Quantum news biweekly vol. 72, 24th April — 9th May
TL;DR
- Quantum mechanical effects such as radioactive decay, or more generally: ‘tunneling’, display intriguing mathematical patterns. Researchers now show that a 40-year-old mathematical discovery can be used to fully encode and understand this structure.
- A recent study delves deeply into the junction of gravity and quantum mechanics, employing ultra-high energy neutrinos detected by a device buried in Antarctica’s glaciers.
- Physicists have devised a method to arrange atoms closer than ever before, down to 50 nanometers, paving the way for magnetic quantum gates crucial for quantum computing.
- Innovative refrigeration technology drastically reduces the time and energy needed to cool materials near absolute zero, benefiting the quantum industry.
- Despite noise interference, scientists achieve near-perfect quantum teleportation, a significant feat for quantum communication.
- Quantum information storage and retrieval are accomplished for the first time, marking a milestone in quantum networking.
- A new technique enhances quantum resistance standards utilizing the Quantum Anomalous Hall effect.
- Atomic-resolution imaging and control of chiral interface states unveil potential applications in quantum computing and efficient electronics.
- Laser-induced quantum behavior at room temperature transforms non-magnetic materials into magnetic ones, opening avenues for faster and more energy-efficient technologies.
- Nanobolometers offer a promising solution for qubit measurement challenges, addressing scalability issues on the path to quantum supremacy.
- 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
Exact instanton transseries for quantum mechanics
by Alexander van Spaendonck, Marcel Vonk in SciPost Physics
Quantum mechanical effects such as radioactive decay, or more generally: ‘tunneling’, display intriguing mathematical patterns. Two researchers at the University of Amsterdam now show that a 40-year-old mathematical discovery can be used to fully encode and understand this structure.
In the quantum world, processes can be separated into two distinct classes. One class, that of the so-called ‘perturbative’ phenomena, is relatively easy to detect, both in an experiment and in mathematical computation. Examples are plentiful: the light that atoms emit, the energy that solar cells produce, and the states of qubits in a quantum computer. These quantum phenomena depend on Planck’s constant, the fundamental constant of nature that determines how the quantum world differs from our large-scale world, but in a simple way. Despite the ridiculous smallness of this constant — expressed in everyday units of kilograms, metres and seconds it takes a value that starts at the 34th decimal place after the comma — the fact that Planck’s constant is not exactly zero is enough to compute such quantum effects.
Then, there are the ‘nonperturbative’ phenomena. One of the best-known is radioactive decay: a process where due to quantum effects, elementary particles can escape the attractive force that ties them to atomic nuclei. If the world were ‘classical’ — that is, if Planck’s constant were exactly zero — this attractive force would be impossible to overcome. In the quantum world, decay does occur, but still only occasionally; a single uranium atom, for example, would on average take over four billion years to decay. The collective name for such rare quantum events is ‘tunneling’: for the particle to escape, it has to ‘dig a tunnel’ through the energy barrier that keeps it tied to the nucleus. A tunnel that can take billions of years to dig, and makes The Shawshank Redemption look like child’s play.
Mathematically, nonperturbative quantum effects are much more difficult to describe than their perturbative cousins. Still, over the century that quantum mechanics has existed, physicists have found many ways to deal with these effects, and to describe and predict them accurately.
“Still, in this century-old problem, there was work left to be done,” says Alexander van Spaendonck, one of the authors of the new publication. “The descriptions of tunneling phenomena in quantum mechanics needed further unification — a framework in which all such phenomena could be described and investigated using a single mathematical structure.”
Surprisingly, such a structure was found in 40-year-old mathematics. In the 1980s, French mathematician Jean Écalle had set up a framework that he dubbed resurgence, and that had precisely this goal: giving structure to nonperturbative phenomena. So why did it take 40 years for the natural combination of Écalle’s formalism and the application to tunneling phenomena to be taken to their logical conclusion? Marcel Vonk, the other author of the publication, explains: “Écalle’s original papers were lengthy — over 1000 pages all combined — highly technical, and only published in French. As a result, it took until the mid-2000s before a significant number of physicists started getting familiar with this ‘toolbox’ of resurgence. Originally, it was mostly applied to simple ‘toy models’, but of course the tools were also tried on real-life quantum mechanics. Our work takes these developments to their logical conclusion.”
That conclusion is that one of the tools in Écalle’s toolbox, that of a ‘transseries’, is perfectly suited to describe tunneling phenomena in essentially any quantum mechanics problem, and does so always in the same way. By spelling out the mathematical details, the authors found that it became possible not only to unify all tunneling phenomena into a single mathematical object but also to describe certain ‘jumps’ in how big the role of these phenomena is — an effect known as Stokes’ phenomenon.
Van Spaendonck: “Using our description Stokes’ phenomenon, we were able to show that certain ambiguities that had plagued the ‘classical’ methods of computing nonperturbative effects — infinitely many, in fact — all dropped out in our method. The underlying structure turned out to be even more beautiful than we originally expected. The transseries that describes quantum tunneling turns out to split — or ‘factorize’ — in a surprising way: into a ‘minimal’ transseries that describes the basic tunneling phenomena that essentially exist in any quantum mechanics problem, and an object that we called the ‘median transseries’ that describes the more problem-specific details, and that depends for example on how symmetric a certain quantum setting is.”
With this mathematical structure completely clarified, the next question is of course where the new lessons can be applied and what physicists can learn from them. In the case of radioactivity, for example, some atoms are stable whereas others decay. In other physical models, the lists of stable and unstable particles may vary as one slightly changes the setup — a phenomenon known as ‘wall-crossing’. What the researchers have in mind next is to clarify this notion of wall-crossing using the same techniques. This difficult problem has again been studied by many groups in many different ways, but now a similar unifying structure might be just around the corner. There is certainly light at the end of the tunnel.
Search for decoherence from quantum gravity with atmospheric neutrinos
by R. Abbasi, M. Ackermann, J. Adams, et al in Nature Physics
Einstein’s theory of general relativity explains that gravity is caused by a curvature of the directions of space and time. The most familiar manifestation of this is the Earth’s gravity, which keeps us on the ground and explains why balls fall to the floor and individuals have weight when stepping on a scale.
In the field of high-energy physics, on the other hand, scientists study tiny invisible objects that obey the laws of quantum mechanics — characterized by random fluctuations that create uncertainty in the positions and energies of particles like electrons, protons and neutrons. Understanding the randomness of quantum mechanics is required to explain the behavior of matter and light on a subatomic scale.
For decades, scientists have been trying to unite those two fields of study to achieve a quantum description of gravity. This would combine the physics of curvature associated with general relativity with the mysterious random fluctuations associated with quantum mechanics.
A new study from physicists at The University of Texas at Arlington reports on a deep new probe into the interface between these two theories, using ultra-high energy neutrino particles detected by a particle detector set deep into the Antarctic glacier at the south pole.
“The challenge of unifying quantum mechanics with the theory of gravitation remains one of the most pressing unsolved problems in physics,” said co-author Benjamin Jones, associate professor of physics. “If the gravitational field behaves in a similar way to the other fields in nature, its curvature should exhibit random quantum fluctuations.”
Jones and UTA graduate students Akshima Negi and Grant Parker were part of an international IceCube Collaboration team that included more than 300 scientists from around the U.S., as well as Australia, Belgium, Canada, Denmark, Germany, Italy, Japan, New Zealand, Korea, Sweden, Switzerland, Taiwan and the United Kingdom.
To search for signatures of quantum gravity, the team placed thousands of sensors throughout one square kilometer near the south pole in Antarctica that monitored neutrinos, unusual but abundant subatomic particles that are neutral in charge and have no mass. The team was able to study more than 300,000 neutrinos. They were looking to see whether these ultra-high-energy particles were bothered by random quantum fluctuations in spacetime that would be expected if gravity were quantum mechanical, as they travel long distances across the Earth.
“We searched for those fluctuations by studying the flavors of neutrinos detected by the IceCube Observatory,” Negi said. “Our work resulted in a measurement that was far more sensitive than previous ones (over a million times more, for some of the models), but it did not find evidence of the expected quantum gravitational effects.”
This non-observation of a quantum geometry of spacetime is a powerful statement about the still-unknown physics that operate at the interface of quantum physics and general relativity.
“This analysis represents the final chapter in UTA’s nearly decade-long contribution to the IceCube Observatory,” said Jones. “My group is now pursuing new experiments that aim to understand the origin and value of the neutrinos mass using atomic, molecular and optical physics techniques.”
Atomic physics on a 50-nm scale: Realization of a bilayer system of dipolar atoms
by Li Du, Pierre Barral, Michael Cantara, Julius de Hond, Yu-Kun Lu, Wolfgang Ketterle in Science
Proximity is key for many quantum phenomena, as interactions between atoms are stronger when the particles are close. In many quantum simulators, scientists arrange atoms as close together as possible to explore exotic states of matter and build new quantum materials.
They typically do this by cooling the atoms to a stand-still, then using laser light to position the particles as close as 500 nanometers apart — a limit that is set by the wavelength of light. Now, MIT physicists have developed a technique that allows them to arrange atoms in much closer proximity, down to a mere 50 nanometers. For context, a red blood cell is about 1,000 nanometers wide.
The physicists demonstrated the new approach in experiments with dysprosium, which is the most magnetic atom in nature. They used the new approach to manipulate two layers of dysprosium atoms, and positioned the layers precisely 50 nanometers apart. At this extreme proximity, the magnetic interactions were 1,000 times stronger than if the layers were separated by 500 nanometers.
What’s more, the scientists were able to measure two new effects caused by the atoms’ proximity. Their enhanced magnetic forces caused “thermalization,” or the transfer of heat from one layer to another, as well as synchronized oscillations between layers. These effects petered out as the layers were spaced farther apart.
“We have gone from positioning atoms from 500 nanometers to 50 nanometers apart, and there is a lot you can do with this,” says Wolfgang Ketterle, the John D. MacArthur Professor of Physics at MIT. “At 50 nanometers, the behavior of atoms is so much different that we’re really entering a new regime here.”
Ketterle and his colleagues say the new approach can be applied to many other atoms to study quantum phenomena. For their part, the group plans to use the technique to manipulate atoms into configurations that could generate the first purely magnetic quantum gate — a key building block for a new type of quantum computer. The study’s co-authors include lead author and physics graduate student Li Du, along with Pierre Barral, Michael Cantara, Julius de Hond, and Yu-Kun Lu — all members of the MIT-Harvard Center for Ultracold Atoms, the Department of Physics, and the Research Laboratory of Electronics at MIT.
To manipulate and arrange atoms, physicists typically first cool a cloud of atoms to temperatures approaching absolute zero, then use a system of laser beams to corral the atoms into an optical trap.
Laser light is an electromagnetic wave with a specific wavelength (the distance between maxima of the electric field) and frequency. The wavelength limits the smallest pattern into which light can be shaped to typically 500 nanometers, the so-called optical resolution limit. Since atoms are attracted by laser light of certain frequencies, atoms will be positioned at the points of peak laser intensity. For this reason, existing techniques have been limited in how close they can position atomic particles, and could not be used to explore phenomena that happen at much shorter distances.
“Conventional techniques stop at 500 nanometers, limited not by the atoms but by the wavelength of light,” Ketterle explains. “We have found now a new trick with light where we can break through that limit.”
The team’s new approach, like current techniques, starts by cooling a cloud of atoms — in this case, to about 1 microkelvin, just a hair above absolute zero — at which point, the atoms come to a near-standstill. Physicists can then use lasers to move the frozen particles into desired configurations.
Then, Du and his collaborators worked with two laser beams, each with a different frequency, or color, and circular polarization, or direction of the laser’s electric field. When the two beams travel through a super-cooled cloud of atoms, the atoms can orient their spin in opposite directions, following either of the two lasers’ polarization. The result is that the beams produce two groups of the same atoms, only with opposite spins.
Each laser beam formed a standing wave, a periodic pattern of electric field intensity with a spatial period of 500 nanometers. Due to their different polarizations, each standing wave attracted and corralled one of two groups of atoms, depending on their spin. The lasers could be overlaid and tuned such that the distance between their respective peaks is as small as 50 nanometers, meaning that the atoms gravitating to each respective laser’s peaks would be separated by the same 50 nanometers. But in order for this to happen, the lasers would have to be extremely stable and immune to all external noise, such as from shaking or even breathing on the experiment. The team realized they could stabilize both lasers by directing them through an optical fiber, which served to lock the light beams in place in relation to each other.
“The idea of sending both beams through the optical fiber meant the whole machine could shake violently, but the two laser beams stayed absolutely stable with respect to each others,” Du says.
As a first test of their new technique, the team used atoms of dysprosium — a rare-earth metal that is one of the strongest magnetic elements in the periodic table, particularly at ultracold temperatures. However, at the scale of atoms, the element’s magnetic interactions are relatively weak at distances of even 500 nanometers. As with common refrigerator magnets, the magnetic attraction between atoms increases with proximity, and the scientists suspected that if their new technique could space dysprosium atoms as close as 50 nanometers apart, they might observe the emergence of otherwise weak interactions between the magnetic atoms.
“We could suddenly have magnetic interactions, which used to be almost negligible but now are really strong,” Ketterle says.
The team applied their technique to dysprosium, first super-cooling the atoms, then passing two lasers through to split the atoms into two spin groups, or layers. They then directed the lasers through an optical fiber to stabilize them, and found that indeed, the two layers of dysprosium atoms gravitated to their respective laser peaks, which in effect separated the layers of atoms by 50 nanometers — the closest distance that any ultracold atom experiment has been able to achieve.
At this extremely close proximity, the atoms’ natural magnetic interactions were significantly enhanced, and were 1,000 times stronger than if they were positioned 500 nanometers apart. The team observed that these interactions resulted in two novel quantum phenomena: collective oscillation, in which one layer’s vibrations caused the other layer to vibrate in sync; and thermalization, in which one layer transferred heat to the other, purely through magnetic fluctuations in the atoms.
“Until now, heat between atoms could only by exchanged when they were in the same physical space and could collide,” Du notes. “Now we have seen atomic layers, separated by vacuum, and they exchange heat via fluctuating magnetic fields.”
The team’s results introduce a new technique that can be used to position many types of atom in close proximity. They also show that atoms, placed close enough together, can exhibit interesting quantum phenomena, that could be harnessed to build new quantum materials, and potentially, magnetically-driven atomic systems for quantum computers.
Dynamic acoustic optimization of pulse tube refrigerators for rapid cooldown
by Ryan Snodgrass, Vincent Kotsubo, Scott Backhaus, Joel Ullom in Nature Communications
By modifying a refrigerator commonly used in both research and industry, researchers at the National Institute of Standards and Technology (NIST) have drastically reduced the time and energy required to cool materials to within a few degrees above absolute zero. The scientists say that their prototype device, which they are now working to commercialize with an industrial partner, could annually save an estimated 27 million watts of power, $30 million in global electricity consumption and enough cooling water to fill 5,000 Olympic swimming pools.
From stabilizing qubits (the basic unit of information in a quantum computer) to maintaining the superconducting properties of materials and keeping NASA’s James Webb Space Telescope cool enough to observe the heavens, ultracold refrigeration is essential to the operation of many devices and sensors. For decades, the pulse tube refrigerator (PTR) has been the workhorse device for achieving temperatures as cold as the vacuum of outer space.
These refrigerators cyclically compress (heat) and expand (cool) high pressure helium gas to achieve the “Big Chill,” broadly analogous to the way a household refrigerator uses the transformation of freon from liquid to vapor to remove heat. For more than 40 years, the PTR has proven its reliability, but it is also power-hungry, consuming more electricity than any other component of an ultralow temperature experiment.
When NIST researcher Ryan Snodgrass and his colleagues took a closer look at the refrigerator, they found that manufacturers had built the device to be energy efficient only at its final operating temperature of 4 kelvin (K), or 4 degrees above absolute zero. The team found that these refrigerators are extremely inefficient at higher temperatures — a big issue because the cooldown process begins at room temperature.
During a series of experiments, Snodgrass, along with NIST scientists Joel Ullom, Vincent Kotsubo and Scott Backhaus, discovered that at room temperature, the helium gas was under such high pressure that some of it was shunted through a relief valve instead of being used for cooling. By changing the mechanical connections between the compressor and the refrigerator, the team ensured that none of the helium would be wasted, greatly improving the efficiency of the refrigerator.
In particular, the researchers continually adjusted a series of valves that control the amount of helium gas flowing from the compressor to the refrigerator. The scientists found that if they allowed the valves to have a larger opening at room temperature and then gradually closed them as cooling proceeded, they could reduce the cooldown time to between one half and one quarter of what it is now. Currently, scientists must wait a day or more for new quantum circuits to be cold enough to test. Since the progress of scientific research can be limited by the time it takes to reach cryogenic temperatures, the faster cooldown provided by this technology could broadly impact many fields, including quantum computing and other areas of quantum research. The technology developed by the NIST team could also allow scientists to replace large pulse tube refrigerators with much smaller ones, which require less supporting infrastructure, Snodgrass said.
The need for these refrigerators will greatly expand as research on quantum computing, along with its reliance on cryogenic technology, continues to grow. The modified PTR would then save a much greater amount of money, electrical energy and cooling water. In addition to supporting a burgeoning quantum economy, the device would also expedite research because scientists would no longer have to wait days or weeks for qubits and other quantum components to cool.
Overcoming noise in quantum teleportation with multipartite hybrid entanglement
by Zhao-Di Liu, Olli Siltanen, Tom Kuusela, Rui-Heng Miao, Chen-Xi Ning, Chuan-Feng Li, Guang-Can Guo, Jyrki Piilo in Science Advances
In teleportation, the state of a quantum particle, or qubit, is transferred from one location to another without sending the particle itself. This transfer requires quantum resources, such as entanglement between an additional pair of qubits. In an ideal case, the transfer and teleportation of the qubit state can be done perfectly. However, real-world systems are vulnerable to noise and disturbances — and this reduces and limits the quality of the teleportation.
Researchers from the University of Turku, Finland, and the University of Science and Technology of China, Hefei, have now proposed a theoretical idea and made corresponding experiments to overcome this problem. In other words, the new approach enables reaching high-quality teleportation despite the presence of noise.
“The work is based on an idea of distributing entanglement — prior to running the teleportation protocol — beyond the used qubits, i.e., exploiting the hybrid entanglement between different physical degrees of freedom,” says Professor Jyrki Piilo from the University of Turku.
Conventionally, the polarisation of photons has been used for the entanglement of qubits in teleportation, while the current approach exploits the hybrid entanglement between the photons’ polarisation and frequency.
“This allows for a significant change in how the noise influences the protocol, and as a matter of fact our discovery reverses the role of the noise from being harmful to being beneficial to teleportation,” Piilo describes.
With conventional qubit entanglement in the presence of noise, the teleportation protocol does not work. In a case where there is initially hybrid entanglement and no noise, the teleportation does not work either.
“However, when we have hybrid entanglement and add noise, the teleportation and quantum state transfer occur in almost perfect manner,” says Dr Olli Siltanen whose doctoral dissertation presented the theoretical part of the current research.
In general, the discovery enables almost ideal teleportation despite the presence of certain type of noise when using photons for teleportation.
“While we have done numerous experiments on different facets of quantum physics with photons in our laboratory, it was very thrilling and rewarding to see this very challenging teleportation experiment successfully completed,” says Dr Zhao-Di Liu from the University of Science and Technology of China, Hefei.
“This is a significant proof-of-principle experiment in the context of one of the most important quantum protocols,” says Professor Chuan-Feng Li from the University of Science and Technology of China, Hefei.
Teleportation has important applications, e.g., in transmitting quantum information, and it is of utmost importance to have approaches that protect this transmission from noise and can be used for other quantum applications. The results of the current study can be considered as basic research that carries significant fundamental importance and opens intriguing pathways for future work to extend the approach to general types of noise sources and other quantum protocols.
Deterministic storage and retrieval of telecom light from a quantum dot single-photon source interfaced with an atomic quantum memory
by Sarah E. Thomas, Lukas Wagner, Raphael Joos, Robert Sittig, Cornelius Nawrath, Paul Burdekin, Ilse Maillette de Buy Wenniger, Mikhael J. Rasiah, Tobias Huber-Loyola, Steven Sagona-Stophel, Sven Höfling, Michael Jetter, Peter Michler, Ian A. Walmsley, Simone L. Portalupi, Patrick M. Ledingham in Science Advances
The ability to share quantum information is crucial for developing quantum networks for distributed computing and secure communication. Quantum computing will be useful for solving some important types of problems, such as optimising financial risk, decrypting data, designing molecules, and studying the properties of materials.
However, this development is being held up because quantum information can be lost when transmitted over long distances. One way to overcome this barrier is to divide the network into smaller segments and link them all up with a shared quantum state.
To do this requires a means to store the quantum information and retrieve it again: that is, a quantum memory device. This must ‘talk’ to another device that allows the creation of quantum information in the first place. For the first time, researchers have created such a system that interfaces these two key components, and uses regular optical fibres to transmit the quantum data. The feat was achieved by researchers at Imperial College London, the University of Southampton, and the Universities of Stuttgart and Wurzburg in Germany.
Co-first author Dr Sarah Thomas, from the Department of Physics at Imperial College London, said: “Interfacing two key devices together is a crucial step forward in allowing quantum networking, and we are really excited to be the first team to have been able to demonstrate this.”
Co-first author Lukas Wagner, from the University of Stuttgart, added: “Allowing long-distance locations, and even to quantum computers, to connect is a critical task for future quantum networks.”
In regular telecommunications — like the internet or phone lines — information can be lost over large distances. To combat this, these systems use ‘repeaters’ at regular points, which read and re-amplify the signal, ensuring it gets to its destination intact. Classical repeaters, however, cannot be used with quantum information, as any attempt to read and copy the information would destroy it. This is an advantage in one way, as quantum connections cannot be ‘tapped’ without destroying the information and alerting the users. But it is a challenge to be tackled for long-distance quantum networking.
One way to overcome this problem is to share quantum information in the form of entangled particles of light, or photons. Entangled photons share properties in such a way that you cannot understand one without the other. To share entanglement over long distances across a quantum network you need two devices: one to create the entangled photons, and one to store them and allow them to be retrieved later.
There are several devices used to create quantum information in the form of entangled photons and to store it, but both generating these photons on demand and having a compatible quantum memory in which to store them eluded researchers for a long time.
Photons have certain wavelengths (which, in visible light, creates different colours), but devices for creating and storing them are often tuned to work with different wavelengths, preventing them from interfacing.
To make the devices interface, the team created a system where both devices used the same wavelength. A ‘quantum dot’ produced (non-entangled) photons, which were then passed to a quantum memory system that stored the photons within a cloud of rubidium atoms. A laser turned the memory ‘on’ and ‘off’, allowing the photons to be stored and released on demand.
Not only did the wavelength of these two devices match, but it is at the same wavelength as telecommunications networks used today — allowing it to be transmitted with regular fibre-optic cables familiar in everyday internet connections.
The quantum dot light source was created by researchers at the University of Stuttgart with support from the University of Wurzburg, and then brought to the UK to interface with the quantum memory device created by the Imperial and Southampton team. The system was assembled in a basement lab at Imperial College London.
While independent quantum dots and quantum memories have been created that are more efficient than the new system, this is the first proof that devices can be made to interface, and at telecommunications wavelengths. The team will now look to improve the system, including making sure all the photons are produced at the same wavelength, improving how long the photons can be stored, and making the whole system smaller.
As a proof of concept however, this is an important step forward, says co-author f from the University of Southampton: “Members of the quantum community have been actively attempting this link for some time. This includes us, having tried this experiment twice before with different memory and quantum dot devices, going back more than five years, which just shows how hard it is to do.
“The breakthrough this time was convening experts to develop and run each part of the experiment with specialist equipment and working together to synchronise the devices.”
A balanced quantum Hall resistor
by Kajetan M. Fijalkowski, Nan Liu, Martin Klement, Steffen Schreyeck, Karl Brunner, Charles Gould, Laurens W. Molenkamp in Nature Electronics
Researchers at the University of Würzburg have developed a method that can improve the performance of quantum resistance standards. It´s based on a quantum phenomenon called Quantum Anomalous Hall effect.
The precise measurement of electrical resistance is essential in industrial production or electronics — for example, in the manufacture of high-tech sensors, microchips and flight controls.
“Very precise measurements are essential here, as even the smallest deviations can significantly affect these complex systems,” explains Professor Charles Gould, a physicist at the Institute for Topological Insulators at the University of Würzburg (JMU). “With our new measurement method, we can significantly improve the accuracy of resistance measurements, without any external magnetic field, using the Quantum Anomalous Hall Effect (QAHE).”
Many people may remember the classic Hall effect from their physics lessons: When a current flows through a conductor and it is exposed to a magnetic field, a voltage is created — the so-called Hall voltage. The Hall resistance, obtained by dividing this voltage by current, increases as the magnetic field strength increases. In thin layers and at large enough magnetic fields, this resistance begins to develop discreet steps with values of exactly h/ne2, where h is the Planck’s constant, e is the elementary charge, and n is an integer number. This is known as the Quantum Hall Effect because the resistance depends only on fundamental constants of nature (h and e), which makes it an ideal standard resistor.
The special feature of the QAHE is that it allows the quantum Hall effect to exist at zero magnetic field. “The operation in the absence of any external magnetic field not only simplifies the experiment, but also gives an advantage when it comes to determining another physical quantity: the kilogram. To define a kilogram, one has to measure the electrical resistance and the voltage at the same time,” says Gould “but measuring the voltage only works without a magnetic field, so the QAHE is ideal for this.”
Thus far, the QAHE was measured only at currents which are far too low for practical metrological use. The reason for this is an electric field that disrupts the QAHE at higher currents. The Würzburg physicists have now developed a solution to this problem.
“We neutralize the electric field using two separate currents in a geometry we call a multi-terminal Corbino device.,” explains Gould. “With this new trick, the resistance remains quantized to h/e2 up to larger currents, making the resistance standard based on QAHE more robust.”
In their feasibility study, the researchers were able to show that the new measurement method works at the precision level offered by basic d.c. techniques. Their next goal is to test the feasibility of this method using more precise metrological tools. To this end, the Würzburg group is working closely with the Physikalisch-Technische Bundesanstalt (German National Metrology Institute, PTB), who specialize in this kind of ultra-precise metrological measurements.
Gould also notes: “This method is not limited to the QAHE. Given that conventional Quantum Hall Effect experiences similar electric field driven limitations at sufficiently large currents, this method can also improve the existing state of the art metrological standards, for applications where even larger currents are useful.”
Manipulation of chiral interface states in a moiré quantum anomalous Hall insulator
by Canxun Zhang, Tiancong Zhu, Salman Kahn, Tomohiro Soejima, Kenji Watanabe, Takashi Taniguchi, Alex Zettl, Feng Wang, Michael P. Zaletel, Michael F. Crommie in Nature Physics
An international research team led by Lawrence Berkeley National Laboratory (Berkeley Lab) has taken the first atomic-resolution images and demonstrated electrical control of a chiral interface state — an exotic quantum phenomenon that could help researchers advance quantum computing and energy-efficient electronics.
The chiral interface state is a conducting channel that allows electrons to travel in only one direction, preventing them from being scattered backwards and causing energy-wasting electrical resistance. Researchers are working to better understand the properties of chiral interface states in real materials but visualizing their spatial characteristics has proved to be exceptionally difficult.
But now, for the first time, atomic-resolution images captured by a research team at Berkeley Lab and UC Berkeley have directly visualized a chiral interface state. The researchers also demonstrated on-demand creation of these resistance-free conducting channels in a 2D insulator. Their work is part of Berkeley Lab’s broader push to advance quantum computing and other quantum information system applications, including the design and synthesis of quantum materials to address pressing technological needs.
“Previous experiments have demonstrated that chiral interface states exist, but no one has ever visualized them with such high resolution. Our work shows for the first time what these 1D states look like at the atomic scale, including how we can alter them — and even create them,” said first author Canxun Zhang, a former graduate student researcher in Berkeley Lab’s Materials Sciences Division and the Department of Physics at UC Berkeley. He is now a postdoctoral researcher at UC Santa Barbara.
Chiral interface states can occur in certain types of 2D materials known as quantum anomalous Hall (QAH) insulators that are insulators in bulk but conduct electrons without resistance at one-dimensional “edges” — the physical boundaries of the material and interfaces with other materials.
To prepare chiral interface states, the team worked at Berkeley Lab’s Molecular Foundry to fabricate a device called twisted monolayer-bilayer graphene, which is a stack of two atomically thin layers of graphene rotated precisely relative to one another, creating a moiré superlattice that exhibits the QAH effect.
In subsequent experiments at the UC Berkeley Department of Physics, the researchers used a scanning tunneling microscope (STM) to detect different electronic states in the sample, allowing them to visualize the wavefunction of the chiral interface state. Other experiments showed that the chiral interface state can be moved across the sample by modulating the voltage on a gate electrode placed underneath the graphene layers. In a final demonstration of control, the researchers showed that a voltage pulse from the tip of an STM probe can “write” a chiral interface state into the sample, erase it, and even rewrite a new one where electrons flow in the opposite direction.
The findings may help researchers build tunable networks of electron channels with promise for energy-efficient microelectronics and low-power magnetic memory devices in the future, and for quantum computation making use of the exotic electron behaviors in QAH insulators.
The researchers intend to use their technique to study more exotic physics in related materials, such as anyons, a new type of quasiparticle that could enable a route to quantum computation.
“Our results provide information that wasn’t possible before. There is still a long way to go, but this is a good first step,” Zhang said.
Terahertz electric-field-driven dynamical multiferroicity in SrTiO3
by M. Basini, M. Pancaldi, B. Wehinger, M. Udina, V. Unikandanunni, T. Tadano, M. C. Hoffmann, A. V. Balatsky, S. Bonetti in Nature
The potential of quantum technology is huge but is today largely limited to the extremely cold environments of laboratories. Now, researchers at Stockholm University, at the Nordic Institute for Theoretical Physics and at the Ca’ Foscari University of Venice have succeeded in demonstrating for the very first time how laser light can induce quantum behavior at room temperature — and make non-magnetic materials magnetic. The breakthrough is expected to pave the way for faster and more energy-efficient computers, information transfer and data storage.
Within a few decades, the advancement of quantum technology is expected to revolutionize several of society’s most important areas and pave the way for completely new technological possibilities in communication and energy. Of particular interest for researchers in the field are the peculiar and bizarre properties of quantum particles — which deviate completely from the laws of classical physics and can make materials magnetic or superconducting. By increasing the understanding of exactly how and why this type of quantum states arise, the goal is to be able to control and manipulate materials to obtain quantum mechanical properties.
So far, researchers have only been able to induce quantum behaviors, such as magnetism and superconductivity, at extremely cold temperatures. Therefore, the potential of quantum research is still limited to laboratory environments.
Now, a research team from Stockholm University and the Nordic Institute of Theoretical Physics (NORDITA)* in Sweden, the University of Connecticut and the SLAC National Accelerator Laboratory in USA, the National Institute for Materials Science in Tsukuba, Japan, the Elettra-Sincrotrone Trieste, the ‘Sapienza’ University of Rome and the Ca’ Foscari University of Venice in Italy, is the first in the world to demonstrate in an experiment how laser light can induce magnetism in a non-magnetic material at room temperature. In the study, the researchers subjected the quantum material strontium titanate to short but intense laser beams of a peculiar wavelength and polarization, to induced magnetism.
“The innovation in this method lies in the concept of letting light move atoms and electrons in this material in circular motion, so to generate currents that make it as magnetic as a refrigerator magnet. We have been able to do so by developing a new light source in the far-infrared with a polarization which has a “corkscrew” shape. This is the first time we have been able to induce and clearly see how the material becomes magnetic at room temperature in an experiment. Furthermore, our approach allows to make magnetic materials out of many insulators, when magnets are typically made of metals. In the long run, this opens for completely new applications in society,” says the research leader Stefano Bonetti at Stockholm University and at the Ca’ Foscari University of Venice
The method is based on the theory of “dynamic multiferroicity,” which predicts that when titanium atoms are “stirred up” with circularly polarized light in an oxide based on titanium and strontium, a magnetic field will be formed. But it is only now that the theory can be confirmed in practice. The breakthrough is expected to have broad applications in several information technologies.
“This opens up for ultra-fast magnetic switches that can be used for faster information transfer and considerably better data storage, and for computers that are significantly faster and more energy-efficient,” says Alexander Balatsky, professor of physics at NORDITA.
Single-shot readout of a superconducting qubit using a thermal detector
by András M. Gunyhó, Suman Kundu, Jian Ma, Wei Liu, Sakari Niemelä, Giacomo Catto, Vasilii Vadimov, Visa Vesterinen, Priyank Singh, Qiming Chen, Mikko Möttönen in Nature Electronics
Chasing ever-higher qubit counts in near-term quantum computers constantly demands new feats of engineering.
Among the troublesome hurdles of this scaling-up race is refining how qubits are measured. Devices called parametric amplifiers are traditionally used to do these measurements. But as the name suggests, the device amplifies weak signals picked up from the qubits to conduct the readout, which causes unwanted noise and can lead to decoherence of the qubits if not protected by additional large components. More importantly, the bulky size of the amplification chain becomes technically challenging to work around as qubit counts increase in size-limited refrigerators.
Cue the Aalto University research group Quantum Computing and Devices (QCD). They have a hefty track record of showing how thermal bolometers can be used as ultrasensitive detectors, and they just demonstrated that bolometer measurements can be accurate enough for single-shot qubit readout.
To the chagrin of many physicists, the Heisenberg uncertainty principle determines that one cannot simultaneously know a signal’s position and momentum, or voltage and current, with accuracy. So it goes with qubit measurements conducted with parametric voltage-current amplifiers. But bolometric energy sensing is a fundamentally different kind of measurement — serving as a means of evading Heisenberg’s infamous rule. Since a bolometer measures power, or photon number, it is not bound to add quantum noise stemming from the Heisenberg uncertainty principle in the way that parametric amplifiers are.
Unlike amplifiers, bolometers very subtly sense microwave photons emitted from the qubit via a minimally invasive detection interface. This form factor is roughly 100 times smaller than its amplifier counterpart, making it extremely attractive as a measurement device.
‘When thinking of a quantum-supreme future, it is easy to imagine high qubit counts in the thousands or even millions could be commonplace. A careful evaluation of the footprint of each component is absolutely necessary for this massive scale-up. We have shown that our nanobolometers could seriously be considered as an alternative to conventional amplifiers. In our very first experiments, we found these bolometers accurate enough for single-shot readout, free of added quantum noise, and they consume 10,000 times less power than the typical amplifiers — all in a tiny bolometer, the temperature-sensitive part of which can fit inside of a single bacterium,’ says Aalto University Professor Mikko Möttönen, who heads the QCD research group.
Single-shot fidelity is an important metric physicists use to determine how accurately a device can detect a qubit’s state in just one measurement as opposed to an average of multiple measurements. In the case of the QCD group’s experiments, they were able to obtain a single-shot fidelity of 61.8% with a readout duration of roughly 14 microseconds. When correcting for the qubit’s energy relaxation time, the fidelity jumps up to 92.7%.
‘With minor modifications, we could expect to see bolometers approaching the desired 99.9% single-shot fidelity in 200 nanoseconds. For example, we can swap the bolometer material from metal to graphene, which has a lower heat capacity and can detect very small changes in its energy quickly. And by removing other unnecessary components between the bolometer and the chip itself, we can not only make even greater improvements on the readout fidelity, but we can achieve a smaller and simpler measurement device that makes scaling-up to higher qubit counts more feasible,’ says András Gunyhó, the first author on the paper and a doctoral researcher in the QCD group.
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