For the first time, physicists have succeeded in measuring the same photon at two different locations within an optical fibre – all without destroying the photon. The new non-destructive technique, which was developed by researchers at the Max Planck Institute of Quantum Optics (MPQ) in Germany, is based on the principles of cavity quantum electrodynamics and could aid the development of quantum communications networks that rely on information-carrying photons.
Although researchers are generally able to detect itinerant photons, the detectors they use invariably destroy the photons being measured. Alternative, non-destructive quantum measurements have important applications in many areas of physics, including quantum sensing, quantum computing and quantum communications.
Quantum non-demolition detector
A team led by Stephan Welte and Emanuele Distante has now developed a “quantum non-demolition” (QND) detector to address this problem. This QND detector consists of a single rubidium atom that has been prepared in a known quantum state that is coupled to a reflective optical cavity. The researchers placed two of these detectors 60 metres apart in an optical fibre. They then used small lengths of additional fibre to connect the detectors to the main fibre, placing “circulators” at the fibre intersections to direct the flow of a beam of laser photons that they sent into the fibre. As a photon enters a circulator, it gets directed towards a detector before being reflected from it and guided back along the main fibre in its original direction.
Improved time resolution
The MPQ team now plan to improve the time resolution of their detection process. This will allow them to more precisely determine the direction in which the measured photon is travelling – information that is only accessible with QND detectors.
They also hope to improve their system so that fewer photons are lost between the two detectors. “Such a non-destructive system could be employed to herald photon loss in a glass fibre,” Welte says. “Once the photon loss has been detected, a given protocol could be stopped and restarted immediately by sending in another new photon,” he tells Physics World. “This way, the rate of the protocol could be increased.”
The world’s first 3D-printed steel footbridge has been unveiled in the centre of Amsterdam. Developed by Imperial College London and the Alan Turing Institute, the 12 m-long bridge took over four years to design and contains a network of sensors to monitor its performance. Data from the sensors will then be used to create a computerised version allowing researchers to analyse the bridge’s behaviour when handling pedestrian traffic.
“A 3D-printed metal structure large and strong enough to handle pedestrian traffic has never been constructed before,” says Leroy Gardner from Imperial. “We have tested and simulated the structure and its components throughout the printing process and upon its completion, and it’s fantastic to see it finally open to the public.”
Requiring only common household items, the experiments include making a balloon hovercraft, a pinhole camera as well as showing the magnetic effect of your breakfast cereal. For the full list of activities, see here.
Keeping with the summer theme, swimsuits and life jackets can be essential items, but if not dried thoroughly after use, they can develop a strong, musty smell. Now researchers have created a buoyant cotton fabric that is also water repellent, which could be used in future to avoid the threat of mould build-up.
Cotton is hydrophilic, allowing liquids and oil inside and previous attempts to make garments repel liquid have often involved spraying the material with “superamphiphobic coatings”. However, this technique is impractical for large-scale manufacturing given that it requires multiple, time-consuming steps.
The researchers, based in China, have created a “one-step” coating process that results in a fabric that is liquid proof and also stays afloat under 35 times its weight. Watch a video of the new material’s hydrophobic properties here.
In 2020, as part of a comprehensive review of defence spending, the UK government underlined the strategic importance of science and technology for national defence and security. The review earmarked an additional £6bn for research and development at the Ministry of Defence (MoD) over the next four years, with an extra £1.1bn allocated to so-called pull-through activities – ensuring that innovations designed initially for the military lead to wider applications in the commercial sector.
That extra funding has led to a major recruitment drive at the Defence Science and Technology Laboratory (Dstl), the scientific division of the MoD. Hundreds of positions for scientists, engineers and project managers need to be filled over the next few months, with more to follow in 2022. In the vanguard are 70 vacancies that are now being advertised for physicists at all stages of their careers, while one of Dstl’s dedicated graduate programmes is also doubling its intake of physicists, up from around 15–20 per year to 40 new graduate positions in both 2021 and 2022.
The skill set offered by physicists is needed for a lot of our current vacancies.
Karen Smith, a talent acquisition and planning adviser at Dstl
“The skill set offered by physicists is needed for a lot of our current vacancies, which need expertise in areas such as lasers, electro-optics and electromagnetic phenomena,” says Karen Smith, a talent acquisition and planning adviser at Dstl. “Focusing on the physicist role in our first tranche of recruitment will build a solid foundation for building up our capabilities in other areas.” All in all, the recruitment drive represents an uplift of around 10% in Dstl’s ranks of scientists and engineers – which already stands at more than 3000 members of staff.
Challenging times: Mark Pickering, Dstl’s technical lead for close-combat guided weapons systems, enjoys the process of turning new ideas into practical solutions (Courtesy: Dstl)
Mark Pickering, a physicist who has worked at Dstl since 2012, stresses the importance of his scientific training for his role as the UK’s technical lead for close-combat guided weapons systems. “I do a lot of work on missile subsystems such as sensors, aerodynamics and rocket physics, and as a result I directly use my physics knowledge and skills to solve a huge variety of physics-based problems,” he says. “It means that I can predict how something might behave from basic principles, and what to investigate as part of the experiment design.”
[At Dstl] you gain a deeper understanding of something, and it’s more satisfying to build up that understanding from scratch.
Omar Sarsah, Dstl
Omar Sarsah, who joined Dstl a couple of years ago as a new physics graduate, has also enjoyed translating the abstract ideas he learnt during his degree into real-world situations. “It turns out that a helicopter has an almost perfect black-body curve, but you have to compensate for various factors, such as the atmosphere and the camera you use to detect the emission,” he explains. “These are all things that you study separately at university, but here you get to put them together. You gain a deeper understanding of something, and it’s more satisfying to build up that understanding from scratch.”
One of the key attractions for both Pickering and Sarsah is the ability to contribute to a variety of different projects. Sarsah, who specializes in electro-optics, might work on several projects at a time – which might involve anything from electronics to highly sensitive quantum-optical systems. For the most part, however, he uses his specific knowledge of cameras and optical modelling to evaluate and improve the performance of different systems or platforms. In the case of the helicopter, this type of analysis revealed that the engine generates bright emission in the infrared, which prompted the use of engine covers to improve stealth and, ultimately, the safety of military personnel.
“You are presented with a problem that no one has yet solved, you think about the best way to tackle it, and then you put it to the test,” explains Pickering. “It’s really rewarding to be involved throughout the entire process of turning what might be quite a nebulous idea into something that makes a real difference.”
As an example, Pickering describes how he was recently trialling some new equipment that needed to be tested onboard a flying aircraft. “I was in a control room directing what trials the aircraft should be doing, all following my experimental plan,” he says. “It was the magical fulfilment of seeing something you’ve worked on for several years coming to fruition. It was an amazing feeling.”
Seeing something you’ve worked on for several years coming to fruition was an amazing feeling.
Mark Pickering, Dstl
An essential part of this project work is the need to join forces with scientists and engineers with different backgrounds and skill sets. “We need to develop things that people can use, and so everyone needs to work together to make sure each element interacts with everything else in the system,” comments Sarsah. “I might be able to design parameters for a specific camera, but when it needs to be integrated into a real-world system we also need to make sure the electronics line-up, that the mechanical stability is fine – and even that it goes in the right way round.”
Close collaboration is also needed with research teams in industry and academia, something that will become even more important with the extra funding for R&D. Pickering already acts as a technical partner on several of these collaborative projects, providing technical expertise and ensuring that any external research meets the objectives set by the MoD. He also serves as the technical lead on several procurement programmes, providing scientific advice to government and front-line commands, and fulfils a similar role with international organizations.
Newer members of staff also have plenty of opportunities to gain different experiences and to get involved with external collaborations. Pickering started his career on secondment to a naval base, and one of his graduates is just about to start a six-month placement with one of Dstl’s industry partners. Meanwhile, Sarsah regularly works off-site, gathering test data from aircraft or from ballistic systems being fired on an outdoor range, and currently sits on one of NATO’s technical panels. He is now considering various options for progressing his career at Dstl, which could include a secondment in the UK or overseas, a move to the MoD headquarters in Whitehall, or studying part-time for a PhD.
From prior experience, Pickering says that it’s incredibly easy to switch domains or to move around the organization. While his sights are now firmly focused on a technical career path, scientists and engineers employed by Dstl can also choose to specialize in people management, project management or operational analysis – which seeks to optimize the performance of a whole system rather than each specific element. “Technical and analytical are pretty interchangeable, and lots of people fly back and forth between the two,” comments Pickering.
Anyone thinking of joining Dstl should not be concerned that they do not have enough knowledge of the technologies they will be working with. “No-one is expected to have direct expertise of these systems because the work you will be involved with is so unique,” says Sarsah. ” I spent two weeks at the Defence Academy in Shrivenham when I started, and other courses and seminars are arranged to help you understand the domain you are working in. You also learn lots of things very quickly by doing project work.”
There is also plenty of support for both new and experienced employees. When he started Sarsah was the only member of his team with expertise in optical modelling, and he was overwhelmed with the amount of work that was coming his way. “My team leader helped to organize which projects took priority,” he says. “She stood by me and said that I couldn’t do everything.”
Skilled operator: Omar Sarsah also spends time working on optical kit in the lab (Courtesy: Dstl)
Sarsah was also surprised and delighted that his team nominated him for a NATO early-career award. “When you start out you can be a bit nervous because you don’t know everything and there’s so much knowledge and expertise around,” he says. “Although I didn’t win the award, it showed me that I was making a valuable contribution to the team.”
Pickering has also been impressed with the help and support that Dstl offers to its employees. He is extremely dyslexic, particularly when it comes to writing, and Dstl has always ensured that additional systems and resources are available to help review and refine his written work. “I have always been impressed at how far they are willing to go to look after people,” he comments. “Dstl really cares about its employees.”
That level of support makes Dstl an open and inclusive place to work. Internal support networks, run by employees for other employees with similar interests and needs, help staff to discuss problems, share advice, and raise any issues with the executive team. Fully flexible working is also available as standard, including alternative working patterns, job share, and variable working hours. “We want all our employees to maximize their potential,” comments Smith.
For their part, it’s clear Pickering and Sarsah are primarily motivated by the diverse opportunities they have to learn new science and new skills, and to work on projects that have a real impact on people’s lives. “It’s been the best job I could ever ask for,” says Pickering. “I’ve really enjoyed everything I’ve done while I have been at Dstl.”
• Some of the names in this article have been changed for privacy reasons.
An ancient star lying on the fringes of the Milky Way likely contains the remnants of a colossal hypernova explosion, which took place early on in the galaxy’s star-forming period. That’s the conclusion of an international team of astronomers, led by David Yong at the Australian National University, who discovered that the star’s abundance of heavy elements could have only been synthesized in the highly energetic “r-process”. Their findings provide the first evidence for magneto-rotational hypernovae, and uncover their role in the changing chemical makeup of the early universe.
Astronomers predict that around half of all heavy atomic nuclei in the universe must have originated in a succession of rapid neutron captures, named the r-process. The sites where these captures take place are still poorly understood, but according to current theories, mergers between neutron stars are thought to play an important role. In the latest models of chemical evolution in galaxies, however, these mergers alone can’t reproduce the abundances of heavy elements that we observe today.
To search for alternative origins, Yong’s team looked to the halo of the Milky Way – which contains an abundance of ancient stars born early on in the galaxy’s star-forming history. The astronomers made their observations using the European Southern Observatory’s Very Large Telescope (VLT) in Chile, and the Australian National University’s SkyMapper telescope in New South Wales, which has previously been used to identify thousands of these chemically primitive stars in the halo.
In a star named SMSS J200322.54−114203.3, Yong and colleagues noted a high abundance of r-process elements, including zinc, uranium, europium and possibly gold, despite it being extremely metal-poor compared with stars of similar ages. Through their analysis, the researchers concluded that these abundances could have only been produced in a colossal explosion named a magneto-rotational hypernova. These as-yet unobserved events are triggered as the core of a rapidly spinning, highly magnetized star, 25 times more massive than our Sun, collapses into a black hole – releasing 10 times more energy than a conventional supernova.
“It’s an explosive death for the star,” says Yong in a press statement. “We calculate that 13 billion-years ago J200322.54-114203.3 formed out of a chemical soup that contained the remains of this type of hypernova. No one’s ever found this phenomenon before.”
Such a dramatic explosion would provide ideal conditions for the r-process, producing an abundance of heavy elements. Alongside this process, the hypernovae would also eject high levels of lighter elements, formed during the progenitor star’s evolution; as well as elements close to the “iron peak” in universal abundance, formed during explosive nuclear burning. As a result, although the remnants should be metal-poor overall, they should contain all stable elements of the periodic table at once.
Based on this evidence, Yong’s team concluded that SMSS J200322.54−114203.3 is made up of the hypernova-ejected remnants of a short-lived, even more ancient star, which underwent a magneto-rotational hypernova just around one billion years after the Big Bang. Their discovery provides key evidence for an as-yet unconsidered site for the r-process, and could lead to better explanations for how heavy elements were first synthesized, early on in the galaxy’s star-forming history.
In 1977 fully automatic timing, as opposed to timing done by people with stopwatches, became mandatory for world records in the 100 m sprint. Immediately after this change, average recorded times of sprinters increased slightly, before decreasing again. How did automatic timing cause this single stepwise increase?
Long jump
Many of the best long-jumpers in the world appear to continue running in the air as they cycle their legs for a few steps after take-off, in a technique called the hitch kick. What is the purpose of this motion?
Shot put
The women’s shot put has a mass of about 4 kg, but the volume varies slightly. If it can be made of solid iron or solid brass, what is the range of possible diameters it could have?
High jump
In the high jump, athletes traditionally keep their body upright as they kick their legs over the bar. But at the 1968 Olympic Games in Mexico City, American high-jumper Dick Fosbury won gold using a new technique he had developed. Now called the Fosbury flop, it involves slinking backwards over the bar and landing on your back. What physical principle does the Fosbury flop use to help an athlete clear a higher bar?
400 m
In the 400 m race, the starting line is staggered across lanes to ensure that all athletes have the same distance to run while staying in their lanes. Why does World Athletics say that the number of lanes for a standard track should be no more than nine?
Day 2
110 m hurdles
The men’s 110 m hurdle event has 10 hurdles spaced at 9.14 m from one another. The take-off foot should touch the ground at 2.1–2.2 m in front of each hurdle, and the athlete normally lands about 1 m from the hurdle. Most athletes take three steps between hurdles (not including the hurdle jump). About how long should each stride be?
Discus
The theoretical optimal angle for throwing an object as far as possible is 45 degrees to the ground. However, most athletes have an optimal angle slightly smaller. Why is this?
Pole vault
For an athlete with a centre of mass 1 m above the ground, who can run at 10 m/s, what is the theoretical limit to the pole vault height they can clear? And why is the pole vault world record slightly above this?
Javelin
In 1986 the men’s javelin was redesigned so that the centre of mass moved 4 cm closer to the tip. The women’s javelin was similarly redesigned in 1999. Which two problems prompted this redesign, and how did it solve them?
100 m: People with stopwatches tend to underestimate sprinters’ times. This is because they start measuring slightly after the runners have taken off, due to non-zero reaction times. Their reaction times to the runners crossing the finish line are not as long, because they are watching the runners and can anticipate when they will get to the end.
Long jump: Long-jumpers generate some angular momentum as they take off, which would cause their body to rotate forwards so that they are leading with their face, making it difficult to land on their feet. By cycling their legs, they take care of the angular momentum while keeping their body upright so that they can land feet-first. This is similar to how you might instinctively swing your arms when you lose balance. Not all long-jumpers use this technique, though. Some use the “hang-style” technique, in which they kick their legs forward after take-off, so that they are horizontal to the ground. They also fold their body forward over their legs. This lengthens their moment of inertia, so that the angular momentum generated causes their body to rotate less.
Shot put: Using density of iron as 7.86 g/cm3 the diameter is 9.91 cm. Using density of brass as 8.73 g/cm3 the diameter is 9.56 cm. The official range in the regulations is given as 95-110 mm. The shot put is not always just one material. It is sometimes made of a smaller lead weight in a metal casing of lower density, leading to a wider range of possible diameters.
High jump: When an athlete slinks over the bar using the Fosbury flop technique, rather than going over it with their body upright, there is no point in time at which their whole body is above the bar. In fact, there is no point in time at which an athlete’s centre of mass goes above the bar – their centre of mass actually goes below the bar. This means that they have to generate less energy to clear the bar than they would if they went over it with their body upright, which would require them to raise their centre of mass higher. Therefore, at the maximum energy they can generate, they can clear a higher bar with the Fosbury flop technique.
400 m: The curved part of the track is sharper on the inside lane than on the outside lane, due to the increasing radius of curvature from the inside to the outside. It is harder to run around a sharper bend, so having a gentler curve may confer an advantage on the runner in the outside lane. However, World Athletics (formerly the IAAF) considers this difference to be mostly negligible, only becoming significant if there are more than nine lanes in the track.
110 m hurdles: The stride length should be about 2 m per step. The spacing of hurdles means that “rhythmic running”, with regular stride lengths, is more important in hurdle events than in flat races.
Discus: The basic model of an object being thrown from ground level with a given force gives 45° as the optimal angle to maximise the distance it will travel before hitting the ground. However, the real-world scenario is more complex. Athletes do not throw from the ground level, but a little above it depending on their height. The object can therefore be considered to be starting at a different point in the parabolic model. You can extrapolate the object’s motion backwards behind the athlete to imagine it starting at ground level. Imagining it taking off from the ground at 45°, you find that by the time it reaches the athlete’s hand, its angle to the horizontal would be less than this value. The athlete should therefore throw it at this lower angle to maximise its distance. There are also biomechanical factors, as athletes may be able to generate a greater force by throwing at an angle closer to the horizontal, due to how our muscles are arranged anatomically. Finally, air resistance also plays a role in real-world scenarios. Since it slows down the object’s horizontal motion and reduces the distance it travels, throwing it with a greater horizontal component may help to counteract this.
Pole vault: Equating kinetic energy with gravitational potential energy, the change in height is about 5.1 m. Adding the initial centre of mass being at about 1 m, this gives a theoretical maximum of about 6.1 m. The world record is currently 6.18 m. The extra height could be because, similarly to high-jumpers, pole-vaulters often use a technique where their centre of mass goes under the bar. Also, as the pole straightens, the athlete pushes off the end of it, putting more energy into the system, which gets added to the kinetic energy they generated in the run-up.
Javelin: The two problems were that 1) athletes were throwing the javelins increasingly far, which started to become dangerous as the javelins could go beyond the field into the audience, and 2) the javelin would often land flat on the ground rather than pointing down into the ground, leading to ambiguity around what was a valid, qualifying throw. When the centre of mass was moved towards the point of the javelin, the javelin’s trajectory changed, so that it started to point downwards sooner in its motion. This reduced the distances it could be thrown to a safer level and reduced the number of instances where it landed flat instead of point-down.
1500 m: The 1500 m is a very tactical race. Rather than trying to run it in the shortest time they can, runners often try to conserve their energy by running slowly throughout the race, staying behind the person in the lead. They then try to win by sprinting at the end. However, this means that no one wants to strike out and be the person in the lead, because that would mean wasting energy that the other athletes conserve, and would probably lead to being overtaken later. So none of the athletes end up running very fast because none of them wants to be in the lead. This leads to an example of a “Nash equilibrium”, which is a concept in game theory where no participant has anything to gain by changing their strategy. In some competitions, pacesetters run parts of the race with the athletes, which encourages them to run faster rather than running strategically. However, no pacesetters run in the Olympic Games, and athletes would usually rather win an Olympic gold medal than beat their own personal best.
Many thanks to Steve Haake, professor of sports engineering at Sheffield Hallam University for checking these answers for accuracy
Fracture tests carried out on hexagonal boron nitride (h-BN) show that this 2D material has an intrinsic toughening mechanism, contradicting its reputation for brittleness. This unexpected behaviour, which was observed by Jun Lou, Huajian Gao and colleagues at Rice University in the US, defies a description of fracture mechanics first put forward by the British engineer A A Griffith in 1921 and still employed today to measure material toughness.
As with most materials, cracks in 2D materials typically form at sites of concentrated stress. The unique structure of 2D materials, however, means that cracks can propagate straight through, opening up the bonds between individual atoms like a zipper.
To investigate this cracking mechanism in h-BN, the Rice researchers subjected samples of single-crystal monolayers of the material to tensile loads in a micromechanical device. They found that in contrast to graphene – a one-atom-thick sheet of carbon that structurally resembles monolayer h-BN – the growth of cracks in h-BN was surprisingly stable, with cracks forming branches as the tensile load increased. This branching means that additional energy is required to drive the crack further, effectively making the material tougher, Lou explains. Overall, h-BN is 10 times more fracture-resistant than graphene, defying Griffith’s formula and leading the Rice University press office to describe it as “the iron man of 2D materials”.
Good news for flexible electronics
The atoms in both graphene and h-BN are arranged almost identically, in a flat hexagonal lattice structure. However, the researchers say that the slight asymmetries that arise in a material containing two elements (boron and nitrogen) instead of just one (carbon) may contribute to the crack branching behaviour in h-BN.
Regardless of the mechanism, h-BN’s newfound toughness is a boon for electronic applications. The material’s resistance to heat, stability to chemicals, and dielectric properties all make it ideal as both a supporting base and an insulating layer for placing between electronic components. The discovery that h-BN is also surprisingly tough means that it could be used to add tear resistance to flexible electronics, which Lou observes is one of the niche application areas for 2D-based materials. For flexible devices, he explains, the material needs to mechanically robust before you can bend it around something. “That h-BN is so fracture-resistant is great news for the 2D electronics community,” he adds.
The team’s findings may also point to a new way of fabricating tough mechanical metamaterials through engineered structural asymmetry. “Under extreme loading, fracture may be inevitable, but its catastrophic effects can be mitigated through structural design,” Gao says.
The way that some plants such as the Venus flytrap and Cape sundew move so quickly and precisely has always fascinated scientists. Plants move with biological necessity, whether it is to feast on insects or to spread their spores far and wide. This video looks at the mechanics of moving plants and how it can inspire innovations in soft robotics.
For a more detailed look at the physics of plant motion, take a look at this feature by science writer Daniel Rayneau-Kirkhope, originally published in the July 2021 issue of Physics World.
A rigorous and independent approach to QA provides an essential audit of the evolving radiotherapy delivery system, complementing the integrated “self-checks” on the treatment machine to ensure that radiation is delivered to the tumour site as intended while minimizing collateral damage to healthy tissues and organs at risk. In this second feature of our series on independent QA, Physics World talks to Jeff Kapatoes, senior director for regulatory and research at Sun Nuclear Corporation, a US-based manufacturer of QA solutions for radiotherapy and diagnostic imaging providers, about the necessity for open data access to drive continuous improvements in patient safety, treatment outcomes and workflow efficiency.
How does independent QA benefit the radiation oncology care team?
The clinical medical physicist owns the critical role of independent oversight of patient treatment in the radiation therapy suite. Independent QA providers like Sun Nuclear equip the medical physics team with the specialist QA devices and software they need to fulfil this role effectively – whether that’s commissioning a new treatment machine or clinical workflow; daily, weekly, monthly or annual machine QA checks; as well as all aspects of patient-specific QA, including in vivo QA. For many of these essential checks, open access to the data generated by the treatment delivery system is fundamental to success while ensuring radiation is delivered to the patient as intended.
In terms of specifics, what do you mean by open data access?
Put simply, we’re referring to the availability of treatment delivery data in its broadest sense. For starters, this includes the standard DICOM objects from the treatment planning system (TPS) – the radiotherapy plan, images, structure sets and dose. There are also the cumulative and time-based data from the electronic portal imaging device (EPID) and the machine log files (i.e. monitor units, leaf positions, couch positions, gantry angle and collimator angle). Finally, there are the imaging data – in particular, the cone-beam CT, kV projection and registration offsets.
Is access to all of these data objects guaranteed?
There are notable concerns – in particular, regarding open access to the data generated by the EPID during treatment delivery. While there have already been attempts to restrict access and monetize these data, it’s worth restating that the data are “owned” by the clinic and must remain freely accessible to enable independent analysis of radiotherapy fulfilment.
Jeff Kapatoes: “We want data access to be part of the clinical community’s collective conversation.”
Right now, though, there are no regulations in the relevant technology standards (e.g. IEC 60601-2-1) mandating the radiotherapy delivery system vendors to provide open data access.
On Elekta’s treatment machines, for example, access to the EPID data is only available to customers via a licence purchase. For Varian, the older C-Series linac allows access, while the newer TrueBeam system only provides access to dosimetric cine data via the service interface. With Varian’s latest Ethos technology, access to the EPID data for patient treatment is not allowed at all, including for Varian’s own portal dosimetry product. This trend is certainly concerning.
Why is data access restricted in this way?
Notwithstanding the lack of hard standards requirements, it’s important to acknowledge the non-trivial engineering and manufacturing challenges of architecting a complex treatment system and bringing it to market. Making the associated data objects readily available and transferable for QA purposes takes resource, time and effort – all of which are at a premium when the development team for a new radiotherapy system is focused on shipping to a demanding commercial schedule. As with all functionality, open data access is a matter of prioritization and business decisions.
So open access to the delivery system data is key for an independent and innovative QA ecosystem?
That’s correct. In our opinion, there should be a goal to ensure interoperability is not impeded for competitive reasons but, rather, that it is expected and enabled in a practical manner. The commercial success of independent QA products, in spite of competitive offerings from the established vendors of radiotherapy treatment delivery systems, is the real-world evidence that independent QA is valued by clinical end-users. Bottom line: we cannot take access to these data for granted and must stay vigilant to ensure that access is maintained.
How does open access to data translate into improvements in patient care?
The growing role of automation within the clinical workflow represents a compelling opportunity for efficiency gains. We are now at a point where it’s feasible to carry out in vivo QA of the treatment delivery and better understand what’s happening – monitoring the actual dose delivered to the patient rather than the dose delivered in theory based on the planning images. This, in turn, creates the possibility to do less pretreatment QA, in particular for highly fractionated treatments.
Sun Nuclear’s SunCHECK platform (for integrated machine and patient QA) is part of this evolving value proposition around automation, pulling data objects in automatically, processing those data in the background, organizing the results, and making the care team aware as and when a problem arises. The one-year study of in vivo patient QA conducted by Evy Bossuyt and colleagues at Iridium Kankernetwerk in Belgium is a great example, wherein adjustments to the patient plan were made in 3.7% of the patients treated in that time period. Such changes were enabled via the information provided automatically to the clinical team by SunCHECK.
What about the role of open data access for clinical research?
There’s a lot of activity in research hospitals and universities to figure out how these data objects can be leveraged to deliver enhanced patient safety and workflow efficiency within the clinic. There’s also an acceptance among radiotherapy delivery system vendors – perhaps not publicly stated – of the value to be had in making these data openly available.
The pioneering work of Ben Mijnheer’s team at the Netherlands Cancer Institute (NKI), Amsterdam, is a case in point, yielding significant advances in EPID-based 3D dosimetry on Elekta treatment systems over the past two decades. Meanwhile, Peter Greer and colleagues at Calvary Mater Newcastle Hospital, Australia, have carried out similar research in collaboration with Varian. Their WatchDog system, which uses EPID images to determine treatment delivery accuracy in real-time, has been evaluated across multiple institutions to optimize for error detection.
Longer term, what needs to happen to ensure an open data environment for independent QA checks?
The ideal scenario is a formal stipulation in the relevant technical standards. In other words, a hard-requirement for a system API to give third parties open access to the data objects associated with treatment delivery – information that rightfully belongs to the clinic and the patient. We are willing to participate in any logical manner possible to help make this happen.
For now, our task as an independent QA provider is to encourage clinical users, whenever they’re making a machine purchase from a treatment delivery system vendor, to reiterate the importance of open data access for their medical physics team and their QA programme. In summary: we want data access to be part of the clinical community’s collective conversation with the radiotherapy delivery system vendors.
When the cosmologist Stephen Hawking published A Brief History of Time in 1988, he quickly became the world’s most famous physicist. In this episode of the Physics World Weekly podcast we talk to science writer Charles Seife about his new book, a biography of the late cosmologist entitled Hawking Hawking: the Selling of a Scientific Celebrity, in which he controversially claims that Hawking’s fame stemmed not from his science — but his mastery at self-promotion.
Our other guest this week is Harry Westfahl Jr, director of Latin America’s only synchrotron light source. Sirius, the new storage ring light at the Brazilian Synchrotron Light Laboratory, is one of the most advanced synchrotron light sources in the world. Westfahl Jr discusses Sirius’s first year of operation, the experiments being performed at the synchrotron and what we can expect next. He also describes the challenges of starting up a major scientific facility in the middle of a pandemic.
Scientists in the US and China have created novel optical tweezers that trap particles at lower temperatures and use weaker lasers than conventional optical tweezer techniques, reducing the risks of photodamage and thermal damage. The novel device takes advantage of optical refrigeration to trap particles via thermophoresis, rather than optical force.
In 2018, a share of the Nobel Prize in Physics was awarded to Arthur Ashkin for inventing optical tweezers. These instruments use a highly focused laser beam to generate forces that can hold and move tiny objects, such as nanoparticles, atoms and biomolecules. They have now been involved in notable breakthroughs in nanotechnology, physics, biological science and chemistry.
But there are issues with optical tweezers. To produce the necessary forces, they require a strongly focused laser beam with high optical intensity. During prolonged interactions, this can cause both photodamage and a build-up of heat that can alter or damage particles and biological samples.
Now researchers at The University of Texas at Austin and Shaanxi Normal University have developed a new technique, which they have dubbed opto-refrigerative tweezers, to overcome these issues. This method relies on thermophoresis (movement of a particle in a temperature gradient) and the use of an optical material that cools when a laser beam is shone on it, a phenomenon known as optical refrigeration.
The idea is to create a cold spot on the material so that the particle migrates towards it. “Because most particles are naturally thermophobic, they tend to move from the hot region to the cold region,” Jingang Li, a physicist at The University of Texas at Austin explains. “So they can be driven to this cold spot created by the laser and then trapped at the cold region.”
As well as stopping overheating and thermal damage, this tweezer technique also uses a weakly focused laser beam, reducing photodamage, because it is not dependent on optical force. “We don’t need a highly focused laser beam, we just need to create a temperature gradient via this optical cooling,” Li tells Physics World.
Cold attraction
To realise their opto-refrigerative tweezers, Li and his colleagues created a substrate from ytterbium-doped yttrium lithium fluoride (Yb:YLF) crystals. When a laser with a wavelength of 1020 nm is shone on these nanocrystals it has an unusual effect, known as anti-Stokes fluorescence, which causes the material to cool. Li explains that materials usually absorb photon energy and convert it to heat, but the electronic properties of Yb:YLF cause it to emit a photon with a higher energy than it absorbed. “By this unique interaction the material actually loses energy, it loses heat and cools,” Li says.
When the researchers shone a 1020-nm laser on their substrate they observed an instant drop in temperature of around 7.5 °C at the laser beam centre. As they increased the intensity of the laser, the temperature dropped further and the temperature gradient across the material increased.
The team used their opto-refrigerative tweezers to attract, trap and release a 200-nm fluorescent polystyrene nanoparticle. The nanoparticle was placed in heavy water above the crystal substrate, chosen because it has low light absorption at 1020 nm. They found that the temperature gradient provided quite a long working range compared with optical tweezers, allowing them to trap nanoparticles that were more than 10 µm from the laser beam. When they increased the intensity of the laser, the nanoparticle became more confined to the laser beam centre. They also trapped a fluorescent protein, achieving similar results.
To test the advantages of a weaker laser beam and colder temperatures, the researchers compared the same 1020-nm laser used as an optical tweezer and an opto-refrigerative tweezer on the fluorescent polystyrene nanoparticle. They found that the conventional optical tweezers caused a marked drop in the fluorescence intensity of the nanoparticle, attributed to photobleaching and thermal bleaching, while the particle trapped by the opto-refrigerative tweezers only showed a slight decrease.
The study, published in Science Advances, demonstrates that opto-refrigerative tweezers are possible. Li and his colleagues now plan to optimize the system. This will include improving the substrate to generate a more uniform cooling spot and enhancing the trapping ability to enable further reductions in the power of the laser.
