Computers Math

Engineers at the University of California San Diego have developed a powerful new tool that monitors the electrical activity inside heart cells, using tiny “pop-up” sensors that poke into cells without damaging them. The device directly measures the movement and speed of electrical signals traveling within a single heart cell — a first — as well as between multiple heart cells. It is also the first to measure these signals inside the cells of 3D tissues.
The device, published Dec. 23 in the journal Nature Nanotechnology, could enable scientists to gain more detailed insights into heart disorders and diseases such as arrhythmia (abnormal heart rhythm), heart attack and cardiac fibrosis (stiffening or thickening of heart tissue).
“Studying how an electrical signal propagates between different cells is important to understand the mechanism of cell function and disease,” said first author Yue Gu, who recently received his Ph.D. in materials science and engineering at UC San Diego. “Irregularities in this signal can be a sign of arrhythmia, for example. If the signal cannot propagate correctly from one part of the heart to another, then some part of the heart cannot receive the signal so it cannot contract.”
“With this device, we can zoom in to the cellular level and get a very high resolution picture of what’s going on in the heart; we can see which cells are malfunctioning, which parts are not synchronized with the others, and pinpoint where the signal is weak,” said senior author Sheng Xu, a professor of nanoengineering at the UC San Diego Jacobs School of Engineering. “This information could be used to help inform clinicians and enable them to make better diagnoses.”
The device consists of a 3D array of microscopic field effect transistors, or FETs, that are shaped like sharp pointed tips. These tiny FETs pierce through cell membranes without damaging them and are sensitive enough to detect electrical signals — even very weak ones — directly inside the cells. To evade being seen as a foreign substance and remain inside the cells for long periods of time, the FETs are coated in a phospholipid bilayer. The FETs can monitor signals from multiple cells at the same time. They can even monitor signals at two different sites inside the same cell.
“That’s what makes this device unique,” said Gu. “It can have two FET sensors penetrate inside one cell — with minimal invasiveness — and allow us to see which way a signal propagates and how fast it goes. This detailed information about signal transportation within a single cell has so far been unknown.”
To build the device, the team first fabricated the FETs as 2D shapes, and then bonded select spots of these shapes onto a pre-stretched elastomer sheet. The researchers then loosened the elastomer sheet, causing the device to buckle and the FETs to fold into a 3D structure so that they can penetrate inside cells. More

A large, unconventional anomalous Hall resistance in a new magnetic semiconductor in the absence of large-scale magnetic ordering has been demonstrated by Tokyo Tech materials scientists, validating a recent theoretical prediction. Their findings provide new insights into the anomalous Hall effect, a quantum phenomenon that has previously been associated with long-range magnetic order.
Charged particles such as electrons can behave in interacting ways when moving under the influence of electric and magnetic fields. For instance, when a magnetic field is applied perpendicular to the plane of a current-carrying conductor, the electrons flowing within start to deviate sideways due to magnetic force and soon enough, a voltage difference appears across the conductor. This phenomenon is famously called the “Hall effect.” However, the Hall effect does not necessarily require fiddling with magnets. In fact, it can be observed in magnetic materials with long-range magnetic order, such as ferromagnets, for free!
Named “anomalous Hall effect” (AHE), this phenomenon appears to be a close cousin of the Hall effect. However, its mechanism is way more involved. Currently, the most accepted one is that the AHE is produced by a property of the electronic energy bands called “Berry curvature,” which results from an interaction between the electron’s spin and its motion inside the material, more commonly known as “spin-orbit interaction.”
Is magnetic ordering necessary for AHE? A recent theory suggests otherwise. “It has been theoretically proposed that a large AHE is possible even above the temperature at which the magnetic order vanishes, especially in magnetic semiconductors with low charge carrier density, strong exchange interaction between electrons, and finite spin chirality, which relates to the spin direction with respect to the direction of motion,” explains Associate Professor Masaki Uchida from Tokyo Institute of Technology (Tokyo Tech), whose research focus lies in condensed matter physics.
Curious, Dr. Uchida and his collaborators from Japan decided to put this theory to the test. In a new study published in Science Advances, they investigated the magnetic properties of a new magnetic semiconductor EuAs that is only known to have a peculiar distorted triangular lattice structure and observed an antiferromagnetic (AFM) behavior (neighboring electron spins aligned in opposite directions) below 23 K. Furthermore, they observed that the material’s electrical resistance dropped dramatically with temperature in the presence of an external magnetic field, a behavior known as “colossal magnetoresistance” (CMR). However, more interestingly, the CMR was observed even above 23 K, where the AFM order vanished.
“It is naturally understood that the CMR observed in EuAs is caused by a coupling between the diluted carriers and localized Eu2+ spins that persist over a wide range of temperatures,” comments Dr. Uchida.
What really stole the show, however, was the rise in Hall resistivity with temperature, which peaked at a temperature of 70 K, far above the AFM ordering temperature, demonstrating that large AHE was indeed possible without magnetic order. To understand what caused this unconventionally large AHE, the team performed model calculations, which showed that the effect could be attributed to a skew scattering of electrons by a spin cluster on the triangular lattice in a “hopping regime” where the electrons did not flow but rather “hopped” from atom to atom.
These results bring us one step closer to understanding the strange behavior of electrons inside magnetic solids. “Our findings have helped shed light on triangular-lattice magnetic semiconductors and could potentially lead to a new field of research targeting diluted carriers coupled to unconventional spin orderings and fluctuations,” comments an optimistic Dr. Uchida.
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A Rice University-led study is forcing physicists to rethink superconductivity in uranium ditelluride, an A-list material in the worldwide race to create fault-tolerant quantum computers.
Uranium ditelluride crystals are believed to host a rare “spin-triplet” form of superconductivity, but puzzling experimental results published this week in Nature have upended the leading explanation of how the state of matter could arise in the material. Neutron-scattering experiments by physicists from Rice, Oak Ridge National Laboratory, the University of California, San Diego and the National High Magnetic Field Laboratory at Florida State University revealed telltale signs of antiferromagnetic spin fluctuations that were coupled to superconductivity in uranium ditelluride.
Spin-triplet superconductivity has not been observed in a solid-state material, but physicists have long suspected it arises from an ordered state that is ferromagnetic. The race to find spin-triplet materials has heated up in recent years due to their potential for hosting elusive quasiparticles called Majorana fermions that could be used to make error-free quantum computers.
“People have spent billions of dollars trying to search for them,” Rice study co-author Pengcheng Dai said of Majorana fermions, hypothetical quasiparticles that could be used to make topological quantum bits free from the problematic decoherence that plagues qubits in today’s quantum computers.
“The promise is that if you have a spin-triplet superconductor, it can potentially be used to make topological qubits,” said Dai, a professor of physics and astronomy and member of the Rice Quantum Initiative. “You can’t do that with spin-singlet superconductors. So, that’s why people are extremely interested in this.”
Superconductivity happens when electrons form pairs and move as one, like couples spinning across a dance floor. Electrons naturally loathe one another, but their tendency to avoid other electrons can be overcome by their inherent desire for a low-energy existence. If pairing allows electrons to achieve a more sloth-like state than they could achieve on their own — something that’s only possible at extremely cold temperatures — they can be coaxed into pairs. More

Researchers at the RIKEN Center for Emergent Matter Science (CEMS) and the RIKEN Cluster for Pioneering Research (CPR) in Japan have developed a technique to improve the flexibility of ultra-thin electronics, such as those used in bendable devices or clothing. Published in Science Advances, the study details the use of water vapor plasma to directly bond gold electrodes fixed onto separate ultra-thin polymer films, without needing adhesives or high temperatures.
As electronic devices get smaller and smaller, and the desire to have bendable, wearable, and on-skin electronics increases, conventional methods of constructing these devices have become impractical. One of the biggest problems is how to connect and integrate multiple devices or pieces of a device that each reside on separate ultra-thin polymer films. Conventional methods that use layers of adhesive to stick electrodes together reduce flexibility and require temperature and pressure that are damaging to super-thin electronics. Conventional methods of direct metal-to-metal bonding are available, but require perfectly smooth and clean surfaces that are not typical in these types of electronics.
A team of researchers led by Takao Someya at RIKEN CEMS/CPR has developed a new method to secure these connections that does not use adhesive, high temperature, or high pressure, and does not require totally smooth or clean surfaces. In fact, the process takes less than a minute at room temperature, followed by about a 12-hour wait. The new technique, called water-vapor plasma-assisted bonding, creates stable bonds between gold electrodes that are printed into ultra-thin — 2 thousandths of a millimeter! — polymer sheets using a thermal evaporator.
“This is the first demonstration of ultra-thin, flexible gold electronics fabricated without any adhesive,” says Senior Research Scientist Kenjiro Fukuda of RIKEN CEMS/CPR. “Using this new direct bond technology, we were able to fabricate an integrated system of flexible organic solar cells and organic LEDs.” Experiments showed that water-vapor plasma-assisted bonding performed better that conventional adhesive or direct bonding techniques. In particular, the strength and consistency of the bonds were greater than what standard surface-assisted direct bonding achieved. At the same time, the material conformed better to curved surfaces and was more durable than what could be achieved using a standard adhesive technique.
According to Fukuda, the method itself is surprisingly simple, which might explain why they discovered it by accident. After fixing the gold electrodes onto polymer sheets, a machine is used to expose the electrode sides of the sheets to water-vapor plasma for 40 seconds. Then, the polymer sheets are pressed together so that the electrodes overlap in the correct location. After waiting 12 hours in room temperature, they are ready to use. Another advantage of this system is that after activation with water-vapor plasma, but before being bonded together, the films can be stored in vacuum packs for days. This is an important practical aspect when considering the potential for ordering and distributing pre-activated components.
As proof of concept, the team integrated ultra-thin organic photovoltaic and LED-light modules that were printed on separate films and connected by five additional polymer films. The devices withstood extensive testing, including being wrapped around a stick and being crumpled and twisted to extremes. Additionally, the power efficiency of the LEDs did not suffer from the treatment. The technique was also able to join pre-packaged LED chips to a flexible surface.
“We expect this new method to become a flexible wiring and mounting technology for next-generation wearable electronics that can be attached to clothes and skin,” says Fukuda. “The next step is to develop this technology for use with cheaper metals, such as copper or aluminum.”
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Quantum effects in superconductors could give semiconductor technology a new twist. Researchers at the Paul Scherrer Institute PSI and Cornell University in New York State have identified a composite material that could integrate quantum devices into semiconductor technology, making electronic components significantly more powerful. They publish their findings today in the journal Science Advances.
Our current electronic infrastructure is based primarily on semiconductors. This class of materials emerged around the middle of the 20th century and has been improving ever since. Currently, the most important challenges in semiconductor electronics include further improvements that would increase the bandwidth of data transmission, energy efficiency and information security. Exploiting quantum effects is likely to be a breakthrough.
Quantum effects that can occur in superconducting materials are particularly worthy of consideration. Superconductors are materials in which the electrical resistance disappears when they are cooled below a certain temperature. The fact that quantum effects in superconductors can be utilised has already been demonstrated in first quantum computers.
To find possible successors for today’s semiconductor electronics, some researchers — including a group at Cornell University — are investigating so-called heterojunctions, i.e. structures made of two different types of materials. More specifically, they are looking at layered systems of superconducting and semiconducting materials. “It has been known for some time that you have to select materials with very similar crystal structures for this, so that there is no tension in the crystal lattice at the contact surface,” explains John Wright, who produced the heterojunctions for the new study at Cornell University.
Two suitable materials in this respect are the superconductor niobium nitride (NbN) and the semiconductor gallium nitride (GaN). The latter already plays an important role in semiconductor electronics and is therefore well researched. Until now, however, it was unclear exactly how the electrons behave at the contact interface of these two materials — and whether it is possible that the electrons from the semiconductor interfere with the superconductivity and thus obliterate the quantum effects.
“When I came across the research of the group at Cornell, I knew: here at PSI we can find the answer to this fundamental question with our spectroscopic methods at the ADRESS beamline,” explains Vladimir Strocov, researcher at the Synchrotron Light Source SLS at PSI.
This is how the two groups came to collaborate. In their experiments, they eventually found that the electrons in both materials “keep to themselves.” No unwanted interaction that could potentially spoil the quantum effects takes place.
Synchrotron light reveals the electronic structures
The PSI researchers used a method well-established at the ADRESS beamline of the SLS: angle-resolved photoelectron spectroscopy using soft X-rays — or SX-ARPES for short. “With this method, we can visualise the collective motion of the electrons in the material,” explains Tianlun Yu, a postdoctoral researcher in Vladimir Strocov’s team, who carried out the measurements on the NbN/GaN heterostructure. Together with Wright, Yu is the first author of the new publication.
The SX-ARPES method provides a kind of map whose spatial coordinates show the energy of the electrons in one direction and something like their velocity in the other; more precisely, their momentum. “In this representation, the electronic states show up as bright bands in the map,” Yu explains. The crucial research result: at the material boundary between the niobium nitride NbN and the gallium nitride GaN, the respective “bands” are clearly separated from each other. This tells the researchers that the electrons remain in their original material and do not interact with the electrons in the neighbouring material.
“The most important conclusion for us is that the superconductivity in the niobium nitride remains undisturbed, even if this is placed atom by atom to match a layer of gallium nitride,” says Vladimir Strocov. “With this, we were able to provide another piece of the puzzle that confirms: This layer system could actually lend itself to a new form of semiconductor electronics that embeds and exploits the quantum effects that happen in superconductors.”
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Scientists and institutions dedicate more resources each year to the discovery of novel materials to fuel the world. As natural resources diminish and the demand for higher value and advanced performance products grows, researchers have increasingly looked to nanomaterials.
Nanoparticles have already found their way into applications ranging from energy storage and conversion to quantum computing and therapeutics. But given the vast compositional and structural tunability nanochemistry enables, serial experimental approaches to identify new materials impose insurmountable limits on discovery.
Now, researchers at Northwestern University and the Toyota Research Institute (TRI) have successfully applied machine learning to guide the synthesis of new nanomaterials, eliminating barriers associated with materials discovery. The highly trained algorithm combed through a defined dataset to accurately predict new structures that could fuel processes in clean energy, chemical and automotive industries.
“We asked the model to tell us what mixtures of up to seven elements would make something that hasn’t been made before,” said Chad Mirkin, a Northwestern nanotechnology expert and the paper’s corresponding author. “The machine predicted 19 possibilities, and, after testing each experimentally, we found 18 of the predictions were correct.”
The study, “Machine learning-accelerated design and synthesis of polyelemental heterostructures,” will be published December 22 in the journal Science Advances.
Mirkin is the George B. Rathmann Professor of Chemistry in the Weinberg College of Arts and Sciences; a professor of chemical and biological engineering, biomedical engineering, and materials science and engineering at the McCormick School of Engineering; and a professor of medicine at the Feinberg School of Medicine. He also is the founding director of the International Institute for Nanotechnology. More

Which factors determine how fast a quantum computer can perform its calculations? Physicists at the University of Bonn and the Technion — Israel Institute of Technology have devised an elegant experiment to answer this question. The results of the study are published in the journal Science Advances.
Quantum computers are highly sophisticated machines that rely on the principles of quantum mechanics to process information. This should enable them to handle certain problems in the future that are completely unsolvable for conventional computers. But even for quantum computers, fundamental limits apply to the amount of data they can process in a given time.
Quantum gates require a minimum time
The information stored in conventional computers can be thought of as a long sequence of zeros and ones, the bits. In quantum mechanics it is different: The information is stored in quantum bits (qubits), which resemble a wave rather than a series of discrete values. Physicists also speak of wave functions when they want to precisely represent the information contained in qubits.
In a traditional computer, information is linked together by so-called gates. Combining several gates allows elementary calculations, such as the addition of two bits. Information is processed in a very similar way in quantum computers, where quantum gates change the wave function according to certain rules.
Quantum gates resemble their traditional relatives in another respect: “Even in the quantum world, gates do not work infinitely fast,” explains Dr. Andrea Alberti of the Institute of Applied Physics at the University of Bonn. “They require a minimum amount of time to transform the wave function and the information this contains.”
More than 70 years ago, Soviet physicists Leonid Mandelstam and Igor Tamm deduced theoretically this minimum time for transforming the wave function. Physicists at the University of Bonn and the Technion have now investigated this Mandelstam-Tamm limit for the first time with an experiment on a complex quantum system. To do this, they used cesium atoms that moved in a highly controlled manner. “In the experiment, we let individual atoms roll down like marbles in a light bowl and observe their motion,” explains Alberti, who led the experimental study. More

Technologies that take advantage of novel quantum mechanical behaviors are likely to become commonplace in the near future. These may include devices that use quantum information as input and output data, which require careful verification due to inherent uncertainties. The verification is more challenging if the device is time dependent when the output depends on past inputs. For the first time, researchers using machine learning dramatically improved the efficiency of verification for time-dependent quantum devices by incorporating a certain memory effect present in these systems.
Quantum computers make headlines in the scientific press, but these machines are considered by most experts to still be in their infancy. A quantum internet, however, may be a little closer to the present. This would offer significant security advantages over our current internet, amongst other things. But even this will rely on technologies that have yet to see the light of day outside the lab. While many fundamentals of the devices that can create our quantum internet may have been worked out, there are many engineering challenges in order to realize these as products. But much research is underway to create tools for the design of quantum devices.
Postdoctoral researcher Quoc Hoan Tran and Associate Professor Kohei Nakajima from the Graduate School of Information Science and Technology at the University of Tokyo have pioneered just such a tool, which they think could make verifying the behavior of quantum devices a more efficient and precise undertaking than it is at present. Their contribution is an algorithm that can reconstruct the workings of a time-dependent quantum device by simply learning the relationship between the quantum inputs and outputs. This approach is actually commonplace when exploring a classical physical system, but quantum information is generally tricky to store, which usually makes it impossible.
“The technique to describe a quantum system based on its inputs and outputs is called quantum process tomography,” said Tran. “However, many researchers now report that their quantum systems exhibit some kind of memory effect where present states are affected by previous ones. This means that a simple inspection of input and output states cannot describe the time-dependent nature of the system. You could model the system repeatedly after every change in time, but this would be extremely computationally inefficient. Our aim was to embrace this memory effect and use it to our advantage rather than use brute force to overcome it.”
Tran and Nakajima turned to machine learning and a technique called quantum reservoir computing to build their novel algorithm. This learns patterns of inputs and outputs that change over time in a quantum system and effectively guesses how these patterns will change, even in situations the algorithm has not yet witnessed. As it does not need to know the inner workings of a quantum system as a more empirical method might, but only the inputs and outputs, the team’s algorithm can be simpler and produce results faster as well.
“At present, our algorithm can emulate a certain kind of quantum system, but hypothetical devices may vary widely in their processing ability and have different memory effects. So the next stage of research will be to broaden the capabilities of our algorithms, essentially making something more general purpose and thus more useful,” said Tran. “I am excited by what quantum machine learning methods could do, by the hypothetical devices they might lead to.”
This work is supported by MEXT Quantum Leap Flagship Program (MEXT Q-LEAP) Grant
Nos. JPMXS0118067394 and JPMXS0120319794.
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