Aurelia Butler

Lead-based vacancy centers in diamonds that form after high-pressure and high-temperature treatment are ideal for quantum networks, find scientists. The modified crystal system could also find applications in spintronics and quantum sensors.
The color in a diamond comes from a defect, or “vacancy,” where there is a missing carbon atom in the crystal lattice. Vacancies have long been of interest to electronics researchers because they can be used as ‘quantum nodes’ or points that make up a quantum network for the transfer of data. One of the ways of introducing a defect into a diamond is by implanting it with other elements, like nitrogen, silicon, or tin.
In a recent study published in ACS Photonics, scientists from Japan demonstrate that lead-vacancy centers in diamond have the right properties to function as quantum nodes. “The use of a heavy group IV atom like lead is a simple strategy to realize superior spin properties at increased temperatures, but previous studies have not been consistent in determining the optical properties of lead-vacancy centers accurately,” says Associate Professor Takayuki Iwasaki of Tokyo Institute of Technology (Tokyo Tech), who led the study.
The three critical properties researchers look for in a potential quantum node are symmetry, spin coherence time, and zero phonon lines (ZPLs), or electronic transition lines that do not affect “phonons,” the quanta of crystal lattice vibrations. Symmetry provides insight into how to control spin (rotational velocity of subatomic particles like electrons), coherence refers to an identicalness in the wave nature of two particles, and ZPLs describe the optical quality of the crystal.
The researchers fabricated the lead-vacancies in diamond and then subjected the crystal to high pressure and high temperature. They then studied the lead vacancies using photoluminescence spectroscopy, a technique that allows you to read the optical properties and to estimate the spin properties. They found that the lead-vacancies had a type of dihedral symmetry, which is appropriate for the construction of quantum networks. They also found that the system showed a large “ground state splitting,” a property that contributes to the coherence of the system. Finally, they saw that the high-pressure high-temperature treatment they inflicted upon the crystals suppressed inhomogeneous distribution of ZPLs by recovering the damage done to the crystal lattice during the implantation process. A simple calculation showed that lead-vacancies had a long spin coherence time at a higher temperature (9K) than previous systems with silicon and tin vacancies.
“The simulation we presented in our study seems to suggest that the lead-vacancy center will likely be an essential system for creating a quantum light-matter interface — one of the key elements in the application of quantum networks,” concludes an optimistic Dr. Iwasaki.
This study paves the way for the future development of large (defective) diamond wafers and thin (defective) diamond films with reliable properties for quantum network applications.
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Engineers and materials scientists studying superconducting quantum information bits (qubits) — a leading quantum computing material platform based on the frictionless flow of paired electrons — have collected clues hinting at the microscopic sources of qubit information loss. This loss is one of the major obstacles in realizing quantum computers capable of stringing together millions of qubits to run demanding computations. Such large-scale, fault-tolerant systems could simulate complicated molecules for drug development, accelerate the discovery of new materials for clean energy, and perform other tasks that would be impossible or take an impractical amount of time (millions of years) for today’s most powerful supercomputers.
An understanding of the nature of atomic-scale defects that contribute to qubit information loss is still largely lacking. The team helped bridge this gap between material properties and qubit performance by using state-of-the-art characterization capabilities at the Center for Functional Nanomaterials (CFN) and National Synchrotron Light Source II (NSLS-II), both U.S. Department of Energy (DOE) Office of Science User Facilities at Brookhaven National Laboratory. Their results pinpointed structural and surface chemistry defects in superconducting niobium qubits that may be causing loss.
“Superconducting qubits are a promising quantum computing platform because we can engineer their properties and make them using the same tools used to make regular computers,” said Anjali Premkumar, a fourth-year graduate student in the Houck Lab at Princeton University and first author on the Communications Materials paper describing the research. “However, they have shorter coherence times than other platforms.”
In other words, they can’t hold onto information very long before they lose it. Though coherence times have recently improved from microseconds to milliseconds for single qubits, these times significantly decrease when multiple qubits are strung together.
“Qubit coherence is limited by the quality of the superconductors and the oxides that will inevitably grow on them as the metal comes into contact with oxygen in the air,” continued Premkumar. “But, as qubit engineers, we haven’t characterized our materials in great depth. Here, for the first time, we collaborated with materials experts who can carefully look at the structure and chemistry of our materials with sophisticated tools.”
This collaboration was a “prequel” to the Co-design Center for Quantum Advantage (C2QA), one of five National Quantum Information Science Centers established in 2020 in support of the National Quantum Initiative. Led by Brookhaven Lab, C2QA brings together hardware and software engineers, physicists, materials scientists, theorists, and other experts across national labs, universities, and industry to resolve performance issues with quantum hardware and software. Through materials, devices, and software co-design efforts, the C2QA team seeks to understand and ultimately control material properties to extend coherence times, design devices to generate more robust qubits, optimize algorithms to target specific scientific applications, and develop error-correction solutions. More

Moving heat around where you want it to go — adding it to houses and hairdryers, removing it from car engines and refrigerators — is one of the great challenges of engineering.
All activity generates heat, because energy escapes from everything we do. But too much can wear out batteries and electronic components — like parts in an aging laptop that runs too hot to actually sit on your lap. If you can’t get rid of heat, you’ve got a problem.
Scientists at the University of Chicago have invented a new way to funnel heat around at the microscopic level: a thermal insulator made using an innovative technique. They stack ultra-thin layers of crystalline sheets on top of each other, but rotate each layer slightly, creating a material with atoms that are aligned in one direction but not in the other.
“Think of a partly-finished Rubik’s cube, with layers all rotated in random directions,” said Shi En Kim, a graduate student with the Pritzker School of Molecular Engineering who is the first author of the study. “What that means is that within each layer of the crystal, we still have an ordered lattice of atoms, but if you move to the neighboring layer, you have no idea where the next atoms will be relative to the previous layer — the atoms are completely messy along this direction.”
The result is a material that is extremely good at both containing heat and moving it, albeit in different directions — an unusual ability at the microscale, and one that could have very useful applications in electronics and other technology.
“The combination of excellent heat conductivity in one direction and excellent insulation in the other direction does not exist at all in nature,” said study lead author Jiwoong Park, professor of chemistry and molecular engineering at the University of Chicago. “We hope this could open up an entirely new direction for making novel materials.”
‘Just amazingly low’ More

Over the last year, handshakes have been replaced by fist or elbow bumps as a greeting. It shows that age-old social conventions can not only change, but do so suddenly. But how does this happen? Robotic engineers and marketing scientists from the University of Groningen joined forces to study this phenomenon, combining online experiments and statistical analysis into a mathematical model that shows how a committed minority can influence the majority to overturn long-standing practices. The results, which were published in Nature Communications on 29 September, may help to stimulate sustainable behaviour.
How does complex human behaviour take shape? This is studied in many ways, mostly relying on lots of data from observations and experiments. Ming Cao, Professor of Networks and Robotics at the Faculty of Science and Engineering at the University of Groningen, has studied complex group behaviour in robots by using agent-based simulations, among other methods. These agents follow a limited number of simple rules, often inspired by nature, which can lead to realistic complex behaviour. ‘Swarming birds or schools of fish are a good example’, Cao explains, ‘their movements can be reproduced by agents that follow a few simple rules on keeping a certain distance and heading in the same direction as their neighbours.’
Game
In parallel, the Marketing research group at the Faculty of Economics and Business, led by Dr Jan Willem Bolderdijk, Dr Hans Risselada, and Prof. Bob Fennis, has carried out various research projects into human behaviour, but not so many using these kinds of agent-based models. After a discussion with Cao and his colleagues, both groups saw possibilities for such models. Consequently, marketing PhD student Zan Mlakar and the two post-doc researchers in Cao’s group, Mengbin Ye and Lorenzo Zino, worked together creating an online experiment to gather data on the social diffusion of new behavioural trends.
They developed an online game in which 12 participants act as board members of a company that plans to launch one of two potential products. The participants have to vote on which product to launch. The catch is that the decision has to be taken unanimously. The participants cannot discuss their choice, they vote in 24 consecutive rounds, and they only see the distribution of votes at the end of each round. If unanimity is reached, the participants receive a reward.
Rules
Unknown to the participants, between two to four participants in the groups studied were computer bots, programmed to stick to their choice. ‘If the majority voted for product A in the first round, the bots were set to vote for B to try and overturn the majority’, explains Ye, who now works as Senior Research Fellow at Curtin University in Australia. Meanwhile, the votes of the human participants over all the rounds studied were registered. The vast majority of over 20 of these online game rounds resulted in a unanimous vote, with humans eventually siding with the bots to vote for product B. The results of all the games were then analysed to look for patterns in the voting decisions of the human participants.
Ye: ‘In quite few cases, we saw a delay before the votes started changing, but when they did, the group would reach unanimity in just a few voting rounds.’ The overall voting behaviour was able to be reproduced in an agent-based model with three simple rules: do as the majority does, stick to your previous decision, and follow the trend. ‘These rules are acknowledged in the literature as group coordination, inertia, and trend-seeking’, explains Ye. ‘They have been separately studied in human behaviour, but never combined in one model; this combination was critical in capturing social change.’
The results of the experiments and the simulations show that new conventions can suddenly arise when the influence of a committed minority reaches a threshold. A small group of ‘activists’ can therefore change social conventions. Cao: ‘However, this only happens if the minority is also able to influence others in their network. And this depends on the amount of risk-taking present among the other voters.’ The team are now interested in exploring what might enhance or inhibit this risk-taking behaviour. ‘We now have a solid framework and a model, which can be used to examine environmental factors that might make people have greater inertia, or be more susceptible to trends’, says Ye.
The three basic rules could help in steering the behaviour of large groups. ‘Of course, we can’t control people’, stresses Cao. ‘But we can provide guidelines, for example on how to nudge people to change their behaviour.’ This could be useful in the energy transition, or in getting people to reduce their meat consumption. ‘Governments already spend money to convince people to adopt more sustainable behaviour. Our research can help them to spend it in a more effective way.’
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Patients using take-home technology following non-elective surgery resulted in significantly greater detection and correction of drug errors, and reduction in patients’ pain, says a national study led by Hamilton researchers.
The study looked at patient outcomes from virtual care and remote automated monitoring (RAM) — video calls with nurses and doctors, and self-monitoring of vital signs using wearable devices.
The research also raised the possibility of a reduction in acute-hospital care as the result of virtual care and RAM.
“We began the study in the first months of the pandemic, when hospitals were challenged to drastically reduce non-emergency care,” said P.J. Devereaux, co-principal investigator of the study.
He is a senior scientist at the Population Health Research Institute (PHRI), professor and director of the division of perioperative care at McMaster University, and a cardiologist and perioperative care physician at Hamilton Health Sciences.
“Our study provides proof of concept that virtual care with RAM can improve outcomes after discharge following non-elective surgery — outcomes that are important to patients,” he said. More

A team led by the Department of Energy’s Oak Ridge National Laboratory has found a rare quantum material in which electrons move in coordinated ways, essentially “dancing.” Straining the material creates an electronic band structure that sets the stage for exotic, more tightly correlated behavior — akin to tangoing — among Dirac electrons, which are especially mobile electric charge carriers that may someday enable faster transistors. The results are published in the journal Science Advances.
“We combined correlation and topology in one system,” said co-principal investigator Jong Mok Ok, who conceived the study with principal investigator Ho Nyung Lee of ORNL. Topology probes properties that are preserved even when a geometric object undergoes deformation, such as when it is stretched or squeezed. “The research could prove indispensable for future information and computing technologies,” added Ok, a former ORNL postdoctoral fellow.
In conventional materials, electrons move predictably (for example, lethargically in insulators or energetically in metals). In quantum materials in which electrons strongly interact with each other, physical forces cause the electrons to behave in unexpected but correlated ways; one electron’s movement forces nearby electrons to respond.
To study this tight tango in topological quantum materials, Ok led the synthesis of an extremely stable crystalline thin film of a transition metal oxide. He and colleagues made the film using pulsed-laser epitaxy and strained it to compress the layers and stabilize a phase that does not exist in the bulk crystal. The scientists were the first to stabilize this phase.
Using theory-based simulations, co-principal investigator Narayan Mohanta, a former ORNL postdoctoral fellow, predicted the band structure of the strained material. “In the strained environment, the compound that we investigated, strontium niobate, a perovskite oxide, changes its structure, creating a special symmetry with a new electron band structure,” Mohanta said.
Different states of a quantum mechanical system are called “degenerate” if they have the same energy value upon measurement. Electrons are equally likely to fill each degenerate state. In this case, the special symmetry results in four states occurring in a single energy level. More