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Astronomers plan huge neutrino observatory in the Pacific Ocean

Astrophysicists in Germany and North America have published plans to build the world’s largest neutrino telescope on the sea floor off the coast of Canada. The Pacific Ocean Neutrino Experiment (P-ONE) is designed to snare very-high-energy neutrinos generated by extreme events from beyond our galaxy.

Neutrino telescopes observe the Cerenkov radiation that is emitted when neutrinos passing through the Earth interact very occasionally with atomic nuclei resulting in the production of fast-moving secondary particles. Being uncharged and exceptionally inert, neutrinos can penetrate gas and dust as they travel through the universe, allowing astronomers in principle to identify the exceptionally energetic phenomena that generate them. Photons from such events, in contrast, are absorbed on their journey.

We are now on the verge of opening up neutrino astronomy
Elisa Resconi

The world’s largest neutrino telescope, known as IceCube, consists of dozens of strings of photomultiplier tubes suspended in holes drilled deep into the ice at the South Pole. Covering a volume of 1 km³, IceCube made history in 2013 when it reported intercepting the first extragalactic neutrinos. Four years later it then recorded an event that could be tied to a very distant, bright galactic nucleus known as a blazar, thanks to concurrent gamma-ray observations.

According to P-ONE head, Elisa Resconi at the University of Munich, IceCube’s 2017 result strictly speaking only constitutes “evidence” for the blazar source. To really claim a discovery and pinpoint the origin of other cosmic neutrinos, she argues, requires the construction of additional neutrino observatories as well as the extension of IceCube. “We are now on the verge of opening up neutrino astronomy,” she says, “but if we base this process on just one telescope it could take a really long time, perhaps decades.”

Heading underwater

P-ONE will consist of seven groups of 10 detector strings creating an instrument volume of about 3 km³. Being larger than IceCube, it will detect rarer, higher-energy neutrinos, and will be most sensitive at a few tens rather than a handful of teraelectronvolts. It will also observe a different part of the sky, mainly capturing neutrinos from the southern hemisphere rather than the north. But there will be some overlap between the two, says Resconi, potentially allowing independent observations of the same event.

The new facility will be located at a depth of about 2.6 km in the Cascadia Basin, some 200 km from the coast of British Columbia. As such, it will take advantage of pre-existing infrastructure – an 800 km-long loop of fibre-optic cable operated by the University of Victoria’s Ocean Networks Canada that supplies power and ferries data to and from existing sea-floor instruments.

Having already confirmed that this site has the necessary optical properties by sending down two initial strings of light emitters and sensors in 2018, the P-ONE collaboration are now planning to deploy a steel cable with additional detectors to investigate the site – including spectrometers, lidars and a muon detector. The plan then, says Resconi, is to install the first part of the observatory – a ring containing seven 1 km-long strings – around the end of 2023. And if that succeeds, the researchers will then apply for the bulk of the $50–100m needed to complete the project, with personnel costs adding roughly $100m more.

Resconi hopes that the full observatory will be installed and taking data by the end of the decade. But she describes this timeline as “very ambitious”. In addition to delays caused by the ongoing COVID- 19 pandemic, she says it will be a challenge to ensure that the detectors work as planned – given the huge pressures and the presence of salt and sea creatures, which together make the seabed such a harsh environment.

Looking at the sky from under water

Visualization of Cherenkov light being detected by KM3NeT

Indeed, scientists had already planned on operating a cubic-kilometre scale neutrino telescope known as KM₃NeT on the floor of the Mediterranean Sea back in 2014, which was delayed to 2020. According to collaboration member Feifei Huang, just two of the 230 strings due to be installed off the coast of southern Italy are so far in place, while another site in French waters currently has six out of a planned 115 strings running – with completion not foreseen until 2026 and 2024 respectively.

Resconi says that part of the problem with that project is a lack of specialist personnel, with the physicists essentially doing everything themselves – for example, their self-built junction boxes, which connect cables on the sea floor, having failed. She hopes that the experience acquired by Ocean Networks Canada will mean a similar fate can be avoided for P-ONE. With 30 or 40 people dedicated to laying cables in the ocean, she says that her team “can focus on the physics”.

Floating oil droplet contains hundreds of degenerate optical modes

Microscopic oil droplets held aloft with optical tweezers can contain more than 200 resonant optical modes of similar energies, creating “hyperdegeneracy” for the first time. That is the claim of researchers in Israel, Spain and the US, who say that their breakthrough could ultimately find application in high-speed optical communications, sensing, quantum data processing and even the creation of dynamic optical circuits.

When optical materials with a high refractive index are formed into certain symmetrical shapes — such as rings, cylinders or spheres —light can be repeatedly reflected around the inside of the material, much in the same way that sound waves pass around the inside edge of St Paul’s Cathedral’s famous “whispering gallery”. The circulating light undergoes constructive interference, forming discrete resonant modes – or so-called degenerate states – with similar energies.

The number of modes is dependent on the ratio between the light’s wavelength and the circumference of the resonator — meaning that, in theory, a spherical object with a circumference tens of microns in size could support hundreds of modes of either visible or near-infrared light. In practice, however, achieving such hyperdegeneracy has proven impossible with conventional fabrication techniques. This is because even a single stem supporting the sphere will reduce the object’s symmetry and thereby reduce the extent of the potential degeneracy.

Clean and unscratched

In a new study, however, mechanical engineer Tal Carmon of the Technion-Israel Institute of Technology and his colleagues have circumvented this issue by supporting a 10 micron spherical droplet of silicone oil within an optical tweezer, thereby removing the need for a disruptive structural support. In fact, the radiation pressure from the laser-based tweezers acts to almost completely preserve the spherical symmetry of the oil droplet – along with the potential for hyperdegeneracy. In addition, the researchers explain, the levitation keeps the surface of the microresonator clean and unscratched.

Writing in the journal Physical Review X ,they say “unlike solids, the liquid droplet does not contain any dislocation, inclinations, and thermally induced stresses, which are typical for solid resonators and reduce their quality”.

To reveal the modes, the team placed a tapered fibre close to the surface of the oil microsphere and passed near-infrared laser light into and out of the droplet by means of an evanescent coupling. In the resulting transmission spectrum, the team observed signals of more than 200 modes – the largest-recorded set of degenerate states to ever be measured. The modes did exhibit slight differences in energy; this was a product of the droplet not being perfectly spherical, but distorted slightly as a result of the pressure from the optical tweezers, the presence of the coupled fibre, and the effect of gravity.

Simulating atomic optics

The technology could have several practical applications, write Peking University physicists Qi-Tao Cao and Yun-Feng Xiao in a commentary on the Physical Review X paper. “As a mesoscopic analogue to a single atom, levitated microresonators could serve as well-controlled platforms for simulating atomic optics,” they explain. Other potential applications could lie in using single-photon versions of the resonators as qubits for quantum processing, the creation of malleable circuits using multiple droplets, and high-capacity optical communications through the use of different modes to form densely packed information channels.

Cao and Xiao also point out that the microspheres could be used in existing sensing applications. “The frequency of hyperdegenerate modes is extremely sensitive to external perturbation, and even a tiny [such disturbance] – such as a biomolecule near the resonator surface — could lead to measurable modulation [of the light modes]”.

Dmitry Skryabin at the University of Bath adds, “The extremely high degeneracy and ability to manipulate it for fundamental studies of many coupled oscillators in linear, nonlinear and quantum regimes links these results to many cross-area ideas”. “Ultra-high finesses and near degeneracies in resonators also link to the cross-disciplinary concept of Arnold tongues and oscillator synchronization in the context of frequency comb research.”

Microswimmers benefit from thermoelectric guidance

Microscopic devices made from so-called Janus particles can be made to “swim” through liquid with the help of light-induced thermoelectric fields. The devices, which can travel 100μm along a straight course in 39 seconds, might find applications in biomedical sensing and the targeted, non-invasive delivery of drugs, according to developers at the University of Texas at Austin, US.

Janus particles – named for the famously two-faced Roman god of beginnings and transitions – are tiny spheres coated with different materials on each side. With the right choice of coatings, such particles will act as “microswimmers”, travelling in a specific direction when placed in a chemical solution and driven by light, magnetic, electric or ultrasonic fields.

Light-driven microswimmers are particularly promising for applications inside the body, as they can be controlled remotely with high spatial and temporal resolution. Their chief drawback is that their direction of travel becomes increasingly erratic over time thanks to rotational Brownian motion – the random motion of particles suspended in a medium.

Asymmetric photothermal response

In designing their microswimmers, Yuebing Zheng and colleagues found a way of overcoming this problem. The researchers made the microswimmers by covering a glass substrate with a single layer of pristine polystyrene beads using a technique called spin coating. They then used physical vapour deposition to cover one side of the beads with a gold film. The resulting Janus particles were freely dispersed in an aqueous solution containing a cation surfactant called CTAC. This surfactant makes the beads positively charged, while also introducing spherical fatty molecules, or micelles, of CTAC into the solution along with Cl^– ions.

While the gold sides of the Janus particles heat up when illuminated with laser light, the uncoated sides do not. The temperature gradient thus produced redistributes the CTAC micelles and Cl^– ions, causing an electric field to build up around the charged particles. According to Zhihan Chen, the study’s co-first author, this opto-thermoelectric force plays a key role in determining the particle’s behaviour.

Comparison with swimming microorganisms

When the researchers illuminated the particles with de-focused laser light, the particles swam in the direction of the optothermally-generated light fields. When they switched to a focused laser beam, however, the particles rotated in-plane. The combination of linear travel and rotations is similar to the “run-and-tumble” motion of swimming microorganisms such as E. Coli bacteria, and it can be maintained thanks to the balance between the opto-thermoelectric, optical and Stokes drag forces.

To keep their Janus particles moving in the right direction, Zheng and colleagues developed a feedback control algorithm to switch between the particles’ swimming and rotating states. By carefully observing the particles in real time, the researchers were able to adjust their control algorithm to make it automatically set the particles rotating whenever they deviate from the desired path. Once the particles realign, the algorithm re-activates their swimming state. Through repeated switching between states, the researchers showed that they could make the Janus particles travel in a straight line – behaviour that could be exploited for non-invasive drug delivery in the body, Chen says.

Improving navigation efficiency

The researchers, who report their work in Light: Science & Applications, now plan to improve the navigation efficiency of their microbots. “In our present study, we showed that 5-μm microswimmers can directionally transport over 110 μm in 39 seconds, but we would like to double this figure and deliver the particles over the same distance in just 18 seconds,” Chen says. “We could achieve this by further improving the response time of our imaging camera and laser shutters.”

The team also plan to further develop their control algorithm so it can steer multiple particles at the same time, while also adding non-collision and path optimization functions.

The half-gold, half-uncoated Janus particles studied in this work are a common type, but in the future, Zheng and colleagues hope to functionalize their polystyrene beads by loading macromolecules onto their uncoated surfaces. “This strategy would enable efficient and targeted cargo delivery totally driven by light,” Chen tells Physics World.

Build a bot: new book covers history and future of robotics

They grow up so fast. It seems like only yesterday that the cherubic little darling was gazing wide-eyed at the world, waving its arms incoherently as it figured out how to move. Now it reaches out with those pudgy little arms, purposefully picking up toys and stacking them on top of each other as it actively plays. However, this change didn’t happen over months, or weeks, or even since “only yesterday” – it’s been mere hours. The reason is that this is not a child, but iCub, a wide-eyed, one-metre tall robot built by researchers at the Italian Institute of Technology in Genova that is designed to resemble a small child, as well as learn like one.

The robotic youngster starts out only able to move its eyes, learning to focus on objects of interest. As time progresses, skills develop and motor restrictions are unlocked, to simulate muscle development, and iCub learns to point, play with toys and even use objects as crude tools to push buttons.

iCub was developed to explore the so-called “embodied cognition hypothesis”, the notion that the development of human-like cognition is dependent on learning to physically interact one’s environment – and that, by extension, the development of a truly human-like artificial intelligence is dependent on it having a physical body. The rationale for the hypothesis is at the heart of computer scientist Mark Lee of the University of Aberystwyth’s new book, How to Grow a Robot: Developing Human-Friendly, Social AI.

The first third of Lee’s work explores the history of and current developments in robotics and artificial intelligence (AI) – from pallet-carrying bots in warehouses to computer chess champions – and highlights the issues that arise from trying to derive generalized AI from the prevailing task-based approach to developing AI. The middle section moves on to how robots like iCub might be taught to grow and learn through developmental interactions with their surroundings.

The final section speculates on the future of robotics, considering where and how fast AI might develop and touching on topics including the risk of the singularity, a concern that Lee emphatically dismisses. These areas are rich enough that more could easily have been made out of them. It was also a pity to see trans-humanism defined reductively as being solely about “downloading the brain”. It strikes me that less extreme concepts, such as technological augmentation to expand the limits of the human body could have been explored in a book that examines how we are in many ways defined by the nature and extent of our embodiment.

warehouse robots — **Back to the future** Mark Lee’s new book spans pallet-carrying bots and the potential for transhumanism. (Courtesy: iStock/Ekkasit919)

Overall, How to Grow a Robot is a rich and comprehensive introduction to robotics and artificial intelligence, with a very clear message at its heart. However, it has one central flaw – in my opinion, it is sadly not a sufficiently engaging read.

Both of Lee’s previous books – on intelligent robotics and assembly systems, respectively – appear to have been intended for a specifically academic readership, rather than the popular audience that this book is marketed at. Perhaps Lee, like many scientists before him, found it a challenge to reshape his material for non-specialists. Yes, the explanations are largely there – and I appreciated the use of periodic jargon-busting fact boxes (although I still remain none-the-wiser as to Lee’s distinction between consciousness and self-awareness). However, the work has the feel of a “recommended class reading”, down to the end-chapter bullet lists repeating key “take-aways” for the reader. Personally, I prefer my casual non-fiction to not be presented as if there might be a pop quiz on the material later. More images of some of the commercial robots described in the opening chapter would also have been welcome for the general reader, and were conspicuous by their absence.

Personally, I prefer my casual non-fiction to not be presented as if there might be a pop quiz on the material later

Other structural aspects, meanwhile, serve to bring to mind a different academic format: that of a dissertation or thesis. On the positive side, the work has one well-explained central argument around which the whole book is constructed – a strength some of its popular-science peers could stand to learn from. Unfortunately, this is countered by repeated instances where the author sets up an interesting area of exploration before punting it to a reference text. The clichéd academic phrase “beyond the scope of the present work” is not used, but it might as well have been – variations on “we do not have the space” recur in its place, and create the looming impression of some word limit narrowly met and avenues curtailed. Taking time to direct the reader to extraneous material is admirable, but such links belong in footnotes, not as disruptions to the main text that leave the average reader feeling underserved – especially if they do not have access to the kind of academic library that would be needed to follow up on such references.

In short, How to Grow a Robot is a detailed and informative read – but one whose style and framing might better recommend it to a computer science syllabus rather than your coffee table.

2020 MIT Press 384pp £22.50hb

Giant Magellan Telescope receives cash injection from the National Science Foundation

The National Science Foundation has awarded the GMTO Corporation — the organization overseeing construction and management of the $1bn Giant Magellan Telescope (GMT) – a grant of $17.5m over the next three years to accelerate the construction of the 25 m-wide telescope.

The GMT will be located at Las Campanas in Chile’s Atacama Desert and is on-track for first light in 2029. Hard rock excavation at the site is complete and in October 2019 the GMTO signed a $135m contract with German company MT Mechatronics and US-based Ingersoll Machine Tools in Illinois to design, build, and install the GMT’s telescope structure. This is set to be delivered to the Chilean site at the end of 2025.

[The NSF award] will enable us to accelerate our progress on critical components of the telescope
Robert Shelton

The GMT will have seven circular mirrors, each 8.4 m in diameter. When put together, they will create a telescope equivalent to one mirror 25.4 m wide. Two of the mirrors are complete and in storage in Arizona, while three are in various stages polishing. The final two haven’t been started yet, but the sixth mirror will be cast in early March 2021.

Acting as one

Each primary mirror is flexible, but they must remain in a precise shape for all seven to function together as one. “The mirror itself is supported, like on a bed of nails, where we have about 160 actuators behind each of these mirrors,” says GMTO project manager James Fanson. “We measure the shape of these mirrors and we adjust them every 30 seconds.” The NSF grant provides the funding to test a full-size primary mirror and actuator system, and adds Fanson, “to demonstrate we can control the primary mirrors the way we need to.”

Each GMT primary mirror reflects light to a corresponding 1 m-diameter secondary mirror with 675 actuators, which alter its shape every millisecond to counteract Earth’s atmospheric blurring effect. With the NSF grant, the GMTO will also build a portion of one of the secondary mirror systems. The grant also provides funding to build a laboratory-bench test to simulate the mirrors, actuators, disturbance sources, and that the seven primary mirrors can all phase together as one — technology not used before.

“One of the areas of great emphasis the team has had from the beginning is to tackle the riskiest, most difficult questions early on to make sure they can be surmounted,” says GMTO president Robert Shelton, who adds that the NSF award will “enable us to accelerate our progress on critical components of the telescope”.

Evidence for life is found on Venus, wider access to the best radiotherapy

In this episode the astronomy writer Keith Cooper is on hand to chat about the surprising discovery of phosphine in the atmosphere of Venus. He explains that here on Earth, microbial life is the only natural source of phosphine – which could mean that life exists in the clouds of Venus. Cooper also speculates about how future missions to the “habitable zone” of the Venusian atmosphere could search for life.

Today, there is a huge disparity in cancer care across the world with people living in low- and middle-income countries having limited access to the best radiotherapy treatments. This imbalance is the focus of the social enterprise company EmpowerRT, which was founded by the University of North Carolina medical physicist Sha Chang. In this episode, Chang and her colleague Cielle Collins talk to Physics World’s Tami Freeman about what can be done to provider greater access to the best treatments.

Industrial lasers generate attosecond light pulses

Studies of ultrafast processes could become more widely accessible thanks to researchers at the University of Central Florida (UCF) in the US, who have shown that commercially available, industrial-grade lasers can generate attosecond pulses of light. Until now, such pulses could only be created at large laboratories boasting complex laser systems.

Researchers make attosecond-scale measurements by passing an attosecond light pulse through a material. When this pulse interacts with electrons inside the material, it gets distorted. By monitoring these distortions, scientists can create 3D maps of the electrons and make movies of their motion. As an example, the classical Bohr model of hydrogen indicates that an electron takes roughly 150 attoseconds (10^-18s) to orbit the hydrogen nucleus. Measurements with attosecond precision therefore enable researchers to study motion at a subatomic scale, which is vital for understanding fundamental physics phenomena such as interactions between light and matter.

Such measurements are, however, currently only possible in world-class laser facilities. While UCF houses such a facility, and another dozen or so exist worldwide, team leader Michael Chini explains that none of them truly operate as user facilities – that is, institutions that allow scientists from other fields to come in for a short period and use their equipment for research. This lack of access creates a barrier for chemists, biologists, materials scientists and others who could benefit from applying attosecond science techniques to their work, he says.

Obtaining few-cycle pulses from industrial-grade lasers

The extremely short light pulses employed in attosecond-scale experiments consist of a single oscillation cycle of an electromagnetic wave. Such cycles are typically generated by propagating femtosecond (10^-15s) laser pulses through tubes filled with noble gases such as argon or neon. The interaction between the light pulses and the gas broadens their spectrum, making it possible to compress them further in time.

Chini and colleagues have now developed a way of obtaining such few-cycle pulses from industrial-grade lasers, which could previously only produce pulses of much longer duration. They achieved their feat by compressing approximately 100-cycle pulses in tubes that contained molecular gases instead of noble gases and varying the length of the pulses sent though the tubes. This procedure made it possible to compress the pulses by a factor of 45, squeezing them down to just 1.6 oscillation cycles. At that point, Chini says, they showed that they could use these compressed pulses to produce attosecond pulses by generating an extreme ultraviolet supercontinuum – something he describes as “a hallmark of attosecond pulse generation”.

Choice of gas and pulse duration are key

Study lead author John Beetar notes that the duration of the initial laser pulse is key. Filling the tube with a molecular gas – and especially a gas of linear molecules, such as the nitrous oxide used in this work – enhances the compression effect because the molecules tend to rotate into alignment with the laser field. However, this alignment-induced enhancement is only present if the pulse is long enough to rotationally align the molecules. The choice of gas is important, too, because the rotational alignment time depends on the molecule’s inertia. To maximise the enhancement, the researchers aim to make this inertia coincide with the duration of their light pulses.

The UCF researchers, who report their work in Science Advances, say that single-cycle pulses are within reach using their technique. With further refinements, Beetar adds that the reduction in complexity associated with commercial, industrial-grade lasers should make attosecond science more approachable and could enable more interdisciplinary applications.

CERN accelerator technology to underpin FLASH radiotherapy facility

How can technology developed at CERN for high-energy physics bring state-of-the-art radiotherapy to a hospital just along the lakeside in Lausanne?

The technologies in question include high-performance electron accelerator components and simulation tools originally designed for CERN’s Compact Linear Collider (CLIC). Now, a collaboration between CERN and Lausanne University Hospital (CHUV) plans to use these to create a system for clinical delivery of FLASH radiotherapy.

FLASH radiotherapy, which involves delivering therapeutic radiation at ultrahigh dose rates of 40 Gy/s and above, vastly decreases normal tissue toxicity while maintaining anti-tumour activity. Of particular note, FLASH should enable dose escalation, potentially offering a new option for cancers that are resistant to treatment. “In all experiments so far, we observed that normal tissues are spared with this type of radiation,” says Jean Bourhis, head of radiation oncology at CHUV. “It’s a really reproducible effect. And there is no sparing of the tumour.”

Bourhis pioneered the development of FLASH radiotherapy, leading the team at CHUV that performed the first FLASH treatment in a human patient in 2018. The patient in question had a resistant superficial skin cancer and was treated with low-energy electrons of roughly 10 MeV. Next, he would like to translate the impressive observations seen in an experimental setting into clinical trials. To treat larger tumours at depths of up to 20 cm in the patient, however, will require much higher energy electron beams.

“A clinical FLASH system must have a high accelerating gradient to achieve the beam energies needed to access deeper-seated tumours, energies in the range of 100 MeV,” explains Walter Wuensch, a senior researcher at CERN. This ability to accelerate beams in a very short distance, he notes, was one of the technologies designed for CLIC. The other key aspect of the high-energy physics study was to deliver a high current in a well-controlled and extremely stable beam – another important requirement for FLASH.

“For some years, CERN has been studying accelerator technology for a possible high-energy physics facility,” says Wuensch. “We have developed prototypes and shown their feasibility and performance. So it was with real excitement that we found out about the needs of CHUV. After some initial discussions it became clear that what we had developed for CLIC seemed an almost perfect match for what is needed for a FLASH facility.”

“The clinical need that we have really converges with the technological answer that CERN has,” adds Bourhis. “This is really powerful.”

The CERN–CHUV partnership has now finished the first phase of its study: moving from an initial idea to creating a conceptual design for the proposed FLASH facility. The next step will be to develop this baseline design in more detail to optimize the system for patient treatments. The team also hopes to collaborate with an industry partner in the radiotherapy field. Alongside, while the machine is being prepared, CHUV will start to prepare the required teams and infrastructure, and submit applications to regulatory agencies so that the treatment can reach patients as soon as possible.

Bourhis predicts that the FLASH facility should be operational within two to three years, at which point the team plans to embark on proof-of-concept in clinical trials. He notes that after these trials, the system could be transferable to other hospitals.

The system will be 2–2.5 times larger than a conventional radiotherapy machine, but should still be compact enough to fit into existing hospital infrastructure. The cost of the first protoype system (being installed in CHUV) is estimated to be about €25m; though if manufacturing scales up, this price should come down. FLASH treatments, however, only require the patient to undergo two or three radiation fractions, compared with 20 or 30 for standard radiotherapy. As such, Wuensch suggests that the eventual cost-per-treatment could be competitive in absolute terms with classical radiotherapy.

“We really appreciate the opportunity to work on something that’s matches so well and is new on both sides,” Wuensch concludes. “It’s a wonderful opportunity to be able to work in the medical field.”

Physics in the pandemic: ‘Our event had grown from a hub for the UK to a truly global event within a matter of days’

Condensed Matter Physics in the City (CMPC) is a long-standing conference series that has become a focal point for researchers in the UK studying strongly correlated materials. First held in 2010, the meeting was conceived by the Hubbard Theory Consortium – a confederation of condensed-matter groups from Royal Holloway, University of London, the University of Kent, the London Centre for Nanoscience at UCL, Imperial College London and the Rutherford Appleton Laboratory (RAL).

This year’s edition of the conference was scheduled to take place its traditional Royal Holloway location at Bedford Square in central London in July 2020, organized by a committee from across the Hubbard Theory Consortium and collaborators. It was to be complemented by a summer school on “Foundations of Quantum Matter” on the Isle of Skye, organized by our UCL colleagues Andrew Green and Frank Krüger.

Then the COVID-19 pandemic struck.

With excellent speakers lined up for both events, we still wanted to go ahead with the conference. But given the uncertainty of whether an in-person meeting would be at all possible, the organizing committee quickly agreed to take the meeting online.

Traditionally, CMPC has provided delegates with lots of time for discussion, offered an informal setting and struck a balance between theory and experiment, while involving junior and senior participants alike. This ethos is epitomized by our colleague Piers Coleman, based at Royal Holloway and Rutgers University, whose enthusiasm for getting theorists and experimentalists together derives from the early days of the field at Bell Labs, Bristol and Cambridge universities, and the Landau Institute.

Thanks to the previous events in the series, including last year’s 10th-anniversary conference, which had events spanning Paris and London, the core group of academics in the Hubbard Theory Consortium felt they were ready to tackle the challenge of recreating some of this conference experience online.

The organizing committee considered how best to ensure that the spirit of free-flowing discussion of our live events could be salvaged online

Through a number of Zoom meetings, our committee considered how best to ensure that the spirit of free-flowing discussion of our live events could be salvaged online. We therefore decided to deliver the conference as an interactive Zoom session, rather than a webinar, which would have allowed only a text-based Q&A channel from participants. To avoid the possibility of any unwanted “Zoom bombing”, we carefully screened conference registrations for authenticity and circulated session links only to registered participants.

From our initial e-mail announcement of the online conference going ahead, the uptake was rapid, with more than 2000 visits to the conference website within two weeks, and registrations from across the globe. To advertise, we only used established e-mail lists from previous events, while organizers e-mailed collaborating groups and people on RAL’s e-mail lists, courtesy of Devashibhai Adroja.

So quick was the response, in fact, that one of my collaborators from Iran had signed up even before I had a chance to mention the meeting in a personal conversation. Lots of registrations came from the UK, the US, India, Germany and Japan, with more from across Europe, Asia, the Americas, the Middle East, Africa and a few from Australia.

Overall, we counted some 670 registrations from people in 36 countries. Unfortunately, we did not collect information about nationalities, which may have been even more diverse. Our event had grown from a hub for the UK to a truly global event within a matter of days.

Encouraging online discussion

So how was the atmosphere of the online format? Unlike a live conference, we scheduled only two or three talks per day, which were held during UK afternoons to make attendance not too difficult for delegates in Asia and the US. Additionally, despite providing talks on YouTube, both live and as recordings, we were overwhelmed by many of our more remote participants’ enthusiasm to brave early mornings or late nights to attend live (the talk listings are still available via our schedule page).

With ample time allocated for discussions, the conversation really did take off despite the lack of face-to-face contact, and the event felt very interactive. There were no technical interruptions to report (except when our Zoom call crashed on the opening day, when numbers soared and we may have encouraged too many cameras to remain open).

The audience was extremely disciplined – especially thanks to the continual efforts of my colleague Sam Carr from the University of Kent to remind everyone of the meeting etiquette (adopted from the PQM group meetings at Kent) and occasional help from co-hosts to mute the odd microphone that had been left open unintentionally.

If anything, the option to ask questions both live by raising a hand, or via text-based chat seems to have lowered the barrier to making interventions, as many participants confirmed in our post-meeting questionnaire, and especially so for the student participants.

To engage students, who made up around 40% of the audience, we created new mechanisms, as the time-limited schedule did not allow the inclusion of additional live talks. Instead, they were allowed to submit pre-recorded talks, with the incentive of a prize for the best talk, which went to Alexandra Ziolkowska from the University of Oxford.

Overall, we were extremely impressed with the quality of student submissions, and there clearly was an audience to be found for them, with several videos being viewed more than 200 times at the time of writing. We also allowed students to lead additional discussion time after the formal sessions, which again drew a good participation, and was found to further lower barriers for participation.

Meeting new people

But what about informal discussions? Online conferences make it trickier to meet new people, get introduced to your long-time physics idols, or to hear or share the latest rumours and gossip. Nonetheless, we found the Zoom session provided a decent workaround, as the list of participants let you see who was there and to then chat to selected individuals or a small group of people.

Zoom provided a decent workaround, as the list of participants let you see who was there and to then chat to selected individuals or a small group of people

We’ve heard that many delegates used this opportunity to talk privately in the background, to follow up on discussion topics raised in the main session, or just make new friends. We also tried to stimulate face-to-face conversation by splitting the conference into break-out rooms during longer breaks, for which Zoom unfortunately only allowed random allocations. Personally, I found this worked surprisingly well on occasion, allowing me to meet a few random people and maybe discover common interests.

However, it really depended on having enough people actually willing to use the break-out rooms, rather than using the online format to “sneak out”, switch to another task or take a comfort break. Unfortunately, only a small number of participants engaged with us on social media, either via our Twitter channel or conference hashtag, though many indicated that our conference notice board had provided them with some helpful information.

From the responses to our post-meeting questionnaire, more than half saw their expectations towards online conferences raised (54%) and only a tiny minority had theirs lowered (4%). Advantages that were often mentioned included not having to spend time travelling and being able to fit in both personal work and conference attendance. Other benefits included not needing to hurry from one conference room to another, not having to worry about disturbing anyone if you arrived late, being able to have a drink, or quickly and discreetly switching to another task if needed. Also, delegates could drop in on the most relevant talks and discussions, while feeling fewer barriers to asking questions.

Several respondents from the developing world highlighted the fact that the online format had given them the chance to attend an event with top experts that they could otherwise not have afforded to attend in person (the event was free of charge). Negative responses were much scarcer, though quite a few people said that the 90 minutes we had allocated for talks – and 30 minutes for breaks – was too long, which meant we may have over-estimated people’s attention span. Excessive screen-time can be quite tiring in the long run, so maybe even shorter conference days would work out better.

Let’s meet again

In the future, we would love to meet colleagues in person again, and I hope our conference funding from the Institute of Complex Adaptive Matter (ICAM-I2CAM), EPSRC and the Institute of Physics can be carried over into 2021 to make this happen. But with the tremendous success of the online meeting this year, any future event, even if held “for real”, will surely have to have an online element associated with it too.

Indeed, our survey indicated a healthy appetite for such a “hybrid” model, with about 30% indicating that they would actually prefer to attend the event online, and 40% wanting to attend at least in part virtually, maybe due to the reduced impact on time and travel commitments. Anyone wanting to keep abreast of announcements for next year can follow our Twitter channel, which also has coverage from the 2020 online edition.

Most of all, what this year’s meeting has confirmed is that there is a global appetite for a deeper understanding of novel materials properties at the quantum level. With topics ranging from spin liquids and neutral Fermi surfaces to novel superconducting states in twisted graphene bilayers and machine learning, we hope we have inspired researchers well beyond the people who usually attend our meetings.

Nifty noise trick makes quantum states live longer

All particles have a wave-like nature, but in the everyday, macroscopic world their quantum behaviour is hidden thanks to interactions with their surroundings – for example via gravity, electromagnetism or heating. Such interactions also mean that a quantum system will quickly lapse into classical behaviour – a process known as decoherence – unless it is isolated from its environment. Scientists at the University of Chicago have now developed a simple strategy that allows quantum systems to fend off decoherence for 10 000 times longer. The technique, which has been tested on solid-state qubits (quantum bits) made from silicon carbide defects, could advance many areas of quantum science, including quantum computing, communications and sensing.

Quantum computing has made significant progress in recent years, and in 2019 researchers at Google unveiled a basic quantum processor that performs certain tasks faster than a conventional supercomputer. In practice, however, the problem of decoherence, which destroys any stored quantum information, must still be overcome before such devices become widespread and able to tackle significant real-world problems. This is because quantum computers work by exploiting the ability of a quantum particle to be in a superposition of two or more states at the same time, and the fragile nature of these superpositions makes them easy to destroy and hard to control.

Silicon carbide point defects

A team of researchers led by David Awschalom recently discovered that silicon carbide – a material already widely employed in high-power electronics – hosts point defects that might help solve this problem. Such defects are attractive because their decoherence time is much longer than the time required to perform a logical operation in a quantum processor – even at room temperature. The defects also contain electron spin states that can be controlled as qubits and manipulated using light.

In their earlier work, the researchers studied a specific crystal structure of silicon carbide known as 4H-SiC that contains naturally-occurring defects called divacancies. These defects correspond to a missing silicon atom next to a missing carbon atom in the material’s crystal lattice, and they are similar to nitrogen-vacancy centres in diamond (which also boast long decoherence times and have already been used as qubits that can be controlled by light at room temperature). Both types of defect form a multi-electron system with a net angular momentum, or spin, that can be aligned either parallel (“1”) or antiparallel (“0”) to an applied magnetic field.

A new trick

Researchers in several groups have explored different strategies for extending the decoherence times of this and other systems. One common approach is to physically isolate the system from its noisy surroundings. Another technique is to make all the materials as pure as possible. Neither task is easy in practice, and Awschalom and colleagues have now devised a very different protocol.

Instead of trying to eliminate noise in the systems’ surroundings, study lead author Kevin Miao says that he and his colleagues, in effect, “trick” the system into thinking that the noise isn’t there. They achieved this by applying an alternating magnetic field to the 4H-SiC divacancy in addition to the electromagnetic pulses (oscillating magnetic fields at microwave frequencies, in this case) employed to control the spin states in quantum systems. These pulses cause the spin of the divacancy to oscillate between its two qubit states (via electron spin resonance), and this oscillation can then be used to “write” quantum information to the sample.

By precisely tuning their additional magnetic field, Awschalom’s team demonstrated that they could rapidly rotate the electron spins in the system and allow it to “tune out” surrounding noise. “It’s like sitting on a merry-go-round with people yelling all around you,” Miao explains. “When the ride is still, you can hear them perfectly, but if you’re rapidly spinning, the noise blurs into a background.”

Divacancy ignores environmental noise

The new technique allowed the researchers to increase the decoherence time of the system to 22 milliseconds – four orders of magnitude longer than it would be without their modification, and far longer than any previously reported electron spin system. This is because the 4H-SiC divacancy, under the influence of the alternating magnetic field, is able to almost completely “ignore” some forms of temperature fluctuations, physical vibrations and electromagnetic noise, all of which are the bane of quantum coherence.

According to Awschalom, the approach creates a pathway to scaling up the numbers of qubits in a quantum processor. “It should make storing quantum information in electron spin practical,” he explains. “Extended storage times will enable more complex operations in quantum computers and allow quantum information transmitted from spin-based devices to travel longer distances in networks.”

The researchers say their approach could also be tested in quantum systems other than 4H-SiC divacancies, such as superconducting quantum bits and molecular quantum structures.

“There are a lot of candidates for quantum technology that were pushed aside because they couldn’t maintain quantum coherence for long periods of time,” Miao stated in a press release issued by the University of Chicago. “Those could be re-evaluated now that we have this way to massively improve coherence.” The best part, he adds, is that “it’s incredibly easy to do. The science behind it is intricate, but the logistics of adding an alternating magnetic field are very straightforward.”

The new coherence protection technique is detailed in Science.

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