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A focus on cutting-edge medical physics research

**Reducing treatment uncertainty** Katia Parodi and her team are developing new techniques to improve the accuracy of particle therapy. (Courtesy: Christoph Olesinski, LMU Munich)

How would you describe Physics in Medicine & Biology?

PMB is one of the most established journals in the field of medical physics and biomedical engineering. Since its foundation in 1956, the journal’s focus has always been on the development and application of theoretical, computational and experimental physics to medicine, physiology and biology, with a major emphasis on biomedical imaging and therapeutic interventions.

What does the journal offer the medical physics community?

It provides an excellent forum to communicate cutting-edge research in the field. In the area of radiation therapy, for example, several of the most influential papers on the development of intensity-modulated radiotherapy were published in PMB. Recently, the journal awarded its citations prize, the Rotblat medal, to a study describing the first patient treatment using a high-field 1.5 T MR-Linac. In this case, PMB published a lot of the initial fundamental and experimental work, right up to this first application in a human. This is a good example of the broad scope of the journal.

What are your plans for the journal?

I am honoured and thrilled to have this opportunity to intensify my commitment to the journal, having served on its editorial board since 2013. The task will be to constantly identify and evaluate new trends in the field, to keep the journal scope up to date, and to offer the best services to our authors and readers. For example, we recently introduced a new format, called roadmap papers, which are invited visionary contributions showing where relevant fields are going. One of the first of these roadmap papers was dedicated to the 10 ps time-of-flight PET challenge – the quest to develop ultrafast time-of-flight PET.

What are the other hot topics emerging in medical physics?

A lot of new trends exploit physical phenomena for biomedical applications, including developments in acoustics, optics and Cherenkov imaging. We are seeing exciting work in nanotechnologies, used for contrast agents or as radiation sensitizers, as well many activities in detector development, such as improvements in technology for ultrafast PET and photon-counting detectors for X-ray imaging. There are also efforts to combine different imaging modalities using hybrid detector technologies, as well as integration of imaging with therapy. In radiotherapy, we see new frontiers under exploration such as FLASH irradiation and micro/minibeam technologies.

In all of these areas there is a new trend of applying artificial intelligence (AI). PMB will not aim to host the development of new AI algorithms, but it will be the journal’s goal to see how new applications, and optimization of existing AI algorithms, could impact what we are doing in the fields of biomedical imaging and therapeutic interventions.

Your own research includes range verification of particle therapy, what are you working on?

Particle therapy is still an emerging radiotherapy technique. Its main advantage is that you can better target energy deposition in the tumour, with less side-effects for healthy tissue and critical organs. But particle therapy also comes with disadvantages: it is highly sensitive to range uncertainties, or in other words, knowing exactly where the beam will stop in the patient. There are a lot of physics processes that one can try to exploit to visualize the stopping position of the beam inside the patient. These include nuclear reaction processes, which can be visualized with PET, prompt gamma imaging and thermoacoustic emissions.

These are all techniques that require a lot of research to make the instruments compatible with a clinical environment and sensitive enough to capture typically very low levels of signal. We are also developing techniques to model the signals and reconstruct images, and also to combine these advanced methods and integrate them into a possible future workflow.

Another interesting research area is to improve in-room imaging of the patient prior to treatment. One of the uncertainties in knowing where the beam stops is due to our limited knowledge of the properties of the tissue interacting with the beam. If you use standard X-ray CT imaging there are relatively large uncertainties. But if other techniques are employed, such as the emerging dual-energy or spectral CT, or even the ion beam itself, to create radiographic or tomographic (so called proton or ion CT) images, then you can reduce this uncertainty and make a far more accurate treatment plan.

Are these in clinical use yet?

PET for range monitoring has been explored clinically for many years by a few institutions, but without dedicated commercial devices, so mostly using research instrumentation. Prompt gamma imaging is now reaching the stage of clinical evaluation, but just using first prototypes, which may not yet fully exploit its potential. Methods such as thermoacoustic imaging are still at the development stage. As for proton imaging, a first prototype is close to reaching clinical application.

What else is your research group investigating?

We have a project funded by the European Research Council to develop a small-animal proton irradiation platform with integrated image guidance. I think this is really exciting because it has been shown that if you want to translate new therapeutic approaches to the clinic, it is good to first perform investigations on animals. However, it is even more challenging to precisely irradiate such small targets.

We are aiming to develop a portable platform that can be integrated in a particle therapy facility’s research room, using a dedicated beamline that will take the clinical beam and focus it to irradiate very small targets. The platform will also include proton imaging for use before treatment, and will integrate PET and thermoacoustic imaging to verify the proton beam range.

There is currently a lot of research into new effects of biology that could be exploited therapeutically. It would be great to have this type of small-animal irradiation platform to test new possible effects in vivo in small-animal tumour models. This could help us figure out better therapeutic approaches that could hopefully be translated to the clinic.

CityU physics: investing in ‘rising stars’ to deliver sustained research excellence

Cool heads and clear thinking are mandatory in the competitive world of front-line research as academic leaders strive for that winning – and sustainable – combination of visibility, recognition and impact that will set their physics programmes apart from the rest of the field. The calculus is simple enough: a virtuous circle in which targeted research funding attracts the brightest and best scientific talent, while the brightest and best, in their turn, go on to secure more (and bigger) research grants. Delivering that sort of win-win doesn’t come easy, though, and requires clarity of vision, clarity of planning and clarity of execution – and especially so for a fast-growing research programme like the City University of Hong Kong Department of Physics.

Although it only came into being in 2017, after the university chose to create distinct disciplinary specialisms from the former combined physics and materials science programme, the CityU Department of Physics demonstrates a unity of purpose and collective endeavour that suggest its ambitious goal – to create one of the leading centres of research excellence for physics in the Asia-Pacific region – is a realistic proposition over the medium term.

The department has already taken small, yet significant, steps towards that objective. In the latest Research Assessment Exercise, commissioned by the University Grants Council of Hong Kong, the CityU physics programme fared well, with an independent international panel rating 38% of its research output as four-star (i.e. “world-leading”). Even so, the growth trajectory is a steep one, with a target of around 30 physics faculty members on board by 2027 (versus a current staff cohort of 23). It’s also the intention that postgraduate numbers, currently at 79, will scale significantly over the same timeframe to around 150 PhD students (and the number of postdoctoral researchers and research assistants, currently 30, is forecast to triple by 2027).

While fast-track expansion brings its own unique hiring challenges – amplified by the ongoing uncertainties associated with Coronavirus pandemic – CityU’s Global Scholar Recruitment Campaign is geared to maximize the chances of success. Underpinning the CityU offer is generous funding – at national, university and department level – to enable incoming faculty to quickly establish new research groups, laboratories and regional collaborations. Within this wider institutional recruitment drive, CityU Department of Physics is currently seeking exceptional scholars at all levels – assistant professors, associate professors, professors and chair professors – and from all regions – whether that’s local institutions in Hong Kong, further afield in mainland China, or leading physics laboratories in Europe and the US.

International perspective

Among the recent arrivals at CityU is Danfeng “Denver” Li, who joined the faculty at the end of last year as an assistant professor of physics pursuing various research themes at the interface between condensed-matter physics and materials science. Specific areas of interest include atomic-scale fabrication of oxide heterostructures and nanomembranes, kinetic-based synthesis of unconventional quantum materials, low-dimensional superconductivity and oxide interfaces for emergent states.

Before taking up his CityU post, Denver Li spent much of the previous decade broadening his research experience in Europe and the US. In 2016, he completed his PhD at the University of Geneva, Switzerland, followed by almost four years as a postdoctoral researcher at Stanford University in California. It was at Stanford, in 2019, where Denver Li and his postdoctoral supervisor, Harold Hwang, led the experimental team that discovered the first nickelate superconductor – a breakthrough that initiated new lines of enquiry on a class of unconventional superconductors that had been a target of materials scientists for more than three decades.

Right now, as he establishes his own research team and laboratory at CityU, Denver Li reckons those formative years spent working across diverse scientific cultures – he’d previously completed his undergraduate training at Zhejiang University in mainland China and a Master’s degree at The Hong Kong Polytechnic University – have equipped him with a unique perspective that informs and shapes his approach to team leadership, student mentoring and research collaboration.

“What I noticed in Europe is that, for the most part, scientific research is still curiosity-driven,” he explains. “For young scientists, in particular, the emphasis is on thinking, discussing and solving interesting problems.” Yet Denver Li also thrived in the high-pressure US research environment, where he relished the competition to deliver new breakthroughs and to communicate and publish results at pace. “In modern science,” he adds, “I guess the optimum approach is to find a balance of these two models – delivering research impact while ensuring that scientists are still able to explore the questions that interest them. That’s certainly one of our big selling points here at CityU.”

At an individual level, adaptability is one of the key attributes that Denver Li is looking for in would-be recruits to his research team – and doubly so given the restrictions that all scientists are facing as a result of the pandemic. The shift from real-world conferences to virtual meetings is a case in point. “Building relationships in a Covid world is hard,” he explains, “so it’s more important than ever for young scientists to be proactive regarding the communication and promotion of their results.” These days, for example, Denver Li takes every opportunity that he can to speak at online seminars and workshops, allowing his peers and potential collaborators to “put a face” to the research he’s doing. “It’s not the same as meeting colleagues over dinner at a real conference,” he concludes, “but it’s the best we’ve got right now.”

The collective conversation

Adaptability is also a priority for Xiao Li, another early-career scientist with a decade of US research experience on his CV when he joined the CityU physics faculty as an assistant professor in January 2019. Xiao Li is mainly interested in novel states of matter that arise due to the interplay between topology, disorder and electron–electron interactions. His research activities span various aspects of non-equilibrium systems (such as many-body localization and quantum information scrambling) as well as the application of machine-learning techniques in condensed-matter physics.

Photo of Xiao Li — Xiao Li: broadening his experience through cross-disciplinary conversations. (Courtesy: CityU Hong Kong)

Prior to CityU, Xiao Li’s career included a four-year contract as a postdoctoral research associate in the Condensed Matter Theory Center at University of Maryland (UMD), College Park, a posting that followed completion of his PhD at the University of Texas at Austin in 2014 (and prior to that an undergraduate degree at Peking University). During his time in the US, Xiao Li was struck by the opportunities available to young scientists to deepen their understanding of how science works – and, in his case, to learn from leading physicists working across a range of subdisciplines.

“They have big and diverse condensed matter groups in Austin and College Park,” he notes, “so there are always a lot of seminars and workshops in the schedule. Just listening to other scientists explain their research means you can gain insights about their methods or pick up ideas on how to present complex scientific ideas with clarity and flair.”

Another defining experience, says Xiao Li, is the collaborative and open working environment that he encountered as a UMD postdoc – a model that he and his faculty colleagues are now seeking to replicate at CityU. “One aspect I treasure from my time at College Park is that all of the postdocs in the Condensed Matter Theory Center make an effort to have lunch together at least once a week,” he explains. “It’s so valuable to talk informally with colleagues in this way – being able to discuss a research problem you’re stuck with, for example, and get fresh insights from graduate students working on often-unrelated research projects.”

For now, Xiao Li continues to build up his team and is particularly keen to engage prospective PhD students and postdocs who – like him – are eager to broaden their international experience and networks. “We are a growing physics department,” he adds, “and we have a well-articulated talent strategy that sees us bringing together a cohort of ambitious young scientists under the guidance of experienced, internationally established research leaders. It’s a synergy that works well.”

Big science, big opportunities

In January of this year, that emerging synergy saw Junzhang Ma, a rising star in experimental condensed-matter physics, join the CityU physics faculty as an assistant professor. Ma’s appointment followed a three-year joint postdoctoral fellowship at the Swiss Light Source (SLS), Paul Scherrer Institut and École Polytechnique Fédérale de Lausanne in Switzerland, prior to which he completed his PhD at the Institute of Physics, Chinese Academy of Sciences (IOP CAS) in Beijing.

Photo of Junzhang Ma — Junzhang Ma: bringing big-science insights from Europe to CityU. (Courtesy: CityU Hong Kong)

“Most of my scientific work in Switzerland was carried out at the SLS, a shared research facility with a very different working model to the typical university department,” Ma explains. During his time at SLS, Ma also learnt a lot about the enabling technologies and infrastructure of a big-science facility, supporting users from around the world with routine maintenance and problem-solving on the synchrotron beam lines and experimental stations. “I became a lot more independent as a researcher,” he adds. “Meanwhile, collaborating with an international cohort of scientists was great preparation for recruiting and leading my own research group here at CityU.”

Ma chose CityU as his next career step based on the offer of generous start-up funding, dedicated laboratory space and the freedom to set his own research agenda. That agenda spans experimental studies of the electronic structure of topological materials, superconductors, low-dimensional materials and correlated materials using synchrotron radiation (and a specific technique called angle-resolved photoemission spectroscopy). Still only in his early 30s, Ma has already contributed to several high-profile research breakthroughs, including the discoveries of the Weyl semimetal, three-component Fermion, hourglass Fermion and the fluctuated magnetic Weyl Fermion.

CityU physics: where regional connections underpin global research ambitions

Photo of CityU Hong Kong and Oak Ridge National Laboratory physicists

Another factor in Ma’s choice of CityU is the university’s close engagement with the Greater Bay Area initiative. This ambitious regional investment programme aims to transform the Pearl River Delta (encompassing Hong Kong, Macau, Shenzhen, Dongguan and nearby cities in Guangdong) into a science and technology powerhouse that, its backers hope, will ultimately rival Silicon Valley in California. Among a raft of big-science facilities under construction in the region, Ma is particularly excited about a next-generation synchrotron light source that’s scheduled to come online within the next decade.

“For now,” Ma concludes, “the priority is to build up my research group and cooperation here at CityU – likely recruiting two PhD students a year for the foreseeable. Longer term, I aim to establish collaborations with the advanced synchrotron source and other physics laboratories in the Greater Bay Area, as well as further afield in mainland China and around the world.”

CityU Global Scholar Recruitment Campaign

CityU Department of Physics is currently seeking applications from outstanding scholars for several open faculty positions. Core areas of interest include (but are not limited to): theoretical and computational physics; spectroscopy and imaging; low-dimensional systems and quantum materials; soft matter and biophysics; and atomic, molecular and optical physics.

The department also invites applications and expressions of interest for its PhD programme. Candidates able to demonstrate outstanding performance versus research capability, communication and interpersonal skills, and leadership potential are encouraged to apply through the Hong Kong PhD Fellowship Scheme.

High-resolution microscope is made from LEGO bricks

A fully functional modular microscope has been built using LEGO bricks and low-cost smartphone lenses. Designed by researchers, teachers and schoolchildren in Germany, the instrument is easy to build, yet it can resolve micrometre-sized objects such as individual living cells.

The idea for a LEGO microscope came to biophysicist Timo Betz (from the Universities of Göttingen and Münster) and his 10 year-old son one weekend while playing with LEGO – modular plastic building blocks that are beloved by children (and many adults) worldwide.

“My first reaction was that this is too hard, because of the precise movements and all the parts that are non-LEGO,” recalls Betz. “But my son came up with a series of great ideas on how to overcome the difficulties that I explained to him; he even figured out a way to use a LEGO light source to illuminate the samples.”

Junior author

By the end of the weekend, the father-son duo had built a prototype. However, even with assistance from Betz’s colleague Bart Vos, it took over a year to fine-tune the design, write construction plans and validate the instrument’s usefulness. Now, the trio has described the microscope in a paper published in The Biophysicist (with Betz’s son co-author, naturally).

The final design resembles a simple LEGO tower, but it hides some clever features (see figure). For example, to provide precise focus adjustment, the team had to “push the limits of the brick system” when designing the objective holder. This incorporates a gear rack with a gear worm screw that achieves approximately 3 mm of travel for every full rotation.

The only non-LEGO parts are the microscope’s lenses. For high magnification, these can be cannibalized from a low-cost replacement iPhone 5 camera module and then attached to a LEGO brick with transparent tape and a glass coverslip. Using these plastic iPhone lenses, the microscope can achieve 254× magnification.

Step-by-step building instructions

Most important to the creators, however, is that the microscope can be built and used in classrooms and homes around the world to learn about optics. To further this aim, they created build instructions and a step-by-step tutorial to guide people through construction while learning about the relevant optical characteristics of a microscope. Moreover, they enlisted eight 9–13-year-olds from a local Münster school who were tasked with building the microscope in an attempt to show how such a hands-on activity assists learning.

Given COVID-19 restrictions, the researchers could not see the children engage with the microscope activity in class. So instead, they created a kit that children could take home alongside a questionnaire to be filled out before and after playing with the kit. Answers revealed that the children’s knowledge of microscopy almost doubled after making the microscope.

Fun and informative activities

These children also conducted several experiments with their newly built microscope. These were suggested by the researchers as interesting, fun and informative activities. The experiments included watching crystallization in real-time as water evaporates from a thin film of salt solution; recording pigmentation changes to red onion epidermal cells exposed to an osmotic shock; and observing the movement of tiny swimming organism such as Artemia shrimp and water fleas.

This is not the first time a microscope has been made of LEGO. In 2013, a group of postgraduate students at the University of California, San Francisco designed a similar microscope called LegoScope. However, this instrument required a custom objective and 3D-printed parts.

Betz argues that the new LEGO microscope’s readily available and reusable parts “lower the barrier, especially for parents”, making it perfect for simply demonstrating the principles of microscopy.

“It was actually a lot of fun to develop this,” he says. “I just hope that children and their parents have a chance to realize here that even with simple tools, one can do amazing things.”

If you would like to build your own LEGO microscope, full instructions are available in English, German, Dutch and Spanish.

Getting children interested in science

My seven-year-old son Aneurin is often given books as gifts, whether he wants them or not, and he’s amassed quite the mini library. He has high standards. As a critic, he’s fierce enough to throw a book in a corner and never look at it again.

So, I’ve allowed him complete licence to give his own verdict on Nano, a new children’s book on the science of tiny materials. The book is written by Jess Wade, Imperial College physicist and champion for representation and inclusivity in science, and illustrated by Melissa Castrillon, known for designing the covers for Philip Pullman’s His Dark Materials trilogy.

The following are Aneurin’s words written entirely independently, lightly edited only for punctuation and to correct the occasional spelling:

“The book is about very, very, very small science stuff. The book lets you learn a new science word called ‘nano’. There are materials like graphene and graphite. You also get to learn about microscopic things.

There are over 100 types of atoms, and everything is made from atoms, even air and water

“The best page was the page about tiny atoms, and there are over 100 types of atoms, and everything is made from atoms, even air and water. Each type of atom is called an element and there are 11 elements in the human body. A few of them are calcium, carbon and sodium. In every living thing there is carbon, from daisies to oak trees, from beetles to blue whales.

“I loved this book. The pictures are good, nice, clear and artistic. The words are the same. I liked the whole book. Five stars. I learned new words like ‘nano’ or ‘graphite’ or ‘sodium’ and why pencils draw on paper.”

Sample pages from Nano showing a graphene water filter — **Science for all** A young girl narrator subtly makes the book inclusive. (Courtesy: Melissa Castrillon)

There you have it. Kids are a tricky audience, unashamedly judgemental, which makes the work of a children’s author perhaps the toughest of all genres. But this book clearly hasn’t been rushed. Appropriately for the theme, even the smallest details feel wisely judged.

Something I personally value as a parent is that, unlike some children’s books released in recent years, Nano isn’t marketed to a particular gender. Although the narrator appears to be a young girl with blue hair, this feels almost incidental. All you see is pure passion for the subject she describes, and this is something any child can relate to. It’s vital to have characters that girls and minorities can see and feel more included in the story of science, and as Nano shows, this can be done implicitly and be just as powerful.

The real focus here is the science itself. Some of the content is quite advanced for a children’s book, but conveyed so skilfully that it remains accessible. Younger readers of five or six will stay for the pictures, but older readers, even aged 10 or 11, will find themselves learning facts that will be new to them. Indeed, Nano is so packed with information that it includes an index at the end.

But it’s not my verdict that matters, of course. The book wasn’t written for readers my age. All you really need to know is that it passed the Aneurin test.

2021 Walker Books 32pp £12.99hb

Trapped-ion clock passes orbital test

A compact atomic clock that has orbited the Earth since 2019 is far more stable than previous space-based clocks, raising hopes that future spacecraft will be able to keep track of time autonomously. Although technical problems have limited the new trapped-ion device’s performance, the US-based scientists who developed it report that it is reliable enough to substantially reduce the need for back-and-forth communication with controllers on the ground, thereby improving navigation.

Stability is a fundamental attribute of good timekeeping. If a clock is unstable on short timescales, the delay from one tick to the next fluctuates and its output must be averaged to achieve the desired precision. If the clock’s output drifts on longer timescales, it needs to be periodically corrected by a more stable device.

This is the situation for the atomic clocks on board today’s global positioning satellites. These clocks allow each satellite to keep track of both time and position, which in turn enables users on the ground to navigate through triangulation. However, because they experience drift, their recorded time must be very slightly tweaked each day to bring it into line with stabler (but also bulkier and more complex) atomic clocks on the ground.

In the latest research, John Prestage, Robert Tjoelker, Eric Burt and colleagues at the Jet Propulsion Laboratory (JPL) in the California Institute of Technology built a clock that is small and rugged enough to withstand the harsh conditions of space yet also stable enough to operate independently. Such a device, they say, would not only make terrestrial satellite systems more autonomous, it should also allow deep-space probes to navigate in close to real-time.

Clock workings

At the core of the clock is a vacuum tube containing two cylindrical traps, each measuring a little over a centimetre in diameter. The first trap is used to load and prepare a small cloud of several million mercury-199 ions, while the second measures the ions’ “ticking”. Both traps generate electric fields to hold the ions in place and prevent them from colliding with the trap walls, which would cause the clock’s signal to drift.

Mercury-199 ions have a “clock” transition at 40.5 GHz that is insensitive to magnetic fields – a useful feature given the high and fluctuating magnetic fields in space. This transition is used to lock the output of a quartz crystal. A frequency synthesizer referenced to this crystal generates microwaves and is tuned to around 40.5 GHz, while a plasma discharge lamp produces ultraviolet light that further raises the energy level of the ions if they have already been excited by the microwaves. The fluorescence these ions generate when they promptly re-release the ultraviolet energy therefore indicates that the oscillator is at the correct frequency.

Time to fly

Researchers at JPL have been working on trapped-ion clock technology since the 1980s and have obtained excellent results in their laboratory, achieving a short-term stability of 2×10⁻¹⁴/τ^1/2 (τ being the averaging time) and drifts as low as 2.7×10⁻¹⁷ per day. What’s more, they have done so without using lasers, cryogenics or microwave cavities, all of which would introduce unwelcome complications for space-based clocks.

With these results in the bag, NASA approved the Deep Space Atomic Clock mission in 2011. Under the leadership of principal investigator Todd Ely, the mission team produced a microwave-oven sized clock that was launched into a 720-km altitude orbit in June 2019 alongside other experiments.

Although the clock has now kept time for almost two years, the mission hasn’t been trouble-free. A problem with levels of neon gas used to cool the ions means the scientists have had to measure the clock frequency with the loading trap, which was not designed to make such measurements. This fault affected the clock’s performance, but Prestage and colleagues nevertheless recorded a short-term stability of 7×10⁻¹³/τ^1/2, a long-term stability of 3×10⁻¹⁵ and an estimated drift of 3×10⁻¹⁶ per day, all without controlling the temperature of the apparatus. The drift result, they say, is better than that of existing satellite-based atomic clocks by around an order of magnitude.

Deep space or bust

The researchers argue that this level of performance is already good enough for autonomous timekeeping. That, in turn, means that future spacecraft equipped with such a clock would need only a single communication link with ground controllers – to receive their timing and positioning data – rather than the three links currently required. The researchers add that the clock might also aid Solar System exploration – making it possible, for example, to plot the gravitational field of Jupiter’s moon Europa to see if there might be an ocean under its icy crust.

Looking ahead, the researchers say they are trying to extend the lifetime of their clock technology from up to five years to at least a decade. Among other things, this will mean working out how to improve the ultraviolet light source and optimize the pressure inside the vacuum chamber so as to lose less neon gas.

However, not everyone is convinced. Liang Liu of the Shanghai Institute of Optics and Fine Mechanics in China, who heads a cold-atom clock experiment that has been in orbit since 2016, welcomes the NASA project but finds its long-term stability of around 3×10⁻¹⁵ per day underwhelming. “For deep space navigation, we might need 10^-16 or better,” he says.

The research is reported in Nature.

Photo captures the subtleties of the magnetic Sun, synchrotron will image millions of insects

The Magnetic Field of our Active Sun © Andrew McCarthy — Subtle beauty: *The Magnetic Field of our Active Sun* by Andrew McCarthy. (Courtesy: Andrew McCarthy)

The UK’s Royal Observatory Greenwich in London has announced the shortlist for its Astronomy Photographer of the Year 13 competition. I’m not sure what the 13 stands for, but it certainly didn’t bring bad luck to the American photographer Andrew McCarthy who has two images on the shortlist. His photograph of the Sun – pictured above – was selected from over 4500 entries this year. Called The Magnetic Field of our Active Sun, the photo was captured in black and white. McCarthy then used false colour to highlight features on the surface of the Sun in red tones – reminiscent of the red hydrogen-alpha emissions that are used to study features on the Sun.

I love the texture of the Sun, which looks a bit like a peeled grapefruit and the subtlety of the structures on the upper right limb of the Sun, which are bits of the chromosphere being tugged up by magnetic field lines.

The winners of competition’s nine categories and two special prizes will be announced on 16 September 2021 – along with the overall winner, who will receive £10,000. The winning photographs will be exhibited at the National Maritime Museum in Greenwich alongside a selection of shortlisted images.

Insect collection

Staying on the theme of stunning images at London museums, scientists at the UK’s Diamond Light Source synchrotron facility have joined forces with the Natural History Museum to create digital 3D images of some of the 35 million insect specimens in the museum’s collection.

Diamond insect image — Larger than life: a 3D visualization of raider ant done by Diamond for the Natural History Museum. (Courtesy: Diamond Light Source Ltd)

Researchers from the two institutions have developed a new way of scanning insect samples rapidly, without any manual input. The technique involves doing a computerized tomography (CT) scan of the insect using coherent X-rays from Diamond. This allows the technique to see details in soft tissue that cannot be resolved when a conventional non-coherent X-ray source is used.

The project is part of an ongoing collaboration between Diamond and the Natural History Museum, which will open a research lab in 2026 on the Harwell Science and Innovation Campus, which is home to Diamond.

Static apertures keep dose on target in pencil-beam scanning proton therapy

Optic glioma plans — Dose distributions for (left) an uncollimated plan and (centre) a plan with apertures for proton therapy of an optic glioma; the right panel shows the dose differences. The use of apertures reduces dose to the eye lenses (blue and magenta contours). (Courtesy: CC BY 4.0/*Front. Oncol.* 10.3389/fonc.2021.599018

Proton therapy can deliver superior dose distributions compared with photon-based radiotherapy, enabling radiation oncologists to precisely target a tumour while reducing the risk of damage to adjacent healthy tissue. Pencil-beam scanning (PBS), the most advanced type of proton therapy, increases this precision by shaping the delivered radiation to a tumour’s exact volume. A team at the West German Proton Therapy Centre Essen (WPE) has now shown that the addition of static apertures to PBS treatments further reduces the dose to nearby normal tissue while maintaining the target dose.

The pencil beams used in PBS proton therapy typically have a spot size of 1 cm (full-width at half maximum) or more, which can impede the sparing of organs-at-risk adjacent to the target volume. The dose fall-off in the lateral direction also limits the options for covering the target while keeping dose to normal tissue at tolerable levels. The WPE team demonstrated that using static apertures to trim these spot-scanning fields reduced the dose to organs-at-risk such as the eye lens and brainstem, and in six brain tumour cases reduced the dose to the brain and hippocampi.

Static apertures enable small air gaps in clinical treatment plans, which is beneficial for the lateral dose gradient. At some proton therapy centres, static apertures are a standard piece of equipment. At WPE, the research team fabricated brass apertures for each individual treatment field using a computerized milling machine. The actual number of apertures used was optimized per treatment field to achieve efficient production and treatment workflow.

Writing in Frontiers in Oncology, Christian Bäumer and colleagues report that “uncollimated” treatment plans for 31 patients with various brain cancers were prepared for conventional PBS delivery. The researchers then inserted apertures for all fields, specifying a lateral margin for the planning target volume (PTV) coverage and blocking adjacent organs-at-risk. They adapted margins for each individual field, and chose the number of fields and their arrangement based on the location of the target.

Quality assurance procedures for treatment plans needed to be modified from the centre’s established procedures to align the X-ray imaging, PBS and aperture systems. The researchers caution that the positioning of the aperture relative to the PBS field is impacted by movement of the snout (the part of the proton delivery nozzle closest to the patient) and the aperture mounting mechanism. The team also performed a robustness evaluation based on perturbed dose scenarios to account for possible deviations of the centre of the pencil beam and the mechanical centre of the aperture holder.

The researchers assessed the dosimetric benefits of the 31 treatment plans delivered using apertures compared with the corresponding plans without apertures. “The volume of the dose gradient surrounding the PTV (evaluated between 80% and 20% dose levels) was decreased on average by 17.6%,” they write. “For the full cohort of 31 cases, the mean brain dose could be reduced on average by 1.2 Gy(RBE) through apertures.”

The most noticeable improvement, they note, was a reduction in dose to the hippocampi of between 1.6 and 4.7 Gy(RBE), with a mean decrease of 2.9 Gy(RBE).

Brain cancer plans — Dose distributions for (left) an uncollimated plan and (centre) a plan with apertures for treatment of a suprasellar craniopharyngioma; the right panel shows the dose differences. Dose sparing of the left thalamus (light blue contour) is seen when apertures are used. (Courtesy: CC BY 4.0/*Front. Oncol.* 10.3389/fonc.2021.599018)

The study included six cases of the brain tumour craniopharyngioma, with targets located near important organs-at-risk including the brainstem, optic nerves, chiasma and hippocampi. The biggest benefit of using PBS with static apertures was achieved for the thalamus, brainstem and hippocampus, where the mean doses were reduced by 5.5, 5.6 and 3.1 Gy(RBE), respectively. For two cases, the mean dose to the eye lens was reduced to below 5 Gy(RBE), which could not have been achieved without using apertures.

In all 31 cases, the dose burden to normal tissue was reduced and the dose target maintained when apertures were used. In four cases, the dose to organs-at-risk was reduced to a level below the tolerance dose, which could only be achieved with the use of the static apertures.

The authors point out that their study was limited to a horizontal beam line. They expect further dosimetric benefits if a gantry is available for the proton treatment. “In the future, we expect an even better dose sparing of normal tissue through the use of dynamic adaptive collimators, which have not yet been clinically released,” says Bäumer.

LIGO mirrors have been cooled to near their quantum ground state

LIGO is designed to detect gravitational waves, but it is also proving to be a fantastic laboratory for pushing the limits of quantum physics. Now, an international team of researchers has cooled the interferometers’ large mirrors close to their quantum ground state. By cooling objects massive enough to potentially feel a detectable gravitational force, the researchers hope to open a new window into gravity’s possible effects on quantum mechanics. The work could also potentially lead to future enhancements in the sensitivity of LIGO to gravitational waves.

The two LIGO observatories (one in Louisiana and the other in Washington state) are famous for their pioneering detection of gravitational waves. In 2015, within days of being switched on, they spotted a signal from merging pair of black holes. Since then, LIGO has detected dozens of black-hole mergers and other events and has been joined in its search by similar instruments in Italy and Japan

Each LIGO detector is laser interferometer with arms 4 km long. Gravitational waves are ripples in space–time that change the relative path lengths in the arms of an interferometer, creating a tiny optical signal. The interferometers must have incredibly low noise to see these signals, which makes them great places to study quantum physics on a macroscopic scale.

Kilograms instead of nanograms

Elsewhere, several groups have published work describing the laser cooling of macroscopic oscillators to their motional ground states, but these have involved trapped objects on the nanogram or picogram scales. In this latest work at LIGO, researchers have looked at a composite object comprising four of an interferometer’s 40 kg mirrors, which together behave as an oscillator with a mass of 10 kg.

Laser cooling would not work for LIGO’s mirrors because the huge optical power required to trap such large objects would itself induce heating. Moreover, the mirrors’ low resonant frequencies would render laser cooling inefficient.

Instead, during what LIGO member Evan Hall of the Massachusetts Institute of Technology (MIT) in the US describes as “some extra time with the instrument when LIGO was on but not actively taking data”, researchers implemented a feedback cooling protocol. They measured the displacement of a mirror from sources such as the radiation pressure from the interferometer laser, before calculating the exact force needed to suppress these oscillations. This force was applied through the silica fibres to keep the mirror almost completely still.

Heisenberg compliant

As radiation pressure comprises the recoil from a stream of photons, applying active feedback to cancel this recoil might seem to involve violating Heisenberg’s uncertainty principle — which forbids knowing both the position and momentum of a particle simultaneously. “It’s not something physicists used to believe was possible 20 years ago,” explains Hall’s MIT colleague Vivishek Sudhir.

The key to experimental success was that the researchers could calculate the force from any given photon by measuring just its phase on impact: “You’ve got a complete record of all the disturbance caused on the mirror, and at that point nothing prevents you from using feedback to suppress the total motion of the mirror,” says Sudhir. This allowed the researchers to cool the mirror from room temperature to 77 nK – which is almost at its motional quantum ground state.

The team now wants to push towards even colder temperatures. “The low hanging fruit in our experiment is to find what we can say about extraneous sources of decoherence that might prevent extremely large objects from being in pure quantum states,” Sudhir says.

Some theoreticians such as Roger Penrose have proposed mechanisms whereby gravitational fields lead to the collapse of quantum states: “One way to test [collapse theories] would be to have a massive system prepared in a pure quantum state, expose it to a gravitational field and see whether it decoheres,” says Sudhir; “With our work, it becomes possible to think of an experiment where you have an object in a pure, or at least reasonably pure quantum state, which is still massive enough to respond appreciably to gravity.”

Beyond this, he says, it might be possible to enhance the sensitivity of LIGO to gravitational waves, for example by placing the mirrors in entangled states. “It is not out of the question, but not any time soon.”

“Very cool”

“I think it’s a very cool [experiment],” says Hendrik Ulbricht of the UK’s University of Southampton; “The LIGO consortium have really studied this system over such a long period of time and they have such a good understanding of this experimental system and how it works.” He is “struggling”, however, with the idea that the system will be sensitive to gravitational effects on quantum mechanics: “If you want to look at quantum evolution, it is a dynamical measurement,” he says; “You need a system that is evolving according to quantum mechanics, and not according to the feedback you’re doing.”

Markus Aspelmeyer of the University of Vienna, whose group is testing for deviations from expected quantum behaviour in increasingly large objects – is more receptive to the notion of quantum behaviour being visible with the feedback switched on. “You constantly measure the system,” he says “The information you get you compare with a model of the system; the model is actually a stochastic Schrödinger equation.” Any deviations from the behaviour predicted by the Schrödinger equation should, therefore, be detectable immediately. He concludes that “it would be so cool to finally lay to rest all these collapse theories”.

The research is described in Science.

James Peebles: a life in cosmology

How has life changed since you won the 2019 Nobel Prize for Physics?

When I got the call from Stockholm, they said that my life will change forever. I am rather unnerved that many people now consider me a god-like figure and that I somehow know everything. I receive many messages from people insisting that their situation could be improved if only I would give attention to their new idea. Maybe among all these theories there are some that are interesting, but who has the time to look?

Throughout your career, you have contributed enormously to several areas of cosmology including the big bang theory, but you dislike the term. Why is that?

The problem I have is that a “bang” connotes an event in space–time. It occurs at a place, at a time. Yet the universe has no special place. There are galaxies everywhere that we can observe. There is no place or time involved, no beginning of time anyway. Instead, this is a theory of what happened as the universe evolved from a dense, hot early state. We do not know with any assurance what the universe was doing before it was expanding.

My new book is an attempt to see the commonality around science, sociology and philosophy

So why then do you still use the term “big bang”?

The name is so embedded in our consciousness that I think there is no chance of changing it. No-one ever said science is logical. Oh, everyone says science is logical, but it’s not true. After all, it is done by people and people are notoriously illogical.

What do you think of current ideas about what the universe was doing before it started expanding?

The standard thinking is that cosmic inflation – a period of accelerated expansion – happened during the universe’s earliest stages. But I believe we need alternative theories about what the universe was doing before it was expanding. In my opinion, inflation is given more weight than it deserves because we have precious little evidence that something like inflation happened, so we should treat it with caution, and we should pay attention to alternative pictures. One is Paul Steinhardt’s ekpyrotic universe, which considers a cyclic universe: expansion, regeneration and expansion again, maybe repeating indefinitely.

If the BICEP2 team’s 2014 detection of gravitational waves had turned out to be correct, would that have convinced you of inflation?

Perhaps not totally convinced, but it would have made a very good case. I am not arguing against inflation, I am only saying we must be cautious because we don’t have a lot of evidence. And in physics evidence is the name of the game.

What do you say to people who believe that disagreement among scientists means that science is not to be trusted?

I think that scientists by and large tend to overestimate the power of their theories. On the other hand, it’s difficult to underestimate them. My favourite example is the mobile phone. It’s an object I hate but you must pause to consider the knowledge of physics that went into its design – it is just gorgeous. How could you distrust science if it could give you such a thing?

On the other hand, although we are right to celebrate the power of science, we should more commonly recognize that all our theories are incomplete.

Does that include Λ-CDM – the current cosmological model?

Yes, absolutely. Λ-CDM is incomplete. We should admit more clearly that although our theories are powerful, they are limited and they are incomplete. That, of course, is why we still have jobs.

Do you think our theories will ever be complete?

No, and that is not a popular opinion. My colleague Steven Weinberg, who wrote the book Dreams of a Final Theory, does not like it when I say that we will never know if we have a final theory because knowledge that a theory is good depends on empirical evidence. And when it comes to the economics, we cannot afford to fund arbitrarily complicated tests of arbitrarily complicated theories.

Last year you wrote Cosmology’s Century: an Inside History of Our Modern Understanding of the Universe. What makes it different from other books of its type?

It’s different for one reason: it is my personal experience. The book is not a popular exposition, which I regret, but I wanted to get the story straight, so I had to be technical. I consider cosmology a good example of how natural science is done because it is relatively simple compared, say, to quantum physics.

I have just finished another book, which is closer to popular science.

Can you tell us more about your new book?

In the late 20th century some sociologists took the position that physical science is a social construction, made up by authority figures who impose their will on students and require students to suspend disbelief and to accept their dogma. Physicists, naturally, were incensed at this proposal. The result is a very serious disconnect between sociology and physical science.

There is a real role for sociology of science, and there is a big role for philosophy in its connection to science, so my new book is an attempt to see the commonality around science, sociology and philosophy. The science, you will not be surprised to learn, is that of cosmology. So I attempted to interweave thinking by sociologists about the way we do science, thinking from philosophers as to what they consider to be science, and how science is really done. It all fascinated me.

You were part of a team in the 1960s, led by Bob Dicke, searching for evidence of a cosmic microwave background. What was the feeling on that day when it was announced by another group – Arno Penzias and Robert Wilson?

I have been asked many times whether I was chagrined at being scooped but the reaction in our group was one of excitement, not chagrin. Here was evidence of something new and for me something to analyse, so it was exciting!

Do you believe that Dicke should have shared the 1978 Nobel Prize for Physics with Penzias and Wilson?

I have never made a secret of it that I was annoyed at the decision to exclude Dicke. The strange thing about that year’s prize was it artificially joined two fields, given that Penzias and Wilson shared the award with Pyotr Kapitza for his work in low-temperature physics. The obvious choice was Penzias, Wilson and Dicke.

When I answered the phone call from Stockholm in 2019 concerning my own Nobel prize, it began very formally: “Are you Professor Phillip James Edwin Peebles?” I said, “Yes,” to which they responded: “We have voted to award you the Nobel Prize for Physics. Do you accept?” At that point I could have been tempted to say, “But first let us discuss the omission of Bob Dicke.” But I did not. I meekly said: “I accept,” and the conversation became a lot more friendly.

Novel sources of tunable laser light

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Widely tunable continuous-wave optical parametric oscillators (CW OPOs) are gaining recognition as novel sources of tunable laser light with great potential – not least due to their unprecedented wavelength coverage. Yet, the overall experimental requirements remain often challenging for the performance of turnkey OPO devices.

In this webinar we will discuss the characteristics of state-of- the-art tunable CW OPO designs and describe tuning schemes that have been tailored for applications like colour centre research and other quantum technologies, nanophotonics, holography, high-resolution spectroscopy, and experiments alike. Several recently published studies in these fields will be discussed in an illustrative fashion to showcase the performance of CW OPOs in the real-world laboratory.

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Left to right: Jaroslaw Sperling, Korbinian Hens, Ole Peters and Niklas Waasem

Jaroslaw Sperling is global productline sales manager for HÜBNER Photonics. He joined the company in 2016, bringing his passion and experience gained in several sales and marketing-related roles across photonics research and industry. With a background in laser spectroscopy, he holds a PhD in physical chemistry from the University of Vienna, Austria.

Korbinian Hens is a product manager for HÜBNER Photonics. He joined the company in 2015, specializing in terahertz spectroscopy and tunable laser systems based on OPO technology. With a background in laser-induced fluorescence spectroscopy for atmospheric research, he carried out his PhD thesis at the Max Planck Institute for Chemistry in Mainz, Germany.

Ole Peters is applications engineer at HÜBNER Photonics. He joined the company recently and has previous experience in terahertz imaging and spectroscopy systems.

Niklas Waasem is regional sales manager and applications specialist for HÜBNER Photonics. He joined the company in 2014, bringing in profound photonics knowledge gained during his PhD thesis at the Fraunhofer Institute for Physical Measurement Techniques in co-operation with the University of Freiburg, Germany.

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What does the journal offer the medical physics community?

What are your plans for the journal?

What are the other hot topics emerging in medical physics?

Your own research includes range verification of particle therapy, what are you working on?

Are these in clinical use yet?

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CityU Global Scholar Recruitment Campaign

Junior author

Step-by-step building instructions

Fun and informative activities

Clock workings

Time to fly

Deep space or bust

Insect collection

Kilograms instead of nanograms

Heisenberg compliant

“Very cool”

How has life changed since you won the 2019 Nobel Prize for Physics?

Throughout your career, you have contributed enormously to several areas of cosmology including the big bang theory, but you dislike the term. Why is that?

So why then do you still use the term “big bang”?

What do you think of current ideas about what the universe was doing before it started expanding?

If the BICEP2 team’s 2014 detection of gravitational waves had turned out to be correct, would that have convinced you of inflation?

What do you say to people who believe that disagreement among scientists means that science is not to be trusted?

Does that include Λ-CDM – the current cosmological model?

Do you think our theories will ever be complete?

Last year you wrote Cosmology’s Century: an Inside History of Our Modern Understanding of the Universe. What makes it different from other books of its type?

Can you tell us more about your new book?

You were part of a team in the 1960s, led by Bob Dicke, searching for evidence of a cosmic microwave background. What was the feeling on that day when it was announced by another group – Arno Penzias and Robert Wilson?

Do you believe that Dicke should have shared the 1978 Nobel Prize for Physics with Penzias and Wilson?