QT/ Evidence of Quantum Gravity’s Existence?
Quantum news biweekly vol. 71, 26th March — 9th April
TL;DR
- An Antarctic experiment probes gravity’s presence at the quantum level using a unique particle’s properties.
- Quantum simulators, vital for deciphering quantum mysteries, now chill even further for enhanced performance.
- Precision standards enable alignment of quantum dots with photonic components, vital for next-gen chip-scale devices.
- The CVQE algorithm emerges as a potent tool for exploring electronic system properties.
- Nobel-winning concepts converge to enhance quantum communication, yielding near-perfect entangled photon pairs.
- Quantum encryption enables secure data transfer over 100 kilometers via fiber optics, a milestone in information security.
- Barkhausen noise, once thought classical, now shows quantum origins, paving the way for novel sensor and electronic devices.
- Innovative window coating blocks unwanted UV and infrared light, cutting cooling costs significantly in hot climates.
- Single-molecule transistor leverages quantum interference for high stability and efficiency, promising smaller, faster devices.
- New research harnesses magnons’ magnetic fields to manipulate qubits, advancing quantum information transduction in networks.
- And more!
Quantum Computing Market
According to the recent market research report by MarketsandMarkets, the Quantum Computing market is expected to grow to USD 5,3 million by 2029, at a CAGR of 32.7%. The 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.
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
Search for decoherence from quantum gravity with atmospheric neutrinos
by R. Abbasi, M. Ackermann, J. Adams, S. K. Agarwalla, et al in Nature Physics
University of Copenhagen team contributes to an Antarctic large-scale experiment striving to find out if gravity also exists at the quantum level. An extraordinary particle able to travel undisturbed through space seems to hold the answer.
Several thousand sensors distributed over a square kilometer near the South Pole are tasked with answering one of the large outstanding questions in physics: does quantum gravity exist? The sensors monitor neutrinos — particles with no electrical charge and almost without mass — arriving at the Earth from outer space. A team from the Niels Bohr Institute (NBI), University of Copenhagen, have contributed to developing the method which exploits neutrino data to reveal if quantum gravity exists.
“If as we believe, quantum gravity does indeed exist, this will contribute to unite the current two worlds in physics. Today, classical physics describes the phenomena in our normal surroundings such as gravity, while the atomic world can only be described using quantum mechanics. The unification of quantum theory and gravitation remains one of the most outstanding challenges in fundamental physics. It would be very satisfying if we could contribute to that end,” says Tom Stuttard, Assistant Professor at NBI.
Tom Stuttard is co-author of a scientific article. The article presents results from a large study by the NBI team and American colleagues. More than 300,000 neutrinos have been studied. However, these are not neutrinos of the most interesting type originating from sources in deep space. The neutrinos in this study were created in the Earth’s atmosphere, as high-energy particles from space collided with Nitrogen or other molecules.
“Looking at neutrinos originating from the Earth’s atmosphere has the practical advantage that they are by far more common than their siblings from outer space. We needed data from many neutrinos to validate our methodology. This has been accomplished now. Thus, we are ready to enter the next phase in which we will study neutrinos from deep space,” says Tom Stuttard.
The IceCube Neutrino Observatory is situated next to the Amundsen-Scott South Pole Station in Antarctica. In contrast to most other astronomy and astrophysics facilities, IceCube works the best for observing space at the opposite side of the Earth, meaning the Northern hemisphere. This is because while the neutrino is perfectly capable of penetrating our planet — and even its hot, dense core — other particles will be stopped, and the signal is thus much cleaner for neutrinos coming from the Northern hemisphere.
The IceCube facility is operated by the University of Wisconsin-Madison, USA. More than 300 scientists from countries around the world are engaged in the IceCube collaboration. University of Copenhagen is one of more than 50 universities having an IceCube center for neutrino studies.
Since the neutrino has no electrical charge and is nearly massless, it is undisturbed by electromagnetic and strong nuclear forces, allowing it to travel billions of lightyears through the Universe in its original state.
The key question is whether the properties of the neutrino are in fact completely unchanged as it travels over large distances or if tiny changes are notable after all.
“If the neutrino undergoes the subtle changes that we suspect, this would be the first strong evidence of quantum gravity,” says Tom Stuttard.
To understand which changes in neutrino properties the team is looking for, some background information is called for. While we refer to it as a particle, what we observe as a neutrino is really three particles produced together, known in quantum mechanics as superposition. The neutrino can have three fundamental configurations — flavors as they are termed by the physicists — which are electron, muon, and tau. Which of these configurations we observe changes as the neutrino travels, a truly strange phenomenon known as neutrino oscillations. This quantum behavior is maintained over thousands of kilometers or more, which is referred to as quantum coherence.
“In most experiments, the coherence is soon broken. But this is not believed to be caused by quantum gravity. It is just very difficult to create perfect conditions in a lab. You want perfect vacuum, but somehow a few molecules manage to sneak in etc. In contrast, neutrinos are special in that they are simply not affected by matter around them, so we know that if coherence is broken it will not be due to shortcomings in the human-made experimental setup,” Tom Stuttard explains.
Asked whether the results of the study were as expected, the researcher replies: “We find ourselves in a rare category of science projects, namely experiments for which no established theoretical framework exists. Thus, we just did not know what to expect. However, we knew that we could search for some of the general properties we might expect a quantum theory of gravity to have.”
“Whilst we did have hopes of seeing changes related to quantum gravity, the fact that we didn’t see them does not exclude at all that they are real. When an atmospheric neutrino is detected at the Antarctic facility, it will typically have travelled through the Earth. Meaning approximately 12,700 km — a very short distance compared to neutrinos originating in the distant Universe. Apparently, a much longer distance is needed for quantum gravity to make an impact, if it exists,” says Tom Stuttard, noting that the top goal of the study was to establish the methodology:
“For years, many physicists doubted whether experiments could ever hope to test quantum gravity. Our analysis shows that it is indeed possible, and with future measurements with astrophysical neutrinos, as well as more precise detectors being built in the coming decade, we hope to finally answer this fundamental question.”
Squeezing Oscillations in a Multimode Bosonic Josephson Junction
by Tiantian Zhang, Mira Maiwöger, Filippo Borselli, Yevhenii Kuriatnikov, Jörg Schmiedmayer, Maximilian Prüfer in Physical Review X
Quantum experiments always have to deal with the same problem, regardless of whether they involve quantum computers, quantum teleportation or new types of quantum sensors: quantum effects break down very easily. They are extremely sensitive to external disturbances — for example, to fluctuations caused simply by the surrounding temperature. It is therefore important to be able to cool down quantum experiments as effectively as possible.
At TU Wien (Vienna), it has now been shown that this type of cooling can be achieved in an interesting new way: A Bose-Einstein condensate is split into two parts, neither abruptly nor particularly slowly, but with a very specific temporal dynamic that ensures that random fluctuations are prevented as perfectly as possible. In this way, the relevant temperature in the already extremely cold Bose-Einstein condensate can be significantly reduced. This is important for quantum simulators, which are used at TU Wien to gain insights into quantum effects that could not be investigated using previous methods.
“We work with quantum simulators in our research,” says Maximilian Prüfer, who is researching new methods at TU Wien’s Atomic Institute with the help of an Esprit Grant from the FWF. “Quantum simulators are systems whose behavior is determined by quantum mechanical effects and which can be controlled and monitored particularly well. These systems can therefore be used to study fundamental phenomena of quantum physics that also occur in other quantum systems, which cannot be studied so easily.”
This means that a physical system is used to actually learn something about other systems. This idea is not entirely new in physics: for example, you can also carry out experiments with water waves in order to learn something about sound waves — but water waves are easier to observe.
“In quantum physics, quantum simulators have become an extremely useful and versatile tool in recent years,” says Maximilian Prüfer. “Among the most important tools for realizing interesting model systems are clouds of extremely cold atoms, such as those we study in our laboratory.” In the current paper, the scientists led by Jörg Schmiedmayer and Maximilian Prüfer investigated how quantum entanglement evolves over time and how this can be used to achieve an even colder temperature equilibrium than before. Quantum simulation is also a central topic in the recently launched QuantA Cluster of Excellence, in which various quantum systems are being investigated.
The decisive factor that usually limits the suitability of such quantum simulators at present is their temperature: “The better we cool down the interesting degrees of freedom of the condensate, the better we can work with it and the more we can learn from it,” says Maximilian Prüfer.
There are different ways to cool something down: For example, you can cool a gas by increasing its volume very slowly. With extremely cold Bose-Einstein condensates, other tricks are typically used: the most energetic atoms are quickly removed until only a collection of atoms remains, which have a fairly uniformly low energy and are therefore cooler.
“But we use a completely different technique,” says Tiantian Zhang, first author of the study, who investigated this topic as part of her doctoral thesis at the Doctoral College of the Vienna Center for Quantum Science and Technology. “We create a Bose-Einstein condensate and then split it into two parts by creating a barrier in the middle.” The number of particles which end up on the right side and on the left side of the barrier is undetermined. Due to the laws of quantum physics, there is a certain amount of uncertainty here. One could say that both sides are in a quantum-physical superposition of different particle number states.
“On average, exactly 50% of the particles are on the left and 50% on the right,” says Maximilian Prüfer. “But quantum physics says that there are always certain fluctuations. The fluctuations, i.e. the deviations from the expected value, are closely related to the temperature.”
The research team at TU Wien was able to show: neither an extremely abrupt nor an extremely slow splitting of the Bose-Einstein condensate is optimal. A compromise must be found, a cleverly tailored way to dynamically split the condensate, in order to control the quantum fluctuations as well as possible. This cannot be calculated: this problem cannot be solved using conventional computers. But with experiments, the research team was able to show: The appropriate splitting dynamics can be used to suppress the fluctuation in the number of particles, and this in turn translates into a reduction the temperature that you want to minimize.
“Different temperature scales exist simultaneously in this system, and we lower a very specific one of them,” explains Maximilian Prüfer. “So you can’t think of it like a mini-fridge that gets noticeably colder overall. But that’s not what we’re talking about: suppressing the fluctuations is exactly what we need to be able to use our system as a quantum simulator even better than before. We can now use it to answer questions from fundamental quantum physics that were previously inaccessible.”
Traceable localization enables accurate integration of quantum emitters and photonic structures with high yield
by Craig R. Copeland, Adam L. Pintar, Ronald G. Dixson, Ashish Chanana, Kartik Srinivasan, Daron A. Westly, B. Robert Ilic, Marcelo I. Davanco, Samuel M. Stavis in Optica Quantum
Traceable microscopy could improve the reliability of quantum information technologies, biological imaging, and more. Devices that capture the brilliant light from millions of quantum dots, including chip-scale lasers and optical amplifiers, have made the transition from laboratory experiments to commercial products. But newer types of quantum-dot devices have been slower to come to market because they require extraordinarily accurate alignment between individual dots and the miniature optics that extract and guide the emitted radiation.
Researchers at the National Institute of Standards and Technology (NIST) and their colleagues have now developed standards and calibrations for optical microscopes that allow quantum dots to be aligned with the center of a photonic component to within an error of 10 to 20 nanometers (about one-thousandth the thickness of a sheet of paper). Such alignment is critical for chip-scale devices that employ the radiation emitted by quantum dots to store and transmit quantum information.
For the first time, the NIST researchers achieved this level of accuracy across the entire image from an optical microscope, enabling them to correct the positions of many individual quantum dots. A model developed by the researchers predicts that if microscopes are calibrated using the new standards, then the number of high-performance devices could increase by as much as a hundred-fold.
That new ability could enable quantum information technologies that are slowly emerging from research laboratories to be more reliably studied and efficiently developed into commercial products. In developing their method, Craig Copeland, Samuel Stavis, and their collaborators, including colleagues from the Joint Quantum Institute (JQI), a research partnership between NIST and the University of Maryland, created standards and calibrations that were traceable to the International System of Units (SI) for optical microscopes used to guide the alignment of quantum dots.
“The seemingly simple idea of finding a quantum dot and placing a photonic component on it turns out to be a tricky measurement problem,” Copeland said.
In a typical measurement, errors begin to accumulate as researchers use an optical microscope to find the location of individual quantum dots, which reside at random locations on the surface of a semiconductor material. If researchers ignore the shrinkage of semiconductor materials at the ultracold temperatures at which quantum dots operate, the errors grow larger. Further complicating matters, these measurement errors are compounded by inaccuracies in the fabrication process that researchers use to make their calibration standards, which also affects the placement of the photonic components.
The NIST method, which the researchers described, identifies and corrects such errors, which were previously overlooked. The NIST team created two types of traceable standards to calibrate optical microscopes — first at room temperature to analyze the fabrication process, and then at cryogenic temperatures to measure the location of quantum dots. Building on their previous work, the room-temperature standard consisted of an array of nanoscale holes spaced a set distance apart in a metal film.
The researchers then measured the actual positions of the holes with an atomic force microscope, ensuring that the positions were traceable to the SI. By comparing the apparent positions of the holes as viewed by the optical microscope with the actual positions, the researchers assessed errors from magnification calibration and image distortion of the optical microscope. The calibrated optical microscope could then be used to rapidly measure other standards that the researchers fabricated, enabling a statistical analysis of the accuracy and variability of the process.
“Good statistics are essential to every link in a traceability chain,” said NIST researcher Adam Pintar, a coauthor of the article.
Extending their method to low temperatures, the research team calibrated an ultracold optical microscope for imaging quantum dots. To perform this calibration, the team created a new microscopy standard — an array of pillars fabricated on a silicon wafer. The scientists worked with silicon because the shrinkage of the material at low temperatures has been accurately measured.
The researchers discovered several pitfalls in calibrating the magnification of cryogenic optical microscopes, which tend to have worse image distortion than microscopes operating at room temperature. These optical imperfections bend the images of straight lines into gnarled curves that the calibration effectively straightens out. If uncorrected, the image distortion causes large errors in determining the position of quantum dots and in aligning the dots within targets, waveguides, or other light-controlling devices.
“These errors have likely prevented researchers from fabricating devices that perform as predicted,” said NIST researcher Marcelo Davanco, a coauthor of the article.
The researchers developed a detailed model of the measurement and fabrication errors in integrating quantum dots with chip-scale photonic components. They studied how these errors limit the ability of quantum-dot devices to perform as designed, finding the potential for a hundred-fold improvement.
“A researcher might be happy if one out of a hundred devices works for their first experiment, but a manufacturer might need ninety-nine out of a hundred devices to work,” Stavis noted. “Our work is a leap ahead in this lab-to-fab transition.”
Beyond quantum-dot devices, traceable standards and calibrations under development at NIST may improve accuracy and reliability in other demanding applications of optical microscopy, such as imaging brain cells and mapping neural connections. For these endeavors, researchers also seek to determine accurate positions of the objects under study across an entire microscope image. In addition, scientists may need to coordinate position data from different instruments at different temperatures, as is true for quantum-dot devices.
Cascaded variational quantum eigensolver algorithm
by Daniel Gunlycke, C. Stephen Hellberg, John P. T. Stenger in Physical Review Research
U.S. Naval Research Laboratory (NRL) scientists published the Cascaded Variational Quantum Eigensolver (CVQE) algorithm in a recent article, expected to become a powerful tool to investigate the physical properties in electronic systems.
The CVQE algorithm is a variant of the Variational Quantum Eigensolver (VQE) algorithm that only requires the execution of a set of quantum circuits once rather than at every iteration during the parameter optimization process, thereby increasing the computational throughput.
“Both algorithms produce a quantum state close to the ground state of a system, which is used to determine many of the system’s physical properties,” said John Stenger, Ph.D., a Theoretical Chemistry Section research physicist. “Calculations that previously took months can now be performed in hours.”
The CVQE algorithm uses a quantum computer to probe the needed probability mass functions and a classical computer to perform the remaining calculations, including the energy minimization.
“Finding the minimum energy is computationally hard as the size of the state space grows exponentially with the system size,” said Steve Hellberg, Ph.D., a Theory of Advanced Functional Materials Section research physicist. “Except for very small systems, even the world’s most powerful supercomputers are unable to find the exact ground state.”
To address this challenge, scientists use a quantum computer with a qubit register, whose state space also increases exponentially, in this case with qubits. By representing the states of a physical system on the state space of the register, a quantum computer can be used to simulate the states in the exponentially large representation space of the system.
Data can subsequently be extracted by quantum measurements. As quantum measurements are not deterministic, the quantum circuit executions must be repeated multiple times to estimate probability distributions describing the states, a process known as sampling. Variational quantum algorithms, including the CVQE algorithm, identify trial states by a set of parameters that are optimized to minimize the energy.
“The key difference between the original VQE method and the new CVQE method is that the sampling and optimization processes have been decoupled in the latter such that the sampling can be performed exclusively on the quantum computer and the parameters processed exclusively on a classical computer,” said Dan Gunlycke, D.Phil., Theoretical Chemistry Section Head, who also leads the NRL quantum computing effort. “The new approach also has other benefits. The form of the solution space does not have to comport with the symmetry requirements of the qubit register, and therefore, it is much easier to shape the solution space and implement symmetries of the system and other physically motivated constraints, which will ultimately lead to more accurate predictions of electronic system properties.”
Quantum computing is a component of quantum science, which has been designated as a Critical Technology Area within the USD(R&E) Technology Vision for an Era of Competition by the Under Secretary of Defense for Research and Engineering Heidi Shyu.
“Understanding the properties of quantum-mechanical systems is essential in the development of new materials and chemistry for the Navy and Marine Corps,” Gunlycke said. “Corrosion, for instance, is an omnipresent challenge costing the Department of Defense billions every year. The CVQE algorithm can be used to study the chemical reactions causing corrosion and provide critical information to our existing anticorrosion teams in their quest to develop better coatings and additives.”
Oscillating photonic Bell state from a semiconductor quantum dot for quantum key distribution
by Matteo Pennacchietti, Brady Cunard, Shlok Nahar, Mohd Zeeshan, Sayan Gangopadhyay, Philip J. Poole, Dan Dalacu, Andreas Fognini, Klaus D. Jöns, Val Zwiller, Thomas Jennewein, Norbert Lütkenhaus, Michael E. Reimer in Communications Physics
Researchers at the University of Waterloo’s Institute for Quantum Computing (IQC) have brought together two Nobel prize-winning research concepts to advance the field of quantum communication.
Scientists can now efficiently produce nearly perfect entangled photon pairs from quantum dot sources.
Entangled photons are particles of light that remain connected, even across large distances, and the 2022 Nobel Prize in Physics recognized experiments on this topic. Combining entanglement with quantum dots, a technology recognized with the Nobel Prize in Chemistry in 2023, the IQC research team aimed to optimize the process for creating entangled photons, which have a wide variety of applications, including secure communications.
“The combination of a high degree of entanglement and high efficiency is needed for exciting applications such as quantum key distribution or quantum repeaters, which are envisioned to extend the distance of secure quantum communication to a global scale or link remote quantum computers,” said Dr. Michael Reimer, professor at IQC and Waterloo’s Department of Electrical and Computer Engineering. “Previous experiments only measured either near-perfect entanglement or high efficiency, but we’re the first to achieve both requirements with a quantum dot.”
By embedding semiconductor quantum dots into a nanowire, the researchers created a source that creates near-perfect entangled photons 65 times more efficiently than previous work. This new source, developed in collaboration with the National Research Council of Canada in Ottawa, can be excited with lasers to generate entangled pairs on command. The researchers then used high-resolution single photon detectors provided by Single Quantum in The Netherlands to boost the degree of entanglement.
“Historically, quantum dot systems were plagued with a problem called fine structure splitting, which causes an entangled state to oscillate over time. This meant that measurements taken with a slow detection system would prevent the entanglement from being measured,” said Matteo Pennacchietti, a PhD student at IQC and Waterloo’s Department of Electrical and Computer Engineering. “We overcame this by combining our quantum dots with a very fast and precise detection system. We can basically take a timestamp of what the entangled state looks like at each point during the oscillations, and that’s where we have the perfect entanglement.”
Long-distance continuous-variable quantum key distribution over 100-km fiber with local local oscillator
by Adnan A. E. Hajomer, Ivan Derkach, Nitin Jain, Hou-Man Chin, Ulrik L. Andersen, Tobias Gehring in Science Advances
Researchers at DTU have successfully distributed a quantum-secure key using a method called Continuous Variable Quantum Key Distribution (CV QKD). The researchers have managed to make the method work over a record 100 km distance — the longest distance ever achieved using the CV QKD method. The advantage of the method is that it can be applied to the existing Internet infrastructure.
Quantum computers threaten existing algorithm-based encryptions, which currently secure data transfers against eavesdropping and surveillance. They are not yet powerful enough to break them, but it’s a matter of time. If a quantum computer succeeds in figuring out the most secure algorithms, it leaves an open door to all data connected via the internet. This has accelerated the development of a new encryption method based on the principles of quantum physics. But to succeed, researchers must overcome one of the challenges of quantum mechanics — ensuring consistency over longer distances. Continuous Variable Quantum Key Distribution has so far worked best over short distances.
“We have achieved a wide range of improvements, especially regarding the loss of photons along the way. In this experiment, we securely distributed a quantum-encrypted key 100 kilometres via fibre optic cable. This is a record distance with this method,” says Tobias Gehring, an associate professor at DTU, who, together with a group of researchers at DTU, aims to be able to distribute quantum-encrypted information around the world via the internet.
“When data needs to be sent from A to B, it must be protected. Encryption combines data with a secure key distributed between sender and receiver so both can access the data. A third party must not be able to figure out the key while it is being transmitted; otherwise, the encryption will be compromised. Key exchange is, therefore, essential in encrypting data.
Quantum Key Distribution (QKD) is an advanced technology that researchers are working on for crucial exchanges. The technology ensures the exchange of cryptographic keys by using light from quantum mechanical particles called photons. When a sender sends information encoded in photons, the quantum mechanical properties of the photons are exploited to create a unique key for the sender and receiver. Attempts by others to measure or observe photons in a quantum state will instantly change their state. Therefore, it is physically only possible to measure light by disturbing the signal.
“It is impossible to make a copy of a quantum state, as when making a copy of an A4 sheet — if you try, it will be an inferior copy. That’s what ensures that it is not possible to copy the key. This can protect critical infrastructure such as health records and the financial sector from being hacked,” explains Tobias Gehring.
The Continuous Variable Quantum Key Distribution (CV QKD) technology can be integrated into the existing internet infrastructure.
“The advantage of using this technology is that we can build a system that resembles what optical communication already relies on.”
The backbone of the internet is optical communication. It works by sending data via infrared light running through optical fibres. They function as light guides laid in cables, ensuring we can send data worldwide. Data can be sent faster and over longer distances via fibre optic cables, and light signals are less susceptible to interference, which is called noise in technical terms.
“It is a standard technology that has been used for a long time. So, you don’t need to invent anything new to be able to use it to distribute quantum keys, and it can make implementation significantly cheaper. And we can operate at room temperature,” explains Tobias Gehring, adding: “But CV QKD technology works best over shorter distances. Our task is to increase the distance. And the 100 kilometres is a big step in the right direction.”
The researchers succeeded in increasing the distance by addressing three factors that limit their system in exchanging the quantum-encrypted keys over longer distances: Machine learning provided earlier measurements of the disturbances affecting the system. Noise, as these disturbances are called, can arise, for example, from electromagnetic radiation, which can distort or destroy the quantum states being transmitted. The earlier detection of the noise made it possible to reduce its corresponding effect more effectively. Furthermore, the researchers have become better at correcting errors that can occur along the way, which can be caused by noise, interference, or imperfections in the hardware.
“In our upcoming work, we will use the technology to establish a secure communication network between Danish ministries to secure their communication. We will also attempt to generate secret keys between, for example, Copenhagen and Odense to enable companies with branches in both cities to establish quantum-safe communication,” Tobias Gehring says.
Continuous Variable Quantum Key Distribution (CV QKD) focuses on measuring the smooth properties of quantum states in photons. It can be compared to conveying information in a stream of all the nuances of colours instead of conveying information step by step in each colour.
Quantum Barkhausen noise induced by domain wall cotunneling
by C. Simon, D.M. Silevitch, P.C.E. Stamp, T.F. Rosenbaum in Proceedings of the National Academy of Sciences
Iron screws and other so-called ferromagnetic materials are made up of atoms with electrons that act like little magnets. Normally, the orientations of the magnets are aligned within one region of the material but are not aligned from one region to the next. Think of packs of tourists in Times Square pointing to different billboards all around them. But when a magnetic field is applied, the orientations of the magnets, or spins, in the different regions line up and the material becomes fully magnetized. This would be like the packs of tourists all turning to point at the same sign.
The process of spins lining up, however, does not happen all at once. Rather, when the magnetic field is applied, different regions, or so-called domains, influence others nearby, and the changes spread across the material in a clumpy fashion. Scientists often compare this effect to an avalanche of snow, where one small lump of snow starts falling, pushing on other nearby lumps, until the entire mountainside of snow is tumbling down in the same direction.
This avalanche effect was first demonstrated in magnets by the physicist Heinrich Barkhausen in 1919. By wrapping a coil around a magnetic material and attaching it to a loudspeaker, he showed that these jumps in magnetism can be heard as a crackling sound, known today as Barkhausen noise.
Now, Caltech researchers have shown that Barkhausen noise can be produced not only through traditional, or classical means, but through quantum mechanical effects. This is the first time quantum Barkhausen noise has been detected experimentally. The research represents an advance in fundamental physics and could one day have applications in creating quantum sensors and other electronic devices.
“Barkhausen noise is the collection of the little magnets flipping in groups,” says Christopher Simon, lead author of the paper and a postdoctoral scholar in the lab of Thomas F. Rosenbaum, a professor of physics at Caltech, the president of the Institute, and the Sonja and William Davidow Presidential Chair. “We are doing the same experiment that has been done many times, but we are doing it in a quantum material. We are seeing that the quantum effects can lead to macroscopic changes.”
Usually, these magnetic flips occur classically, through thermal activation, where the particles need to temporarily gain enough energy to jump over an energy barrier. However, the new study shows that these flips can also occur quantum mechanically through a process called quantum tunneling.
In tunneling, particles can jump to the other side of an energy barrier without having to actually pass over the barrier. If one could scale up this effect to everyday objects like golf balls, it would be like the golf ball passing straight through a hill rather than having to climb up over it to get to the other side.
“In the quantum world, the ball doesn’t have to go over a hill because the ball, or rather the particle, is actually a wave, and some of it is already on the other side of the hill,” says Simon.
In addition to quantum tunneling, the new research shows a co-tunneling effect, in which groups of tunneling electrons are communicating with each other to drive the electron spins to flip in the same direction.
“Classically, each one of the mini avalanches, where groups of spins flip, would happen on its own,” says co-author Daniel Silevitch, research professor of physics at Caltech. “But we found that through quantum tunneling, two avalanches happen in sync with each other. This is a result of two large ensembles of electrons talking to each other and, through their interactions, they make these changes. This co-tunneling effect was a surprise.”
For their experiments, members of the team used a pink crystalline material called lithium holmium yttrium fluoride cooled to temperatures near absolute zero (equivalent to minus 273.15 degrees Celsius). They wrapped a coil around it, applied a magnetic field, and then measured brief jumps in voltage, not unlike what Barkhausen did in 1919 in his more simplified experiment. The observed voltage spikes indicate when groups of electron spins flip their magnetic orientations. As the groups of spins flip, one after the other, a series of voltage spikes is observed, i.e. the Barkhausen noise.
By analyzing this noise, the researchers were able to show that a magnetic avalanche was taking place even without the presence of classical effects. Specifically, they showed that these effects were insensitive to changes in the temperature of the material. This and other analytical steps led them to conclude that quantum effects were responsible for the sweeping changes.
According to the scientists, these flipping regions can contain up to 1 million billion spins, in comparison to the entire crystal that contains approximately 1 billion trillion spins.
“We are seeing this quantum behavior in materials with up to trillions of spins. Ensembles of microscopic objects are all behaving coherently,” Rosenbaum says. “This work represents the focus of our lab: to isolate quantum mechanical effects where we can quantitively understand what is going on.”
Another recent PNAS paper from Rosenbaum’s lab similarly looks at how tiny quantum effects can lead to larger-scale changes. In this earlier study, the researchers studied the element chromium and showed that two different types of charge modulation (involving the ions in one case and the electrons in the other) operating at different length scales can interfere quantum mechanically.
“People have studied chromium for a long time,” says Rosenbaum, “but it took until now to appreciate this aspect of the quantum mechanics. It is another example of engineering simple systems to reveal quantum behavior that we can study on the macroscopic scale.”
Wide-angle spectral filter for energy-saving windows designed by quantum annealing-enhanced active learning
by Seongmin Kim, Serang Jung, Alexandria Bobbitt, Eungkyu Lee, Tengfei Luo in Cell Reports Physical Science
Windows welcome light into interior spaces, but they also bring in unwanted heat. A new window coating blocks heat-generating ultraviolet and infrared light and lets through visible light, regardless of the sun’s angle. The coating can be incorporated onto existing windows or automobiles and can reduce air-conditioning cooling costs by more than one-third in hot climates.
“The angle between the sunshine and your window is always changing,” said Tengfei Luo, the Dorini Family Professor for Energy Studies at the University of Notre Dame and the lead of the study. “Our coating maintains functionality and efficiency whatever the sun’s position in the sky.”
Window coatings used in many recent studies are optimized for light that enters a room at a 90-degree angle. Yet at noon, often the hottest time of the day, the sun’s rays enter vertically installed windows at oblique angles. Luo and his postdoctoral associate Seongmin Kim previously fabricated a transparent window coating by stacking ultra-thin layers of silica, alumina and titanium oxide on a glass base. A micrometer-thick silicon polymer was added to enhance the structure’s cooling power by reflecting thermal radiation through the atmospheric window and into outer space.
Additional optimization of the order of the layers was necessary to ensure the coating would accommodate multiple angles of solar light. However, a trial-and-error approach was not practical, given the immense number of possible combinations, Luo said. To shuffle the layers into an optimal configuration — one that maximized the transmission of visible light while minimizing the passage of heat-producing wavelengths — the team used quantum computing, or more specifically, quantum annealing, and validated their results experimentally. Their model produced a coating that both maintained transparency and reduced temperature by 5.4 to 7.2 degrees Celsius in a model room, even when light was transmitted in a broad range of angles.
“Like polarized sunglasses, our coating lessens the intensity of incoming light, but, unlike sunglasses, our coating remains clear and effective even when you tilt it at different angles,” Luo said.
The active learning and quantum computing scheme developed to create this coating can be used to design of a broad range of materials with complex properties.
Quantum interference enhances the performance of single-molecule transistors
by Zhixin Chen, Iain M. Grace, Steffen L. Woltering, Lina Chen, Alex Gee, Jonathan Baugh, G. Andrew D. Briggs, Lapo Bogani, Jan A. Mol, Colin J. Lambert, Harry L. Anderson, James O. Thomas in Nature Nanotechnology
An international team of researchers from Queen Mary University of London, the University of Oxford, Lancaster University, and the University of Waterloo have developed a new single-molecule transistor that uses quantum interference to control the flow of electrons. The transistor, which is described in a paper, opens new possibilities for using quantum effects in electronic devices.
Transistors are the basic building blocks of modern electronics. They are used to amplify and switch electrical signals, and they are essential for everything from smartphones to spaceships. However, the traditional method of making transistors, which involves etching silicon into tiny channels, is reaching its limits. As transistors get smaller, they become increasingly inefficient and susceptible to errors, as electrons can leak through the device even when it is supposed to be switched off, by a process known as quantum tunnelling. Researchers are exploring new types of switching mechanisms that can be used with different materials to remove this effect.
In the nanoscale structures that Professor Jan Mol, Dr James Thomas, and their group study at Queen Mary’s School of Physical and Chemical Sciences, quantum mechanical effects dominate, and electrons behave as waves rather than particles. Taking advantage of these quantum effects, the researchers built a new transistor. The transistor’s conductive channel is a single zinc porphyrin, a molecule that can conduct electricity. The porphyrin is sandwiched between two graphene electrodes, and when a voltage is applied to the electrodes, electron flow through the molecule can be controlled using quantum interference.
Interference is a phenomenon that occurs when two waves interact with each other and either cancel each other out (destructive interference) or reinforce each other (constructive interference). In the new transistor’s case, researchers switched the transistor on and off by controlling whether the electrons interfere constructively (on) or destructively (off) as they flow through the zinc porphyrin molecule.
The researchers found that the new transistor has a very high on/off ratio, meaning that it can be turned on and off very precisely. Destructive quantum interference plays a crucial role in this by eliminating the leaky electron flow from quantum tunneling through the transistor when it is supposed to be switched off. They also found that the transistor is very stable. Previous transistors made from a single molecule have only been able to demonstrate a handful of switching cycles, however this device can be operated for hundreds of thousands of cycles without breaking down.
“Quantum interference is a powerful phenomenon that has the potential to be used in a wide variety of electronics applications,” said lead author Dr James Thomas, Lecturer in Quantum Technologies at Queen Mary. “We believe that our work is a significant step towards realizing this potential.”
“Our results show that quantum interference can be used to control the flow of electrons in transistors, and that this can be done in a way that is both efficient and reliable,” said co-author Professor Jan Mol. “This could lead to the development of new types of transistors that are smaller, faster, and more energy-efficient than current devices.”
The researchers also found that the quantum interference effects could be used to improve the transistor’s subthreshold swing, which is a measure of how sensitive the transistor is to changes in the gate voltage. The lower the subthreshold swing, the more efficient the transistor is. The researchers’ transistors had a subthreshold swing of 140 mV/dec, which is better than subthreshold swings reported for other single-molecule transistors, and comparable to larger devices made from materials such as carbon nanotubes.
The research is still in its initial stages, but the researchers are optimistic that the new transistor could be used to create a new generation of electronic devices. These devices could be used in a variety of applications, starting from computers, smartphones, and ending with medical devices.
Parametric magnon transduction to spin qubits
by Mauricio Bejarano, Francisco J. T. Goncalves, Toni Hache, Michael Hollenbach, Christopher Heins, Tobias Hula, Lukas Körber, Jakob Heinze, Yonder Berencén, Manfred Helm, Jürgen Fassbender, Georgy V. Astakhov, Helmut Schultheiss in Science Advances
Quantum computers promise to tackle some of the most challenging problems facing humanity today. While much attention has been directed towards the computation of quantum information, the transduction of information within quantum networks is equally crucial in materializing the potential of this new technology. Addressing this need, a research team at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) is now introducing a new approach for transducing quantum information: the team has manipulated quantum bits, so called qubits, by harnessing the magnetic field of magnons — wave-like excitations in a magnetic material — that occur within microscopic magnetic disks.
The construction of a programmable, universal quantum computer stands as one of the most challenging engineering and scientific endeavors of our time. The realization of such a computer holds great potential for diverse industry fields such as logistics, finance, and pharmaceutics. However, the construction of a practical quantum computer has been hindered by the intrinsic fragility of how the information is stored and processed in this technology. Quantum information is encoded in qubits, which are extremely susceptible to the noise in their environment. Tiny thermal fluctuations, a fraction of a degree, could entirely disrupt the computation.
This has prompted researchers to distribute the functionalities of quantum computers among distinct separate building blocks, in an effort to reduce error rates, and harness complementary advantages from their constituents.
“However, this poses the problem of transferring the quantum information between the modules in a way that the information doesn’t go missing,” says HZDR researcher Mauricio Bejarano, first author of the publication. “Our research lies precisely in this specific niche, transducing communication between distinct quantum modules.”
The currently established method to transfer quantum information and addressing qubits is through microwave antennas. This is the approach used by Google and IBM in their superconducting chips, the technological platform standing at the forefront in this quantum race.
“We, on the other hand, address the qubits with magnons,” says HZDR physicist Helmut Schultheiß, who supervised the work. “These can be thought of as magnetic excitation waves that pass through a magnetic material. The advantage here is that the wavelength of magnons lies in the micrometer range and is significantly shorter than the centimeter waves of conventional microwave technology. Consequently, the microwave footprint of magnons costs less space in the chip.”
The HZDR group investigated the interaction of magnons and qubits formed by vacancies of silicon atoms in the crystal structure of silicon carbide, a material commonly used in high-power electronics. Such types of qubits are typically called spin qubits, given the quantum information is encoded in the spin state of the vacancy. But how can magnons be utilized to control these types of qubits? “Typically, magnons are generated with microwave antennas. This poses the problem that it is very difficult to separate the microwave drive coming from the antenna from the one coming from the magnons,” explains Bejarano.
To isolate the microwaves from the magnons, the HZDR team used an exotic magnetic phenomena observable in microscopic magnetic disks of a nickel-iron alloy. “Due to a nonlinear process, some magnons inside the disk possess a much lower frequency than the driving frequency of the antenna. We manipulate qubits only with these lower frequency magnons.” The research team emphasizes they did not perform any quantum calculations yet. However, they showed that it is fundamentally feasible to address qubits exclusively with magnons.
“To date, the quantum engineering community has not yet realized that magnons can be used to control qubits,” stresses Schultheiß. “But our experiments demonstrate that these magnetic waves could indeed be useful.” In order to further develop their approach, the team is already preparing for their future plans: they want to try to control several closely spaced individual qubits in such a way that magnons mediate their entanglement process — a prerequisite for performing quantum computations.
Their vision is that, in the long term, magnons could be excited by direct electrical currents with such precision that they specifically and exclusively address a single qubit in an array of qubits. This would make it possible to use magnons as a programmable quantum bus to address qubits in an extremely effective manner. While there is plenty of work ahead, the group’s research highlights that combining magnonic systems with quantum technologies could provide useful insights for the development of a practical quantum computer in the future.
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