data fabrication, falsification and plagiarism.
Maimone told Physics World that they used strict selection criteria to ensure that the two groups of academics were comparable and that the accusations against them were public. This meant her team only used scholars whose misconduct allegations have been reported in the media and had “detailed accounts of the allegations online”.
Maimone’s team concluded that papers by scientists accused of sexual misconduct experienced a significant drop in citations in the three years after allegations become public compared with a “control” group of academics of a similar professional standing. Those accused of scientific fraud, meanwhile, saw no statistically significant change in the citation rates of their papers.
Further work
To further explore attitudes towards sexual and scientific misconduct, the researchers surveyed 231 non-academics and 240 academics. The non-academics considered sexual misconduct more reprehensible than scientific misconduct and more deserving of punishment, while academics claimed that they would more likely continue to cite researchers accused of sexual misconduct as compared to scientific misconduct. “Exactly the opposite of what we observe in the real data,” adds Maimone.
According to the researchers, there are two possible explanations for this discrepancy. One is that academics, according to Maimone, “overestimate their ability to disentangle the scientists from the science”. Another is that scientists are aware that they would not cite sexual harassers, but they are unwilling to admit it because they feel they should take a harsher professional approach towards scientific misconduct.
Maimone says they would now like to explore the longer-term consequences of misconduct as well as the psychological mechanisms behind the citation drop for those accused of sexual misconduct. “Do [academics] simply want to distance themselves from these allegations or are they actively trying to punish these scholars?” she asks.
From the Global Physics Summit in Anaheim, California
Some of the most fascinating people that you meet at American Physical Society meetings are not actually physicists, and Bruce Rosenbaum is no exception. Based in Massachusetts, Rosenbaum is a maker of beautiful steampunk objects and he is in Anaheim with a quantum-related creation (see figure).
At first glance Rosenbaum’s sculpture of a “quantum engine” fits in nicely at a conference exhibition that features gleaming vacuum chambers and other such things. However, this lovely artistic object is meant to be admired, rather than being a functioning machine.
At the centre of the object is a small vacuum chamber that could hold a single trapped ion – which could be operated as a quantum engine. Lasers are pointed at the ions through the chamber windows and the chamber is surrounded by a spherical structure that represents both the Bloch sphere of quantum physics and an armillary sphere. The latter being used to demonstrate the motions of celestial objects in the days before computers. But as someone who, many years ago, did some electron spectroscopy, the rings are more reminiscent of Helmholtz coils that would screen the ion from Earth’s magnetic field.
I should make it clear that the neither the vacuum chamber, nor the lasers are real — and there is no trapped ion. However, a real quantum engine based on a trapped ion has been created in a real physics lab. So, in principle, the sculpture could be made into a functional device by using “real components”.
Past and future connections
In my mind, the object symbolizes the connection between the state-of-the-art today (the trapped-ion qubit) and the many technologies that have come before (armillary sphere).
While Rosenbaum does not have a background in physics, I think he has a kinship with the thousands of experimental physicists who have built devices that bear a striking resemblance to this object. But some physicists were involved in the development of this beautiful object. They include Nicole Yunger Halpern of the University of Maryland. Yunger Halpern is a theorist who uses the ideas of quantum information to study thermodynamics. She describes the field as “quantum steampunk” because like the artistic genre of steampunk, it combines 19th century concepts (thermodynamics) with the 21st century concepts of quantum science and technology.
I had a lovely chat with Rosenbaum and he had some very interesting things to say about the intersection of creativity and technology – things that are highly relevant to physicists. I hope to have him and perhaps one of his physicist colleagues on a future episode of the Physics World Weekly podcast.
When physicists got their first insights into the quantum world more than a century ago, they found it puzzling to say the least. But gradually, and through clever theoretical and experimental work, a consistent quantum theory emerged.
Two physicists that who played crucial roles in this evolution were Albert Einstein and John Bell. In this episode of the Physics World Weekly podcast the theoretical crypto-physicist Artur Ekert explains how a quantum paradox identified by Einstein and colleagues in 1935 inspired a profound theoretical breakthrough by Bell three decades later.
Ekert, who splits his time between the University of Oxford and the National University of Singapore, describes how he used Bell’s theorem to create a pioneering quantum cryptography protocol and he also chats about current research in quantum physics and beyond.
The European Space Agency (ESA) has released the first batch of survey data from its €1.4bn Euclid mission. The release includes a preview of its “deep field” where in just one week of observations, Euclid already spotted 26 million galaxies as well as many transient phenomena such as supernovae and gamma-ray bursts. The dataset is published along with 27 scientific papers that will be submitted to the journal Astronomy & Astrophysics.
The dataset also features a catalogue of 380,000 galaxies that have been detected by artificial intelligence or “citizen-science” efforts. They include those with spiral arms, central bars and “tidal tails”, inferring merging galaxies.
There has also been the discovery of 500 gravitational-lens candidates thanks to AI and citizen science. Gravitational lensing in when light from more distant galaxies is bent around closer galaxies due to gravity and it can help identify where dark matter is located and its properties.
“For the past decade, my research has been defined by painstakingly analysing the same 50 strong gravitational lenses, but with the data release, I was handed 500 new strong lenses in under a week,” says astronomer James Nightingale from Newcastle University. “It’s a seismic shift – transforming how I do science practically overnight.”
The data released today still represents only 0.4% of the total number of galaxies that Euclid is expected to image over its lifetime. Euclid will capture images of more than 1.5 billion galaxies over six years, sending back around 100 GB of data every day.
Light curves: a collection of gravitational lenses that Euclid captured in its first observations of the deep field areas. (Courtesy: ESA/Euclid/Euclid Consortium/NASA, image processing by M Walmsley, M Huertas-Company, J-C Cuillandre)
“Euclid shows itself once again to be the ultimate discovery machine. It is surveying galaxies on the grandest scale, enabling us to explore our cosmic history and the invisible forces shaping our universe,” says ESA’s science director, Carole Mundell. “With the release of the first data from Euclid’s survey, we are unlocking a treasure trove of information for scientists to dive into and tackle some of the most intriguing questions in modern science”.
More data to come
Euclid was launched in July 2023 and is currently located in a spot in space called Lagrange Point 2 – a gravitational balance point some 1.5 million kilometres beyond the Earth’s orbit around the Sun. The Euclid Consortium comprises some 2600 members from more than 15 countries.
Euclid has a 1.2 m-diameter telescope, a camera and a spectrometer that it uses to plot a 3D map of the distribution of galaxies. The images it takes are about four times as sharp as current ground-based telescopes.
Researchers have demonstrated that they can remotely detect radioactive material from 10 m away using short-pulse CO2 lasers – a distance over ten times farther than achieved via previous methods.
Conventional radiation detectors, such as Geiger counters, detect particles that are emitted by the radioactive material, typically limiting their operational range to the material’s direct vicinity. The new method, developed by a research team headed up at the University of Maryland, instead leverages the ionization in the surrounding air, enabling detection from much greater distances.
The study may one day lead to remote sensing technologies that could be used in nuclear disaster response and nuclear security.
Using atmospheric ionization
Radioactive materials emit particles – such as alpha, beta or gamma particles – that can ionize air molecules, creating free electrons and negative ions. These charged particles are typically present at very low concentrations, making them difficult to detect.
Senior author Howard Milchberg and colleagues – also from Brookhaven National Laboratory, Los Alamos National Laboratory and Lawrence Livermore National Laboratory – demonstrated that CO2 lasers could accelerate these charged particles, causing them to collide with neutral gas molecules, in turn creating further ionization. These additional free charges would then undergo the same laser-induced accelerations and collisions, leading to a cascade of charged particles.
This effect, known as “electron avalanche breakdown”, can create microplasmas that scatter laser light. By measuring the profile of the backscattered light, researchers can detect the presence of radioactive material.
The team tested their technique using a 3.6-mCi polonium-210 alpha particle source at a standoff distance of 10 m, significantly longer than previous experiments that used different types of lasers and electromagnetic radiation sources.
“The researchers successfully demonstrated 10-m standoff detection of radioactive material, significantly surpassing the previous range of approximately 1 m,” she says.
Milchberg and collaborators had previously used a mid-infrared laser in a similar experiment in 2019. Changing to a long-wavelength (9.2 μm) CO2 laser brought significant advantages, he says.
“You can’t use any laser to do this cascading breakdown process,” Milchberg explains. The CO2 laser’s wavelength was able to enhance the avalanche process, while being low energy enough to not create its own ionization sources. “CO2 is sort of the limit for long wavelengths on powerful lasers and it turns out CO2 lasers are very, very efficient as well,” he says. “So this is like a sweet spot.”
Imaging microplasmas
The team also used a CMOS camera to capture visible-light emissions from the microplasmas. Milchberg says that this fluorescence around radioactive sources resembled balls of plasma, indicating the localized regions where electron avalanche breakdowns had occurred.
By counting these “plasma balls” and calibrating them against the backscattered laser signal, the researchers could link fluorescence intensity to the density of ionization in the air, and use that to determine the type of radiation source.
The CMOS imagers, however, had to be placed close to the measured radiation source, reducing their applicability to remote sensing. “Although fluorescence imaging is not practical for field deployment due to the need for close-range cameras, it provides a valuable calibration tool,” Milchberg says.
Scaling to longer distances
The researchers believe their method can be extended to standoff distances exceeding 100 m. The primary limitation is the laser’s focusing geometry, which would affect the regions in which it could trigger an avalanche breakdown. A longer focal length would require a larger laser aperture but could enable kilometre-scale detection.
Choi points out, however, that deploying a CO2 laser may be difficult in real-world applications. “A CO₂ laser is a bulky system, making it challenging to deploy in a portable manner in the field,” she says, adding that mounting the laser for long-range detection may be a solution.
Milchberg says that the next steps will be to continue developing a technique that can differentiate between different types of radioactive sources completely remotely. Choi agrees, noting that accurately quantifying both the amount and type of radioactive material continues to be a significant hurdle to realising remote sensing technologies in the field.
“There’s also the question of environmental conditions,” says Milchberg, explaining that it is critical to ensure that detection techniques are robust against the noise introduced by aerosols or air turbulence.
The Square Kilometre Array (SKA) Observatory has released the first images from its partially built low-frequency telescope in Australia, known as SKA-Low.
The new SKA-Low image was created using 1024 two-metre-high antennas. It shows an area of the sky that would be obscured by a person’s clenched fist held at arm’s length.
Observed at 150 MHz to 175 MHz, the image contains 85 of the brightest known galaxies in that region, each with a black hole at their centre.
“We are demonstrating that the system as a whole is working,” notes SKA Observatory director-general Phil Diamond. “As the telescopes grow, and more stations and dishes come online, we’ll see the images improve in leaps and bounds and start to realise the full power of the SKAO.”
SKA-Low will ultimately have 131 072 two-metre-high antennas that will be clumped together in arrays to act as a single instrument.
These arrays collect the relatively quiet signals from space and combine them to produce radio images of the sky with the aim of answering some of cosmology’s most enigmatic questions, including what dark matter is, how galaxies form, and if there is other life in the universe.
When the full SKA-Low gazes at the same portion of sky as captured in the image released yesterday, it will be able to observe more than 600,000 galaxies.
“The bright galaxies we can see in this image are just the tip of the iceberg,” says George Heald, lead commissioning scientist for SKA-Low. “With the full telescope we will have the sensitivity to reveal the faintest and most distant galaxies, back to the early universe when the first stars and galaxies started to form.”
‘Milestone’ achieved
SKA-Low is one of two telescopes under construction by the observatory. The other, SKA-Mid, which observes mid-frequency range, will include 197 three-storey dishes and is being built in South Africa.
The telescopes, with a combined price tag of £1bn, are projected to begin making science observations in 2028. They are being funded through a consortium of member states, including China, Germany and the UK.
University of Cambridge astrophysicist Eloy de Lera Acedo, who is principal Investigator at his institution for the observatory’s science data processor, says the first image from SKA-Low is an “important milestone” for the project.
“It is worth remembering that these images now require a lot of work, and a lot more data to be captured with the telescope as it builds up, to reach the science quality level we all expect and hope for,” he adds.
Rob Fender, an astrophysicist at the University of Oxford, who is not directly involved in the SKA Observatory, says that the first image “hints at the enormous potential” for the array that will eventually “provide humanity’s deepest ever view of the universe at wavelengths longer than a metre”.
“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.
Physics at work Damage to underground infrastructure costs millions of pounds a year in the UK alone. That’s why there is a need to develop new methods to image the subsurface that don’t require holes to be dug or rely on electromagnetic pulses whose penetration depth is highly variable. (Courtesy: iStock/mikeuk)
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.
Out and about The quantum-based gravity sensor, pictured outside on the University of Birmingham campus. The blue tube houses the two interferometers and the black box houses the lasers and control electronics. (CC BY 4.0 Nature602 590)
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’.”
(a) A schematic of the 2021 test of the gravity gradiometer, with the hollow utility tunnel pictured to scale. (b) The hourglass configuration of the quantum gravity gradiometer. The atom clouds (green dots) are laser-cooled (red arrows) in magneto-optical traps formed using mirrors (blue). To measure the gravity gradient the atoms are subject to interferometry laser pulses (yellow arrows) under freefall (purple dots).
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 (Nature602 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 Communications15 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 Delta.g (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. A105 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.
From the Global Physics Summit in Anaheim, California
The greatest pleasure of being at a huge physics conference is learning about the science of something that’s familiar, but also a little bit quirky. That’s why I always try to go to sessions given by undergraduate students, because for some reason they seem to do research projects that are the most fun.
I was not disappointed by the talk given this morning by Atharva Lele, who is at the Georgia Institute of Technology here in the US. He spoke about the physics of manu jumping, a competitive sport that originates from the Māori and Pasifika peoples of New Zealand.
The general idea will be familiar to anyone who messed around at swimming pools as a child: who can make the highest splash when they jump into the water.
Cavity creation
According to Lele, the best manu jumpers enter the water back first, creating a V-shape with their legs and upper body. The highest splashes are made when a jumper creates a deep and wide air cavity that quickly closes, driving water upwards in a jet – often to astonishing heights.
Lele and colleagues discovered that a 45° angle between the legs and torso afforded the highest splashes. This is probably because this angle results in a cavity that is both deep and wide. An analysis of videos of manu jumpers revealed that the best ones entered the water at an angle of about 46°, corroborating the teams findings. This is good news for jumpers, because there is risk of injury at higher angles (think belly flop).
Another important aspect of the study looked at what jumpers did when they entered the water – which is to roll and kick. To study the effect of this motion, the team created a “manu bot”, which unfolded as it entered the water. They found that there was an optimal opening time for making the highest splashes – it is a mere 0.26 s.
I was immediately taken back to my childhood in Canada and realized that we were doing our own version of manu from the high diving board at the local pool. The most successful technique that we discovered was to keep our bodies straight, but entering the water at an angle. This would consistently produce a narrow jet of water. I realize now that by entering the water at an angle, we must have been creating a relatively deep and wide cavity – although probably not as efficiently and manu jumpers. Maybe Lele and colleagues could do a follow-up study looking at alternative versions of manu around the world.
A new study probing quantum phenomena in neurons as they transmit messages in the brain could provide fresh insight into how our brains function.
In this project, described in the Computational and Structural Biotechnology Journal, theoretical physicist Partha Ghose from the Tagore Centre for Natural Sciences and Philosophy in India, together with theoretical neuroscientist Dimitris Pinotsis from City St George’s, University of London and the MillerLab of MIT, proved that established equations describing the classical physics of brain responses are mathematically equivalent to equations describing quantum mechanics. Ghose and Pinotsis then derived a Schrödinger-like equation specifically for neurons.
Our brains process information via a vast network containing many millions of neurons, which can each send and receive chemical and electrical signals. Information is transmitted by nerve impulses that pass from one neuron to the next, thanks to a flow of ions across the neuron’s cell membrane. This results in an experimentally detectable change in electrical potential difference across the membrane known as the “action potential” or “spike”.
When this potential passes a threshold value, the impulse is passed on. But below the threshold for a spike, a neuron’s action potential randomly fluctuates in a similar way to classical Brownian motion – the continuous random motion of tiny particles suspended in a fluid – due to interactions with its surroundings. This creates the so-called “neuronal noise” that the researchers investigated in this study.
Previously, “both physicists and neuroscientists have largely dismissed the relevance of standard quantum mechanics to neuronal processes, as quantum effects are thought to disappear at the large scale of neurons,” says Pinotsis. But some researchers studying quantum cognition hold an alternative to this prevailing view, explains Ghose.
“They have argued that quantum probability theory better explains certain cognitive effects observed in the social sciences than classical probability theory,” Ghose tells Physics World. “[But] most researchers in this field treat quantum formalism [the mathematical framework describing quantum behaviour] as a purely mathematical tool, without assuming any physical basis in quantum mechanics. I found this perspective rather perplexing and unsatisfactory, prompting me to explore a more rigorous foundation for quantum cognition – one that might be physically grounded.”
As such, Ghose and Pinotsis began their work by taking ideas from American mathematician Edward Nelson, who in 1966 derived the Schrödinger equation – which predicts the position and motion of particles in terms of a probability wave known as a wavefunction – using classical Brownian motion.
Firstly they proved that the variables in the classical equations for Brownian motion that describe the random neuronal noise seen in brain activity also obey quantum mechanical equations, deriving a Schrödinger-like equation for a single neuron. This equation describes neuronal noise by revealing the probability of a neuron having a particular value of membrane potential at a specific instant. Next, the researchers showed how the FitzHugh-Nagumo equations, which are widely used for modelling neuronal dynamics, could be re-written as a Schrödinger equation. Finally, they introduced a neuronal constant in these Schrödinger-like equations that is analogous to Planck’s constant (which defines the amount of energy in a quantum).
“I got excited when the mathematical proof showed that the FitzHugh-Nagumo equations are connected to quantum mechanics and the Schrödinger equation,” enthuses Pinotsis. “This suggested that quantum phenomena, including quantum entanglement, might survive at larger scales.”
“Penrose and Hameroff have suggested that quantum entanglement might be related to lack of consciousness, so this study could shed light on how anaesthetics work,” he explains, adding that their work might also connect oscillations seen in recordings of brain activity to quantum phenomena. “This is important because oscillations are considered to be markers of diseases: the brain oscillates differently in patients and controls and by measuring these oscillations we can tell whether a person is sick or not.”
Going forward, Ghose hopes that “neuroscientists will get interested in our work and help us design critical neuroscience experiments to test our theory”. Measuring the energy levels for neurons predicted in this study, and ultimately confirming the existence of a neuronal constant along with quantum effects including entanglement would, he says, “represent a big step forward in our understanding of brain function”.
