I am a 22-year-old physicist working in cryogenics and magnetics at the Rutherford Appleton Laboratory in Oxfordshire. Earlier this year, I did something totally different from my day-to-day working life and entered the Institute of Physics’ Three Minute Wonder competition, which challenges early-career physicists to present their work or research in just three minutes.
Despite being initially hesitant to enter the competition, I began to think about what I could present and concluded doing something about the European Radioisotope Stirling Generator – a proof-of-concept project that we are working on for the European Space Agency.
We are attempting to use a Stirling engine to convert radioactive-decay heat from americium to electricity. If successful, Stirling converters could be used to generate electricity for satellites on missions deep into the solar system, where generating electricity from solar panels becomes increasingly difficult.
This project seemed perfect for a Three Minute Wonder talk, given the topic has a clear objective and the physics of a Stirling engine can be easily demonstrated.
After I got through the regional heats, the final of the competition was held on 17 May at the world-famous Faraday lecture theatre at the Royal Institution (RI). Arriving at the RI was intimidating, especially knowing that I would be presenting to science communication experts such as the physicist and broadcaster Jim Al-Khalili.
I also felt a bit out of my depth as I was the only presenter not studying for or who had a PhD. However, meeting everyone beforehand relaxed the atmosphere and it felt in the end more like showing your work to friends.
My presentation was eventful, to say the least. At one point I stood on the presenter’s desk in the lecture hall to demonstrate how power produced from a solar panel decreases as you travel further from a light source. This was done with a torch and a solar-powered merry-go-round – a very visual demonstration. Unfortunately, my Stirling engine, which shows how to convert heat to electricity, didn’t work. While being comedic, it at least allowed me to speak about potential issues and challenges with this technology.
Despite the hiccup, the presentation was a fantastic experience and I gained so much confidence speaking in public. I also must have done something right as I won the audience vote as well as the overall competition.
I would happily enter again and if you might be thinking of doing the same then here are some tips for presenting your work to a general audience:
Make a clear narrative I had three clear sections in my talk: “The problem”, “The experiment”, and “The solution”. This gives cohesiveness to your presentation and helps the audience follow it.
Don’t stress about perfection Things go wrong, demos can break or you could get heckled. Personally, I found it easier to go through topics or bullet points instead of remembering a whole script.
Have a demo Even if it is loosely related to your presentation, having something you can show or interact with brings your presentation to life and makes it more relatable and memorable.
Details about how to get involved in next year’s Three Minute Wonder competition are expected soon.
Physicists have discovered a new quantum state in a material with the chemical formula Mn3SiTe6. The new state forms due to long-theorized but never previously observed internal currents that flow in loops around the material’s honeycomb-like structure. According to its discoverers, this new state could have applications for quantum sensors and memory storage devices for quantum computers.
Mn3SiTe6 is a ferrimagnet, meaning that its component atoms have opposing but unequal magnetic moments. It usually behaves like an insulator, but when physicists led by Gang Cao of the University of Colorado, Boulder, US, exposed it to a magnetic field applied along a certain direction, they found that it became dramatically more conducting – almost like it had morphed from being a rubber to a metal.
This effect, known as colossal magnetoresistance (CMR), is not itself new. Indeed, physicists have known about it since the 1950s, and it is now employed in computer disk drives and many other electronic devices, where it helps electric currents shuttle across along distinct trajectories in a controlled way.
The thing that differentiates the insulator–CMR transition Cao and colleagues observed is the mechanism behind it. In Mn3SiTe6, this transition is driven by internal loops of current with a specific handedness, or chirality, that form within the material’s octahedral structures. Cao explains that these internal currents are extraordinarily susceptible to external currents, which disrupt and eventually “melt” the loop currents. The result is a transition to a different quantum state via a mechanism rather like the transition from solid to liquid.
Long-term stability
“Such a transition takes seconds or minutes to occur because the loop currents extend over multiple atomic sites that allow the new quantum to remain metastable over strikingly long time scales,” Cao reports. “This stability is set by atomic motion rather than by electrons alone, which would make the transition otherwise occur much quicker, on the order of picoseconds.”
Mn3SiTe6’s CMR behaviour makes it an intriguing exception to more common CMR patterns, Cao adds. “In this material, CMR only occurs when magnetic polarization is absent,” he tells Physics World. “The phenomenon we observed defies all existing theoretical models and experimental precedents.”
According to Cao and his colleagues, in the absence of a magnetic field, the loop currents in Mn3SiTe6 tend to remain disordered, flowing in both clockwise and anticlockwise directions. Cao compares this effect to cars driving through a roundabout in both directions at once, and says that the resulting disorder causes “traffic jams” for electrons travelling in the material, increasing the resistance and making it an insulator.
“In the presence of a magnetic field, however, the loop currents will begin to circulate only in one direction, which creates an ordered state,” he continues. “The traffic jams disappear and the electrons can propagate through the material with drastically reduced electron scattering as if it was a metal wire.”
In Cao’s view, the newly-discovered quantum state raises many fundamental questions yet unanswered. However, he suggests that the extraordinary susceptibility of the new quantum state to small external currents, along with its controllable, time-dependent switching, could offer a paradigm for quantum devices such as memories and sensors.
For now, the effect has only been observed at low temperatures, but Cao and colleagues are now searching for materials that show the same behaviour at higher temperatures. “No quantum state exists only in one particular material,” Cao says. “The quantum state we have observed should therefore not be an exception.”
Mitigating and reversing the effects of climate change is the most important scientific challenge facing humanity. Carbon sequestration describes a range of technologies with the potential to reduce the concentration of carbon dioxide (CO2) in the atmosphere. Most of these schemes involve storing the gas underground, however, this is not without risk, and scientists are concerned that underground storage could lead to increased seismic activity (a phenomenon known as “induced seismicity”).
Now, researchers in the US and Switzerland have studied microseismicity, the small seismic events caused by carbon injection into host rock, at the Illinois Basin Decatur Project (IBDP) in the midwestern US. In 2011–2014, the IBDP injected one million tonnes of CO2 into an underground reservoir just above a rhyolite crystalline basin. Nikita Bondarenko and Roman Makhnenko at the University of Illinois and Yury Podladchikov at the University of Lausanne have used a combination of field observations and computer simulations to show how microseismicity at the IBDP is highly dependent on the microscale structure of the host rock.
Mohr’s circle
The foundation of the researchers’ approach is a concept called “Mohr’s circle,” which describes the graph that can be drawn to depict a stress tensor. Integral to many geoengineering endeavors, Mohr’s circles can be plotted to describe the response of soils, minerals, and other geophysical materials to stress in multiple directions. The goal of the researchers was to develop a deeper understanding of the local microseismicity, considering only events of magnitude 2.0 or less on the Richter scale, during the injection of CO2 into the IBDP rock reservoir.
To complement their Mohr’s circle calculations, the group has considered how the CO2 behaves as a fluid and fills the cracks and pores of the host rock. Their results from the observation of the IBDP’s seismic activity indicate that the injection of CO2 into the “crystalline basement” (the rock layer below a sediment deposit) can exacerbate existing cracks and faults, thereby destabilizing the basin. In addition, injection-induced cracking can occur in the rigid layer directly above the crystalline basement, also known as the “stiff competent layer.”
At IBDP, CO2 is injected into the lower unit of Mt. Simon sandstone within the stratigraphy of the Illinois Basin (see figure). Due to the presence of intraformational seals (impermeable mineral veins in the rock) in the Mt. Simon complex, the injected CO2 affects the faults in the crystalline basement beneath the reservoir, making possible the reactivation of any fault structures that are favorably oriented.
Poroelastic effect
Another phenomenon that needs to be addressed during CO2 injection is the pororoelastic effect, which is related to pore pressure and mechanical stress. This part of the study focused on Argenta sandstone and Precambrian rhyolite from the T.R. McMillen #2 well, which is 25 km southwest of the IBDP injection site. The goal was to measure the site’s poromechanical properties. Cores of Argenta sandstone and Precambrian rhyolite were both extracted within the depth range of 1900–2000 m.
Precambrian rhyolite, the crystalline basement rock, is known to have fractures that permit internal fluid migration, thereby weakening the rock and lowering its elastic modulus. Intact, or heterogeneous, samples were obtained via lab-scale experiments on the specimen with size on the order of 10-100 mm. The measurements obtained on this minuscule scale were then run through the team’s “fully coupled hydromechanical numerical code,” based on the set of partial derivative Biot equations for pore fluid and behavior, to model the seismicity induced by CO2 injection at the IBDP.
Numerical modelling
In addition to laboratory measurements, some numerical modeling was done to relate the stratigraphy of the sandstone and rhyolite to the microseismicity taking place at the injection site. Results of seismic surveys conducted by the Illinois State Geological Survey show some uneven sedimentation in the stratigraphic layers below IBDP, which might result in a change in the stress within the rock. In addition, the strength of the rock was measured, and comparison of the friction angle to the tangent line to Mohr’s circle allowed the researchers to understand the threshold for injection-induced cracking and rock failure. In short, they conclude that the injection of CO2 is unlikely to result in significant seismic activity.
The researchers describe their results in Scientific Reports, and the main takeaway from their paper is that seismicity is a highly complex phenomenon. Local stratigraphic features complicate the analysis of injection-generated seismicity. As a result, the IBDP injection site cannot be effectively described by a single Mohr’s circle, nor can a microseismic response be explained only by the changes in pore pressure. Hydromechanical coupling, two-phase flow, stratigraphic effects, and temperature must be considered as part of the bigger picture of the IBDP’s seismicity profile. Indeed, more work must be done to reconcile the need for carbon sequestration with the continued prevalence of industry; seismicity portends a safety hazard, which impacts people’s perception of carbon sequestration measures. Until we reach a better understanding of induced seismicity from carbon injection, hazard mitigation is the best course of action.
From Brunel to Banksy, the UK city of Bristol has a history of people unafraid to think big while doing things on their own terms. One place that typifies Bristol’s free-spirited attitude is the Centre for Print Research (CFPR) at the University of the West of England.
At the CFPR, scientists and artists work side-by-side to reimagine old print and manufacturing techniques for modern applications, such as anti-forgery labelling and aircraft components. CFPR researchers are also working at the vanguard of wearable electronics, designing e-textiles using graphene and other 2D materials for health and lifestyle applications.
Find out more about this print innovation hub in this article by Joe McEntee, which features in the December issue of Physics World.
In this episode of the Physics World Weekly podcast we meet Adrian Mancuso, who is the new Physical Science Director at the UK’s Diamond Light Source. The physicist talks about his plans for the national synchrotron lab, and chats about the myriad research that is done at synchrotrons and related facilities called free electron lasers.
Also in this podcast, the science journalist Keith Cooper talks about plans for the next generation of NASA’s space telescopes – and how these instruments will be used by astronomers. He also chats about his latest book, which looks at how humans could respond to contact from extraterrestrial intelligent life.
How many electrical charges does a platinum nanoparticle have? Thanks to an improved high-precision electron holography technique, it is now possible to answer this question by counting the charges directly, down to the level of a single electron. The technique, developed by researchers at Kyushu University and Hitachi Ltd in Japan, could help scientists create more efficient catalysts.
Removing just one or two negative charges from a nanoparticle can significantly change its behaviour as a catalyst. For this reason, determining the charge state of individual nanoparticles on a metal oxide surface is an important task for catalyst engineering, explains team leader Yasukazu Murakami, a quantum materials scientist at Kyushu. The problem is that current techniques to do this, such as X-ray photoemission spectroscopy, only provide charge information averaged over many nanoparticles.
Electron holography
In the new work, the researchers employed electron holography (a type of transmission electron microscopy) to directly identify the electrostatic potential created by nanoparticles of platinum on a surface of titanium oxide – a combination of materials frequently used as a catalyst to speed up chemical reactions. In electron holography, an electron interacting with electric and magnetic fields produces a phase shift in the electron’s wavefunction that can then be identified by comparing it to a reference electron that hasn’t interacted with a field.
By measuring the fields around the platinum nanoparticles, Murakami and colleagues determined the number of “extra” or “missing” electrons associated with them. Their measurements showed that a nanoparticle could gain or lose anywhere between one and six electrons.
The researchers say that the mechanism behind platinum charging involves a difference in the work functions (the energy required to completely withdraw an electron from a metal surface) of platinum and titanium dioxide (TiO2). This difference depends on the orientation of the nanoparticles on the TiO2 and the distortion of the crystal lattice.
Reducing mechanical and electrical noise
A central element in the researchers’ achievements was a series of improvements made to a 1.2-MV atomic-resolution holography microscope developed and operated by Hitachi. This instrument reduces mechanical and electrical noise and then processes the data to further tease out the signal from the noise, Murakami explains.
“High-precision electron holography could be applied to cutting-edge studies in condensed-matter physics, inorganic chemistry, including catalysis, spintronic/semiconductor devices, new types of batteries and other subjects in which a comprehensive electromagnetic field analysis is essential,” he tells Physics World.
In this study, which is detailed in Science, the researchers measured the charge on single nanoparticles in a vacuum. However, in the future they hope to repeat their experiments in a gaseous environment. “Such studies would reflect the conditions in which working catalysts are employed,” Murakami says.
Why do scientists at top universities publish more papers than their peers at less prestigious institutions? According to a new study, it’s because faculty at leading universities are more likely to form large research groups, which in turn are more productive (Sci. Advs. 8 eabq705). Such groups essentially have the money to employ lots of postgraduates and postdocs, who churn out lots of work.
Carried out by a team led by Sam Zhang – a computational social scientist from the University of Colorado at Boulder – the study examined 1.6 million publications written by 78 802 tenured or tenure-track faculty members at 4492 departments in the US. The papers spanned 25 disciplines, which were divided into two types: those (such as the physical sciences) where group leaders usually add co-authors on papers, and those (such as economics) where such “group collaboration norms” do not exist.
After examining the affiliations of each paper’s co-authors, Zhang’s team worked out if faculty members had – or had not – written the articles jointly with their graduate students or post-docs. Papers that had been written together with those junior staff were counted as the faculty member’s “group productivity”, while articles written without their input were described as “individual productivity”.
Faculty in group-norm and non-group-norm disciplines were found to have similar individual productivity – averaging 0.74 and 0.78 papers per year respectively. But when it comes to group productivity, the group-norm disciplines fare better, pumping out 1.92 papers per year compared with 1.05 for non-group-norm subjects. Group productivity also increases with the prestige of an author’s institute, yet individual productivity remains roughly the same.
Zhang and colleagues then looked at how productivity is linked to the numbers of graduate students or postdoctoral researchers at universities, finding that labour is unevenly distributed by prestige in all disciplines. The physical sciences have a very wide imbalance, with the top 10% of institutes having an average of 4.5 funded graduate and postdoctoral researchers per faculty member, while the bottom decile has just 0.5.
Feedback loop
Given that research groups are often evaluated by how many papers they publish, Zhang is concerned this metric could lead to a positive feedback loop. Large groups, in other words, write lots of papers, which brings them bigger research grants. That extra money lets them recruit additional researchers who write even more papers, further entrenching inequalities.
The authors believe this mechanism gives researchers in elite departments undue dominance over scientific discourse. Furthermore, the research shows that topics vary with institutional prestige, so a more equitable distribution of labour could enrich the breadth of research being done.
“The presence of funded researchers in a department tends to translate into productivity for the faculty and this labour is unequally distributed by prestige,” Zhang told Physics World. “So what questions aren’t being studied because of these disparities? Our work suggests that increasing funded labour in less prestigious institutions can reduce inequalities across science, and to us, that is a worthwhile outcome to strive for.”
Thousands of scientists and engineers will be converging in Boston at the end of November for the Fall Meeting of the Materials Research Society, the largest international scientific gathering for materials research. More than 50 technical symposia during the event will showcase leading interdisciplinary research in both fundamental and applied areas, presented by scientists from all over the world.
This year’s conference maintains the hybrid approach introduced in 2021, with the live meeting kicking off at the Hynes Convention Center in Boston on 27 November. A dedicated virtual event will run on 6–8 December, with online delegates also able to tune into live streams of featured talks during the in-person event.
This year also sees the return of the iMatSci Innovation Showcase, which provides a platform for scientists and engineers to demonstrate the practical applications of materials-based technologies. iMatSci aims to connect these innovators with early-stage investors, corporate technology leaders and potential partners, fostering collaborations that will accelerate the adoption of new materials technologies for real-world applications.
Alongside the wide-ranging programme of technical presentations, tutorials and professional-development sessions, the technical exhibit offers delegates the opportunity to connect with more than 150 companies showcasing the latest innovations for advancing materials research. A few of the highlights are detailed below.
Probe insert offers integrated solution for Hall analysis
In addition to reducing the time needed to perform Hall effect measurements, Lake Shore’s MeasureReady M91-HR FastHall measurement controller can be used with any type of magnet, including superconducting devices. One such magnet system is the Physical Property Measurement System (PPMS) from Quantum Design, which through a new probe insert from Lake Shore can now be easily integrated with the M91-HR. A specialized version of the insert enables high-resistance measurements up to 200 GΩ, while a standard kit for measurements between 10 mΩ and 10 MΩ is also available.
A new probe insert enables Lake Shore’s M91-HR FastHall controller to be used for high-resistance Hall measurements within a PPMS environment. (Courtesy: Lake Shore Cryotronics)
The new insert works with both van der Pauw and Hall bar geometries, with samples wired to specially designed sample boards. Fully guarded connections from the PPMS-inserted probe to the M91 instrument ensure ultralow noise measurements. The solution is simple to implement, with the M91-HR’s control software integrating easily with the MultiVu system installed on the PPMS. Preloaded scripts enable complete Hall measurement sequences to be executed quickly within the PPMS environment.
The M91-HR combines all necessary Hall measurement functions into a single instrument, automating the measurement process and directly reporting the calculated parameters. Its speed of measurements results from Lake Shore’s patented FastHall technique, which fundamentally changes the way the Hall effect is measured by eliminating the need to switch the polarity of the applied magnetic field during a measurement. This results in faster, more precise measurements, allowing the analysis time in some cases to be reduced by a factor of 100. Most commonly measured materials can be analysed in a few seconds, and even low-mobility (down to approximately 0.001 cm2/V s) samples can generally be measured.
Visit Lake Shore Cryotronics at booth #908
Correlative microscope combines AFM and SEM capabilities
Quantum Design has released the FusionScope, an innovative correlative microscope that combines the measurement power of AFM with the benefits of SEM imaging. Designed from the ground up to seamlessly integrate these two powerful techniques, the FusionScope exploits a shared co-ordinate system that automatically aligns both AFM and SEM operations. This shared mapping system makes it quick and easy to identify the area of interest, measure the sample, and combine the imaging data in real time.
The FusionScope from Quantum Design has emerged from almost a decade of research in correlative microscopy. (Courtesy: Quantum Design)
“The ability to scan and image across differing magnification scales in the FusionScope is the system’s major enabling attribute,” said Stefano Spagna, the company’s chief technology officer. “It allows smooth image transitions between millimetre, micron, and sub-nanometre scales, allowing you to see new correspondences in your data from specific sample areas.”
FusionScope supports most standard AFM measurement modes. It also offers the Finite Impulse Response Excitation (FIRE) mode, a novel off-resonance intermittent contact scanning force microscopy technique that characterizes nanomechanical properties such as sample stiffness and tip adhesion. Advanced AFM techniques include conductive atomic force microscopy and magnetic force microscopy, and switching to these specialized measurement modes can be achieved simply by swapping the self-sensing cantilevers available with the system.
The software provided with the FusionScope can be used to interactively overlay AFM imaging data onto SEM images during operation, allowing researchers to create 2D and 3D visualizations with nanoscale resolution. The software also provides automation for most routine functions, as well as intelligent data handling to make it easy to store and retrieve experimental results. Visit fusionscope.com to learn more.
Visit Quantum Design at booth #300
Hall system offers single measurement solution for complex materials
Semilab has announced the commercial release of its PDL-1000 Parallel Dipole Line Hall Measurement System with integrated temperature control. This tool enables the measurement of sheet resistance, carrier concentration, and electron and hole mobilities for challenging electronic materials, including those with very low mobility or highly resistivity.
The PDL-1000 system from Semilab exploits a novel Hall measurement technique to reveal the properties of challenging electronic materials. (Courtesy: Semilab)
Building on work published in Nature by Oki Gunawan from IBM Research, the PDL-1000 system can differentiate between the Hall effect mobilities of holes and electrons in a material. This novel approach, called the Carrier Resolved Photo-Hall (CRPH) technique, unlocks information about cutting-edge materials that would otherwise require combining together several different characterization techniques. The CRPH technique has proven successful for studying a range of advanced materials, including perovskites, kesterites, thermoelectric compounds, transparent conducting oxides, organic semiconductors, as well as more traditional semiconductor materials.
In addition to the novel CRPH capability, the PDL-1000 can be equipped for mobility and carrier concentration measurements at cryogenic temperatures, opening up a new set of material characterization applications. This cryogenic option supports the full CRPH capability of the tool. The PDL-1000 system also supports both AC and DC Hall measurement modes, with the AC field measurement particularly useful for characterizing samples with low mobilities, including semiconductor, photovoltaic and thermoelectric materials.
The PDL-1000 is now commercially available and shipping to customers. To learn more, contact Semilab at info.usa@semilab.com.
Last year and after about a decade of trying, physicists working at the mammoth National Ignition Facility (NIF) in the US finally succeeded in generating a self-sustaining fusion reaction. But having since struggled to reproduce the feat, they have been busy trying to work out what makes the results of their experiments so variable. Now, a new finding at NIF may provide a clue – ions in what is known as a burning plasma have an unexpected kinetic energy distribution, which encourages fusion.
The $3.5bn NIF at the Lawrence Livermore National Laboratory in California was designed to recreate the conditions inside nuclear warheads, with the primary aim of maintaining the US weapons stockpile without testing. The facility is also used to develop a new clean, abundant source of fusion energy. It does so by firing exceptionally powerful laser pulses at a 1 cm-long hollow metal cylinder. This generates X-rays that then irradiate a peppercorn-sized capsule at the cylinder’s centre made from industrial diamond and containing the hydrogen isotopes deuterium and tritium. With some of the diamond blasted off, the capsule rapidly implodes and for a fraction of a second creates a plasma with the extreme temperatures and pressures needed to fuse the light nuclei together – yielding alpha particles, neutrons and excess energy.
Since NIF switched on in 2009, scientists have struggled to produce significant fusion yields – getting nowhere close to generating more energy than is needed to power the laser. But in early 2021 they had good news, reporting the observation of a burning plasma in which the alpha particles are the main source of heat for the plasma. This was followed by an even more eye-grabbing result in August last year: the ignition of the plasma. In this case the alpha-particle heating was enough to outstrip all energy losses within the plasma and enabled a whopping 1.37 MJ output. This is more than 70% of the1.92 MJ delivered by the laser and about eight times as much as their previous best shot.
Researchers then tried to reproduce that result but another four shots in the following few months yielded at best about half of the record-breaking output. They had better luck in September this year when they achieved about 1.2 MJ from a 2.08 MJ laser pulse. This greater input allowed them to use a thicker capsule, which was less sensitive to the problem plaguing the previous shots – tiny defects in the diamond that can cause carbon to enter the fusion “hot spot” and cool the reactions.
Surprising kinetic distribution
Now, new work by Edward Hartouni of Lawrence Livermore and colleagues in the US and UK could shed light on NIF’s inconsistent performance when it comes to fusion. They found that the kinetic energy of the reacting deuterium–tritium ion pairs in NIF’s burning plasma does not follow the expected Maxwell–Boltzmann distribution that is characteristic of thermal plasmas.
The researchers measured the ions’ properties by carrying out spectroscopy of the neutrons produced alongside the alpha particles. Although the neutrons are emitted at a single energy in all directions within the reference frame of the fusing nuclei, seen from the lab they are jostled around by thermal fluctuations and the plasma’s bulk motion. This means that the energy spectrum of the emitted neutrons provides information about the behaviour of the ions.
Hartouni and colleagues collected isotropic neutron data using five time-of-flight spectrometers positioned at different points around the fusion target. After plotting the results, they were able to work out the ions’ mean velocity – and hence kinetic energy – by measuring the offset between the neutrons’ spectral peak and the energy they are known to acquire in deuterium–tritium fusion reactions. In a similar way, they calculated the temperature of the ions.
Steeper slope
The researchers analysed the results from numerous implosion experiments at NIF by plotting them all on a graph of ion velocity against temperature. Doing so, they found that the data points clustered together in two distinct groups. Within the bounds of experimental error, those shots with a lower fusion output followed the gently diminishing slope characteristic of thermal plasmas. Whereas the shots yielding higher fusion energies instead veered off at a steeper angle.
The output of the latter shots in fact corresponded to ion temperatures matching the shots’ ion velocity rather than their measured (lower) ion temperatures. The researchers say that the ions could have a “suprathermal energy spectrum”, and that more of them have energies favourable for fusion reactions than would be the case for a thermal plasma.
Hartouni and colleagues have still to establish what is causing this departure from a Maxwell–Boltzmann distribution. They point to several potential explanations, but each has its shortcomings. For example, they suggest that there may be fewer neutrons from beyond the hot spot reaching their detectors than those emitted on the near side. This would tend to artificially inflate the mean neutron energy. But they say that other diagnostic observations imply that the hot spot is not dense enough to explain this effect. Likewise, they conclude that measurement distortions due to a spatial offset between the hotspot and capsule centre can be ruled out because X-ray images of the plasma show no such offset.
The researchers say they are continuing to carry out experiments, revise theory and perform computer simulations to try and pin down the cause of the anomaly. If they are successful, according to Stefano Atzeni of the University of Rome “La Sapienza”, they might be in a better position to control NIF’s fusion reactions so that it becomes possible to achieve ignition on demand. “More accurate models of basic physical processes make simulation more predictive,” he says.
The new clock’s frequency instability is less than 4×10–19. (Courtesy: J Pan)
Physicists have transferred time and frequency information over a distance of more than 100 km in free space, far exceeding the previous record. The technique, which makes it possible to synchronize and monitor optical clocks in environments where optical-fibre-based connections are impractical, could be used to set higher standards for metrology, navigation and positioning. It also has applications for basic physics studies such as searching for dark matter, redefining fundamental constants and testing relativity.
An optical clock has three main components. The first is a sample of atoms or ions that transition between energy levels at a well-defined and highly stable reference frequency in the optical region of the electromagnetic spectrum. The second element is a feedback system that “locks” the output of a laser (called the local oscillator) to this reference frequency. The third component provides a very precise measurement of the frequency of the laser, usually via a device known as an optical frequency comb (OFC).
One second in 100 billion years
In the new work, researchers led by Jianwei Pan of the University of Science and Technology of China demonstrated time-frequency dissemination between a feedback system and an OFC separated by a record-breaking distance of 113 km. After 10,000 seconds, the clock’s frequency instability was less than 4 × 10–19, which implies that the clock’s comparison errors would be kept within one second after 100 billion years. The researchers note that this value surpasses the benchmark required to redefine the fundamental unit of the second, which is due to be discussed at the 2026 General Conference on Weights and Measures.
Previous attempts at free-space dissemination of time and frequency at such high precision did not extend beyond dozens of kilometres, which the researchers note is insufficient for high-precision transmission in satellite-to-ground links. “This work opens the path to satellite-ground time-frequency dissemination,” says Pan, “and we anticipate that long-haul free-space OFC links, combined with fibre-based and satellite-based time-frequency links, will become important parts of future optical clock networks.”
Pan adds that the new clock builds on the free-space optical tracking technology developed for the Micius quantum communications satellite, which launched in 2016. “We then developed high power, highly stable OFCs operating at 1 W, high-stability and high-efficiency optical transceiver systems and high-sensitivity linear optical sampling detection operating at nanowatts,” he tells Physics World.
The researchers, who report their work in Nature, now plan to develop a Medium Earth Orbit-to-Geosynchronous Equatorial Orbit (MEO-to-GEO) quantum science experiment satellite that can realize both a GEO satellite-based optical frequency standard and satellite-ground time-frequency transfer. “We hope this system will have a time-frequency instability of less than 5 × 10–18 at 10,000 seconds,” Pan says. “Two-way comparison links are being established with the station in China we worked with for this study and the overseas station to realize an intercontinental optical clock comparison. This satellite is planned to launch in 2026.”
