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Quantum treatment sheds fresh light on triboelectricity

Photo of Robert Alicki and Alejandro Jenkins with a bust of Thales — **Ancient observations, modern explanation** Robert Alicki (left) and Alejandro Jenkins (right). Alicki holds a bust of Thales, the pre-Socratic philosopher who described magnetic and “amber” effects as evidence of a material’s soul. (Credit: Maria Alicki)

Shuffling around on a carpet to give someone an electric shock might seem like the oldest trick in the book, yet scientists know surprisingly little about why it happens. “I believed – like I think most physicists – that these phenomena were understood by the experts,” says Robert Alicki, a mathematical and theoretical physicist at the University of Gdansk, Poland. “But it was not the case. It was still an open question.”

Thanks to Alicki and his colleague Alejandro Jenkins of the Universidad de Costa Rica, the mystery surrounding triboelectricity (as the “charging by rubbing” effect is known) may be clearing up. According to Alicki and Jenkins, a major barrier to understanding triboelectricity is that physicists tend to view the phenomenon in terms of electrostatic potentials, even though “from a potential effect, you are never going to sustain a current that is going around a circuit,” Jenkins says. “It’s like the problem of perpetual motion.”

Alicki and Jenkins formulated their alternative description by incorporating the concept of pumping into a new, quantum model of a system undergoing triboelectric processes. “Pumping can replenish a potential, but it is not describable by a potential,” Jenkins explains. “It can do something that no potential can do, and that is to drive something around on a closed path.”

Using this pumping-based model, the pair successfully reproduced several experimentally observed characteristics of triboelectricity, such as its dependence on material surface and geometry and the speed of rubbing. In particular, the model accurately predicts that the most electrically negative and electrically positive materials will have symmetrical maximum charge densities when rubbed – something that models based on electric potentials models cannot explain. The new model also predicts a maximum tribovoltage in terms of the sliding velocity of the two surfaces, which Alicki and Jenkins say could be tested using an experimental set-up with sufficient control over a constant sliding velocity.

From lasing bosons to fermions

While Alicki has been working on quantum thermodynamics for decades, Jenkins is a more recent recruit, having started out in high-energy theory. As their paths converged – Jenkins has just begun a fellowship at Gdansk’s Institute for Theory of Quantum Technologies (ICTQT) – they discovered that they shared an interest in systems found in motors and engines that operate away from equilibrium, where energy is irreversibly converted from one form to another. While such systems are the bread and butter of engineers, and Alicki and collaborators started working on them as far back as the late 1970s, Jenkins says that on the whole, they have attracted less attention from theorists than systems at equilibrium, fluctuating around equilibrium or relaxing to equilibrium.

At first, this common interest in out-of-equilibrium systems led Alicki and Jenkins to formulate a mathematical description of “superradiance”, or the enhanced effects of radiation associated with rotating objects. Such effects were first described in 1971 by Yakov Zel’dovich, whose suggestion that superradiance ought to apply to a spinning gravitational mass led to follow-up work by Jacob Bekenstein and Stephen Hawking on the thermodynamics of black holes.

By describing rotating systems in terms of quantum fields, and treating the moving object as a heat bath, Alicki and Jenkins showed how work could be extracted via stimulated emission, similar to a laser’s operation. But while their laser analogy offered a new perspective on such systems, the underlying process, while exotic, was already pretty well-understood. It was only later that they realized that their formulation of a quantum field and two heat baths could lead to something “qualitatively new”: a description for the motion that drives an active current of fermionic electrons from one material to another in the humble triboelectric effect.

“The soul of inanimate objects”

The pair built up their model by defining Hamiltonians with creation and annihilation operators for electron states on the surface of the moving material (where the population inversion takes place) and within the interior of the two rubbed materials (which act as the heat baths). They then defined the pumping of the system in terms of the rate of change of the populations of these electron states. Although the Pauli exclusion principle forbids fermions such as electrons to exhibit superradiance, Alicki and Jenkins were able to show that with the bulk bodies of the two surfaces acting as two heat baths, a motion-induced population inversion of fermions could nevertheless result and sustain a macroscopic current.

Can flexoelectricity explain the charging by friction conundrum?

While magnetic and triboelectric effects have been known to scientists since antiquity – in the 6th century BCE, the pre-Socratic philosopher Thales of Miletus referred to them as “evidence of a kind of soul” – Jenkins notes that “the interesting point is neither can be described classically”. The need for quantum mechanics to explain the behaviour of permanent magnets was established by Niels Bohr and Hendrika Johanna van Leeuwen over 100 years ago, and Jenkins says that his and Alicki’s latest work shows that the same is true of triboelectricity. Although pumping and work cycles exist in classical thermodynamics, the pair insist that only a quantum treatment can make sense of electrons’ fermionic behaviour as they move between surfaces in the triboelectric effect. In effect, Alicki says, “Quantum mechanics is the soul of inanimate objects.”

Alicki and Jenkins are now considering ways to further investigate dry friction to explore how it relates to the triboelectric effect. They are also interested in understanding details of energy transduction in active devices such as batteries, solar cells and thermoelectric generators, as well as active processes in various applications from astrophysics and cosmology, to fundamental physics.

Full details of the work are reported in Physical Review Letters

Roger Penrose, Reinhard Genzel and Andrea Ghez bag the Nobel Prize for Physics

Nobel Prize for Physics 2020 winners — New laureates: Roger Penrose, Reinhard Genzel and Andrea Ghez have won the the 2020 Nobel Prize for Physics. (Courtesy: IOP Publishing/Tushna Commissariat; CC-BY-SA H Garching; UCLA/Christopher Dibble)

The 2020 Nobel Prize for Physics has been awarded to Roger Penrose, Reinhard Genzel and Andrea Ghez for their work on black holes.

The prize is worth 10 million Swedish krona (about $1.1 million) and half goes to Penrose, with Genzel and Ghez sharing the other half of the prize.

The Nobel Committee cites Penrose “for the discovery that black hole formation is a robust prediction of the general theory of relativity”, and Genzel and Ghez “for the discovery of a supermassive compact object at the centre of our galaxy”.

After the announcement was made this morning by the Royal Swedish Academy of Sciences, Ghez answered questions remotely from the US.

Doubt and excitement

When asked what went through her mind when she first thought there was a huge black hole lurking in the middle of the Milky Way, Ghez replied “The first thing was doubt that you’re really seeing what you think you’re seeing. Doubt and excitement – that feeling that you’re at the frontier of research.”

On being the fourth woman to win the physics Nobel prize, Ghez said “I’m thrilled to receive the prize and take very seriously the responsibility with being the fourth woman to win the Nobel prize. I hope I can inspire other young women into the field – it’s a field that has so many pleasures. And if you’re passionate about science there are so many things that can be done.”

Since the 18th century, physicists have speculated about the existence of objects so massive that even light cannot escape their gravitational pull. However, it was not until the early 20th century when Albert Einstein created his general theory of relativity that scientists had the mathematical tools to investigate black holes with mathematical precision.

Can black holes form?

But even then, there was confusion over whether a black hole could form in nature. One concern at the time was the idea that any departure from perfect spherical symmetry of an object could prevent it from collapsing to a singularity – a single point in space and time. This was an important consideration because rotating stars do not have spherical symmetry.

In 1965 Penrose developed new mathematical tools for describing how a star could collapse to a black hole and devised a rigorous proof that the formation of a black hole is entirely consistent with general relativity. In particular, he introduced the concept of the “trapped surface” – a closed 2D surface with the property that all light rays orthogonal to the surface converge when traced toward the future. A trapped surface is formed in the early stages of the gravitational collapse of a star and once it has formed, the system must collapse to a singularity, creating a black hole. Crucially, Penrose showed that this applies irrespective of the symmetry of the collapsing object.

As well as being the first major contribution to general relativity since Einstein, Penrose’s work inspired generations of astrophysicists and astronomers to work towards observing black holes.

“Renaissance in relativity”

“It was Penrose, more than anyone else, who triggered the renaissance in relativity in the 1960s through his introduction of new mathematical techniques,” says the UK’s Astronomer Royal, Martin Rees.

Also in the mid-1960s, astronomers and astrophysicists were beginning to think that light emitted from bright regions at the centres of some galaxies was created by matter falling into black holes that were millions or even billions of times more massive than the Sun. However, verifying that these active galactic nuclei (AGNs) contained black holes proved to be very difficult because telescopes did not have the resolution to distinguish between a black hole and a tight cluster of stars – which could also be lurking at the centres of galaxies.

A way around this problem is to study the motions of stars that orbit close to the AGN. If the stars are orbiting a black hole, their speeds should have a specific relationship with their distance from the black hole – as do planets orbiting the Sun. However, if the stars are orbiting a cluster of stars, a different speed–distance relationship is expected.

Highly elliptical orbit

Teams led by Ghez and Genzel studied a star that takes about 16 years to orbit the AGN at the centre of the Milky Way. The star has a highly elliptical orbit and gets to within 17 light-hours from the AGN. Independent analyses of the motion of the star by both teams suggests that it is orbiting an extremely compact object that with a mass of about 4 million Suns. The only reasonable interpretation of this is that there is a supermassive black hole at the centre of the galaxy.

Laura Nuttall, who studies black-hole mergers at Portsmouth University told Physics World “It’s great to see Penrose, Ghez and Genzel recognized with the Nobel prize. Penrose is synonymous with black holes. His work in proving how black holes form, as well as their centre being a singularity, has opened so many fields, including that of searching for gravitational waves.”

She adds, “Ghez and Genzel’s work has also inspired many, such as the Event Horizon Telescope, which only released an image of a supermassive black hole last year. It’s wonderful, that their work is very much taken as a given today – of course black holes form from the collapse of matter and of course there’s a black hole at the centre of the galaxy. It’s easy to forget that this has not always been the case!”

Ghez was born in 1965 in the New York City, US. She received a BS in physics from the Massachusetts Institute of Technology in 1987 and a PhD at the California Institute of Technology in 1992. After a year at the University of Arizona, she moved to the University of California, Los Angeles in 1994 where she has remained since.

Genzel was born in 1952 in Bad Homburg vor der Höhe, Germany. He studied physics at the University of Freiburg before completing a PhD in radio astronomy at the University of Bonn in 1978. He then moved to the US working first at the Harvard-Smithsonian Center for Astrophysics until 1980 and then the Space Sciences Laboratory at the University of California, Berkeley until 1985. After a year as a professor at the University of California, Berkeley, he became a director of the Max Planck Institute for Extraterrestrial Physics in 1986. Since 1999 Genzel has held a joint appointment between the Max Planck Institute for Extraterrestrial Physics and the University of California, Berkeley.

Penrose was born in 1931 in Colchester, UK. He did a BSc in mathematics at University College London before completing a PhD in algebraic geometry at the University of Cambridge in 1957. After spending time at Princeton and Syracuse universities in the US in 1959–1961, he returned to England to King’s College London before heading to the University of Texas at Austin in 1963–1964. Penrose then moved to Birkbeck College, London until 1973 before heading to the University of Oxford, where he has remained since.

Flexible electronics make their way into the operating room

Electrode array — Electrodes fixed to an inflated balloon catheter (Courtesy: John Rogers, Northwestern University)

A research team led by engineers at the George Washington University and Northwestern University has developed a new surgical tool containing advanced flexible electronics that could improve diagnosis and treatment of cardiac diseases.

Balloon catheters are often used during minimally invasive surgery or ablation procedures, where they are relied upon to carry out measurements and perform therapeutic functions when inserted through small incisions. They can also be inserted into the heart to treat cardiac arrhythmias by locating and ablating the region of tissue causing the arrhythmia. Currently, however, most balloon catheters are rigid, which means they cannot conform well to the soft surfaces in the heart. In addition, these devices can only perform one function at a time, requiring doctors to use multiple catheters throughout a procedure.

Using their experience in flexible and stretchable electronics, the researchers sought to create an elastic system that conforms to tissue surfaces and can act as both a diagnostic and therapeutic device in one.

Flexible arrangements

The device is made up of stretchable gold interconnects sandwiched between a flexible polyimide sheet to form a flexible surface. The catheter is not only flexible but can also stretch up to 30% in both directions without causing damage to the material.

The researchers employed existing manufacturing techniques commonly used in the semiconductor industry to produce each array on a temporary silicon wafer. They then transferred the arrays to the soft elastomeric surface.

One of the features that makes this catheter unique is its multilayer design, with each layer having a unique purpose. The layer on the outside, in contact with the skin, contains electrodes that carry out electrical readings and electrical stimulation of tissues. The next layer down, separated by an insulating layer of polyimide, contains temperature sensors. These could allow surgeons to track changes in temperature of tissues in specific areas. Finally, at the bottom is a layer containing pressure sensors, which measure local forces between heart tissue and the device.

New device could change surgery

The team tested the balloon catheter using computational models, plastic heart models, and real human and animal hearts. They found that the catheter had advantages over current devices in both physical form and functionality.

The multilayered nature of the new catheter means that a wide range of diagnostic and therapeutic functions can be integrated into one device, allowing doctors to perform several measurements simultaneously and map them to specific areas. This opens up the possibility that the device could automatically regulate properties like temperature throughout surgery.

“We have taken new breakthrough materials and fabrication techniques typically employed by the semiconductor industry and applied them to the medical field, in this case cardiology, to advance a new class of medical instruments that will improve cardiac outcomes for patients and allow physicians to deliver better, safer and more patient-specific care,” says Igor Efimov, a senior author of the study.

Details of the catheter are reported in Nature Biomedical Engineering.

Overlooked for the Nobel: Lise Meitner

The discovery of nuclear fission in 1938 is among the most momentous events in 20^th-century physics. Within seven years, this experimental and theoretical breakthrough – made jointly by Otto Hahn and Fritz Strassmann, who obtained the data, and by Lise Meitner and Otto Frisch, who interpreted it – led to the first atomic weapons. Less than a decade later, it led to the first nuclear power plants. If ever there was a discovery that should have won its instigators a Nobel Prize in Physics, nuclear fission is surely it.

But that’s not what happened. Though the terms of Alfred Nobel’s bequest would have allowed three of Hahn, Frisch, Meitner and Strassmann to share a prize, only Hahn got the nod, becoming the sole recipient of the 1944 Nobel Prize in Chemistry “for his discovery of the fission of heavy nuclei”. The contributions of Frisch, Meitner and Strassmann were relegated to a few lines in the official Nobel presentation speech, which took place in December 1945. That same speech, incidentally, claims that Hahn “never dreamed of giving Man control over atomic energy” – which is a bit rich, given that Hahn worked on the Nazi atomic weapons programme and was in fact still incarcerated by the British authorities at the time.

The 1944 Nobel Prize in Chemistry represents a strong challenge to anyone who claims that the Nobels are fair or reflective of how collaborative science works. Strassmann, whom the presentation speech patronizingly called “one of [Hahn’s] young colleagues”, had in fact been his assistant for the best part of a decade. For much of that period, Strassmann worked for half wages or none; his opposition to the Nazi regime meant that he was blacklisted from other jobs, leaving him dependent on Hahn and unable to develop a solo career. Meitner fared slightly better in the speech, since it did at least acknowledge her as Hahn’s collaborator for more than 30 years. Not mentioned, though, is the reason she was absent during the crucial 1938 experiments: Meitner, like her nephew Frisch, was an ethnic Jew, and her conversion to Protestantism 30 years earlier did not protect her from Nazi predation. In the summer of 1938, both Meitner and Frisch were forced to flee Germany. They made their seminal contributions to fission in exile, communicating with the Berlin-based Hahn and Strassmann by letter and telephone.

Of the three researchers left out of the 1944 Nobel Prize in Chemistry, the injustice done to Meitner is the most severe. Unlike the other “overlooked” physicists in this series, the records of her Nobel nominations are now public. They show that Meitner’s male colleagues (the scientists in the Nobel nomination pools were all male then, notwithstanding the existence of contemporary female luminaries like Ida Noddack and Iréne Joliot-Curie) nominated her for the physics Nobel 29 times, and for the chemistry Nobel 19 times. Her earliest nomination came from the Norwegian chemist Heinrich Goldschmidt in 1924. Her last was in 1965, three years before her death, when Max Born made her his fourth choice after Pyotr Kapitsa (who went on to win in 1978), Cornelis Gorter (who never won) and Walter Heitler (ditto).

The records do not entirely explain why none of these nominations were successful. However, they do suggest that Meitner has something other than gender in common with two other entries in this series. The chemistry Nobel committee in 1944 was as divided about the relative importance of theory and experiment as the physics committee was in 1957, when Chien-Shiung Wu was denied a share of the parity-violation prize that went to Chen Ning Yang and Tsung-Dao Lee. The 1944 committee also failed to appreciate the major role that Meitner, Frisch and Strassmann played in the fission collaboration, much as a later committee failed to understand that Jocelyn Bell Burnell was not merely Antony Hewish’s assistant in discovering pulsars. On top of that, a whole suite of prejudices – racial, sexual, political and disciplinary – seems to have made it impossible for the parochial chemists of neutral Sweden to see the contributions of a refugee Jewish woman physicist in their proper light.

Early in her career, Meitner faced and overcame a considerable degree of personal prejudice. In one laughable example, the Nobel laureate Emil Fischer refused to let her work in his lab because he thought women’s long hair was a fire hazard (apparently Fischer’s massive beard was perfectly fine). Meitner’s subsequent achievements – as well as nuclear fission, she also discovered the element protactinium – won her legions of admirers, 26 of whom went on to nominate her for a Nobel Prize at least once. In the end, though, neither her efforts nor theirs were enough to counterbalance the subtle, structural forces that helped (and still do help) to keep the Nobel prizes overwhelmingly white and male, 82 years after Lise Meitner’s earth-shattering discovery.

New microwave bolometers could boost quantum computers

Two graphene-based bolometers that are sensitive to detect single microwave photons have been built by independent teams of physicists. The devices could find a range of applications in quantum technologies, radio astronomy and even in the search for dark matter.

One bolometer was created in Finland by Mikko Möttönen and colleagues at Aalto University and VTT Technical Research Centre of Finland, while the other was created by an international team led by Kin Chung Fong at Raytheon BBN Technologies in the US.

A bolometer measures the energy of incoming radiation by determining how much the radiation heats up a material. Bolometers capable of detecting single microwave photons would be very useful in creating quantum computers and other technologies that use superconducting quantum bits (qubits). This is because superconducting qubits interact via microwaves and single photons provide a very efficient way of transferring quantum information between qubits.

Too slow

So far, however, creating single-photon microwave detectors has been difficult because of the relatively low energies of microwave photons. The Finnish team had addressed the low-energy problem by creating a bolometer that used a gold-palladium alloy to absorb photons. While this device operates at very low noise levels, it is not fast enough to be useful when it comes to measuring the state of a superconducting qubit.

Now, Möttönen and colleagues have replaced the gold-palladium absorber with one made from graphene, which has a very low heat capacity. This makes graphene ideal for making a bolometer because heat capacity is a measure of the energy required to raise the temperature of a material by one degree.

“Changing to graphene increased the detector speed by 100 times, while the noise level remained the same,” explains team member Pertti Hakonen. Indeed, the new device can make a measurement in less than microsecond, which is on par with technology that is currently used to measure the state of qubits. Hakonen adds, “After these initial results, there is still a lot of optimization we can do to make the device even better”.

Josephson junction

Meanwhile, Fong and colleagues created a bolometer in which graphene is integrated within a superconducting device called a Josephson junction. When the graphene warms up by absorbing a microwave photon, it affects the electrical current flowing through the Josephson junction – thus creating the detection signal. This device is a whopping 100,000 times faster than microwave bolometers based on other materials.

Team member Dmitri Efetov at ICFO in Barcelona comments “such achievements were thought impossible with traditional materials, and graphene did the trick again. This opens entirely new avenues for quantum sensors for quantum computation and quantum communication.”

Fong and colleagues and Hakonen and colleagues describe their bolometers in separate papers in Nature.

Climate change threatens future astronomical observations

Rising global temperatures could worsen seasonal El Niño events and cause telescope images to lose their quality. That is the finding of an interdisciplinary team of researchers in Germany, who predict that climate-change-induced alterations in temperature, wind speed, humidity and “seeing” – atmospheric turbulence that causes blurring in images – will reduce the clarity of observations at ground-based telescopes around the world. The researchers, led by Faustine Cantalloube of the Max Planck Institute for Astronomy, argue that climate change therefore needs to be taken into account when building new observatories such as the Extremely Large Telescope (ELT), which is currently under construction at Cerro Armazones in Chile’s Atacama desert.

In their study, the researchers focused on the European Southern Observatory’s facility at Cerro Paranal, which is about 20 km from Cerro Armazones. There are three critical parameters for operating astronomical instruments: integrated water vapour (IWV), relative humidity and cloud coverage. They found that high IWV levels at Paranal, which affect astronomical observations in the infrared, were associated with high central equatorial sea surface temperatures during El Niño events. Although such events are a natural, periodic phenomenon, increases in atmospheric CO₂ concentration could make them more vicious, thanks to the associated global rise in humidity. A rise in humidity could also cause previously rare catastrophes such as the March 2015 flooding of the Atacama region to become more frequent.

Atmospheric turbulence affecting images

The study also examined the direct effects of increasing temperatures on the four telescopes that make up Paranal’s Very Large Telescope (VLT) array. The dome enclosures that house these telescopes are kept cool during the day so that when the dome opens at sunset, the telescopes are at a similar temperature to the outside air. This cooling is necessary because any temperature difference between the inside of the dome, where the telescopes are found, and its environment can cause air turbulence. The air turbulence distorts the starlight that reaches the telescopes, much like how stars appear to twinkle to the naked eye thanks to turbulence in the higher atmosphere.

The VLT’s current thermal active control system, however, cannot exceed target temperatures of over 16 °C. Since the build-up of atmospheric CO₂ is already linked to an increase of 1.5 °C in average temperatures at Paranal to over the past 40 years – 0.5 °C more than the global average – additional local increases in surface temperature could further impair observations.

The researchers used worst-case climate change scenarios on models such as the Coupled Model Intercomparison Project Phase 6 (CMIP6) multi-model ensemble to determine climate projections for the region. The results suggest that by the end of the 21^st century, Paranal could experience a further 4 °C increase in average temperatures. This would cause a greater difference between the temperatures inside and outside of telescope domes, and generate more air turbulence.

Adaptive optics can help correct for atmospheric turbulence. However, the time lag associated with these corrections creates an imaging artefact known as a wind-driven halo. Such halos are caused by winds from the southern subtropical jet stream, they are visible in ~30-40% of images, and they dramatically reduce the image’s contrast. Many wind-driven halos occur during El Niño events, which are likely to become more frequent as well as more intense as global temperatures rise.

Mitigating the impact of climate change on research

On a global level, Cantalloube notes that increased humidity and worse seeing are far from the most pressing threats to astronomy. “Fires in the US and in Australia already destroyed or were about to destroy observatories,” she explains. “This is the most imminent threat that is a consequence of climate change.” Cantalloube adds that hurricanes in Hawaii, which have become stronger in recent years “could also threaten the Hawaiian-based observatories.” As for Chilean observatories, she says, “the effects are there”.

According to Cantalloube, interdisciplinary teams like hers will become more prevalent over time as scientists seek to understand the many ways that climate change could impact research capabilities. She notes that her team’s paper, which appears in Nature Astronomy, is a preliminary study, focusing solely on the effects of climate change on observations conducted in Chile. Since climate change will likely affect different regions of the globe in different ways, the team hopes to extend its analysis to cover additional sites. In the meantime, Cantalloube urges members of the astronomy community to “investigate whether climate change and/or environmental signals are also detectable in the observations of other observatories/others fields of research” and “convince political leaders to act now against climate change”.

Nobel prize sizzle: building excitement in the run-up to the physics award

Nobel topics infographic — Nobel Prize disciplines timeline (click to enlarge): The thickness of the coloured line denotes how many laureates shared the prize in that year. In years when the prize was split between two disciplines, two lines are used, with each denoting the number of laureates in that discipline. (This timeline was originally published in 2014 and was updated in 2019. © IOP Publishing)

Tomorrow, the winner(s) of this year’s Nobel Prize for Physics will be announced. This will the 14th Nobel prize that I have covered for Physics World, and I can honestly say that the excitement has never waned.

A big challenge for journalists on Physics World, however, is coming up with new ways of creating a bit of “sizzle” in the run-up to the announcement. This is difficult because unlike some other prizes, there is no public shortlist for the Nobel prize to whet the appetite. Indeed, who has been nominated and the deliberations of the Nobel committee are kept secret for at least 50 years – so it is impossible to know who is in the running and very difficult to understand how decisions are currently being made by the committee.

However, I am not complaining about this information deficit. I think it is one of the many things that keeps the Nobel prize fresh and exciting every year.

Last year I was very fortunate to gain some insights into the selection process when I interviewed the Swedish physicist Lars Brink, who served on the Nobel Committee for Physics on eight separate occasions. So if you are keen to learn about the finer points of how winners are selected, check out my article “Inside the Nobels: Lars Brink reveals how the world’s top physics prize is awarded”.

Predicting winners

One way of generating a bit of excitement in the run-up to the Nobel is to predict who will bag the prize. The problem with predictions is that mine are almost always wrong. And after a few years, I also run out of new people to predict as winners.

To see how good our Physics World predictions have been, I looked back at a blog I wrote in late September 2009 (“Nobel predictions”). The predictions were made by Physics World editors and the Nobel laureate Albert Fert. Here are the 13 predictions we made (all men, I’m afraid) and the years that some of them actually won a Nobel prize: Alain Aspect, David Wineland (2012), Peter Zoller, Juan Ignacio Cirac, Anton Zeilinger, Michel Mayor (2019), Didier Queloz (2019), Andre Geim (2010), Konstantin Novoselov (2010), Yakir Aharanov, Michael Berry, Saul Perlmutter (2011) and Brian Schmidt (2011). That is better than 50%, so maybe I am underselling our ability to predict winners (I say “our”, because I only got one right).

Not content to rely on intuition to pick winners, in 2014 I took a close look at whether there is a temporal pattern in how prizes are awarded. I divided physics into seven disciplines and created an infographic that shows when prizes were awarded in those fields.

Quantum information this year?

The figure “Nobel Prize disciplines timeline” is version of the infographic updated to 2019 and if you squint at it for long enough you may convince yourself that there is a pattern in the prizes. For example, there has not been an award in the field of quantum physics for eight years – so, my only prediction for this year will be a prize to physicists working in the field of quantum information.

Migration world map — Migration routes: the US is the champion when it comes to attracting Nobel laureates. (Courtesy: Paul Matson)

Another reasonable bet is that at least one of the 2020 laureates will be an immigrant. That prediction comes courtesy of our 2015 Nobel sizzle, when I created an infographic that reveals that more than 25% of physics laureates can be classed as immigrants. That was updated in 2019 and you can read more in “More than one-quarter of physics Nobel laureates are immigrants, reveal updated infographics”.

I think that our best bit of Nobel sizzle so far was last year’s series of pieces about our favourite Nobel prizes. My pick was the 1982 prize to Kenneth Wilson “for his theory for critical phenomena in connection with phase transitions”. I have always been fascinated by phase transitions, but I have to admit that it was a bit daunting to try to describe Wilson’s work in a few hundred words: (“My favourite Nobel prize: a universal theory for phase transitions”).

This year we are focussing on people who we think missed out on a Nobel. You can find those pieces on the Physics World website in a panel called “Overlooked for the Nobel”. I have contributed two pieces to this series: one championing the physicists at CERN who discovered the Higgs boson, and the other asking why the Chinese-American physicist Chien-Shiung Wu did not win a Nobel for her pioneering measurement of parity violation.

Compton imaging opens up new avenues for diagnostic imaging

Compton imaging, originally developed by astronomers for detecting gamma ray sources, is now also under investigation for clinical imaging. In particular, a high-performance Compton camera could prove invaluable for applications within nuclear medicine and molecular imaging.

Unlike the established medical imaging technique of PET, which can only visualize positron emitters, Compton imaging has the potential to visualize a variety of gamma ray sources. To date, however, Compton image quality has not matched up to that of typical PET scans. To investigate its potential further, researchers at the National Institute of Radiological Sciences (NIRS) in Japan have created a combined whole gamma imaging (WGI) platform to directly compare the two modalities.

“Compton imaging has potential to provide better images than conventional SPECT and PET, in particular for radionuclides emitting high-energy gamma rays,” explains first author Hideaki Tashima. “We expect to explore new radionuclides that have never been used for nuclear medicine.”

A Compton camera incorporates two detectors that work in synchrony. For an individual gamma emission, Compton scattering occurs in the first detector (the scatterer) and photoelectric absorption in the second (the absorber). Both detectors record the interaction positions and corresponding deposited energies, enabling reconstruction of a Compton cone that indicates the emission point.

To create a WGI system that can perform both PET and Compton imaging, the researchers inserted a scatterer ring inside a whole-body depth-of-interaction PET scanner (the absorber ring). To enable small-animal imaging experiments, they remodelled a previous WGI prototype by halving the diameter of the scatterer ring. As the spatial resolution of Compton imaging reduces in proportion to the source–detector distance, this modification should also improve the resulting images.

Design sketches of the modified WGI prototype (a, b); the scatterer detector and absorber detector (c); the developed prototype (d). (Courtesy: Phys. Med. Biol. 10.1088/1361-6560/abb92e)

System evaluation

Tashima and colleagues tested the WGI platform using ⁸⁹Zr as the imaging source, as it decays via emission of a positron and a 909 keV gamma ray, enabling direct comparison of PET and Compton imaging. Alongside, they developed a 3D image reconstruction method incorporating detector response function (DRF) modelling, random correction and normalization. They report their findings in Physics in Medicine & Biology.

The researchers first evaluated the uniformity of the WGI prototype, by imaging a cylindrical phantom filled with ⁸⁹Zr solution. When reconstructed without normalization, PET images exhibited ring artefacts and stripe patterns, while Compton images had higher coefficients of variation (COV) in the central region. The normalized PET and Compton images were of comparable quality, showing uniform radioactivity intensity throughout the phantom, and with COVs of 3.7% and 4.8% for PET and Compton images, respectively.

To assess the spatial resolution of the modalities, the researchers imaged a small rod phantom with clusters of cylindrical holes filled with ⁸⁹Zr solution. The PET image clearly resolved the 2.2-mm diameter holes. The Compton image resolved the 3.0-mm holes in the peripheral region, with lower spatial resolution in the central region.

Finally, the team injected a mouse with 9.8 MBq ⁸⁹Zr-oxalate and, one day later, used the WGI prototype to image the animal for 1 hr. The absorber ring (containing the PET detectors) had an axial length of 214 mm – enough to cover the whole animal. The PET image thus showed the entire mouse, revealing that ⁸⁹Zr localized in its bones.

PET and Compton images — Reconstructed PET (left) and Compton (right) images of a mouse injected with ⁸⁹Zr-oxalate. (Courtesy: Phys. Med. Biol. 10.1088/1361-6560/abb92e)

The scatterer ring was only 52 mm long, so the researchers positioned the animal’s torso inside the ring and its head and tail outside. Nevertheless, in Compton imaging mode, the WGI system could reconstruct the distribution outside the ring and create an image of almost the entire body. The Compton images agreed well with the PET images, clearly showing ⁸⁹Zr in the mouse bones, although the image quality was better for regions inside the scatterer ring than those outside.

The researchers conclude that the WGI prototype could achieve Compton imaging with a quality approaching that of PET. They attribute this success to four key factors: the high energy of the gamma ray emitted from ⁸⁹Zr, which improves the spatial resolution of Compton imaging; the DRF model used for image reconstruction, which further improved spatial resolution; the normalization step, essential for image uniformity; and the full-ring geometry of the WGI system.

Future work will focus on improving Compton imaging such that it outperforms PET. But the team’s ultimate goal is to unite the two techniques and implement a combined image reconstruction method. “Simultaneous measurement of different tracers with PET and Compton imaging can improve the efficiency of diagnosis,” explains Tashima. “In addition, reconstruction of a single tracer image by combining two types of signals can improve image quality through high sensitivity.”

“We are now exploring detector technologies for better energy resolution,” adds project leader Taiga Yamaya. “We are looking ahead to the realization of a clinical WGI system.”

D-Wave launches 5000 qubit system, solar-powered nanobeads could clean-up oil sands waste

D-Wave Advantage — New Advantage: D-Wave’s latest quantum computer. (Courtesy: D-Wave Systems)

The Canadian quantum computer maker D-Wave Systems has unveiled its latest platform, which contains a whopping 5000 qubits. Called Advantage, the system can be accessed via the company’s Leap 2 cloud service, which was launched earlier this year. The system is designed for use by businesses and D-Wave already counts several companies as customers, including the carmaker Volkswagen.

Your can read more about D-Wave system in “D-Wave’s 5,000-qubit quantum computing platform handles 1 million variables”.

Staying in Canada, petroleum production from Alberta’s oil sands produces effluent that is stored in tailings ponds – which are notoriously toxic. Now, researchers have used glass bubbles coated in titanium dioxide nanocrystals to deal with naphthenic acids, which are a particularly nasty group of chemicals found in the ponds.

The bubbles were developed at the University of Waterloo and their efficacy has been tested by a team led by Diane Orihel at Queens University in Kingston, Ontario. The team found that as the bubbles float on a pond, these use energy form the Sun to create radicals that destroy naphthenic acids. The coated bubbles are nontoxic and because they float, they can be gathered up and used again.

The research is described in the journal Facets and you can listen to Orihel explain the results to the CBC’s Bob McDonald on the science programme Quirks and Quarks.

Overlooked for the Nobel: Chien-Shiung Wu

The 1957 Nobel Prize for Physics was shared by Chen Ning Yang and Tsung-Dao Lee “for their penetrating investigation of the so-called parity laws which has led to important discoveries regarding the elementary particles”. However, some physicists argue that the Chinese-American physicist Chien-Shiung Wu should have shared the prize for providing the experimental evidence for Lee and Yang’s theoretical prediction of parity violation. Furthermore, some believe that Wu was denied the prize because she was a woman.

Based at Columbia University in the late 1950s, Wu designed an experiment that tested parity laws by observing beta decay at ultracold temperatures. While Wu was an expert in measuring beta decay, she collaborated with scientists at the National Bureau of Standards (NBS) in Washington DC (now NIST) to meet the cryogenic requirements of what is now known as the “Wu experiment”.

In an 2012 article for Physics World (“Credit where credit’s due?”) the Hungarian chemist and historian of science Magdolna Hargittai dug deep into the claims and counter claims of who, if anybody, should have shared the 1957 prize with Lee and Yang.

Three competing groups

In a nutshell, it is complicated. According to Hargittai’s article, Wu and colleagues saw the first hints of parity violation on 27 December 1956. Shortly after news of this preliminary result got out, an independent group working at the Columbia cyclotron did a quick measurement in early January and observed parity violation in muons – at about the same time as Wu and colleagues were making their definitive measurements. What is more, a third group at the University of Chicago had been looking for parity violation since the summer of 1956 and had also found preliminary evidence by December.

The Wu–NBS and Columbia cyclotron teams published their results in the February 1957 issue of the Physical Review and the Chicago paper was published in the March issue of the same journal.

And therein lies a major barrier to Wu sharing the 1957 prize. According to the Nobel rules, the 1957 prize cannot be awarded for work published in 1957. Indeed, the Nobel Prize Nomination Archive shows that neither Wu, nor anyone else who had measured parity violation in 1956-57, had been nominated for the 1957 prize.

So what is Hargittai’s conclusion on whether Wu was overlooked for a Nobel?

“My view is that Wu made an outstanding contribution to bringing down the axiom of parity conservation in weak interactions,” writes Hargittai. “But to say it was an injustice that she did not win a Nobel prize is an oversimplification of a complex story.”

Eminent supporters

It is worth pointing out that some of the most eminent physicists of the day did champion Wu’s case for a prize – including the Nobel laureates Willis Lamb, Polykarp Kusch and Emilio Segrè. Starting in 1958, she was nominated for the Nobel prize at least seven times until her death in 1997 (only nominations made before 1966 are currently available online to the public, which means there could, in fact, have been more).

So regardless of whether it was the rules or sexism that denied Wu a Nobel prize, there is no doubt in my mind that she deserved one – if not in 1957, then certainly thereafter. Indeed, writing in Physics World after Hargittai’s article was published, Yang said that Wu’s contribution went well beyond her considerable experimental prowess and included her deep perception of why parity must be tested. In that same issue, Herwig Schopper, who was one of those to have nominated Wu for a Nobel prize, expressed the view that Wu and indeed others who made the experimental measurements were denied a prize because of the rule that only three winners can be named.

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