QT/ Functional semiconductor made from graphene
Quantum news biweekly vol.66, 2nd January — 18th January
TL;DR
- Researchers have created the first functional semiconductor made from graphene, a single sheet of carbon atoms held together by the strongest bonds known. The breakthrough throws open the door to a new way of doing electronics
- Scientists have developed a method to extract the spectral density for molecules in solvent using simple resonance Raman experiments — a method that captures the full complexity of chemical environments.
- Engineers pair vibrating particles, called phonons, with particles of light, called photons, to enhance the nonlinear optical properties of hexagonal boron nitride.
- Researchers have fabricated a new high-performance shortwave infrared (SWIR) image sensor based on non-toxic colloidal quantum dots. They report on a new method for synthesizing functional high-quality non-toxic colloidal quantum dots integrable with complementary metal-oxide-semiconductor (CMOS) technology.
- Perovskite nanosheets show distinctive characteristics with significant applications in science and technology. In a recent study, researchers achieved enhanced signal amplification in CsPbBr3 perovskite nanosheets with a unique waveguide pattern, which enhanced both gain and thermal stability. These advancements carry wide-ranging implications for laser, sensor, and solar cell applications, and can potentially influence areas like environmental monitoring, industrial processes, and healthcare.
- An international research team recently demonstrated how magnetism can be actively changed by pressure.
- Lights could soon use the full color suite of perfectly efficient organic light-emitting diodes, or OLEDs, that last tens of thousands of hours, thanks to an innovation from physicists and engineers at the University of Michigan.
- Researchers developed a way to quickly calculate the transition state structure of a chemical reaction, using machine-learning models.
- For the first time, scientists publish results on a new chip composed of diamond and lithium niobate. The results demonstrate the combination as a promising candidate for quantum devices.
- In a study that could help fill some holes in quantum theory, the team recreated a ‘quantum bomb tester’ in a classical droplet test.
- And more!
Quantum Computing Market
According to the recent market research report ‘Quantum Computing Market with COVID-19 impact by Offering (Systems and Services), Deployment (On Premises and Cloud Based), Application, Technology, End-use Industry and Region — Global Forecast to 2026’, published by MarketsandMarkets, the Quantum Computing market is expected to grow from USD 472 million in 2021 to USD 1,765 million by 2026, at a CAGR of 30.2%. The early adoption of quantum computing in the banking and finance sector is expected to fuel the growth of the market globally. Other key factors contributing to the growth of the quantum computing market include rising investments by governments of different countries to carry out research and development activities related to quantum computing technology. Several companies are focusing on the adoption of QCaaS post-COVID-19. This, in turn, is expected to contribute to the growth of the quantum computing market. However, stability and error correction issues are expected to restrain the growth of the market.
According to ‘Quantum Computing Market Research Report: By Offering, Deployment Type, Application, Technology, Industry — Industry Share, Growth, Drivers, Trends and Demand Forecast to 2030’ report, the quantum computing market is projected to reach $64,988 million by 2030. Machine learning (ML) is expected to progress at the highest CAGR, during the forecast period, among all application categories, owing to the fact that quantum computing is being integrated in ML for improving the latter’s use case.
Latest Research
Ultrahigh-mobility semiconducting epitaxial graphene on silicon carbide
by Jian Zhao, Peixuan Ji, Yaqi Li, Rui Li, Kaimin Zhang, Hao Tian, Kaicheng Yu, Boyue Bian, Luzhen Hao, Xue Xiao, Will Griffin, Noel Dudeck, Ramiro Moro, Lei Ma, Walt A. de Heer in Nature
Researchers at the Georgia Institute of Technology have created the world’s first functional semiconductor made from graphene, a single sheet of carbon atoms held together by the strongest bonds known. Semiconductors, which are materials that conduct electricity under specific conditions, are foundational components of electronic devices. The team’s breakthrough throws open the door to a new way of doing electronics.
Their discovery comes at a time when silicon, the material from which nearly all modern electronics are made, is reaching its limit in the face of increasingly faster computing and smaller electronic devices. Walter de Heer, Regents’ Professor of physics at Georgia Tech, led a team of researchers based in Atlanta, Georgia, and Tianjin, China, to produce a graphene semiconductor that is compatible with conventional microelectronics processing methods — a necessity for any viable alternative to silicon.
In this latest research, de Heer and his team overcame the paramount hurdle that has been plaguing graphene research for decades, and the reason why many thought graphene electronics would never work. Known as the “band gap,” it is a crucial electronic property that allows semiconductors to switch on and off. Graphene didn’t have a band gap — until now.
“We now have an extremely robust graphene semiconductor with 10 times the mobility of silicon, and which also has unique properties not available in silicon,” de Heer said. “But the story of our work for the past 10 years has been, ‘Can we get this material to be good enough to work?’”
De Heer started to explore carbon-based materials as potential semiconductors early in his career, and then made the switch to exploring two-dimensional graphene in 2001. He knew then that graphene had potential for electronics.
“We were motivated by the hope of introducing three special properties of graphene into electronics,” he said. “It’s an extremely robust material, one that can handle very large currents, and can do so without heating up and falling apart.”
De Heer achieved a breakthrough when he and his team figured out how to grow graphene on silicon carbide wafers using special furnaces. They produced epitaxial graphene, which is a single layer that grows on a crystal face of the silicon carbide. The team found that when it was made properly, the epitaxial graphene chemically bonded to the silicon carbide and started to show semiconducting properties. Over the next decade, they persisted in perfecting the material at Georgia Tech and later in collaboration with colleagues at the Tianjin International Center for Nanoparticles and Nanosystems at Tianjin University in China. De Heer founded the center in 2014 with Lei Ma, the center’s director and a co-author of the paper.
In its natural form, graphene is neither a semiconductor nor a metal, but a semimetal. A band gap is a material that can be turned on and off when an electric field is applied to it, which is how all transistors and silicon electronics work. The major question in graphene electronics research was how to switch it on and off so it can work like silicon. But to make a functional transistor, a semiconducting material must be greatly manipulated, which can damage its properties. To prove that their platform could function as a viable semiconductor, the team needed to measure its electronic properties without damaging it.
They put atoms on the graphene that “donate” electrons to the system — a technique called doping, used to see whether the material was a good conductor. It worked without damaging the material or its properties. The team’s measurements showed that their graphene semiconductor has 10 times greater mobility than silicon. In other words, the electrons move with very low resistance, which, in electronics, translates to faster computing. “It’s like driving on a gravel road versus driving on a freeway,” de Heer said. “It’s more efficient, it doesn’t heat up as much, and it allows for higher speeds so that the electrons can move faster.” The team’s product is currently the only two-dimensional semiconductor that has all the necessary properties to be used in nanoelectronics, and its electrical properties are far superior to any other 2D semiconductors currently in development.
“A long-standing problem in graphene electronics is that graphene didn’t have the right band gap and couldn’t switch on and off at the correct ratio,” said Ma. “Over the years, many have tried to address this with a variety of methods. Our technology achieves the band gap, and is a crucial step in realizing graphene-based electronics.”
Epitaxial graphene could cause a paradigm shift in the field of electronics and allow for completely new technologies that take advantage of its unique properties. The material allows the quantum mechanical wave properties of electrons to be utilized, which is a requirement for quantum computing.
“Our motivation for doing graphene electronics has been there for a long time, and the rest was just making it happen,” de Heer said. “We had to learn how to treat the material, how to make it better and better, and finally how to measure the properties. That took a very, very long time.”
According to de Heer, it is not unusual to see yet another generation of electronics on its way. Before silicon, there were vacuum tubes, and before that, there were wires and telegraphs. Silicon is one of many steps in the history of electronics, and the next step could be graphene.
“To me, this is like a Wright brothers moment,” de Heer said. “They built a plane that could fly 300 feet through the air. But the skeptics asked why the world would need flight when it already had fast trains and boats. But they persisted, and it was the beginning of a technology that can take people across oceans.”
Mapping electronic decoherence pathways in molecules
by Ignacio Gustin, Chang Woo Kim, David W. McCamant, Ignacio Franco in Proceedings of the National Academy of Sciences
In quantum mechanics, particles can exist in multiple states at the same time, defying the logic of everyday experiences. This property, known as quantum superposition, is the basis for emerging quantum technologies that promise to transform computing, communication, and sensing. But quantum superpositions face a significant challenge: quantum decoherence. During this process, the delicate superposition of quantum states breaks down when interacting with its surrounding environment.
To unlock the power of chemistry to build complex molecular architectures for practical quantum applications, scientists need to understand and control quantum decoherence so that they can design molecules with specific quantum coherence properties. Doing so requires knowing how to rationally modify a molecule’s chemical structure to modulate or mitigate quantum decoherence. To that end, scientists need to know the “spectral density,” the quantity which summarizes how fast the environment moves and how strongly it interacts with the quantum system.
Until now, quantifying this spectral density in a way that it accurately reflects the intricacies of molecules has remained elusive to theory and experimentation. But a team of scientists has developed a method to extract the spectral density for molecules in solvent using simple resonance Raman experiments — a method that captures the full complexity of chemical environments. Led by Ignacio Franco, an associate professor of chemistry and of physics at the University of Rochester, the team published their findings.
Using the extracted spectral density, it is possible not only to understand how fast the decoherence happens but also to determine which part of the chemical environment is mostly responsible for it. As a result, scientists can now map decoherence pathways to connect molecular structure with quantum decoherence.
“Chemistry builds up from the idea that molecular structure determines the chemical and physical properties of matter. This principle guides the modern design of molecules for medicine, agriculture, and energy applications. Using this strategy, we can finally start to develop chemical design principles for emerging quantum technologies,” says Ignacio Gustin, a chemistry graduate student at Rochester and the first author of the study.
The breakthrough came when the team recognized that resonance Raman experiments yielded all the information needed to study decoherence with full chemical complexity. Such experiments are routinely used to investigate photophysics and photochemistry, but their utility for quantum decoherence had not been appreciated. The key insights emerged from discussions with David McCamant, an associate professor in the chemistry department at Rochester and an expert in Raman spectroscopy, and with Chang Woo Kim, now on the faculty at Chonnam National University in Korea and an expert in quantum decoherence, while he was a postdoctoral researcher at Rochester.
The team used their method to show, for the first time, how electronic superpositions in thymine, one of the building blocks of DNA, unravel in just 30 femtoseconds (one femtosecond is one millionth of one billionth of a second) following its absorption of UV light. They found that a few vibrations in the molecule dominate the initial steps in the decoherence process, while solvent dominates the later stages. In addition, they discovered that chemical modifications to thymine can significantly alter the decoherence rate, with hydrogen-bond interactions near the thymine ring leading to more rapid decoherence.
Ultimately, the team’s research opens the way toward understanding the chemical principles that govern quantum decoherence. “We are excited to use this strategy to finally understand quantum decoherence in molecules with full chemical complexity and use it to develop molecules with robust coherence properties,” says Franco.
Phonon-enhanced nonlinearities in hexagonal boron nitride
by Jared S. Ginsberg, M. Mehdi Jadidi, Jin Zhang, Cecilia Y. Chen, Nicolas Tancogne-Dejean, Sang Hoon Chae, Gauri N. Patwardhan, Lede Xian, Kenji Watanabe, Takashi Taniguchi, James Hone, Angel Rubio, Alexander L. Gaeta in Nature Communications
Engineers at Columbia and theoretical collaborators at the Max Planck for the Structure and Dynamics of Matter find that pairing laser light to crystal lattice vibrations can enhance the nonlinear optical properties of a layered 2D material.
Cecilia Chen, a Columbia Engineering PhD student and co-author of the recent paper, and her colleagues from Alexander Gaeta’s Quantum and Nonlinear Photonics group used hexagonal boron nitride (hBN). hBN is a 2D material similar to graphene: its atoms are arranged in a honey-combed-shaped repeating pattern and can be peeled into thin layers with unique quantum properties. Chen noted that hBN is stable at room temperature, and its constituent elements — boron and nitrogen — are very light. That means they vibrate very quickly.
Atomic vibrations occur in all materials above absolute zero. That movement can be quantized into quasiparticles called phonons with particular resonances; in hBN’s case, the team was interested in the optical phonon mode vibrating at 41 THz, corresponding to a wavelength of 7.3 μm, which is in the mid-infrared regime of the electromagnetic spectrum.
While mid-IR wavelengths are considered short, and thus, high energy, in the picture of crystal vibrations, they are considered very long and low energy in most optics research with lasers, where the overwhelming majority of experiments and studies are performed in the visible to near-IR range of approximately 400 nm to 2 um.
When they tuned their laser system to hBN’s frequency corresponding to 7.3 μm, Chen, along with fellow PhD student Jared Ginsberg (now a data scientist at Bank of America) and postdoc Mehdi Jadidi (now a Team Lead at quantum computing company PsiQuantum), were able to coherently and simultaneously drive the phonons and electrons of the hBN crystal to efficiently generate new optical frequencies from the medium — an essential goal of nonlinear optics.
Theoretical work led by Professor Angel Rubio’s group at Max Planck helped the experimental team understand their results. Using commercially available, table-top mid-infrared lasers, they explored the phonon-mediated nonlinear optical process of four-wave mixing to generate light close to even harmonics of an optical signal. They also observed greater than a 30-fold increase in third-harmonic generation over what is achieved without exciting the phonons.
“We’re excited to show that amplifying the natural phonon motion with laser driving can enhance nonlinear optical effects and generate new frequencies,” said Chen. The team plans to explore how they might be able to modify hBN and materials like it using light in future work.
Silver telluride colloidal quantum dot infrared photodetectors and image sensors
by Yongjie Wang, Lucheng Peng, Julien Schreier, Yu Bi, Andres Black, Aditya Malla, Stijn Goossens, Gerasimos Konstantatos in Nature Photonics
ICFO and Qurv researchers have fabricated a new high-performance shortwave infrared (SWIR) image sensor based on non-toxic colloidal quantum dots. In their study published in Nature Photonics, they report on a new method for synthesizing functional high-quality non-toxic colloidal quantum dots integrable with complementary metal-oxide-semiconductor (CMOS) technology.
Invisible to our eyes, shortwave infrared (SWIR) light can enable unprecedented reliability, function and performance in high-volume, computer vision first applications in service robotics, automotive and consumer electronics markets. Image sensors with SWIR sensitivity can operate reliably under adverse conditions such as bright sunlight, fog, haze and smoke. Furthermore, the SWIR range provides eye-safe illumination sources and opens up the possibility of detecting material properties through molecular imaging.
Colloidal quantum dots (CQD) based image sensor technology offers a promising technology platform to enable high-volume compatible image sensors in the SWIR. CQDs, nanometric semiconductor crystals, are a solution-processed material platform that can be integrated with CMOS and enables accessing the SWIR range.
However, a fundamental roadblock exists in translating SWIR-sensitive quantum dots into key enabling technology for mass-market applications, as they often contain heavy metals like lead or mercury (IV-VI Pb, Hg-chalcogenide semiconductors). These materials are subject to regulations by the Restriction of Hazardous Substances (RoHS), a European directive that regulates their use in commercial consumer electronic applications.
In a new study published in Nature Photonics, ICFO researchers Yongjie Wang, Lucheng Peng, and Aditya Malla led by ICREA Prof. at ICFO Gerasimos Konstantatos, in collaboration with researchers Julien Schreier, Yu Bi, Andres Black, and Stijn Goossens, from Qurv, have reported on the development of high-performance infrared photodetectors and a shortwave infrared (SWIR) image sensor operating at room temperature based on non-toxic colloidal quantum dots. The study describes a new method for synthesizing size tuneable, phosphine-free silver telluride (Ag2Te) quantum dots while preserving the advantageous properties of traditional heavy-metal counterparts paving the way to the introduction of SWIR colloidal quantum dot technology in high-volume markets.
While investigating how to synthetize silver bismuth telluride (AgBiTe2) nanocrystals to extent the spectral coverage of the AsBiS2 technology to enhance the performance of photovoltaic devices, the researchers obtained silver telluride (Ag2Te) as a by-product. This material showed a strong and tuneable quantum confined absorption akin to quantum dots. They realized its potential for SWIR photodetectors and image sensors and pivoted their efforts to achieve and control a new process to synthesize phosphine-free versions of silver telluride quantum dots, as phosphine was found to have a detrimental impact on the optoelectronic properties of the quantum dots relevant to photodetection.
In their new synthetic method, the team used different phosphine-free complexes such as a tellurium and silver precursors that led them to obtain quantum dots with a well-controlled size distribution and excitonic peaks over a very broad range of the spectrum. After fabricating and characterizing them, the new synthesized quantum dots exhibited remarkable performances, with distinct excitonic peaks over 1500 nm — an unprecedented achievement compared to previous phosphine-based techniques for quantum dot fabrication.
The researchers decided then to implement the obtained phosphine-free quantum dots to fabricate a simple laboratory scale photodetector on the common standard ITO (Indium Tin Oxide)-coated glass substrate to characterize the devices and measure its properties.
“Those lab-scale devices are operated with shining light from the bottom. For CMOS integrated CQD stacks, light comes from the top, whereas the bottom part of the device is taken by the CMOS electronics,” comments Yongjie Wang, postdoc researcher at ICFO and first author of the study. “So, the first challenge we had to overcome was reverting the device setup. A process that in theory sounds simple, but in reality proved to be a challenging task.”
Initially, the photodiode exhibited a low performance in sensing SWIR light, prompting a redesign that incorporated a buffer layer. This adjustment significantly enhanced the photodetector performance, resulting in a SWIR photodiode exhibiting a spectral range from 350nm to 1600nm, a linear dynamic range exceeding 118 dB, a -3dB bandwidth surpassing 110 kHz and a room temperature detectivity of the order 1012 Jones.
“To the best of our knowledge, the photodiodes reported here have for the first time realized solution processed, non-toxic shortwave infrared photodiodes with figures of merit on par with other heavy-metal containing counterparts,” Gerasimos Konstantatos, ICREA Prof. at ICFO and leading author of the study mentions. “These results further support the fact that Ag2Te quantum dots emerge as a promising RoHS-compliant material for low-cost, high-performance SWIR photodetectors applications.”
With the successful development of this heavy-metal-free quantum dot based photodetector, the researchers went further and teamed up with Qurv, an ICFO spin-off, to demonstrate its potential by constructing a SWIR image sensor as a case study. The team integrated the new photodiode with a CMOS based read-out integrated circuit (ROIC) focal plane array (FPA) demonstrating for the first time a proof-of-concept, non-toxic, room temperature-operating SWIR quantum dot based image sensor. The authors of the study tested the imager to prove its operation in the SWIR by taking several pictures of a target object. In particular, they were able to image the transmission of silicon wafers under the SWIR light as well as to visualize the content of plastic bottles that were opaque in the visible light range.
“Accessing the SWIR with a low-cost technology for consumer electronics will unleash the potential of this spectral range with a huge range of applications including improved vision systems for automotive industry (cars) enabling vision and driving under adverse weather conditions,” says Gerasimos Konstantatos.
“SWIR band around 1.35–1.40 µm, can provide an eye-safe window, free of background light under day/night condition, thus, further enable long-range light detection and ranging (LiDAR), three-dimensional imaging for automotive, augmented reality and virtual reality applications.”
Now the researchers want to increase the performance of photodiodes by engineering the stack of layers that comprise the photodetector device. They also want to explore new surface chemistries for the Ag2Te quantum dots to improve the performance and the thermal and environmental stability of the material on its way to the market.
Gain enhancement of perovskite nanosheets by a patterned waveguide: excitation and temperature dependence of gain saturation
by Inhong Kim, Ga Eul Choi, Ming Mei, Min Woo Kim, Minju Kim, Young Woo Kwon, Tae-In Jeong, Seungchul Kim, Suck Won Hong, Kwangseuk Kyhm, Robert A. Taylor in Light: Science & Applications
Perovskite nanosheets show distinctive characteristics with significant applications in science and technology. In a recent study, researchers from Korea and UK achieved enhanced signal amplification in CsPbBr3 perovskite nanosheets with a unique waveguide pattern, which enhanced both gain and thermal stability. These advancements carry wide-ranging implications for laser, sensor, and solar cell applications, and can potentially influence areas like environmental monitoring, industrial processes, and healthcare.
Perovskite materials are still attracting a lot of interest in solar cell applications. Now, the nanostructures of perovskite materials are being considered as a new laser medium. Over the years, light amplification in perovskite quantum dots has been reported, but most of the works present inadequate quantitative analysis. To assess the light amplification ability, “gain coefficient” is necessary, whereby the essential characteristic of a laser medium is revealed. An efficient laser medium is one that has a large gain.
Scientists have been exploring ways to boost this gain. Now, in a recent study, a team of researchers, led by Professor Kwangseuk Kyhm from the Department of Optics & Mechatronics at Pusan National University in Korea, has managed to enhance signal amplification in perovskite nanosheets of CsPbBr3 with a unique waveguide pattern.
Perovskite nanosheets are two-dimensional structures arranged in sheet-like configurations on the nanoscale and possess characteristics that make them valuable for various applications. Their achievement overcomes the shortcomings of CsPbBr3 quantum dots, whose gain is inherently limited due to the Auger process, which essentially shortens the decay time for population inversion (a state in which more members of the system are in higher, excited states than in lower, unexcited energy states).
Prof. Kyhm explains: “Perovskite nanosheets can be a new laser medium, and this work has demonstrated that light amplification can be achieved based on tiny perovskite nanosheets that are synthesized chemically.”
The researchers also proposed a new gain analysis of “gain contour” to overcome the limit of earlier gain analysis. While the old method provides a gain spectrum, it is unable to analyze the gain saturation for long optical stripe lengths. Because the “gain contour” illustrates the variation of the gain with respect to spectrum energy and optical stripe length, it is very convenient to analyze the local gain variation along spectrum energy and optical stripe length.
The researchers also studied the excitation and temperature dependence of the gain contour and the patterned waveguide, based on polyurethane-acrylate, which boosted both the gain and thermal stability of perovskite nanosheets. This enhancement was attributed to improved optical confinement and heat dissipation, which was facilitated by the two-dimensional center-of-mass confined excitons and localized states arising from the inhomogeneous sheet thickness and the defect states. The implementation of such a patterned waveguide is promising for efficient and controlled signal amplification and can contribute to the development of more reliable and versatile devices based on perovskite nanosheets, including lasers, sensors, and solar cells. In addition, it could also impact industries related to encryption and decryption of information, neuromorphic computing, and visible light communication. Furthermore, enhanced amplification and increased efficiencies can help perovskite solar cells compete better with traditional silicon-based solar cells.
The study is also poised to significantly influence optics and photonics. The insights gained can help optimize laser operation, enhance signal transmission in optical communication, and improve sensitivity in photodetectors. This, in turn, could allow devices to operate more reliably. In the long term, when intense light is needed at the nanoscale, perovskite nanosheets can be combined with other nanostructures, allowing the amplified light to serve as an optical probe. However, the successful application of perovskite nanosheets in diverse areas, including consumer products like smartphones and lighting, would depend on overcoming challenges related to their stability, scalability, and toxicity.
“So far, perovskite quantum dots have been studied for lasers, but such zero-dimensional structures have fundamental limits. In this regard, our work suggests that the two-dimensional structure of perovskite nanosheets can be an alternative solution,” concludes Prof. Kyhm.
Controlled Frustration Release on the Kagome Lattice by Uniaxial-Strain Tuning
by Jierong Wang, M. Spitaler, Y.-S. Su, K. M. Zoch, C. Krellner, P. Puphal, S. E. Brown, A. Pustogow in Physical Review Letters
Magnetism occurs depending on how electrons behave. For example, the elementary particles can generate an electric current with their charge and thereby induce a magnetic field. However, magnetism can also arise through the collective alignment of the magnetic moments (spins) in a material. What has not been possible until now, however, is to continuously change the type of magnetism in a crystal.
An international research team led by TU Wien professor Andrej Pustogow has now succeeded in doing just that: Changing magnetism “by pushing a button.” For that, the team continuously changed the magnetic interactions in a single crystal by applying pressure.
People have been fascinated by magnetism for thousands of years and it has made many technical applications possible in the first place. From compasses and electric motors to generators — these and other devices would not exist without ferromagnetism. While ferromagnetism is already well studied, fundamental research is increasingly interested in other forms of magnetism. These are of particular interest for secure data storage and as potential platforms for quantum computers.
“However, searching for novel forms of magnetism and controlling them fully is an extremely difficult endeavour,” says the study leader Andrej Pustogow.
Spins can be visualised as small compass needles that can align themselves in an external magnetic field and have a magnetic field themselves. In case of ferromagnetism, which is used in permanent magnets, all electron spins align parallel to each other. In some arrangements of electron spins, for example in ordinary square, checkerboard-type crystal lattices, an anti-parallel alignment of the spins is also possible: neighbouring spins always point alternately in opposite directions.
With triangular lattices (or lattices in which triangular structures occur, such as the more complex kagome lattice), a completely antiparallel arrangement is not possible: If two corners of a triangle have opposite spin directions, the remaining side must match one of the two directions. Both options — spin up or spin down — are then exactly equivalent.
“This possibility of multiple identical alternatives is known as ‘geometrical frustration’ and occurs in crystal structures with electron spins arranged in triangular, kagome or honeycomb lattices,” explains Pustogow.
As a result, randomly arranged spin pairs are formed, with some spins not finding a partner at all.
“The remaining unpaired magnetic moments could be entangled with each other, manipulated with external magnetic fields and thus used for data storage or computational operations in quantum computers,” says the solid-state physicist Pustogow.
“In real materials, we are still far from such a state of ideal frustration. First of all, we need to be able to precisely control the symmetry of the crystal lattice and thus the magnetic properties,” says Andrej Pustogow.
Although materials with strong geometrical frustration can already be produced, a continuous change from weak to strong frustration and vice versa has not been possible yet, especially not in one and the same crystal. In order to change the magnetism in the material investigated “by pushing a button,” the researchers put the crystal under pressure. Starting from a kagome structure, the crystal lattice was deformed by uniaxial stress, which changed the magnetic interactions between the electrons.
“We use mechanical pressure to force the system into a preferred magnetic direction. As sometimes in real life, stress reduces frustration because a decision is forced upon us and we don’t have to make it ourselves,” says Andrej Pustogow.
The team succeeded in increasing the temperature of the magnetic phase transition by more than ten per cent.
“This may seem not much at first glance, but if the freezing point of water were increased by ten per cent, for example, it would freeze at 27 °C — with serious consequences for the world as we know it,” explains Pustogow.
While in the current case geometrical frustration was reduced by mechanical pressure, the research team is now targeting an increase in frustration in order to completely eliminate antiferromagnetism and realise a quantum spin liquid as described above. “The possibility of actively controlling geometric frustration through uniaxial mechanical stress opens the door to undreamt-of manipulations of material properties ‘by pushing a button’,” summarises Andrej Pustogow.
Stable blue phosphorescent organic LEDs that use polariton-enhanced Purcell effects
by Haonan Zhao, Claire E. Arneson, Dejiu Fan, Stephen R. Forrest in Nature
Lights could soon use the full color suite of perfectly efficient organic light-emitting diodes, or OLEDs, that last tens of thousands of hours, thanks to an innovation from physicists and engineers at the University of Michigan.
The U-M team’s new phosphorescent OLEDs, commonly referred to as PHOLEDs, can maintain 90% of the blue light intensity for 10–14 times longer than other designs that emit similar deep blue colors. That kind of lifespan could finally make blue PHOLEDs hardy enough to be commercially viable in lights that meet the Department of Energy’s 50,000-hour lifetime target. Without a stable blue PHOLED, OLED lights need to use less-efficient technology to create white light.
The lifetime of the new blue PHOLEDs currently is only long enough to use as lighting, but the same design principle could be combined with other light-emitting materials to create blue PHOLEDs hardy enough for TVs, phone screens and computer monitors. Display screens with blue PHOLEDs could potentially increase a device’s battery life by 30%.
“Achieving long-lived blue PHOLEDs has been a focus of the display and lighting industries for over 20 years. It is probably the most important and urgent challenge facing the field of organic electronics,” said Stephen Forrest, the Peter A. Franken Distinguished University Professor of Electrical and Computer Engineering at the University of Michigan.
PHOLEDs have nearly 100% internal quantum efficiency, meaning all of the electricity entering the device is used to create light. As a result, lights and display screens equipped with PHOLEDs can run brighter colors for longer periods of time with less power and carbon emissions. Before the U-M team’s research, the best blue PHOLEDs weren’t durable enough to be used in either lighting or displays. Only red and green PHOLEDs are stable enough to use in devices today, but blue is needed to complete the trio of colors in OLED “RGB” displays and white OLED lights. Red, green and blue light can be combined at different relative brightness to produce any color desired in display pixels and light panels.
So far, the workaround in OLED displays has been to use older, fluorescent OLEDs to produce the blue colors, but the internal quantum efficiency of that technology is much lower. Only a quarter of the electric current entering the fluorescent blue device produces light.
“A lot of the display industry’s solutions are upgrades to fluorescent OLEDs, which is still an alternative solution,” said study first author Haonan Zhao, a doctoral student in physics and electrical and computer engineering. “I think a lot of companies would prefer to use blue PHOLEDs, if they had the choice.”
To make blue light, electricity excites heavy metal-containing phosphorescent organic molecules. Sometimes, the excited molecules come into contact before emitting the light, transferring all of the pair’s stored energy into one molecule. Because the energy of blue light is so high, the transferred energy, which is double that of the single excited molecule, can break chemical bonds and degrade the organic material. One way around this problem is to use materials that emit a broader spectrum of colors, which lowers the total amount of energy in the excited states. But such materials appear cyan or even green, rather than a deep blue.
The U-M team got around this issue by sandwiching cyan material between two mirrors. By perfectly tuning the space between the mirrors, only the deepest blue light waves can persist and eventually emit from the mirror chamber. Further tuning the optical properties of the organic, light-emitting layer to an adjacent metal electrode introduced a new quantum mechanical state called a plasmon-exciton-polariton, or PEP. This new state allows the organic material to emit light very fast, thus further decreasing the opportunity for excited states to collide and destroy the light-emitting material.
“In our device, the PEP is introduced because the excited states in the electron transporting material are synchronized with the light waves and the electron vibrations in the metal cathode,” said study co-author Claire Arneson, a doctoral student in physics and electrical and computer engineering.
Accurate transition state generation with an object-aware equivariant elementary reaction diffusion model
by Chenru Duan, Yuanqi Du, Haojun Jia, Heather J. Kulik in Nature Computational Science
During a chemical reaction, molecules gain energy until they reach what’s known as the transition state — a point of no return from which the reaction must proceed. This state is so fleeting that it’s nearly impossible to observe it experimentally.
The structures of these transition states can be calculated using techniques based on quantum chemistry, but that process is extremely time-consuming. A team of MIT researchers has now developed an alternative approach, based on machine learning, that can calculate these structures much more quickly — within a few seconds. Their new model could be used to help chemists design new reactions and catalysts to generate useful products like fuels or drugs, or to model naturally occurring chemical reactions such as those that might have helped to drive the evolution of life on Earth. Chenru Duan PhD ’22 is the lead author of a paper describing the work. Cornell University graduate student Yuanqi Du and MIT graduate student Haojun Jia are also authors of the paper.
“Knowing that transition state structure is really important as a starting point for thinking about designing catalysts or understanding how natural systems enact certain transformations,” says Heather Kulik, an associate professor of chemistry and chemical engineering at MIT, and the senior author of the study.
For any given chemical reaction to occur, it must go through a transition state, which takes place when it reaches the energy threshold needed for the reaction to proceed. The probability of any chemical reaction occurring is partly determined by how likely it is that the transition state will form.
“The transition state helps to determine the likelihood of a chemical transformation happening. If we have a lot of something that we don’t want, like carbon dioxide, and we’d like to convert it to a useful fuel like methanol, the transition state and how favorable that is determines how likely we are to get from the reactant to the product,” Kulik says.
Chemists can calculate transition states using a quantum chemistry method known as density functional theory. However, this method requires a huge amount of computing power and can take many hours or even days to calculate just one transition state.
Recently, some researchers have tried to use machine-learning models to discover transition state structures. However, models developed so far require considering two reactants as a single entity in which the reactants maintain the same orientation with respect to each other. Any other possible orientations must be modeled as separate reactions, which adds to the computation time.
“If the reactant molecules are rotated, then in principle, before and after this rotation they can still undergo the same chemical reaction. But in the traditional machine-learning approach, the model will see these as two different reactions. That makes the machine-learning training much harder, as well as less accurate,” Duan says.
The MIT team developed a new computational approach that allowed them to represent two reactants in any arbitrary orientation with respect to each other, using a type of model known as a diffusion model, which can learn which types of processes are most likely to generate a particular outcome. As training data for their model, the researchers used structures of reactants, products, and transition states that had been calculated using quantum computation methods, for 9,000 different chemical reactions.
“Once the model learns the underlying distribution of how these three structures coexist, we can give it new reactants and products, and it will try to generate a transition state structure that pairs with those reactants and products,” Duan says.
The researchers tested their model on about 1,000 reactions that it hadn’t seen before, asking it to generate 40 possible solutions for each transition state. They then used a “confidence model” to predict which states were the most likely to occur. These solutions were accurate to within 0.08 angstroms (one hundred-millionth of a centimeter) when compared to transition state structures generated using quantum techniques. The entire computational process takes just a few seconds for each reaction.
“You can imagine that really scales to thinking about generating thousands of transition states in the time that it would normally take you to generate just a handful with the conventional method,” Kulik says.
Although the researchers trained their model primarily on reactions involving compounds with a relatively small number of atoms — up to 23 atoms for the entire system — they found that it could also make accurate predictions for reactions involving larger molecules.
“Even if you look at bigger systems or systems catalyzed by enzymes, you’re getting pretty good coverage of the different types of ways that atoms are most likely to rearrange,” Kulik says.
The researchers now plan to expand their model to incorporate other components such as catalysts, which could help them investigate how much a particular catalyst would speed up a reaction. This could be useful for developing new processes for generating pharmaceuticals, fuels, or other useful compounds, especially when the synthesis involves many chemical steps.
“Traditionally all of these calculations are performed with quantum chemistry, and now we’re able to replace the quantum chemistry part with this fast generative model,” Duan says.
Another potential application for this kind of model is exploring the interactions that might occur between gases found on other planets, or to model the simple reactions that may have occurred during the early evolution of life on Earth, the researchers say.
Efficient Photonic Integration of Diamond Color Centers and Thin-Film Lithium Niobate
by Daniel Riedel, Hope Lee, Jason F. Herrmann, Jakob Grzesik, Vahid Ansari, Jean-Michel Borit, Hubert S. Stokowski, Shahriar Aghaeimeibodi, Haiyu Lu, Patrick J. McQuade, Nicholas A. Melosh, Zhi-Xun Shen, Amir H. Safavi-Naeini, Jelena Vučković in ACS Photonics
Quantum information scientists are always on the hunt for winning combinations of materials, materials that can be manipulated at the molecular level to reliably store and transmit information.
Following a recent proof-of-principle demonstration, researchers are adding a new combination of compounds to the quantum materials roster. In a study, researchers combined two nanosized structures — one made of diamond and one of lithium niobate — onto a single chip. They then sent light from the diamond to the lithium niobate and measured the fraction of light that successfully made it across.
The greater that fraction, the more efficient the coupling of the materials, and the more promising the pairing as a component in quantum devices. The result: An extraordinary 92% of the light made the jump from diamond to lithium niobate. The research was supported in part by Q-NEXT, a U.S. Department of Energy (DOE) National Quantum Information Science Research Center led by DOE’s Argonne National Laboratory. Stanford University’s Amir Safavi-Naeini and Jelena Vuckovic led the study.
“It was an exciting result to get 92% efficiency from this device,” said Hope Lee, paper co-author and a Ph.D. student at Stanford University and researcher who worked with Q-NEXT Director David Awschalom while an undergraduate at the University of Chicago. “It showed the advantages of the platform.”
Quantum technologies harness special features of matter at the molecular scale to process information. Quantum computers, networks and sensors are expected to have an enormous impact on our lives in areas such as medicine, communication and logistics. Quantum information is delivered in packets called qubits, which can take many forms. In the research team’s new platform, qubits transmit information as particles of light. Reliable qubits are critical for technologies such as quantum communication networks. As in traditional networks, information in quantum networks travels from one node to another. Stationary qubits store information within a node; flying qubits carry information between nodes. The research team’s new chip would form the basis of a stationary qubit. The more robust the stationary qubit, the more reliable the quantum network, and the greater the distance that networks can cover. A quantum network spanning a continent is well within reach.
Diamond has long been touted as a great home for qubits. For one, diamond’s molecular structure can be easily manipulated to host stationary qubits. For another, a diamond-hosted qubit can maintain information for a relatively long time, meaning more time for performing computations. Also, computations performed using diamond-hosted qubits exhibit high accuracy.
Diamond’s partner in the group’s study, lithium niobate, is another star performer when it comes to processing quantum information. Its special properties give scientists versatility by allowing them to change the frequency of the light passing through it. For example, researchers can apply an electric field or a mechanical strain to the lithium niobate to adjust how it channels light. It’s also possible to flip the orientation of its crystal structure. Doing this at regular intervals is another way to shape light’s passage through the material.
“You can use these properties of the lithium niobate to convert and change the light coming from the diamond, modulating it in ways that are useful for different experiments,” said Jason Herrmann, paper co-author and a Ph.D. student at Stanford. “For instance, you can basically convert the light into a frequency used by existing communications infrastructure. So those properties of lithium niobate are really beneficial.”
Traditionally, light from diamond-hosted qubits is channeled into either a fiber-optic cable or free space. In both cases, the experimental setup is unwieldy. Fiber-optic cables are long, dangly and floppy. And transmitting qubits into free space requires bulky equipment. All that equipment goes away when light from the diamond’s qubits is instead channeled into lithium niobate. Nearly every component can be placed on one tiny chip.
“There’s an advantage to having as many of your devices and your functionalities as possible on a single chip,” Lee said. “It’s more stable. And it really allows you to miniaturize your setups.”
Not only that, but because the two devices are connected by a whisper-thin filament — 1/100 of the width of a human hair — the quantum light is squeezed into the narrow passage that leads to lithium niobate, increasing the light’s interaction with the material and making it easier to manipulate light’s properties.
“When all the different light particles are interacting together in such a small volume, you get a much higher efficiency in the conversion process,” Herrmann said. “Being able to do this in the integrated platform will hopefully give rise to much higher efficiencies compared to the setup with fibers or free space.”
One of the challenges of developing the platform was manipulating the diamond — a mere 300 nanometers wide — to align with the lithium niobate.
“We had to poke at the diamond with tiny little needles to shift it around until it visibly looked like it was in the correct spot on this plate,” Lee said. “It’s almost like you’re poking at it with little chopsticks.”
Measuring the transferred light was another painstaking process.
“We have to really make sure we’re accounting for all the places where light is transmitted or lost to be able to say, ‘This is how much is going from diamond to lithium niobate,’” Herrmann said. “That calibration measurement took a lot of back and forth to make sure we were doing it correctly.”
The team is planning further experiments that leverage the quantum-information advantages offered by diamond and lithium niobate, both separately and together. Their latest success is only one milestone in what they hope will be a diverse menu of devices based on the two materials.
“By putting these two material platforms together and channeling light from one to the other, we show that, instead of working with just one material, you can really have the best of both worlds,” Lee said.
Misinference of interaction-free measurement from a classical system
by Valeri Frumkin, John W. M. Bush in Physical Review A
In our everyday classical world, what you see is what you get. A ball is just a ball, and when lobbed through the air, its trajectory is straightforward and clear. But if that ball were shrunk to the size of an atom or smaller, its behavior would shift into a quantum, fuzzy reality. The ball would exist as not just a physical particle but also a wave of possible particle states. And this wave-particle duality can give rise to some weird and sneaky phenomena.
One of the stranger prospects comes from a thought experiment known as the “quantum bomb tester.” The experiment proposes that a quantum particle, such as a photon, could act as a sort of telekinetic bomb detector. Through its properties as both a particle and a wave, the photon could, in theory, sense the presence of a bomb without physically interacting with it.
The concept checks out mathematically and is in line with what the equations governing quantum mechanics allow. But when it comes to spelling out exactly how a particle would accomplish such a bomb-sniffing feat, physicists are stumped. The conundrum lies in a quantum particle’s inherently shifty, in-between, undefinable state. In other words, scientists just have to trust that it works. But mathematicians at MIT are hoping to dispel some of the mystery and ultimately establish a more concrete picture of quantum mechanics. They have now shown that they can recreate an analog of the quantum bomb tester and generate the behavior that the experiment predicts. They’ve done so not in an exotic, microscopic, quantum setting, but in a seemingly mundane, classical, tabletop setup.
In a paper, the team reports recreating the quantum bomb tester in an experiment with a study of bouncing droplets. The team found that the interaction of the droplet with its own waves is similar to a photon’s quantum wave-particle behavior: When dropped into a configuration similar to what is proposed in the quantum bomb test, the droplet behaves in exactly the same statistical manner that is predicted for the photon. If there were actually a bomb in the setup 50 percent of the time, the droplet, like the photon, would detect it, without physically interacting with it, 25 percent of the time.
The fact that the statistics in both experiments match up suggests that something in the droplet’s classical dynamics may be at the heart of a photon’s otherwise mysterious quantum behavior. The researchers see the study as another bridge between two realities: the observable, classical world and the fuzzier quantum realm.
“Here we have a classical system that gives the same statistics as arises in the quantum bomb test, which is considered one of the wonders of the quantum world,” says study author John Bush, professor of applied mathematics at MIT. “In fact, we find that the phenomenon is not so wonderful after all. And this is another example of quantum behavior that can be understood from a local realist perspective.” Bush’s co-author is former MIT postdoc Valeri Frumkin.
To some physicists, quantum mechanics leaves too much to the imagination and doesn’t say enough about the actual dynamics from which such weird phenomena supposedly arise. In 1927, in an attempt to crystallize quantum mechanics, physicist Louis de Broglie presented the pilot wave theory — a still-controversial idea that poses a particle’s quantum behavior is determined not by an intangible, statistical wave of possible states but by a physical “pilot” wave of its own making, that guides the particle through space.
The concept was mostly discounted until 2005, when physicist Yves Couder discovered that de Broglie’s quantum waves could be replicated and studied in a classical, fluid-based experiment. The setup involves a bath of fluid that is made to subtly vibrate up and down, though not quite enough to generate waves on its own. A millimeter-sized droplet of the same fluid is then dispensed over the bath, and as it bounces off the surface, the droplet resonates with the bath’s vibrations, creating what physicists know as a standing wave field that “pilots,” or pushes the droplet along. The effect is of a droplet that appears to walk along a rippled surface in patterns that turn out to be in line with de Broglie’s pilot wave theory.
For the last 13 years, Bush has worked to refine and extend Couder’s hydrodynamic pilot wave experiments and has successfully used the setup to observe droplets exhibiting emergent, quantum-like behavior, including quantum tunneling, single-particle diffraction, and surreal trajectories.
“It turns out that this hydrodynamic pilot-wave experiment exhibits many features of quantum systems which were previously thought to be impossible to understand from a classical perspective,” Bush says.
In their new study, he and Frumkin took on the quantum bomb tester. The thought experiment begins with a conceptual interferometer — essentially, two corridors of the same length that branch out from the same starting point, then turn and converge, forming a rhombus-like configuration as the corridors continue on, each ending in a respective detector.
According to quantum mechanics, if a photon is fired from the interferometer’s starting point, through a beamsplitter, the particle should travel down one of the two corridors with equal probability. Meanwhile, the photon’s mysterious “wave function,” or the sum of all its possible states, travels down both corridors simultaneously. The wave function interferes in such a way to ensure that the particle only appears at one detector (let’s call this D1) and never the other (D2). Hence, the photon should be detected at D1 100 percent of the time, regardless of which corridor it traveled through.
If there is a bomb in one of the two corridors, and a photon heads down this corridor, it predictably triggers the bomb and the setup is blown to bits, and no photon is detected at either detector. But if the photon travels down the corridor without the bomb, something weird happens: Its wave function, in traveling down both corridors, is cut short in one by the bomb. As it’s not quite a particle, the wave does not set off the bomb. But the wave interference is altered in such a way that the particle will be detected with equal probability at D1 and D2. Any signal at D2 therefore would mean that a photon has detected the presence of the bomb, without physically interacting with it. If the bomb is present 50 percent of the time, then this weird quantum bomb detection should occur 25 percent of the time.
In their new study, Bush and Frumkin set up an analogous experiment to see if this quantum behavior could emerge in classical droplets. Into a bath of silicon oil, they submerged a structure similar to the rhombus-like corridors in the thought experiment. They then carefully dispensed tiny oil droplets into the bath and tracked their paths. They added a structure to one side of the rhombus to mimic a bomb-like object and observed how the droplet and its wave patterns changed in response.
In the end, they found that 25 percent of the time a droplet bounced through the corridor without the “bomb,” while itspilot waves interacted with the bomb structure in a way that pushed the droplet away from the bomb. From this perspective, the droplet was able to “sense” the bomb-like object without physically coming into contact with it. While the droplet exhibited quantum-like behavior, the team could plainly see that this behavior emerged from the droplet’s waves, which physically helped to keep the droplet away from the bomb. These dynamics, the team says, may also help to explain the mysterious behavior in quantum particles.
“Not only are the statistics the same, but we also know the dynamics, which was a mystery,” Frumkin says. “And the inference is that an analogous dynamics may underly the quantum behavior.”
“This system is the only example we know which is not quantum but shares some strong wave-particles properties,” says theoretical physicist Matthieu Labousse, at CNRS, ESPCI Paris PSL, who was not involved in the study. “It is very surprising that many examples thought to be peculiar to the quantum world can be reproduced by such a classical system. It enables to understand the barrier between what it is specific to a quantum system and what is not. The latest results of the group at MIT pushes the barrier very far.”
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