This year has been dominated by one event: the SARS-CoV-2 coronavirus, which was first reported in Wuhan, China, before quickly spreading throughout the world. The severe impact of COVID-19, the disease caused by the coronavirus, has been felt by all and physicists are no exception. Universities and research facilities all shut their doors earlier this year as scientists headed home under lockdown.
In this year’s Physics World ChinaBriefing, we report how universities in China – the first to be affected – are now beginning to cautiously reopen. Despite a few localized outbreaks in the country, which seem to have been contained, things appear to be getting back on track. Without a vaccine on the immediate horizon, however, progress will remain slow and cautious and with international travel limited, collaborations will likely remain online-only for the foreseeable future.
Indeed, the impact of COVID-19 has already hit many scientific conferences, some of which have had to switch at short notice to online platforms. One example is Quantum 2020 – a major international conference in quantum technology organized by the Institute of Physics and IOP Publishing in partnership with the Chinese Physical Society and the University of Science and Technology of China. Due to be held in Shanghai, it will now take place entirely online on 19–22 October. For this year’s briefing, which is free to read, we talk to USTC physicist Chaoyang Lu about managing this virtual shift as well as the future of quantum technologies.
The concept of quantum communication, with security guaranteed by the laws of physics, took the world by storm when first unveiled in 1984. The traditional protocol, however, allows only two people to communicate securely. Attempts to extend this to “quantum networking” have usually proved either insecure or impracticably complex. Now, however, researchers in the UK and Austria have demonstrated secure information exchange between eight users spaced all around a city.
The canonical quantum communication protocol relies on two parties generating a secure key by exchanging polarized photons. The security of the link is guaranteed by the fact a third party cannot make a measurement of their state without disturbing them and being detected. Though remarkable, this approach is fundamentally limited to pairwise communication: it does not provide a blueprint for the multi-dimensional quantum network, or “quantum internet” that some researchers have dreamed of, in which multiple users connected together can all communicate simultaneously and securely with any other member of the network.
One obvious way to extend the scheme beyond two people is for the second person to simply act as a link in the chain, repeating the procedure and communicating securely with a third person, who in turn passes the message on until the message reaches its ultimate destination. Quantum networking schemes based on such “trusted nodes” have been developed. The security of such schemes is no longer absolute, however, because a trusted node may not be totally secure. Schemes that avoid trusted nodes have generally proved unfeasibly complex and hardware-intensive or have suffered other problems, such as restricting which users can communicate at any one time.
In 2018, researchers at the Institute for Quantum Optics and Quantum Information in Vienna led by Rupert Ursin demonstrated a scheme in which four users received pairs of entangled photons from a single, laser-pumped crystal source. “I generate these two-by-two, but I generate several such two-by-two pairs in a tiny amount of time,” explains project leader Siddarth Koduru Joshi.
Project leader Siddarth Joshi optimizes the central quantum network hub. Instead of making a physical connection between each and every user, the researchers created a scheme where every user only has a single glass fibre connected to a source of quantum entanglement. (Courtesy: Soeren Wengerowsky)
This constant stream of entangled photon pairs from a single, central source allowed each of the four parties to become pairwise entangled with each of the other three parties. “If I’m talking to you, I look at stream one, if I’m talking to somebody else, I look at stream two, and so on,” says Joshi, now at the University of Bristol. The researchers believed that this scheme provided a simpler, more scalable architecture for secure, trusted-node free information exchange between multiple parties.
In the new work, researchers at the University of Bristol, in collaboration with the Austrian scientists, have confirmed that the technique works, demonstrating simultaneous and secure exchange of information between eight users spaced up to 12.6 km away from the central source around the city of Bristol. “This time, we actually demonstrated quantum communication, and we did this through deployed fibres across the city to show compatibility with existing infrastructure,” says Joshi.
Moreover, the researchers added additional multiplexing to simplify the hardware required by each user and make the protocol even more scalable: whereas their original protocol would have required 56 wavelength channels to fully interconnect eight users, their improved version required only 16. The researchers believe their network is the largest trusted-node free quantum network to date.
Quantum and computer engineer Wolfgang Tittel of QuTech at Delft University of Technology in the Netherlands describes the paper as “nice and important work” and is especially impressed by the absence of trusted nodes. An important next step, he says, is the scheme’s integration with quantum repeater technology, which to mitigates photon loss and decoherence and allows entanglement distribution over long distances. This, he says, could “extend the network beyond metropolitan size”.
What are your main areas of research at the Institute of High Energy Physics (IHEP), Chinese Academy of Sciences?
My interest focuses on radiation imaging and its applications in medical physics and industry. This includes the design of X-ray and gamma-ray detectors, imaging algorithms as well as systems development.
How did you get into medical imaging?
For my PhD, I worked on IHEP’s Beijing Electron Positron Collider, which gave me experience in radiation measurements of positron annihilation. When I graduated in 2006, I began research on radiation-imaging technologies such as positron emission tomography (PET), single-photon emission computed tomography (SPECT) and computerized tomography (CT). Since 2008 research into radiation-imaging technology that has been carried out at IHEP’s division of nuclear technology and applications has achieved a number of breakthroughs in PET and SPECT detector development. Such work has especially benefited from the development of electronics and detection technology during the construction of big-science facilities at IHEP.
Do you have an example?
With the rapid development of the Chinese medical-imaging market, our team has co-operated with companies to develop devices such as a dedicated breast-PET (PEMi) scanners. We have now completed more than 500 clinical test runs and in 2015 received certification from the China Food and Drug Administration. Now it is being used for early breast cancer diagnosis in clinical settings – an important addition to improving breast cancer survival rates.
How important is technology transfer at IHEP?
It is very important. In IHEP’s strategic outlook, technology transfer is as critical as the construction of big-science facilities and basic scientific research. Most research centres in IHEP have been involved in technology transfer to some degree, with IHEP also setting up a dedicated technology-transfer office and a spin-out company.
Each of IHEP’s big-science facilities has led to the development of multiple disciplines and technologies.
What are some of the technologies that IHEP has spun out of its research?
The main technologies that have been successfully transferred are accelerator and medical-imaging technologies, such as the PEMi scanner. As IHEP makes large accelerators and their core components, we have also developed high-power electronic irradiation accelerators that are being used for the sterilization of food and medical equipment. These accelerators have already been installed in Yantai, Wuhan, Tianjin and other regions around China.
Many of the technologies at IHEP seem suited for medical applications. Why is this?
The dominant technologies of IHEP are accelerator, nuclear detection and nuclear electronics technologies, which are exactly the type of technologies that are needed in nuclear medicine, radiotherapy and medical diagnosis. National funding projects give support for these areas of medical physics and there are many companies in this area that we collaborate with.
What kind of innovations do you see in the future?
Given advanced detector and electronics technology, we see developments such as high resolution – i.e. less than 100 picoseconds – time-of-flight PET technology that can increase the resolution of images and reduce the radiation dose to patients. Another aspect in medical applications is “static spectral” CT scanners, which will also allow for high-resolution imaging. In addition, the integration of diagnosis and treatment, especially in precision-particle therapy is another development I see in the future.
Do you collaborate on technology transfer with other countries?
Not yet, but international co-operation is an aspect that we are interested to grow.
With China designing a 100 km collider, do you envisage a similar emphasis on technology transfer when planning that project?
Each of IHEP’s big-science facilities has led to the development of multiple disciplines and technologies. Pixel-detector technology, for example, is involved in almost every scientific project, so we will accumulate more technical knowledge in this field as we advance. Combining the emphasis on detector development with medical-imaging equipment, we plan to focus on pixel-type semiconductor-detector technology by developing an advanced sensing chip and utilizing it in the next generation of CT and SPECT scanners.
Has COVID-19 affected your research?
COVID-19 has not really had any effect on our lab and we have continued to carry on with our research during the pandemic.
Physicists in France have made small objects float upside-down on the underside of a layer of viscous liquid levitating in air. Although their apparently gravity-defying demonstration breaks no laws of physics, they say it could shed new light on the interaction between air and liquids.
Archimedes’ principle says that an object fully or partially immersed in a liquid experiences the upward force of buoyancy, which is equal to the weight of liquid it displaces. By opposing the force of gravity due to the object’s own weight, buoyancy will cause an object to float if it is less dense (overall) than the liquid – while denser objects will sink.
This is a very familiar phenomenon in our everyday world in which water and other liquids naturally exist at a lower gravitational potential than less dense gases. People on a boat travel through the air, with the sea below them. What the latest research shows is that the same principle would hold for people in an upside-down world – one in which, in effect, the sky lies beneath the sea.
Counter-intuitive effects
The secret to bringing about this topsy-turvy floating is vibration. Since the 1950s, scientists have demonstrated a range of counter-intuitive effects by vibrating fluids at high frequencies. Gas bubbles, for example, can be made to sink, while heavier particles rise. On a larger scale, whole layers of fluid can levitate in air. This is because more dense fluids tend to drip under the action of gravity, eventually displacing the air beneath them. By preventing the formation of drops, vibration keeps the fluid’s lower surface flat and allows it to hover.
In the new experiment, Emmanuel Fort and colleagues at the PSL University in Paris fixed a plexiglass container to a vibrating platform and then filled the container with either silicon oil or glycerol. These viscous liquids have surfaces that remain stable even at high accelerations. After turning the shaker on, the researchers used a syringe with a long needle to inject air towards the bottom of the liquid. With the resulting bubble sinking and growing, it eventually created a layer of air across the bottom of the container – causing the viscous liquid to levitate.
This fluid levitation is nothing new, but what Fort and co-workers then did was to place objects upside-down on the lower interface between the liquid and the air. As they explain in a paper in Nature, this buoyancy is governed by the same basic physics that would be at work on the upper interface. The object’s gravity tending to pull it down while the disturbance its downward motion creates in the liquid tends to pull it up.
Slight displacement from equilibrium
There is an important difference between the normal and inverted flotation, however. In the former, an object is in stable equilibrium because any attempt to move it up or down will be met by a restoring force. When pushed down it displaces a greater amount of liquid and so experiences more buoyancy, while any force pushing it out of the liquid will be opposed by gravity. But in the upside-down case, any slight displacement from equilibrium will see the object accelerate away – thanks to gravity below and the mass of liquid above.
Fort and colleagues found that the vibrations needed to invert the air and liquid also in fact stabilized the exotic buoyancy – at least for objects up to a certain density. They vibrated the container at 100 Hz and showed they could pin a series of small plastic spheres to the lower liquid-air interface, with the spheres remaining in place even when they poked them. To make the demonstration more eye-catching they repeated the feat with a little plastic boat, while floating an identical boat on the upper interface (see figure).
The researchers then found that 2.5 cm-diameter plastic spheres weighing no more than about 6 g only fell to the bottom of the chamber once they had reduced the amplitude of the vibrations to the point where the liquid layer itself succumbed to gravity. Heavier spheres, in contrast, fell first. This behaviour, they say, is consistent with a simple theoretical model that they developed to explain the inverted buoyancy. Only when the spheres’ density approached that of the liquid – meaning a mass of about 8 g – did significant discrepancies occur. In that case, they add, other more complex effects play a role.
The team based its model on the concept of time-averaged forces, which tend to stabilize the equilibrium states. As they point out, this is the same basic idea underlying what is known as a Stephenson–Kapitza pendulum. In that case, vibrations allow a pendulum fixed from below to oscillate back and forth in a small arc – when otherwise it would fall to the ground.
Writing a commentary piece in Nature, Vladislav Sorokin of the University of Auckland in New Zealand and Iliya Blekhman of the Russian Academy of Science in St Petersburg, Russia, argue that several assumptions – such as a linear relationship between the pressure and height of the air layer – “somewhat limit the accuracy” of the French group’s model. But they reckon that the work might nevertheless lead to practical applications – pointing out that gravity-defying effects in fluids have previously been used to improve chemical reactions and aid mineral processing – and that it suggests other “remarkable phenomena” remain to be discovered in vibrating mechanical systems.
A new collision detector that mimics the neurological mechanisms that stops swarming locusts from crashing into each other has been developed by Saptarshi Das at Pennsylvania State University and colleagues. The team’s compact, low-power device could lead to a boost in efficiency for collision detection mechanisms in robots and autonomous vehicles.
Today’s most advanced robots can safely navigate through unfamiliar environments using algorithms that allow them to avoid collisions with surrounding objects. These algorithms can be used for general, non-specific purposes, but this makes them computationally expensive – meaning their hardware requires large energy budgets and spatial footprints. In their study, Das’ team investigated whether navigation could be achieved using more task-specific algorithms.
The researchers turn to the tried-and-tested method of seeking inspiration from nature. In this case, they considered swarms of locusts, which are best known for the widespread devastation they can unleash on crops. The insects fly in dense groups containing millions of individuals, but very rarely collide with each other. This requires individual locusts to carry out complex mathematical calculations in real time, despite their extremely limited brain sizes.
Specialized neuron
Locusts make up for their lack of brain power with a single specialized neuron named the “lobula giant movement detector” (LGMD), which operates using two visual stimuli: the angular sizes and relative angular velocities of approaching insects. The resulting firing frequency of the neuron peaks immediately before collision, driving the locust to change direction.
To mimic this behaviour, Das’ team started from an equation linking the stimuli with the LGMD’s time-varying firing rate. They then incorporated this mathematics into a device containing a molybdenum disulphide monolayer, stacked on top of a programable memory architecture. While the photoconductor increased the device’s current as objects approached, introducing an “excitatory” signal, the architecture underneath decreased its current when no visual stimuli were present – creating an “inhibitory” signal. The signals competed with each other, with the excitatory signal winning out immediately before a collision – creating a signal spike.
Like the LGMD, the team’s device consumes a tiny amount of energy (just a few nanojoules) and occupies a modest spatial footprint of just 1×5 µm. At the same time, it can identify potential collisions from a variety of objects, approaching at a range of different speeds with an efficiency that is lacking in current general-purpose devices.
Das and colleagues now hope to extend the responses of their device beyond head-on collisions, and to incorporate multi-pixel detectors for predicting collisions in 3D. Through these improvements, their technology could be an important step towards safe, affordable autonomous vehicles, and robotics applications including manufacturing and medical surgery.
Task Group 233 provides new guidance for CT performance assessment and addresses the challenges inherent in established CT imaging quality metrics. The Mercury 4.0 Phantom, designed by Dr Ehsan Samei at Duke University and commercialized by Gammex, now Sun Nuclear Corporation, assists with evaluations outlined in TG-233.
During this webinar, Timothy Szczykutowicz, PhD, DABR, of the University of Wisconsin-Madision, Department of Medical Physics, will present on how his department uses the Mercury 4.0 Phantom for common clinical CT tasks. You will hear about how the Mercury 4.0 Phantom addresses advanced features including automatic exposure control and tube current modulation. This webinar will enable you to understand how this new phantom lets the user check patient size for protocol optimization and proper dose management.
Timothy Szczykutowicz is an associate professor in the University of Wisconsin School of Medicine and Public Health Departments of Radiology, Medical Physics and Biomedical Engineering. His clinical and research activities include: optimizing CT scan protocols, monitoring patient dose, developing new metrics to define image quality in the clinical setting, developing protocol management methodologies, fluence field modulated CT, dual energy CT, and radiology department workflow and quality metrics.
Timothy is also the author of the book The CT Handbook: Optimizing protocols for today’s feature-rich scanners. He is an associate/section editor or on the board for multiple journals including: Medical Physics, Radiographics, Contemporary Diagnostic Radiology, and the Journal of Computer Assisted Tomography.
Chosen one: 81 Nobel laureates have voiced their support for Joe Biden in this year’s US presidential election on 3 November. (Courtesy: Michael Stokes/CC BY 2.0)
Over 80 US Nobel laurates have issued an open letter endorsing Democrat presidential candidate Joe Biden in the US election scheduled for 3 November. Biden – the vice-president under Barack Obama from 2009 to 2017 – will go up against president Donald Trump, who is seeking a second four-year term. The signatories of the open letter include 26 physics laureates, 31 medicine and physiology awardees and 24 chemistry Nobel winners.
The letter – signed by 81 laureates who received their Nobel prizes between 1975 and 2019 – focuses on Biden’s attitude to science. “At no time in our nation’s history has there been a greater need for our leaders to appreciate the value of science in formulating public policy,” the letter states. “During his long record of public service, Joe Biden has consistently demonstrated his willingness to listen to experts, his understanding of the value of international collaboration in research, and his respect for the contribution that immigrants make to the intellectual life of our country.”
They recognize the harm being done by ignoring science in public policy
Bill Foster
One of those to sign the letter is Barry Barish, who shared the 2017 physics prize for his contributions to the LIGO detector. “I support Joe Biden because of his long record of making policy, informed by science, to deal with large, complicated issues like cancer, climate change and nuclear proliferation,” Barish, who is at the California Institute of Technology, told CNN, adding that “we absolutely must elect Joe with his science-based approach to successfully lead us out of the COVID-19 pandemic”.
Facts and science
Nobel laureates’ support of Democratic presidential candidates isn’t new. Hillary Clinton received 70 such endorsements in 2016, while former president Barack Obama received the support of 76 in 2008. Carol Greider, who shared the 2009 prize in physiology or medicine for discovering the enzyme telomerase, asserted that elected leaders “should be making decisions based on facts and science,” adding that she “strongly endorses” Biden, in particular because of his “commitment to putting public health professionals, not politicians, back in charge”.
Democratic Representative Bill Foster of Illinois, the only physicist in Congress, organized the open letter saying it would be an “important” development for the Biden campaign. He says that “a core group” of laureates decided on which issues to raise in the letter. Foster, who received the endorsement of 31 laureates when he ran for Congress in 2007, says that when he started calling the laureates to back the intiative, “it was like pushing at an open door”. He adds that “there was a lot of enthusiasm because of the difference [the laureates] perceive in the scientific understanding” between the two candidates.
Foster believes that the letter reflects the view of much of the US scientific community. “They recognize the harm being done by ignoring science in public policy,” he says. “And it’s not only science; it’s logic and integrity. The scientific community wants to get to a situation in which they trust people’s word.” Foster sees the COVID-19 pandemic as a factor in refocusing voters on the importance of science. “The only reason we’re in a position to develop vaccines rapidly is decades of scientific research,” he says. “This may be an opportunity for the scientific community to remind everyone about long-term investment in science.”
Average magnetic forces per unit mass of the AYFFF aromatic peptide and a control non-aromatic peptide, IIIGK, in dissolved, dispersed and assembled states. (Courtesy: Haijun Yang, Feng Zhang and Haiping Fang)
Peptides exhibit surprisingly strong diamagnetism when they join together to form microfibres, say researchers in China. Haijun Yang, at the Chinese Academy of Sciences (CAS) in Shanghai, and colleagues measured the force exerted by peptide-containing liquids placed in a static magnetic field. They found that samples in which the peptides had been allowed to self-assemble had a diamagnetic mass susceptibility 11 times higher than samples in which the peptides were dissolved, and 175 times that of pure water. The unexpected result, reported in Chinese Physics Letters, helps to explain the origin of magnetism in biomolecules, and could have applications in magnetically controlled microfabrication, medical imaging and brain–computer interfaces.
A diverse range of biological structures – from entire cells down to DNA and other subcellular polymers – are known to change their orientation in response to magnetic fields. This effect is behind the ability of some animals to navigate using the Earth’s magnetic field. However, many of the biological structures that respond to such fields apparently contain too little iron for the phenomenon to be explained by either ferromagnetism or ferrimagnetism. This suggests that the magnetic sensitivity of these structures is instead due to more subtle magnetic effects.
In an amino-acid chain called a peptide, for example, an external magnetic field induces a weak secondary field with the opposite direction, which pushes the peptide away. This effect, known as diamagnetism, has been attributed to the alignment of electrons in the peptide bonds between amino acids, or to electron currents flowing around cyclic structures called aromatic rings.
To investigate this phenomenon, Yang and colleagues prepared three different samples of a pentapeptide with the sequence AYFFF. A pentapeptide is a polymer consisting specifically of five amino acid units – in this case, one unit of alanine (A), one of tyrosine (Y) and three of phenylalanine (F).
In the first sample, the researchers dissolved the peptide in the organic solvent dimethyl sulfoxide (DMSO). In the second sample, they dispersed the powdered peptide in water. In the third sample, they also dispersed the powdered peptide in water, but then left it to stand for 16 hours. Over this 16-hour period, the peptides self-assembled into fibres more than 10 µm long.
AFM images of the self-assembled fibres of AYFFF aromatic peptides (left) and fibres of a control non-aromatic peptide IIIGK (right). (Courtesy: Haijun Yang, Feng Zhang and Haiping Fang)
The researchers weighed the samples, and then repeated the measurement in the presence of a magnetic field, which they applied by placing a permanent magnet above the apparatus. They assumed that any difference between measurements with and without the magnet was due to the diamagnetic interaction between the external magnetic field and the magnetic fields induced within the sample.
They found that the peptide dissolved in DMSO exhibited the weakest diamagnetic response, while the sample in which the peptide had been allowed to self-assemble exhibited the strongest. The strength of diamagnetism in the water-dispersed sample was somewhere between the two, which the researchers put down to the occurrence of a limited degree of self-assembly.
When the team performed the same experiment using a different pentapeptide – IIIGK, comprising isoleucine (I), glycine (G) and lysine (K) – they found only a weak diamagnetic response, even for the sample that had been left to self-assemble into fibril structures. Whereas tyrosine and phenylalanine in the AYFFF sequence incorporate aromatic rings in their structures, this is not the case for any of the components of the IIIGK peptide. Yang and colleagues therefore interpret their results as confirmation that the electron dynamics of the aromatic rings, rather than the peptide bonds, are responsible for strong diamagnetism in peptides.
Although the role of aromatic ring currents was not unexpected in itself, the researchers were surprised by how strong the diamagnetism was in the self-assembled peptide samples. They are still investigating the physics underlying the effect, but they do already have an idea of how such anomalously strong diamagnetism might arise.
“Each peptide molecule has an induced magnetic moment in the presence of an applied magnetic field,” says Haiping Fang, of the CAS and East China University of Science and Technology in Shanghai, who led the research. “Thermal forces randomize the orientation of these magnetic moments, weakening the diamagnetism. A group of aromatic peptides in the assembled state – with interactions between them – might reduce the effect of these thermal fluctuations, resulting in the strong diamagnetism.”
Whatever its origin, the researchers foresee several possible applications for the effect. “Peptides with various sequences could be easily assembled into different nanostructures under magnetic fields, widening fabrication strategies,” says Feng Zhang, a collaborator from Guangzhou Medical University. “Assembled peptides modified with biomarkers could also be used as biological tracers, enhancing the contrast between normal and abnormal tissues in magnetic resonance imaging. Or, in novel brain–computer interfaces, the assembled peptides’ strong diamagnetism could be used as a non-invasive sensor to detect weak variations in magnetic signals from brain activity.”
Simulated mountain and valley landscape created by buckling in graphene. Credit: Yuhang Jiang
An international team led by researchers at Rutgers University in the US has found a way to create “flat” electronic bands – that is, electron states in which there is no relationship between the electrons’ energy and velocity – in graphene simply by causing the material to buckle. This new strategy could be used to produce so-called “superlattice” systems that serve as platforms for exploring the collective behaviour of electrons in strongly interacting quantum systems. Such behaviour is known to be linked to high-temperature superconductivity, but a complete understanding is still lacking.
Flat bands are especially interesting for physicists because electrons become “dispersionless” in these bands – that is, their kinetic energy is suppressed. As the electrons slow down almost to a halt, their effective mass approaches infinity, leading to exotic topological phenomena as well as strongly correlated states of matter associated with high-temperature superconductivity, magnetism and other quantum properties of solids.
A fine-tuning challenge
Flat bands are, however, difficult to engineer, and researchers have only observed them in a handful of physical systems. An example is twisted bilayer graphene, which is created by placing two sheets of graphene on top of each other and slightly misaligning them. Under these conditions, the atoms in the graphene sheets form a quasi-periodic moiré pattern with a period that is determined by the relative twist between the sheets’ crystallographic axes, rather than the spacing between individual atoms.
The result is a “superlattice” in which the material’s unit cell (that is, the simple repetition of carbon atoms in its crystal structure) expands to a huge extent – as if the 2D crystal were being stretched 100 times in all directions. This stretching dramatically changes the material’s interactions and properties. Notably, it undergoes a transition from an insulator to a superconductor at a “magic” twist angle of 1.1° and a temperature of 1.7 K.
“Magic angle” graphene has been studied extensively since its discovery in 2018. However, because the “magic” effect disappears at slightly larger or smaller twists, very accurate fine-tuning of the material is required to achieve the desired electronic band structure.
Pseudo-magnetic fields
A team led by Eva Andrei of the Department of Physics and Astronomy at Rutgers has now developed an alternative means of producing flat electronic bands. She and her colleagues began by placing graphene on an atomically flat substrate of niobium diselenide or hexagonal boron nitride. Using scanning tunnelling microscope topography and computer simulations, they found that the graphene sheet buckled when cooled to 4 degrees above absolute zero. This buckling is driven by compressive strain within the graphene sample, which develops when ridges that formed during the sample’s fabrication collapse as it cools.
As the graphene buckles, a “mountain and valley” landscape forms that electrons in the material experience as pseudo-magnetic fields. “These fields are an electronic illusion, but they act as real magnetic fields,” Andrei explains. The result, she says, is a dramatic change in the material’s electronic properties – including the emergence of flat bands.
Unlike earlier realizations of a pseudo-magnetic fields that were mostly local in their extent, the buckling transition observed in this work produces a global change in the electronic structure of graphene, with a sequence of flat bands spread throughout the material.
According to the team, the new technique could thus become a general strategy for creating other superlattice systems and using them to explore interaction phenomena characteristic of flat bands.
The researchers, who report their work in Nature, say they would now like to develop ways of engineering buckled 2D materials with novel electronic and mechanical properties for use in applications such as nanorobotics and quantum computing.
Simulating chemical processes is one of the most promising applications of quantum computers, but problems with noise have prevented nascent quantum systems from outperforming conventional computers on such tasks. Now, researchers at Google have taken a major step towards this goal by using the most powerful quantum computer yet built to successfully implement a protocol for calculating the electronic structure of a molecule. The results may form a blueprint for complex, useful calculations on quantum computers affected by noise.
In October 2019, Google announced to great fanfare that its 53-qubit Sycamore computer had achieved quantum advantage. This means that a quantum computer can solve at least one problem much faster than any conventional supercomputer. However, Google researchers openly acknowledged that the problem Sycamore solved (sampling the outcome of a random quantum circuit) is easy for a quantum computer but difficult for a conventional supercomputer — and had little practical use.
What researchers would really like to do is use quantum computers to solve useful problems more effectively than possible with conventional computers: “Sycamore is extremely programmable and, in principle, you really can run any algorithm on it…In this sense, it’s a universal quantum computer,” explains team member Ryan Babbush of Google Research, “However, there’s a heavy caveat: there’s still noise affecting the device and as a result we’re still limited in the size of circuit we can implement.” Such noise, which results from classical sources such as thermal interference, can destroy the fragile superpositions crucial to quantum computation: “We can implement a completely universal circuit before the noise catches up and eventually destroys the computation,” says Babbush.
Hartree-Fock procedure
In the new research, the team used Sycamore to implement the Hartree-Fock procedure – a well-established method for calculating the electronic structure of molecular systems – and applied it to the isomerization of diazene. This is significantly more complex than previous simulations on quantum computers, increasing the maximum number of qubits used from six to 12.
Each successive qubit brings additional potential for noise: “You need to perform some type of error mitigation,” says Google Research’s Nick Rubin: “The extreme way – and this is something that the Google team is building towards – is to build an error corrected quantum computer, which involves turning a quantum computation into a digital quantum computation and correcting any errors that occur along the way”.
In the absence of this, however, Rubin developed a mathematical error mitigation technique that allowed noise to be identified and discarded. Using this technique and others, Rubin explains, the researchers could “lower the error rates of Sycamore through calibration and then apply algorithmic error mitigation to see the – in this case – chemistry at high fidelity.” Their results matched those from a conventional computer.
More complex simulations
Ironically, the structures predicted by the simple Hartree-Fock procedure for this molecule do not agree with measurements in the lab, but Babbush explains that the “notable deficiencies” of the Hartree-Fock procedure for structure prediction are irrelevant. “We certainly do want to build more complex simulations on top of it,” he says, “but even those will use the sub-routines we’ve developed here as a stepping stone.” Whether or not, with further improvements, it will prove possible to solve classically intractable problems in quantum chemistry on a so-called “noisy intermediate-scale quantum computer” (NISQ) using quantum error mitigation remains unknown: “Error mitigation will only take you so far,” says Babbush. “At some point all you’re going to be getting is noise, so it doesn’t matter whether you can tell whether it’s noise or not. To get past that point, you really need error correction.”
“This work pushes the needle in quantum computing for chemistry,” says quantum chemist and computer scientist Alán Aspuru-Guzik of the University of Toronto in Canada. “It shows in an honest way what quantum computers can do today in a device. Compare this to what they could do a couple of years ago, and the progress is extraordinary. It is also worth saying that there is still a large open space for hardware improvements and clever algorithmic tricks.”
“We’re in this era of NISQ,” says quantum information scientist Barry Sanders at Canada’s University of Calgary. “I appreciate what they’re doing now, which is to say ‘let’s forget about universal quantum computing and let’s just push our technology to answer relevant problems.”
This year has been dominated by one event: the SARS-CoV-2 coronavirus, which was first reported in Wuhan, China, before quickly spreading throughout the world. The severe impact of COVID-19, the disease caused by the coronavirus, has been felt by all and physicists are no exception. Universities and research facilities all shut their doors earlier this year as scientists headed home under lockdown.
Task Group 233 provides new guidance for CT performance assessment and addresses the challenges inherent in established CT imaging quality metrics. The Mercury 4.0 Phantom, designed by Dr Ehsan Samei at Duke University and commercialized by Gammex, now Sun Nuclear Corporation, assists with evaluations outlined in TG-233.
