QT/ Quantum computers can cut through the noise
Quantum news biweekly vol.55, 27th June — 14th July
TL;DR
- The new device, introduced by a team of scientists, includes two qubits, which are a quantum computer’s analog to the logic bits in a classical computer’s processing chip. The new strategy relies on a “toggle switch” device that connects the qubits to a circuit called a “readout resonator” that can read the output of the qubits’ calculations.
- Scientists have announced a significant advancement in developing fault-tolerant qubits for quantum computing. They report that, in experiments with flakes of semiconductor materials they detected signatures of ‘fractional quantum anomalous Hall’ (FQAH) states. The discoveries mark a first and promising step in constructing a type of fault-tolerant qubit because FQAH states can host anyons — strange ‘quasiparticles’ that have only a fraction of an electron’s charge.
- The photons can carry quantum information and offer exceptional capabilities that can’t be achieved any other way. Unlike binary computing, where bits can only represent a 0 or 1, qubit behavior exists in the realm of quantum mechanics. Through “superpositioning,” a qubit can represent a 0, a 1, or any proportion between. This vastly increases a quantum computer’s processing speed compared to today’s computers. Experts are now investigating the inside of a quantum-dot-based light emitter.
- Quantum researchers twist double bilayers of an antiferromagnet to demonstrate tunable moiré magnetism.
- Scientists using one of the world’s most powerful quantum microscopes have made a discovery that could have significant consequences for the future of computing. Researchers have discovered a spatially modulating superconducting state in a new and unusual superconductor Uranium Ditelluride (UTe2). This new superconductor may provide a solution to one of quantum computing’s greatest challenges.
- Researchers have demonstrated the principle of using spatial correlations in quantum entangled beams of light to encode information and enable its secure transmission.
- Scientists investigated numerically the interaction between a quantized vortex and a normal-fluid. Based on the experimental results, researchers decided the most consistent of several theoretical models. They found that a model that accounts for changes in the normal-fluid and incorporates more theoretically accurate mutual friction is the most compatible with the experimental results.
- Researchers have demonstrated a novel concept for exciting and probing coherent phonons in crystals of a transiently broken symmetry. The key of this concept lies in reducing the symmetry of a crystal by appropriate optical excitation, as has been shown with the prototypical crystalline semimetal bismuth (Bi).
- A team of physicists recently announced that they have discovered a new phase of matter. Called the ‘chiral bose-liquid state,’ the discovery opens a new path in the age-old effort to understand the nature of the physical world.
- Researchers have discovered how to design materials that necessarily have a point or line where the material doesn’t deform under stress, and that even remember how they have been poked or squeezed in the past. These results could be used in quantum computers.
- 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
Strong parametric dispersive shifts in a statically decoupled two-qubit cavity QED system
by T. Noh, Z. Xiao, X. Y. Jin, K. Cicak, E. Doucet, J. Aumentado, L. C. G. Govia, L. Ranzani, A. Kamal, R. W. Simmonds in Nature Physics
What good is a powerful computer if you can’t read its output? Or readily reprogram it to do different jobs? People who design quantum computers face these challenges, and a new device may make them easier to solve.
The device, introduced by a team of scientists at the National Institute of Standards and Technology (NIST), includes two superconducting quantum bits, or qubits, which are a quantum computer’s analogue to the logic bits in a classical computer’s processing chip. The heart of this new strategy relies on a “toggle switch” device that connects the qubits to a circuit called a “readout resonator” that can read the output of the qubits’ calculations.
This toggle switch can be flipped into different states to adjust the strength of the connections between the qubits and the readout resonator. When toggled off, all three elements are isolated from each other. When the switch is toggled on to connect the two qubits, they can interact and perform calculations. Once the calculations are complete, the toggle switch can connect either of the qubits and the readout resonator to retrieve the results. Having a programmable toggle switch goes a long way toward reducing noise, a common problem in quantum computer circuits that makes it difficult for qubits to make calculations and show their results clearly.
“The goal is to keep the qubits happy so that they can calculate without distractions, while still being able to read them out when we want to,” said Ray Simmonds, a NIST physicist and one of the paper’s authors. “This device architecture helps protect the qubits and promises to improve our ability to make the high-fidelity measurements required to build quantum information processors out of qubits.”
Quantum computers, which are still at a nascent stage of development, would harness the bizarre properties of quantum mechanics to do jobs that even our most powerful classical computers find intractable, such as aiding in the development of new drugs by performing sophisticated simulations of chemical interactions. However, quantum computer designers still confront many problems. One of these is that quantum circuits are kicked around by external or even internal noise, which arises from defects in the materials used to make the computers. This noise is essentially random behavior that can create errors in qubit calculations.
Present-day qubits are inherently noisy by themselves, but that’s not the only problem. Many quantum computer designs have what is called a static architecture, where each qubit in the processor is physically connected to its neighbors and to its readout resonator. The fabricated wiring that connects qubits together and to their readout can expose them to even more noise. Such static architectures have another disadvantage: They cannot be reprogrammed easily. A static architecture’s qubits could do a few related jobs, but for the computer to perform a wider range of tasks, it would need to swap in a different processor design with a different qubit organization or layout.
The team’s programmable toggle switch sidesteps both of these problems. First, it prevents circuit noise from creeping into the system through the readout resonator and prevents the qubits from having a conversation with each other when they are supposed to be quiet.
“This cuts down on a key source of noise in a quantum computer,” Simmonds said.
Second, the opening and closing of the switches between elements are controlled with a train of microwave pulses sent from a distance, rather than through a static architecture’s physical connections. Integrating more of these toggle switches could be the basis of a more easily programmable quantum computer. The microwave pulses can also set the order and sequence of logic operations, meaning a chip built with many of the team’s toggle switches could be instructed to perform any number of tasks.
“This makes the chip programmable,” Simmonds said. “Rather than having a completely fixed architecture on the chip, you can make changes via software.”
One last benefit is that the toggle switch can also turn on the measurement of both qubits at the same time. This ability to ask both qubits to reveal themselves as a couple is important for tracking down quantum computational errors.
The qubits in this demonstration, as well as the toggle switch and the readout circuit, were all made of superconducting components that conduct electricity without resistance and must be operated at very cold temperatures. The toggle switch itself is made from a superconducting quantum interference device, or “SQUID,” which is very sensitive to magnetic fields passing through its loop. Driving a microwave current through a nearby antenna loop can induce interactions between the qubits and the readout resonator when needed.
At this point, the team has only worked with two qubits and a single readout resonator, but Simmonds said they are preparing a design with three qubits and a readout resonator, and they have plans to add more qubits and resonators as well. Further research could offer insights into how to string many of these devices together, potentially offering a way to construct a powerful quantum computer with enough qubits to solve the kinds of problems that, for now, are insurmountable.
Programming correlated magnetic states with gate-controlled moiré geometry
by Eric Anderson, Feng-Ren Fan, Jiaqi Cai, William Holtzmann, Takashi Taniguchi, Kenji Watanabe, Di Xiao, Wang Yao, Xiaodong Xu in Science
Quantum computing could revolutionize our world. For specific and crucial tasks, it promises to be exponentially faster than the zero-or-one binary technology that underlies today’s machines, from supercomputers in laboratories to smartphones in our pockets. But developing quantum computers hinges on building a stable network of qubits — or quantum bits — to store information, access it and perform computations.
Yet the qubit platforms unveiled to date have a common problem: They tend to be delicate and vulnerable to outside disturbances. Even a stray photon can cause trouble. Developing fault-tolerant qubits — which would be immune to external perturbations — could be the ultimate solution to this challenge.
A team led by scientists and engineers at the University of Washington has announced a significant advancement in this quest. They report that, in experiments with flakes of semiconductor materials — each only a single layer of atoms thick — they detected signatures of “fractional quantum anomalous Hall” (FQAH) states. The team’s discoveries mark a first and promising step in constructing a type of fault-tolerant qubit because FQAH states can host anyons — strange “quasiparticles” that have only a fraction of an electron’s charge. Some types of anyons can be used to make what are called “topologically protected” qubits, which are stable against any small, local disturbances.
“This really establishes a new paradigm for studying quantum physics with fractional excitations in the future,” said Xiaodong Xu, the lead researcher behind these discoveries, who is also the Boeing Distinguished Professor of Physics and a professor of materials science and engineering at the UW.
FQAH states are related to the fractional quantum Hall state, an exotic phase of matter that exists in two-dimensional systems. In these states, electrical conductivity is constrained to precise fractions of a constant known as the conductance quantum. But fractional quantum Hall systems typically require massive magnetic fields to keep them stable, making them impractical for applications in quantum computing. The FQAH state has no such requirement — it is stable even “at zero magnetic field,” according to the team.
Hosting such an exotic phase of matter required the researchers to build an artificial lattice with exotic properties. They stacked two atomically thin flakes of the semiconductor material molybdenum ditelluride (MoTe2) at small, mutual “twist” angles relative to one another. This configuration formed a synthetic “honeycomb lattice” for electrons. When researchers cooled the stacked slices to a few degrees above absolute zero, an intrinsic magnetism arose in the system. The intrinsic magnetism takes the place of the strong magnetic field typically required for the fractional quantum Hall state. Using lasers as probes, the researchers detected signatures of the FQAH effect, a major step forward in unlocking the power of anyons for quantum computing.
The team — which also includes scientists at the University of Hong Kong, the National Institute for Materials Science in Japan, Boston College and the Massachusetts Institute of Technology — envisions their system as a powerful platform to develop a deeper understanding of anyons, which have very different properties from everyday particles like electrons. Anyons are quasiparticles — or particle-like “excitations” — that can act as fractions of an electron. In future work with their experimental system, the researchers hope to discover an even more exotic version of this type of quasiparticle: “non-Abelian” anyons, which could be used as topological qubits. Wrapping — or “braiding” — the non-Abelian anyons around each other can generate an entangled quantum state. In this quantum state, information is essentially “spread out” over the entire system and resistant to local disturbances — forming the basis of topological qubits and a major advancement over the capabilities of current quantum computers.
“This type of topological qubit would be fundamentally different from those that can be created now,” said UW physics doctoral student Eric Anderson, who is lead author of the Science paper and co-lead author of the Nature paper. “The strange behavior of non-Abelian anyons would make them much more robust as a quantum computing platform.”
Three key properties, all of which existed simultaneously in the researchers’ experimental setup, allowed FQAH states to emerge:
- Magnetism: Though MoTe2 is not a magnetic material, when they loaded the system with positive charges, a “spontaneous spin order” — a form of magnetism called ferromagnetism — emerged.
- Topology: Electrical charges within their system have “twisted bands,” similar to a Möbius strip, which helps make the system topological.
- Interactions: The charges within their experimental system interact strongly enough to stabilize the FQAH state.
“The observed signatures of the fractional quantum anomalous Hall effect are inspiring,” said UW physics doctoral student Jiaqi Cai, co-lead author on the Nature paper and co-author of the Science paper. “The fruitful quantum states in the system can be a laboratory-on-a-chip for discovering new physics in two dimensions, and also new devices for quantum applications.”
“Our work provides clear evidence of the long-sought FQAH states,” said Xu, who is also a member of the Molecular Engineering and Sciences Institute, the Institute for Nano-Engineered Systems and the Clean Energy Institute, all at UW. “We are currently working on electrical transport measurements, which could provide direct and unambiguous evidence of fractional excitations at zero magnetic field.”
Spin texture and chiral coupling of circularly polarized dipole field
by Yu Shi, Hong Koo Kim in Nanophotonics
We interact with bits and bytes everyday — whether that’s through sending a text message or receiving an email.
There’s also quantum bits, or qubits, that have critical differences from common bits and bytes. These photons — particles of light — can carry quantum information and offer exceptional capabilities that can’t be achieved any other way. Unlike binary computing, where bits can only represent a 0 or 1, qubit behavior exists in the realm of quantum mechanics. Through “superpositioning,” a qubit can represent a 0, a 1, or any proportion between. This vastly increases a quantum computer’s processing speed compared to today’s computers.
“Learning about the capabilities of qubits has been a driving force for the emerging field of quantum technologies, opening up new and unexplored applications like quantum communication, computing and sensing,” said Hong Koo Kim, Professor of Electrical and Computer Engineering at the University of Pittsburgh Swanson School of Engineering.
Quantum technologies are important for a number of fields, like for banks protecting financial information or providing researchers with the speed needed to mimic all aspects of chemistry. And through quantum “entanglement,” qubits could “communicate” across vast distances as a single system. Kim and his graduate student, Yu Shi, made a discovery that may help quantum technology take a quantum leap.
Photon-based quantum technologies rely on single photon sources that can emit individual photons. These single photons can be generated from nanometer scale semiconductors, more commonly known as quantum dots. Similar to how microwave antennas broadcast mobile phone signals, a quantum dot acts as an antenna that radiates light.
“By performing rigorous analysis, we discovered that a quantum dot emitter — or a nanometer scale dipole antenna — traps a large amount of energy,” Kim explained. “The outer regime operation of a dipole emitter is well understood, but this is really the first time a dipole has been studied on the inside.”
Photons from those quantum dots come out with handedness, like us a right-handed or left-handed person, and quantum information is carried by this handedness of individual photons. As such, sorting them out to different pathways is an important task for quantum information processing. Kim’s team has developed a new way of separating differently-handed photons and efficiently harvesting them for further processing down the road.
“The findings of this work are expected to contribute to developing high-speed single photon sources, a critical component needed in quantum photonics,” Kim said.
Electrically tunable moiré magnetism in twisted double bilayers of chromium triiodide
by Guanghui Cheng, Mohammad Mushfiqur Rahman, Andres Llacsahuanga Allcca, Avinash Rustagi, Xingtao Liu, Lina Liu, Lei Fu, Yanglin Zhu, Zhiqiang Mao, Kenji Watanabe, Takashi Taniguchi, Pramey Upadhyaya, Yong P. Chen in Nature Electronics
Twistronics isn’t a new dance move, exercise equipment, or new music fad. No, it’s much cooler than any of that. It is an exciting new development in quantum physics and material science where van der Waals materials are stacked on top of each other in layers, like sheets of paper in a ream that can easily twist and rotate while remaining flat, and quantum physicists have used these stacks to discover intriguing quantum phenomena.
Adding the concept of quantum spin with twisted double bilayers of an antiferromagnet, it is possible to have tunable moiré magnetism. This suggests a new class of material platform for the next step in twistronics: spintronics. This new science could lead to promising memory and spin-logic devices, opening the world of physics up to a whole new avenue with spintronic applications. A team of quantum physics and materials researchers at Purdue University has introduced the twist to control the spin degree of freedom, using CrI3, an interlayer-antiferromagnetic-coupled van der Waals (vdW) material, as their medium.
“In this study, we fabricated twisted double bilayer CrI3, that is, bilayer plus bilayer with a twist angle between them,” says Dr. Guanghui Cheng, co-lead author of the publication. “We report moiré magnetism with rich magnetic phases and significant tunability by the electrical method.”
“We stacked and twisted an antiferromagnet onto itself and voila got a ferromagnet,” says Chen. “This is also a striking example of the recently emerged area of ‘twisted’ or moiré magnetism in twisted 2D materials, where the twisting angle between the two layers gives a powerful tuning knob and changes the material property dramatically.”
“To fabricate twisted double bilayer CrI3, we tear up one part of bilayer CrI3, rotate and stack onto the other part, using the so-called tear-and-stack technique,” explains Cheng. “Through magneto-optical Kerr effect (MOKE) measurement, which is a sensitive tool to probe magnetic behavior down to a few atomic layers, we observed the coexistence of ferromagnetic and antiferromagnetic orders, which is the hallmark of moiré magnetism, and further demonstrated voltage-assisted magnetic switching. Such a moiré magnetism is a novel form of magnetism featuring spatially varying ferromagnetic and antiferromagnetic phases, alternating periodically according to the moiré superlattice.”
Twistronics up to this point have mainly focused on modulating electronic properties, such as twisted bilayer graphene. The Purdue team wanted to introduce the twist to spin degree of freedom and chose to use CrI3, an interlayer-antiferromagnetic-coupled vdW material. The result of stacked antiferromagnets twisting onto itself was made possible by having fabricated samples with different twisting angles. In other words, once fabricated, the twist angle of each device becomes fixed, and then MOKE measurements are performed. Theoretical calculations for this experiment were performed by Upadhyaya and his team. This provided strong support for the observations arrived at by Chen’s team.
“Our theoretical calculations have revealed a rich phase diagram with non-collinear phases of TA-1DW, TA-2DW, TS-2DW, TS-4DW, etc.,” says Upadhyaya.
This research folds into an ongoing research avenue by Chen’s team. This work follows several related recent publications by the team related to novel physics and properties of “2D magnets,” such as “Emergence of electric-field-tunable interfacial ferromagnetism in 2D antiferromagnet heterostructures,” which was recently published in Nature Communications. This research avenue has exciting possibilities in the field of twistronics and spintronics.
“The identified moiré magnet suggests a new class of material platform for spintronics and magnetoelectronics,” says Chen. “The observed voltage-assisted magnetic switching and magnetoelectric effect may lead to promising memory and spin-logic devices. As a novel degree of freedom, the twist can be applicable to the vast range of homo/heterobilayers of vdW magnets, opening the opportunity to pursue new physics as well as spintronic applications.”
Detection of a pair density wave state in UTe2
by Qiangqiang Gu, Joseph P. Carroll, Shuqiu Wang, Sheng Ran, Christopher Broyles, Hasan Siddiquee, Nicholas P. Butch, Shanta R. Saha, Johnpierre Paglione, J. C. Séamus Davis, Xiaolong Liu in Nature
Scientists using one of the world’s most powerful quantum microscopes have made a discovery that could have significant consequences for the future of computing.
Researchers at the Macroscopic Quantum Matter Group laboratory in University College Cork (UCC) have discovered a spatially modulating superconducting state in a new and unusual superconductor Uranium Ditelluride (UTe2). This new superconductor may provide a solution to one of quantum computing’s greatest challenges. Lead author Joe Carroll, a PhD researcher working with UCC Prof. of Quantum Physics Séamus Davis, explains the subject of the paper.
“Superconductors are amazing materials which have many strange and unusual properties. Most famously they allow electricity to flow with zero resistance. That is, if you pass a current through them they don’t start to heat up, in fact, they don’t dissipate any energy despite carrying a huge current. They can do this because instead of individual electrons moving through the metal we have pairs of electrons which bind together. These pairs of electrons together form macroscopic quantum mechanical fluid.”
“What our team found was that some of the electron pairs form a new crystal structure embedded in this background fluid. These types of states were first discovered by our group in 2016 and are now called Electron Pair-Density Waves. These Pair Density Waves are a new form of superconducting matter the properties of which we are still discovering.”
“What is particularly exciting for us and the wider community is that UTe2 appears to be a new type of superconductor. Physicists have been searching for a material like it for nearly 40 years. The pairs of electrons appear to have intrinsic angular momentum. If this is true, then what we have detected is the first Pair-Density Wave composed of these exotic pairs of electrons.”
When asked about the practical implications of this work Mr. Carroll explained: “There are indications that UTe2 is a special type of superconductor that could have huge consequences for quantum computing.”
“Typical, classical, computers use bits to store and manipulate information. Quantum computers rely on quantum bits or qubits to do the same. The problem facing existing quantum computers is that each qubit must be in a superposition with two different energies — just as Schrödinger’s cat could be called both ‘dead’ and ‘alive’. This quantum state is very easily destroyed by collapsing into the lowest energy state — ‘dead’ — thereby cutting off any useful computation.
“This places huge limits on the application of quantum computers. However, since its discovery five years ago there has been a huge amount of research on UTe2 with evidence pointing to it being a superconductor which may be used as a basis for topological quantum computing. In such materials there is no limit on the lifetime of the qubit during computation opening up many new ways for more stable and useful quantum computers. In fact, Microsoft have already invested billions of dollars into topological quantum computing so this is a well-established theoretical science already.” he said.
“What the community has been searching for is a relevant topological superconductor; UTe2 appears to be that.”
“What we’ve discovered then provides another piece to the puzzle of UTe2. To make applications using materials like this we must understand their fundamental superconducting properties. All of modern science moves step by step. We are delighted to have contributed to the understanding of a material which could bring us closer to much more practical quantum computers.”
Information encoding in the spatial correlations of entangled twin beams
by Gaurav Nirala, Siva T. Pradyumna, Ashok Kumar, Alberto M. Marino in Science Advances
Researchers at the University of Oklahoma led a study that proves the principle of using spatial correlations in quantum entangled beams of light to encode information and enable its secure transmission.
Light can be used to encode information for high-data rate transmission, long-distance communication and more. But for secure communication, encoding large amounts of information in light has additional challenges to ensure the privacy and integrity of the data being transferred.
Alberto Marino, the Ted S. Webb Presidential Professor in the Homer L. Dodge College of Arts, led the research with OU doctoral student and the study’s first author Gaurav Nirala and co-authors Siva T. Pradyumna and Ashok Kumar. Marino also holds positions with OU’s Center for Quantum Research and Technology and with the Quantum Science Center, Oak Ridge National Laboratory.
“The idea behind the project is to be able to use the spatial properties of the light to encode large amounts of information, just like how an image contains information. However, to be able to do so in a way that is compatible with quantum networks for secure information transfer. When you consider an image, it can be constructed by combining basic spatial patterns know as modes, and depending on how you combine these modes, you can change the image or encoded information,” Marino said.
“What we’re doing here that is new and different is that we’re not just using those modes to encode information; we’re using the correlations between them,” he added. “We’re using the additional information on how those modes are linked to encode the information.”
The researchers used two entangled beams of light, meaning that the light waves are interconnected with correlations that are stronger than those that can be achieved with classical light and remain interconnected despite their distance apart.
“The advantage of the approach we introduce is that you’re not able to recover the encoded information unless you perform joint measurements of the two entangled beams,” Marino said. “This has applications such as secure communication, given that if you were to measure each beam by itself, you would not be able to extract any information. You have to obtain the shared information between both of the beams and combine it in the right way to extract the encoded information.”
Through a series of images and correlation measurements, the researchers demonstrated results of successfully encoding information in these quantum-entangled beams of light. Only when the two beams were combined using the methods intended did the information resolve into recognizable information encoded in the form of images.
“The experimental result describes how one can transfer spatial patterns from one optical field to two new optical fields generated using a quantum mechanical process called four-wave mixing,” said Nirala. “The encoded spatial pattern can be retrieved solely by joint measurements of generated fields. One interesting aspect of this experiment is that it offers a novel method of encoding information in light by modifying the correlation between various spatial modes without impacting time-correlations.”
“What this could enable, in principle, is the ability to securely encode and transmit a lot of information using the spatial properties of the light, just like how an image contains a lot more information than just turning the light on and off,” Marino said. “Using the spatial correlations is a new approach to encode information.”
Imaging quantized vortex rings in superfluid helium to evaluate quantum dissipation
by Yuan Tang, Wei Guo, Hiromichi Kobayashi, Satoshi Yui, Makoto Tsubota, Toshiaki Kanai in Nature Communications
Liquid helium-4, which is in a superfluid state at cryogenic temperatures close to absolute zero (-273°C), has a special vortex called a quantized vortex that originates from quantum mechanical effects. When the temperature is relatively high, the normal fluid exists simultaneously in the superfluid helium, and when the quantized vortex is in motion, mutual friction occurs between it and the normal-fluid. However, it is difficult to explain precisely how a quantized vortex interacts with a normal-fluid in motion. Although several theoretical models have been proposed, it has not been clear which model is correct.
A research group led by Professor Makoto Tsubota and Specially Appointed Assistant Professor Satoshi Yui, from the Graduate School of Science and the Nambu Yoichiro Institute of Theoretical and Experimental Physics, Osaka Metropolitan University respectively in cooperation with their colleagues from Florida State University and Keio University, investigated numerically the interaction between a quantized vortex and a normal-fluid. Based on the experimental results, researchers decided on the most consistent of several theoretical models. They found that a model that accounts for changes in the normal-fluid and incorporates more theoretically accurate mutual friction is the most compatible with the experimental results.
“The subject of this study, the interaction between a quantized vortex and a normal-fluid, has been a great mystery since I began my research in this field 40 years ago,” stated Professor Tsubota. “Computational advances have made it possible to handle this problem, and the brilliant visualization experiment by our collaborators at Florida State University has led to a breakthrough. As is often the case in science, subsequent developments in technology have made it possible to elucidate, and this study is a good example of this.”
Quantum pathways of carrier and coherent phonon excitation in bismuth
by Azize Koç, Isabel Gonzalez-Vallejo, Matthias Runge, Ahmed Ghalgaoui, Klaus Reimann, Laurenz Kremeyer, Fabian Thiemann, Michael Horn-von Hoegen, Klaus Sokolowski-Tinten, Michael Woerner, Thomas Elsaesser in Physical Review B
Atoms in a crystal form a regular lattice, in which they can move over small distances from their equilibrium positions. Such phonon excitations are represented by quantum states. A superposition of phonon states defines a so-called phonon wavepacket, which is connected with collective coherent oscillations of the atoms in the crystal. Coherent phonons can be generated by excitation of the crystal with a femtosecond light pulse and their motions in space and time be followed by scattering an ultrashort x-ray pulse from the excited material. The pattern of scattered x-rays gives direct insight in the momentary position of and distances between the atoms. A sequence of such patterns provides a ‘movie’ of the atomic motions.
The physical properties of coherent phonons are determined by the symmetry of the crystal, which represents a periodic arrangement of identical unit cells. Weak optical excitation does not change the symmetry properties of the crystal. In this case, coherent phonons with identical atomic motions in all unit cells are excited . In contrast, strong optical excitation can break the symmetry of the crystal and make atoms in adjacent unit cells oscillate differently. While this mechanism holds potential for accessing other phonons, it has not been explored so far.
Researchers from the Max-Born-Institute in Berlin in collaboration with researchers from the University of Duisburg-Essen have demonstrated a novel concept for exciting and probing coherent phonons in crystals of a transiently broken symmetry. The key of this concept lies in reducing the symmetry of a crystal by appropriate optical excitation, as has been shown with the prototypical crystalline semimetal bismuth (Bi).
Ultrafast mid-infrared excitation of electrons in Bi modifies the spatial charge distribution and, thus, reduces the crystal symmetry transiently. In the reduced symmetry, new quantum pathways for the excitation of coherent phonons open up. The symmetry reduction causes a doubling of the unit-cell size from the red framework with two Bi atoms to the blue framework with four Bi atoms. In addition to the unidirectional atomic motion, the unit cell with 4 Bi atoms allows for coherent phonon wave packets with bidirectional atomic motions.
Probing the transient crystal structure directly by femtosecond x-ray diffraction reveals oscillations of diffracted intensity, which persist on a picosecond time scale. The oscillations arise from coherent wave packet motions along phonon coordinates in the crystal of reduced symmetry. Their frequency of 2.6 THz is different from that of phonon oscillations at low excitation level. Interestingly, this behavior occurs only above a threshold of the optical pump fluence and reflects the highly nonlinear, so-called non-perturbative character of the optical excitation process.
In summary, optically induced symmetry breaking allows for modifying the excitation spectrum of a crystal on ultrashort time scales. These results may pave the way for steering material properties transiently and, thus, implementing new functions in optoacoustics and optical switching.
Single-photon absorption and emission from a natural photosynthetic complex
by Quanwei Li, Kaydren Orcutt, Robert L. Cook, Javier Sabines-Chesterking, Ashley L. Tong, Gabriela S. Schlau-Cohen, Xiang Zhang, Graham R. Fleming, K. Birgitta Whaley in Nature
Using a complex cast of metal-studded pigments, proteins, enzymes, and co-enzymes, photosynthetic organisms can convert the energy in light into the chemical energy for life. And now, we know that this organic chemical reaction is sensitive to the smallest quantity of light possible — a single photon.
The discovery solidifies our current understanding of photosynthesis and will help answer questions about how life works on the smallest of scales,where quantum physics and biology meet.
“A huge amount of work, theoretically and experimentally, has been done around the world trying to understand what happens after a photon is absorbed. But we realized that nobody was talking about the first step. That was still a question that needed to be answered in detail,” said co-lead author Graham Fleming, a senior faculty scientist in the Biosciences Area at Lawrence Berkeley National Laboratory (Berkeley Lab) and professor of chemistry at UC Berkeley.
In their study, Fleming, co-lead author Birgitta Whaley, a senior faculty scientist in the Energy Sciences Area at Berkeley Lab, and their research groups showed that a single photon can indeed initiate the first step of photosynthesis in photosynthetic purple bacteria. Because all photosynthetic organisms use similar processes and share an evolutionary ancestor, the team is confident that photosynthesis in plants and algae works the same way. “Nature invented a very clever trick,” Fleming said.
Based on how efficient photosynthesis is at converting sunlight into energy-rich molecules, scientists have long assumed that a single photon was all it took to initiate the reaction, wherein photons pass energy to electrons that then trade places with electrons in different molecules, eventually creating the precursor ingredients for the production of sugars. After all, the sun doesn’t provide that many photons — only a thousand photons arrive at a single chlorophyll molecule per second on a sunny day — yet the process occurs reliably across the planet. However, “no one had ever backed up that assumption with a demonstration,” said first author Quanwei Li, a joint postdoctoral researcher who develops new experimental techniques with quantum light in the Fleming and Whaley groups. And, further complicating matters, a great deal of the research that has unraveled precise details about later steps of photosynthesis was performed by triggering photosynthetic molecules with powerful, ultra-fast laser pulses.
“There’s a huge difference in intensity between a laser and sunlight — a typical focused laser beam is a million times brighter than sunlight,” said Li. Even if you manage to produce a weak beam with an intensity matching that of sunlight, they are still very different due to quantum properties of light called photon statistics. Since no one has seen the photon get absorbed, we don’t know what difference it makes what kind of photon it is, he explained. “But just like you need to understand each particle to build a quantum computer, we need to study the quantum properties of living systems to truly understand them, and to make efficient artificial systems that generate renewable fuels.”
Photosynthesis, like other chemical reactions, was first understood in bulk — meaning that we knew what the overall inputs and outputs were, and from that we could infer what interactions between individual molecules might look like. In the 1970s and 80s, advances in technology allowed scientists to directly study individual chemicals during reactions. Now, scientists are beginning to explore the next frontier, the individual atom and subatomic particle scale, using even more advanced technologies.
Designing an experiment that would allow for observation of individual photons meant bringing together a unique team of theorists and experimentalists who combined cutting-edge tools from quantum optics and biology. “It was new for people who study photosynthesis, because they don’t normally use these tools, and it was new for people in quantum optics because we don’t normally think about applying these techniques to complex biological systems,” said Whaley, who is also a professor of chemical physics at UC Berkeley.
The scientists set up a photon source that generates a single pair of photons through a process called spontaneous parametric down-conversion. During each pulse, the first photon — “the herald” — was observed with a highly sensitive detector, which confirmed that the second photon was on its way to the assembled sample of light absorbing molecular structures taken from photosynthetic bacteria. Another photon detector near the sample was set up to measure the lower-energy photon that is emitted by the photosynthetic structure after it absorbed the second “heralded” photon of the original pair.
The light absorbing structure used in the experiment, called the LH2, has been studied extensively. It is known that photons at the 800 nanometer (nm) wavelength get absorbed by a ring of 9 bacteriochlorophyll molecules in LH2, causing energy to be passed to a second ring of 18 bacteriochlorophyll molecules which can emit fluorescent photons at 850 nm. In the native bacteria, the energy from the photons would continue transferring to subsequent molecules until it is used to initiate the chemistry of photosynthesis. But in the experiment, when the LH2s had been separated from other cellular machinery, the detection of the 850 nm photon served as definitive sign that the process had been activated.
“If you’ve only got one photon, it’s awfully easy to lose it. So that was the fundamental difficulty in this experiment and that’s why we use the herald photon,” said Fleming. The scientists analyzed more than 17.7 billion herald photon detection events and 1.6 million heralded fluorescent photon detection events to ensure that the observations could only be attributed to single-photon absorption, and that no other factors were influencing the results.
“I think the first thing is that this experiment has shown that you can actually do things with individual photons. So that’s a very, very important point,” said Whaley. “The next thing is, what else can we do? Our goal is to study the energy transfer from individual photons through the photosynthetic complex at the shortest possible temporal and spatial scales.”
Non-orientable order and non-commutative response in frustrated metamaterials
by Xiaofei Guo, Marcelo Guzmán, David Carpentier, Denis Bartolo, Corentin Coulais in Nature
Researchers from the UvA Institute of Physics and ENS de Lyon have discovered how to design materials that necessarily have a point or line where the material doesn’t deform under stress, and that even remember how they have been poked or squeezed in the past. These results could be used in robotics and mechanical computers, while similar design principles could be used in quantum computers.
The outcome is a breakthrough in the field of metamaterials: designer materials whose responses are determined by their structure rather than their chemical composition. To construct a metamaterial with mechanical memory, physicists Xiaofei Guo, Marcelo Guzmán, David Carpentier, Denis Bartolo and Corentin Coulais realised that its design needs to be ‘frustrated’, and that this frustration corresponds to a new type of order, which they call non-orientable order.
A simple example of a non-orientable object is a Möbius strip, made by taking a strip of material, adding half a twist to it and then gluing its ends together. You can try this at home with a strip of paper. Following the surface of a Möbius strip with your finger, you’ll find that when you get back to your starting point, your finger will be on the other side of the paper. A Möbius strip is non-orientable because there is no way to label the two sides of the strip in a consistent manner; the twist makes the entire surface one and the same. This is in contrast to a simple cylinder (a strip without any twists whose ends are glued together), which has a distinct inner and outer surface.
Guo and her colleagues realised that this non-orientability strongly affects how an object or metamaterial responds to being pushed or squeezed. If you place a simple cylinder and a Möbius strip on a flat surface and press down on them from above, you’ll find that the sides of the cylinder will all bulge out (or in), while the sides of the Möbius strip cannot do the same. Instead, the non-orientability of the latter ensures that there is always a point along the strip where it does not deform under pressure.
Excitingly, this behaviour extends far beyond Möbius strips. ‘We discovered that the behaviour of non-orientable objects such as Möbius strips allows us to describe any material that is globally frustrated. These materials naturally want to be ordered, but something in their structure forbids the order to span the whole system and forces the ordered pattern to vanish at one point or line in space. There is no way to get rid of that vanishing point without cutting the structure, so it has to be there no matter what,’ explains Coulais, who leads the Machine Materials Laboratory at the University of Amsterdam.
The research team designed and 3D-printed their own mechanical metamaterial structures which exhibit the same frustrated and non-orientable behaviour as Möbius strips. Their designs are based on rings of squares connected by hinges at their corners. When these rings are squeezed, neighbouring squares will rotate in opposite directions so that their edges move closer together. The opposite rotation of neighbours makes the system’s response analogous to the anti-ferromagnetic ordering that occurs in certain magnetic materials.
Rings composed of an odd number of squares are frustrated, because there is no way for all neighbouring squares to rotate in opposite directions. Squeezed odd-numbered rings therefore exhibit non-orientable order, in which the rotation angle at one point along the ring must go to zero. Being a feature of the overall shape of the material makes this a robust topological property. By connecting multiple metarings together, it is even possible to emulate the mechanics of higher-dimensional topological structures such as the Klein bottle.
Having an enforced point or line of zero deformation is key to endowing materials with mechanical memory. Instead of squeezing a metamaterial ring from all sides, you can press the ring at distinct points. Doing so, the order in which you press different points determines where the zero deformation point or line ends up. This is a form of storing information. It can even be used to execute certain types of logic gates, the basis of any computer algorithm. A simple metamaterial ring can thus function as a mechanical computer. Beyond mechanics, the results of the study suggest that non-orientability could be a robust design principle for metamaterials that can effectively store information across scales, in fields as diverse as colloidal science, photonics, magnetism, and atomic physics. It could even be useful for new types of quantum computers.
Coulais concludes: ‘Next, we want to exploit the robustness of the vanishing deformations for robotics. We believe the vanishing deformations could be used to create robotic arms and wheels with predictable bending and locomotion mechanisms.’
MISC
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