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Gender gap in physics entrenched by biased collaboration networks, study finds

Biased collaboration and citation patterns are responsible for driving the gender gap in physics. That is according to a new study, which finds that poor female representation persists due to established male physicists preferring to work with early-career male researchers. The study’s authors say that integrating young female physicists into established networks could help to tackle the under-representation of women (Communications Physics 7 309).

The gender gap in physics is one of the largest in science and recent research suggests that it could take couple of centuries until there are equal numbers of senior male and female physicists.

Keen to understand the network dynamics behind the gap, Fariba Karimi at the Complexity Science Hub in Austria and colleagues analysed 668,028 papers published in American Physical Society journals between 1893 to 2020 and 8.5 million citations.

They deduced with “high confidence” the genders of 136,598 first authors in the APS dataset and used this data to construct citation and co-authorship networks.

Despite rising overall numbers of female physicists and female-led papers, the authors find that the ratio of male to female first authors and researchers has remained stable for decades. In fact, the gender gap in absolute numbers appears to be growing.

The researchers then developed a model of the citation and co-authorship networks to explore how the “adoption” of new members by established members impacts network growth.

Small changes

The model focused on two mechanisms. One is “asymmetric mixing” – the inclination of people to adopt people like themselves. The other is general preferential attachment, or the idea that established network members attract more connections.

The model mirrors real-world dynamics and shows that these mechanisms and adoption behaviours cause group ratio inequalities to persist. In the case of physicists, the gender imbalance continues because male physicists are more likely to collaborate with and cite their male counterparts.

Compared with women, men entering the network are more likely to be adopted by those who are already well established in the network, which tends to be men. This trend has been shown elsewhere with research in 2022 finding that male-led papers are more likely to cite male-led work.

Gender gap in physics among highest in science

The team then used their model to show how small changes to a two-group system can alter the group balance. They find that if the simulation’s mixing values – such as adoption behaviours – are altered slightly in favour of a smaller, less dominant group, that group’s size quickly catches up with that of the dominant group.

Karimi says that it is “not just about having more women” but also about how they are integrated into networks. “In real systems, it’s not as simple as someone coming and connecting to others in a network,” adds Karimi. “It is also a matter of who takes in the newcomer and adopts him or her into their personal network.”

To alter the network dynamics, the study authors suggest interventions such as creating opportunities for junior women to collaborate with senior men and giving female researchers more opportunities for funding and promotion. “If we don’t take these interventions soon, this gap will not close very easily,” says Karimi.

Nobel predictions and humorous encounters with physics laureates

In this episode of the Physics World Weekly podcast, our very own Matin Durrani and Hamish Johnston explain why they think that this year’s Nobel Prize for Physics could be awarded for work in condensed-matter physics – and who could be in the running. They also reminisce about some of the many Nobel laureates that they have met over the years and the excitement that comes every October when the winners are announced.

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Celebrating with a new Nobel laureate in Canada’s ‘Steeltown’

For nearly two decades I have been covering the Nobel prize for Physics World and every October I tune into to the announcement that’s made live from Stockholm. But, the frisson that I feel with each announcement brings me straight back to a day 30 years ago when Bertram Brockhouse bagged the award.

Three decades ago I was living in Hamilton, an industrial city at the western end of Lake Ontario. About 70 km from downtown Toronto and staunchly blue collar, Hamilton was famous for its smoke-belching steel mills and its beloved Tiger-Cats of the Canadian Football League. In addition to steel, the city has been home to myriad manufacturing companies and in the days of Empire it had been dubbed the “Birmingham of Canada”.

So it’s safe to say that Hamilton in the 1990s was not the sort of place where you would expect to run into a Nobel laureate.

But that changed one day in October 1994. I began that day listening to a news bulletin on CBC radio – and the lead item was that the Canadian physicist Bertram Brockhouse had won half of the 1994 Nobel Prize for Physics for his pioneering work on inelastic neutron scattering.

In 1994 Brockhouse was an emeritus professor of physics at McMaster University in Hamilton – where I was doing a PhD. What’s more, I had been an undergraduate intern at Chalk River Laboratories, where I worked at the Neutron Physics Branch – which was founded by Brockhouse in 1960 before he left for McMaster.

“Son of a gun”

Needless to say, I was very excited to get to the physics department and join in the celebrations that morning. And I was not disappointed. As I arrived, the normally mild-mannered theorist Jules Carbotte was skipping along the corridor shouting “Bert Brockhouse, son of a gun” as he punched the air.

I don’t remember seeing Brockhouse that day, but everyone else was in very good spirits. Indeed, it was the start of celebrations at the university that seemed very inclusive to me – with faculty, students and members of the wider community invited to what seemed like endless parties and receptions. This was understandable because Brockhouse was McMaster’s first Nobel prize winner. There have been three more since – including another in physics, with the 2018 laureate Donna Strickland having done her degree in engineering physics at McMaster.

At one of those receptions I was introduced to Brockhouse and discovered that he lived in one of my favourite parts of Hamilton – a semi-rural and heavily-wooded portion of the Niagara Escarpment nestled between the former towns of Ancaster and Dundas. Instead of talking about neutrons, I believe we chatted about the growing number of deer in the area and how they were wreaking havoc in people’s gardens.

Coffee lounge gang

Brockhouse had retired a decade earlier, but he was often at the university where he shared a small office with other emeriti professors – a gang that I would often see in the coffee lounge. As I recall, he was very quickly given an office of his own (and perhaps a personal assistant) to help him cope with his new fame.

While writing this piece, I was surprised to discover that Brockhouse was just 76 when he bagged his Nobel for work he had done 40 years previously. Perhaps because 30 years have passed, 76 no longer seems old to me – but I don’t think this is just my perception. Today, as mandatory retirement fades into the past and people are encouraged to remain physically and mentally active, 76 is not that old for a working physicist. Many people that age and older continue to make important contributions to physics.

Indeed, one of Brockhouse’s colleagues at McMaster – Tom Timusk – remains active in research into his 90s. In 2003 Timusk published an obituary of Brockhouse in Nature and it reminded me of what Brockhouse said to a gathering of students after he won the prize: “I used to think that my work was not important, but recently I have had to change my mind.”

How nice to be able look back on one’s work and find value. I suspect that like Brockhouse, many people underestimate their contributions to the greater good. But unlike, Brockhouse, some will never stand corrected.

Camera takes inspiration from cats’ eyes to improve imaging performance

**Features of feline eyes** . (A, B) Illustration showing the camouflage-breaking ability of a feline under diverse light conditions. Illustrations of the tapetum lucidum in the retina (C) and the anatomy of a cat’s eye (D). (Courtesy: Kim *et al. Sci. Adv.* 10 eadp2809 (2024))

A novel camera inspired by structures within cats’ eyes could be employed in autonomous vehicles, drones and surveillance systems – applications where precise object detection in varied light conditions and complex backgrounds is critical.

One key feature of the new device is the use of a vertically elongated slit, like the pupils of cats’ eyes, which are different from those of other mammals, explains Minseok Kim of the Gwangju Institute of Science and Technology in Korea. As in a cat’s eye, this pupil creates an asymmetric depth of focus when it dilates and contracts, allowing the camera to blur out backgrounds and focus sharply on objects. Another feature is a metal reflector that enables more efficient light absorption in low-light settings. This mimics the tapetum lucidum, a mirror-like structure that gives cats’ eyes their characteristic glow. It reflects incident light back into the retina, allowing it to amplify light.

“The result is a camera that works well in both bright and low-light environments, allowing it to capture high-sensitivity images without the need for complex software post-processing,” Kim says.

Mimicking animal eyes

Kim and colleagues have been working on mimicking the eyes of various animals for several years. Some of their recent studies include structures inspired by fish eyes, fiddler crab eyes, cuttlefish eyes and avian eyes. They decided to work on this latest project with the aim of overcoming the limitations of current cameras systems, in particular, their difficulty in handling very low or very bright lighting conditions. They also wanted to do away with the post-processing image software required to better distinguish objects from their backgrounds.

One of the main difficulties that the researchers had to overcome in this study was to simplify the intricate structure of the tapetum lucidum. Instead of replicating it exactly, they used a metal reflector placed beneath a hemispherical silicon photodiode array, which reduces excessive light and enhances photosensitivity. This design allows for clear focusing under bright light and improved sensitivity in dim conditions.

“Another challenge was to create a vertical pupil that could mimic the cat’s ability to focus sharply on an object while blurring the background,” says Kim. “We were able to construct the vertical aperture using a 3D printer, but our future work will focus on making this pupil dynamic so it can automatically adjust its size in response to changing light conditions.”

Many application areas

The research could significantly improve technologies that rely on high-performance imaging in difficult lighting conditions, Kim tells Physics World. The team expects the system to be highly useful in autonomous vehicles, where precise object detection is critical for safe navigation.

“It could also be applied to drones and surveillance systems that operate in various lighting environments, as well as in military applications where camouflage-breaking capabilities are essential,” Kim adds. “The system could also find use in medical imaging, where the ability to capture high-sensitivity, real-time images without extensive software processing is crucial.”

The researchers now plan to further optimize their camera’s pixel density – which they admit is quite low at the moment – and its resolution to improve image quality. “We also aim to conduct more real-world tests, particularly in applications such as autonomous driving and robotic surveillance, to evaluate how the system performs in practical settings,” says Kim. “Lastly, we are looking into binocular object recognition systems so that the camera can handle more complex visual tasks.”

The study is detailed in Science Advances.

Robert Laughlin: the Nobel interview that became an impromptu press conference

As a science journalist, some interviews you do go well, some don’t, but at least they usually have a distinct start and end. That wasn’t the case with Robert Laughlin, whom I once met at the annual Lindau conference for Nobel-prize-winners in Germany.

Most of the conference involves Nobel laureates giving lectures to a select band of PhD students from around the world. But Laughlin, who’d shared the 1998 Nobel Prize for Physics for his work on quantum fluids with fractional charges, had agreed to speak to me in a private room at the conference venue on the shores of Lake Constance.

Things started sensibly enough (he was ostensibly talking about a new book he was writing) but after about 20 minutes, a conference official barged in.

There’d be an over-booking and no, we weren’t allowed to stay. We were two people in the wrong place at the wrong time – and the fact that one of us was a Nobel-prize-winning physicist didn’t cut any mustard. Out we went.

Laughlin and I packed up our stuff and reconvened at an outside terrace in the summer sun, where we tried to pick up the thread of our conversation.

Now, laureates like Laughlin are the big draw of the Lindau conference – in fact, they’re the whole reason the meeting takes place. If Lindau were a music festival, they’d be the artists everyone’s come to see.

Before I knew what was going on, first one then two then three students had sidled up to our table. Like electrons around a nucleus, they’d been attracted by the presence of a Nobel laureate and weren’t going to miss out.

Laughlin didn’t appear fazed by the unexpected turn of events; in fact, I’m sure Nobel laureates love nothing better than being the centre of attention. Within minutes, the entire table had been surrounded by a phalanx of hangers-on.

Our one-to-one interview had become an impromptu one-man press conference with me seemingly serving as Laughlin’s minder. As he held court to his gaggle of fawning students, apparently oblivious that I was still there, Laughlin was in his element.

Laughlin probably doesn’t remember the encounter: Nobel laureates, who are the only real celebrities in physics, meet hundreds of people all the time. The students, however, appeared to be enjoying themselves, so the conference organizers must have been happy.

But I just ended up squirming in my seat. I put my notebook back in my bag and let Laughlin take over.

Steven Weinberg: the Nobel laureate who liked nuts

It was 2003 and Steven Weinberg was sitting with me in the lobby of a hotel in Geneva, explaining his research into fundamental physics, when he paused to grab a handful of peanuts from a bowl on the table in front of us.

I had been speaking to Weinberg as he’d come to Switzerland to give a lecture at CERN on the development of the Standard Model of particle physics, in which he’d played a key part, and had agreed to an interview with Physics World during a break in his schedule.

The old-fashioned Dictaphone on which I recorded our interview on has gone missing so I’ve only got a hazy recollection of what he said. I do remember that Weinberg was charming, friendly and witty, but it was pretty clear he felt he was in the company of some kind of intellectual buffoon.

Turning round, he asked me: “Do you like nuts?”

You see, the only time Weinberg properly interacted with me was to reveal how he enjoyed those little bags of nuts you get on plane journeys (he was obviously used to flying business class); it was then that he wanted my view of them too. It was as if Weinberg doubted I could handle anything deeper than airline snacks and was just trying to be kind.

That’s what happens when you an interview a Nobel laureate. Apart from them enjoying the sound of their own voice, they obviously know they know several orders of magnitude more than you do about their specialist subject.

You’re left squirming and feeling ever so slightly inadequate, trying to absorb a whirlwind of high-level information while at the same time desperately wondering what your next question should be.

His opinion of me certainly must have dipped further a few weeks later. Despite some misgivings, I decided to write up our interview and e-mail Weinberg my draft, which covered his life, research and career.

Stupidly, I’d made a few schoolboy errors near the start, prompting Weinberg to write back, explaining he didn’t have the time or energy to check my nonsense any further (I paraphrase slightly) and, no, he wasn’t going to spend time pointing out my mistakes either.

At least Weinberg was polite, which is more than you could say for the late Subrahmanyan Chandrasekhar, who shared the 1983 Nobel Prize for Physics for his theoretical work on the structure and evolution of stars. Robert P Crease takes up the story in this memorable article.

CERN celebrates 70 years at the helm of particle physics in lavish ceremony

Officials gathered yesterday for an official ceremony to celebrate 70 years of the CERN particle-physics lab, which was founded in 1954 in Geneva less than a decade after the end of the Second World War.

The ceremony was attended by 38 national delegations including the heads of state and government from Bulgaria, Italy, Latvia, Serbia, Slovakia and Switzerland as well as Her Royal Highness Princess Astrid of Belgium and the president of the European Commission. It marked the culmination of a year of events that showcased the lab’s history and plans for the future as it looks beyond the Large Hadron Collider.

Created to foster peace between nations and bring scientists together, CERN’s origins can be traced back to 1949 when the French Nobel-prize-winning physicist Louis de Broglie first proposed the idea a European laboratory. A resolution to create the European Council for Nuclear Research (CERN) was adopted at a UNESO conference in Paris in 1951, with 11 countries signing an agreement to establish the CERN council the year after.

CERN Council met for the first time in May 1952 and in October of that year chose Geneva as the site for a 25–30 GeV proton synchrotron. The formal convention establishing CERN was signed at a meeting in Paris in 1953 by the lab’s 12 founding member states: Belgium, Denmark, France, West Germany, Greece, Italy, the Netherlands, Norway, Sweden, Switzerland, the UK and Yugoslavia.

On 29 September 1954 CERN was formed and the provisional CERN council was dissolved. That year also saw the start of construction of the lab in which the proton synchrotron, with a circumference of 628 m, accelerated protons for the first time on 24 November 1959 with an energy of 24 GeV, becoming the world’s highest-energy particle accelerator.

A proud moment

Today CERN has 23 member states with 10 associate member states. Some 17,000 people from 100 nationalities work at CERN, mostly on the LHC but the lab also does research into antimatter research and theory. CERN is now planning on building on that success through a Future Circular Collider, which if funded, would include a 91 km circumference collider to study the Higgs boson in unprecedented detail.

As part of the celebrations, this year has seen over 100 events organized in 63 cities in 28 countries. The first public event at CERN, held on 30 January, combined science, art and culture, and featured scientists discussing the evolution of particle physics and CERN’s significant contributions in advancing this field.

Other events over the past months have focused on open questions in physics and future directions; the link between fundamental science and technology; CERN’s role as a model for international collaboration; and training, education and accessibility.

The meeting yesterday, the culmination of this year-long celebration, was held in the auditorium of CERN’s Science Gateway, which was inaugurated in October 2023.

“CERN is a great success for Europe and its global partners, and our founders would be very proud to see what CERN has accomplished over the seven decades of its life,” noted CERN director general Fabiola Gianotti. “The aspirations and values that motivated those founders remain firmly anchored in our organization today: the pursuit of scientific knowledge and technological developments for the benefit of humanity; training and education; collaboration across borders, diversity and inclusion; knowledge, technology and education accessible to society at no cost; and a great dose of boldness and determination to pursue paths that border on the impossible.”

You can watch a recording of the second instalment of Physics World Live, in which Tara Shears, Philip Burrows and Tulika Bose discuss what is in store for particle physics in the coming decades.

Rambling tour of Europe explores the backstory of the Scientific Revolution

Sixteenth-century Europe was a place of great change. Religious upheaval swept the continent, empires expanded and the mystic practices of the medieval world slowly began shifting toward modern science.

Copernicus’s heliocentric model of the universe, introduced in 1543, is often considered the origin of this so-called “Scientific Revolution”. However, with her latest book Inside the Stargazer’s Palace: the Transformation of Science in 16th-Century Northern Europe, historian and writer Violet Moller gives the story behind this transformation, putting lesser-known figures at the fore. She looks at the effect of religious and geopolitical events in northern Europe, starting from the late 15th century, and shows how the scholars of this period drew together strands of scientific thought that had been developing for decades.

Beginning in the German town of Nuremberg in 1471, the book is a sweeping tour of the continent, visiting the ancient university city of Louvain in what is now Belgium, the London suburb of Mortlake, Kassel in Germany and the formerly Danish island of Hven. She concludes this journey in Prague with the deposition of the Holy Roman Emperor and scientific patron Rudolf II in 1611, an event that broke apart Europe’s flourishing community of scientific minds.

As a scientist, I was disappointed to find the book fairly light on scientific detail. Inside the Stargazer’s Palace is first and foremost a history book, but I felt that some more scientific context would help most readers grasp the significance of the events Moller describes.

Nonetheless, it was fascinating to see how politics and economics across the continent shaped scientific study. In the 15th century, the scientific community in northern Europe was exceedingly small, with scholarly knowledge restricted to those who could travel to the great knowledge centres in Italy, Greece and beyond. However, the development of the printing press in 1440 and the founding of the first scientific print house in Nuremberg changed the way information was shared forever. As scientific knowledge became more accessible, interest in understanding the natural world began to grow.

Through the closely connected tales of a number of individuals – from cartographer and instrument maker Gemma Frisius to the renowned alchemist Tycho Brahe – we see the beginnings of a scientific community. As Moller says, “Everyone, it seems, knew everyone,” with theories, techniques and instruments shared across a growing network of enthusiastic practitioners.

The development of the printing press mid-century and the founding of the first scientific print house in Nuremberg changed the way information was shared forever

This complexity did not come without its challenges. Moller introduces so many significant figures, each with their own niche, that by chapter four it’s difficult to keep track of who everyone is. The emphasis on personal stories also creates a slightly muddled narrative. In the introduction, Moller tells us “This narrative is based around places,” but at times the location seems incidental at best, if not entirely irrelevant. For example, chapter five ostensibly focuses on the Danish (now Swedish) island of Hven, home to Tycho Brahe. However, over the first 20 pages, we instead follow Brahe on his travels around Europe, and the description of his famous castle-come-laboratory Uraniborg at the end of the chapter feels rather compressed. Other locations, notably Kassel and Prague, are only relevant during the lifetime of a single enthusiastic patron, begging the question of whether it was the place or the person that really mattered.

Despite this sometimes rambling focus, Moller expertly guides the reader through the significant cultural and political events of the century. Beginning in the 1510s, the spread of Lutheranism across Europe brought with it an intellectual revolution, with its fiercest proponents encouraging followers to “think in innovative ways … and focus on praising God through studying his creation”. The conflict between the new Protestant denominations and the traditional Catholic faith drove the migration of great minds, who converged on the places most supportive of their scientific endeavours.

During this period, new observations also directly challenged long-held beliefs. In the early 16th century, astronomy and astrology were one and the same, and astrological predictions underpinned everything from medicine to political decisions. However, a series of astronomical phenomena towards the end of the century – the appearance of a new star in 1572 (later confirmed as a supernova), a comet in 1577, and the conjunction of Saturn and Jupiter in 1583 – triggered a shift away from divinatory thinking in the following decades. Measurements made from these observations conflicted with accepted theories about the universe, showing that the stars and planets were much further away than previously thought.

The discussion of these phenomena is a welcome one, introducing one of surprisingly few scientific details in the book. We are still left to guess many of the basic particulars of this scientific study: what was being measured and how, and why the results were significant. Moller instead provides a list of instruments – astrolabes, quadrants, sextants, torquetums and astronomy rings – with little or no explanation of what they are or how they work.

Moller is a historian, specializing in 16th-century England, so perhaps these subjects are beyond the scope of her expertise. However, a further frustration is the almost exclusive focus on astronomy; there is scant mention of other topics such as alchemy or botany, although this was promised by the book’s synopsis. Occasionally it also seems that Moller indulges her personal enthusiasm over the needs of the reader, placing an undue emphasis on inconsequential details and characters – John Dee, for example, continues to crop up long after his relevant contributions have passed.

The lack of scientific detail and loose focus made this a sometimes frustrating read. However, I can see that for non-scientists and those who prefer a more fluid approach, the book presents an intriguing alternative view of the Scientific Revolution. By the end of Inside the Stargazer’s Palace and, correspondingly, the 16th century, the stage has been set for the discoveries to come, but it feels like we’ve taken a circuitous route to get there.

2024 Oneworld 304pp £25.00hb

Nuclear clock ticks ever closer

Could a new type of clock potentially be more accurate than today’s best optical atomic clocks? Such a device is now nearing reality, thanks to new work by researchers at JILA and their collaborators who have successfully built all the elements necessary for a fully functioning nuclear clock. The clock might not only outperform the best time-keepers today, it could also revolutionize fundamental physics studies.

Today’s most accurate clocks rely on optically trapped ensembles of atoms or ions, such as strontium or ytterbium. They measure time by locking laser light into resonance with the frequencies of specific electronic transitions. The oscillations of the laser then behave like (very high-frequency) pendulum swings. Such clocks can be stable to within one part in 10²⁰, which means after nearly 14 billion years (or the age of the universe), they will be out by just 10 ms.

As well as accurately keeping time, atomic clocks can be used to study fundamental physics phenomena. Nuclear clocks should be even more accurate than their atomic counterparts since they work by probing nuclear energy levels rather than electronic energy levels. They are also less sensitive to external electromagnetic fluctuations that could affect clock accuracy.

Detecting tiny temporal variations

A nucleus measures between 10^-14 and 10^-15 m across, while an atom is 10^-10 m. Shifts between nuclear energy levels are thus higher in energy and would be resonant with a higher-frequency laser. This translates into more wave cycles per second — and can be thought of as a greater number of pendulum swings per second.

Such a nuclear transition probes fundamental particles and interactions differently to existing atomic clocks. Comparing a nuclear clock with a precise atomic clock could therefore help to unearth new discoveries related to very tiny temporal variations, such as those in the values of the fundamental constants of nature. Any detected changes would point to physics beyond the Standard Model.

The problem is that the high-frequency lasers needed to excite the nuclear transitions in most elements are not easy to come by. To excite nuclear transitions, most atomic nuclei need to be hit by high-energy X-rays. In the late 1970s, however, physicists identified thorium-229 as having the smallest energy gap of all atoms and found that it could thus be excited by lower-energy, ultraviolet light. In 2003, Ekkehard Peik and Christian Tamm at the Physikalisch-Technische Bundesanstalt (Germany’s National Metrology Institute), proposed that this transition could be used to make a nuclear clock. But, it was only in 2016 that this transition was directly observed for the first time.

In the new study, an international team led by Jun Ye at JILA, a joint institute of NIST and the University of Colorado Boulder, have fabricated all of the components needed to create a nuclear clock made from thorium-229. These are: a coherent laser for resolving different nuclear states; a “high concentration” thorium-229 sample embedded in a solid-state calcium fluoride host crystal; and a “frequency comb” referenced to an established atomic standard for precisely measuring the frequency of these transitions.

A frequency comb is a special type of laser that acts like a measuring stick for light. It works using laser light that comprises up to 10⁶ equidistant, phase-stable frequencies (which look like the teeth of a comb) to measure other unknown frequencies with high precision and absolute traceability when compared with a radiofrequency standard. The researchers used a frequency comb operating in the infrared part of the spectrum, which they upconverted (through a cavity-enhanced high harmonic generation process) to produce a vacuum-ultraviolet frequency comb whose frequency is linked to the infrared comb. They then used one line in the comb laser to drive the thorium nuclear transition.

Comparisons for fundamental physics studies

And that is not all: the team also succeeded in directly comparing the ultraviolet frequency to the optical frequency employed in one of today’s best atomic clocks made from strontium-87. This last feat will be the starting point for future nuclear–atomic clock comparisons for fundamental physics studies. “For example, we’ll be able to precisely test if some fundamental constants (like the fine structure alpha) are constant or slowly varying over time,” says Chuankun Zhang, a graduate student in Ye’s group.

Looking forward, the researchers say that they eventually hope to use their technology to make portable solid-state nuclear clocks that can be deployed outside the laboratory for practical applications. They also want to investigate how the clock transitions shift depending on temperature and different crystal environments.

“We also plan to develop faster readout schemes of the excited nuclear states for actual clock operation,” Zhang tells Physics World.

The study is detailed in Nature.

Fluctuations suppress condensation in 1D photon gas

The narrower the parabola shape, the more one-dimensionally the gas behaves — **Smooth transition** The polymers applied to the reflective surface trap the photon gas in a parabola of light. The narrower this parabola is, the more one-dimensionally the gas behaves. (Artistic illustration courtesy: IAP/Uni Bonn)

By tuning the spatial dimension of an optical quantum gas from 2D to 1D, physicists at Germany’s University of Bonn and University of Kaiserslautern-Landau (RPTU) have discovered that it does not condense suddenly, but instead undergoes a smooth transition. The result backs up an important theory prediction concerning this exotic state of matter, allowing it to be studied in detail for the first time in an optical quantum gas.

Decreasing the number of dimensions from three to two to one dramatically influences the physical behaviour of a system, causing different states of matter to emerge. In recent years, physicists have been using optical quantum gases to study this phenomenon.

In the new study, conducted in the framework of the collaborative research centre OSCAR, a team led by Frank Vewinger of the Institute of Applied Physics (IAP) at the University of Bonn looked at how the behaviour of a photon gas changed as it went from being 2D to 1D. The researchers prepared the 2D gas in an optical microcavity, which is a structure in which light is reflected back and forth between two mirrors. The cavity was filled with dye molecules. As the photons repeatedly interact with the dye, they cool down and the gas eventually condenses into an extended quantum state called a Bose–Einstein condensate.

Parabolic-shaped protrusions

To make the gas 1D, they modified the reflective surface of the optical cavity by laser-printing a transparent polymer nanostructure on top of one of the mirrors. This patterning created parabolic-shaped protrusions that could be elongated and made narrower – and in which the photons could be trapped.

As the gas transitioned between the 2D and 1D structures, Vewinger and colleagues measured its thermal properties as it was allowed to come back to room temperature – by coupling it to a heat bath. Usually, there is a precise temperature at which condensation occurs – think of water freezing at precisely 0°C. The situation is different when a 1D gas instead of a 2D one is created, however, explains Vewinger. “So-called thermal fluctuations take place in photon gases but they are so small in 2D that they have no real impact. However, in 1D these fluctuations can – figuratively speaking – make big waves.”

These fluctuations destroy the order in 1D systems, meaning that different regions within the gas begin to behave differently, he adds. The phase transition therefore becomes more diffuse.

A difficult experiment

The experiment was not an easy one to set up, he says. The main challenge was to adapt the direct laser writing method to create small and steep structures in which to confine the photons so that it worked for the dye-filled microcavity. “We then had to analyse the photons emitted from the microcavity.”

“Our colleagues in Kaiserslautern eventually succeeded in fabricating new tiny polymer structures with high resolution, sticking to our ultra-smooth dielectric cavity mirrors (with a roughness of around 0.5 Å) that were robust to both the chemical solvent in our dye solution and the laser irradiation employed to inject photons into the cavity,” he tells Physics World.

It is often the case in physics that theories and predictions are based on simple toy models, and these models are powerful in building robust theoretical framework, he explains. “But nature is far from simple; it is extremely difficult to build these ideal platforms to test these foundational concepts since real-world systems are usually interacting, driven-dissipative or coupled to some other system. For photon condensates, it is known that they very closely resemble an ideal Bose gas coupled to a heat bath, so we were interested in using this platform to study the effect of the dimension on the phase transition to a Bose–Einstein condensate.”

Looking forward, the researchers say they will now use their novel technique to study more elaborate forms of photon confinement – such as logarithmic or Coulomb-like confinement. They also plan to study photons confined in large lattice structures in which stable vortices can form without particle–particle interactions. “For example, in one-dimensional chains, there are predictions of an exotic zig-zag phase, induced by incoherent hopping between lattice sites,” says Vewinger. “In essence, the structuring opens up a large playground for us in which to study interesting physics.”

The present study is detailed in Nature Physics.

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