QT/ Riddle of Kondo effect solved in ultimately thin wires
Quantum news biweekly vol.63, 8th November — 24th November
TL;DR
- A research team has now directly measured the so-called Kondo effect, which governs the behavior of magnetic atoms surrounded by a sea of electrons: New observations with a scanning tunneling microscope reveal the effect in one-dimensional wires floating on graphene.
- Experts are preparing a laser experiment intended to verify the vacuum quantum fluctuations in a novel way, which could potentially provide clues to new laws in physics. A research team has developed a series of proposals designed to help conduct the experiment more effectively — thus increasing the chances of success.
- Quantum scientists have discovered a rare phenomenon that could hold the key to creating a ‘perfect switch’ in quantum devices which flips between being an insulator and superconductor.
- In 1973, physicist Phil Anderson hypothesized that the quantum spin liquid, or QSL, state existed on some triangular lattices, but he lacked the tools to delve deeper. Fifty years later, a team has confirmed the presence of QSL behavior in a new material with this structure, KYbSe2.
- Distributed cloud storage is a hot topic for security researchers, and a team is now merging quantum physics with mature cryptography and storage techniques to achieve a cost-effective cloud storage solution.
- Physicists have trapped electrons in a pure crystal, marking the first achievement of an electronic flat band in a three-dimensional material. The results provide a new way for scientists to explore rare electronic states in 3D materials.
- Single-photon emitters quantum mechanically connect quantum bits (or qubits) between nodes in quantum networks. They are typically made by embedding rare-earth elements in optical fibers at extremely low temperatures. Now, researchers have developed an ytterbium-doped optical fiber at room temperature. By avoiding the need for expensive cooling solutions, the proposed method offers a cost-effective platform for photonic quantum applications.
- Researchers turned a paramagnetic material into a magnet by manipulating electrons’ spin via atomic motion.
- Scientists have discovered how superfluid helium 3He would feel if you could put your hand into it. The interface between the exotic world of quantum physics and classical physics of the human experience is one of the major open problems in modern physics. Nobody has been able to answer this question during the 100-year history of quantum physics.
- Physicists have started the countdown on developing a new generation of timepieces capable of shattering records by providing accuracy of up to one second in 300 billion years, or about 22 times the age of the universe.
- And more!
Quantum Computing Market
According to the recent market research report ‘Quantum Computing Market with COVID-19 impact by Offering (Systems and Services), Deployment (On Premises and Cloud Based), Application, Technology, End-use Industry and Region — Global Forecast to 2026’, published by MarketsandMarkets, the Quantum Computing market is expected to grow from USD 472 million in 2021 to USD 1,765 million by 2026, at a CAGR of 30.2%. The early adoption of quantum computing in the banking and finance sector is expected to fuel the growth of the market globally. Other key factors contributing to the growth of the quantum computing market include rising investments by governments of different countries to carry out research and development activities related to quantum computing technology. Several companies are focusing on the adoption of QCaaS post-COVID-19. This, in turn, is expected to contribute to the growth of the quantum computing market. However, stability and error correction issues are expected to restrain the growth of the market.
According to ‘Quantum Computing Market Research Report: By Offering, Deployment Type, Application, Technology, Industry — Industry Share, Growth, Drivers, Trends and Demand Forecast to 2030’ report, the quantum computing market is projected to reach $64,988 million by 2030. Machine learning (ML) is expected to progress at the highest CAGR, during the forecast period, among all application categories, owing to the fact that quantum computing is being integrated in ML for improving the latter’s use case.
Latest Research
Modulated Kondo screening along magnetic mirror twin boundaries in monolayer MoS2
by Camiel van Efferen, Jeison Fischer, Theo A. Costi, Achim Rosch, Thomas Michely, Wouter Jolie in Nature Physics
A team of physicists at the University of Cologne has solved a long-standing problem of condensed matter physics: they have directly observed the Kondo effect (the re-grouping of electrons in a metal caused by magnetic impurities) visible in a single artificial atom. This has not been done successfully in the past, since the magnetic orbitals of atoms usually cannot be directly observed with most measurement techniques. However, the international research team led by Dr Wouter Jolie at the University of Cologne’s Institute for Experimental Physics used a new technique to observe the Kondo effect in an artificial orbital inside a one-dimensional wire floating above a metallic sheet of graphene.
When electrons moving through a metal encounter a magnetic atom, they are affected by the atom’s spin — the magnetic pole of elementary particles. In trying to screen the effect of the atomic spin, the electron sea groups together close to the atom, forming a new many-body state which is called the Kondo resonance. This collective behaviour is known as the Kondo effect and is often used to describe metals interacting with magnetic atoms. However, other types of interactions can lead to very similar experimental signatures, questioning the role of the Kondo effect for single magnetic atoms on surfaces.
The physicists used a new experimental approach to show that their one-dimensional wires are also subject to the Kondo effect: the electrons trapped in the wires form standing waves, which can be thought as extended atomic orbitals. This artificial orbital, its coupling to the electron sea, as well as the resonant transitions between orbital and sea can be imaged with the scanning tunnelling microscope. This experimental technique uses a sharp metallic needle to measure electrons with atomic resolution. This has allowed the team to measure the Kondo effect with unparalleled precision.
“With magnetic atoms on surfaces, it is like with the story about the person who has never seen an elephant and tries to imagine its shape by touching it once in a dark room. If you only feel the trunk, you imagine a completely different animal than if you are touching the side,” said Camiel van Efferen, the doctoral student who conducted the experiments. “For a long time, only the Kondo resonance was measured. But there could be other explanations for the signals observed in these measurements, just like the elephant’s trunk could also be a snake.”
The research group at the Institute of Experimental Physics specializes in the growth and exploration of 2D materials — crystalline solids consisting of just a few layers of atoms — such as graphene and monolayer molybdenum disulfide (MoS2). They found that at the interface of two MoS2 crystals, one of which is the mirror image of the other, a metallic wire of atoms forms. With their scanning tunnelling microscope, they could simultaneously measure the magnetic states and the Kondo resonance, at an astonishingly low temperature of -272.75 degrees C (0.4 Kelvin), at which the Kondo effect emerges.
“While our measurement left no doubts that we observed the Kondo effect, we did not yet know how well our unconventional approach could be compared to theoretical predictions,” Jolie added. For that, the team enlisted the help of two theoretical physicists, Professor Dr Achim Rosch from the University of Cologne and Dr Theo Costi from Forschungszentrum Jülich, both experts in the field of Kondo physics. After crunching the experimental data in the supercomputer in Jülich, it turned out that the Kondo resonance could be exactly predicted from the shape of the artificial orbitals in the magnetic wires, validating a decades-old prediction from one of the founding fathers of condensed matter physics, Philip W. Anderson.
The scientists are now planning to use their magnetic wires to investigate even more exotic phenomena.
“Placing our 1D wires on a superconductor or on a quantum spin-liquid, we could create many-body states emerging from other quasiparticles than electrons,” explained Camiel van Efferen. “The fascinating states of matter that arise from these interactions can now be seen clearly, which will allow us to understand them on a completely new level.”
Detection schemes for quantum vacuum diffraction and birefringence
by N. Ahmadiniaz, T. E. Cowan, J. Grenzer, S. Franchino-Viñas, A. Laso Garcia, M. Šmíd, T. Toncian, M. A. Trejo, R. Schützhold in Physical Review D
Absolutely empty — that is how most of us envision the vacuum. Yet, in reality, it is filled with an energetic flickering: the quantum fluctuations. Experts are currently preparing a laser experiment intended to verify these vacuum fluctuations in a novel way, which could potentially provide clues to new laws in physics. A research team from the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) has developed a series of proposals designed to help conduct the experiment more effectively — thus increasing the chances of success.
The physics world has long been aware that the vacuum is not entirely void but is filled with vacuum fluctuations — an ominous quantum flickering in time and space. Although it cannot be captured directly, its influence can be indirectly observed, for example, through changes in the electromagnetic fields of tiny particles. However, it has not yet been possible to verify vacuum fluctuations without the presence of any particles. If this could be accomplished, one of the fundamental theories of physics, namely quantum electrodynamics (QED), would be proven in a hitherto untested area. Should such an experiment reveal deviations from the theory, however, it would suggest the existence of new, previously undiscovered particles.
The experiment intended to accomplish this is planned as part of the Helmholtz International Beamline for Extreme Fields (HIBEF), a research consortium led by the HZDR at the HED experimental station of the European XFEL in Hamburg, the largest X-ray laser in the world. The underlying principle is that an ultra-powerful laser fires short, intense flashes of light into an evacuated stainless steel chamber. The aim is to manipulate the vacuum fluctuations so that they, seemingly magically, change the polarization of an X-ray flash from the European XFEL, i.e., rotate its direction of oscillation.
“It would be like sliding a transparent plastic ruler between two polarizing filters and bending it back and forth,” explains HZDR theorist Prof. Ralf Schützhold. “The filters are originally set up so that no light passes through them. Bending the ruler would now change the direction of the light’s oscillation in such a way that something could be seen as a result.” In this analogy, the ruler corresponds to the vacuum fluctuations while the ultra-powerful laser flash bends them.
The original concept involved shooting just one optical laser flash into the chamber and using specialized measurement techniques to register whether it changes the X-ray flash’s polarization. But there is a problem: “The signal is likely to be extremely weak,” explains Schützhold. “It is possible that only one in a trillion X-ray photons will change its polarization.”
But this might be below the current measurement limit — the event could simply fall through the cracks undetected. Therefore, Schützhold and his team are relying on a variant: instead of just one, they intend to shoot two optical laser pulses simultaneously into the evacuated chamber. Both flashes will strike there and literally collide. The X-ray pulse of the European XFEL is set to fire precisely into their collision point. The decisive factor: The colliding laser flashes affect the X-ray pulse like a type of crystal. Just as X-rays are diffracted, i.e., deflected, when passing through a natural crystal, the XFEL X-ray pulse should also be deflected by the briefly existing “light crystal” of the two colliding laser flashes.
“That would not only change the polarization of the X-ray pulse but also slightly deflect it at the same time,” explains Ralf Schützhold. This combination could increase the chances of actually being able to measure the effect — so the researchers hope. The team has calculated various options for the striking angle of the two laser flashes colliding in the chamber. Experiments will show which variant proves to be most suitable.
The prospects could even be improved further if the two laser flashes shot into the chamber were not of the same color but of two different wavelengths. This would also allow the energy of the X-ray flash to change slightly, which would, likewise, help to measure the effect. “But this is technically quite challenging and may only be implemented at a later date,” says Schützhold.
The project is currently in the planning stages in Hamburg together with the European XFEL team at the HED experimental station, and the first trials are scheduled to launch in 2024. If successful, they could confirm QED once more.
But perhaps the experiments will reveal deviations from the established theory. This could be due to previously undiscovered particles — for example, ultra-light ghost particles known as axions. “And that,” says Schützhold, “would be a clear indication of additional, previously unknown laws of nature.”
Emergent symmetry in a low-dimensional superconductor on the edge of Mottness
by P. Chudzinski, M. Berben, Xiaofeng Xu, N. Wakeham, B. Bernáth, C. Duffy, R. D. H. Hinlopen, Yu-Te Hsu, S. Wiedmann, P. Tinnemans, Rongying Jin, M. Greenblatt, N. E. Hussey in Science
Quantum scientists have discovered a rare phenomenon that could hold the key to creating a ‘perfect switch’ in quantum devices which flips between being an insulator and superconductor.
The research, led by the University of Bristol, found these two opposing electronic states exist within purple bronze, a unique one-dimensional metal composed of individual conducting chains of atoms.
Tiny changes in the material, for instance prompted by a small stimulus like heat or light, may trigger an instant transition from an insulating state with zero conductivity to a superconductor with unlimited conductivity, and vice versa. This polarised versatility, known as ‘emergent symmetry’, has the potential to offer an ideal On/Off switch in future quantum technology developments.
Lead author Nigel Hussey, Professor of Physics at the University of Bristol, said: “It’s a really exciting discovery which could provide a perfect switch for quantum devices of tomorrow. “The remarkable journey started 13 years ago in my lab when two PhD students, Xiaofeng Xu and Nick Wakeham, measured the magnetoresistance — the change in resistance caused by a magnetic field — of purple bronze.”
In the absence of a magnetic field, the resistance of purple bronze was highly dependent on the direction in which the electrical current is introduced. Its temperature dependence was also rather complicated. Around room temperature, the resistance is metallic, but as the temperature is lowered, this reverses and the material appears to be turning into an insulator. Then, at the lowest temperatures, the resistance plummets again as it transitions into a superconductor. Despite this complexity, surprisingly, the magnetoresistance was found to be extremely simple. It was essentially the same irrespective of the direction in which the current or field were aligned and followed a perfect linear temperature dependence all the way from room temperature down to the superconducting transition temperature.
“Finding no coherent explanation for this puzzling behaviour, the data lay dormant and published unpublished for the next seven years. A hiatus like this is unusual in quantum research, though the reason for it was not a lack of statistics,” Prof Hussey explained.
“Such simplicity in the magnetic response invariably belies a complex origin and as it turns out, its possible resolution would only come about through a chance encounter.”
In 2017, Prof Hussey was working at Radboud University and saw advertised a seminar by physicist Dr Piotr Chudzinski on the subject of purple bronze. At the time few researchers were devoting an entire seminar to this little-known material, so his interest was piqued.
Prof Hussey said: “In the seminar Chudzinski proposed that the resistive upturn may be caused by interference between the conduction electrons and elusive, composite particles known as ‘dark excitons’. We chatted after the seminar and together proposed an experiment to test his theory. Our subsequent measurements essentially confirmed it.”
Buoyed by this success, Prof Hussey resurrected Xu and Wakeham’s magnetoresistance data and showed them to Dr Chudzinski. The two central features of the data — the linearity with temperature and the independence on the orientation of current and field — intrigued Chudzinski, as did the fact that the material itself could exhibit both insulating and superconducting behaviour depending on how the material was grown.
Dr Chudzinski wondered whether rather than transforming completely into an insulator, the interaction between the charge carriers and the excitons he’d introduced earlier could cause the former to gravitate towards the boundary between the insulating and superconducting states as the temperature is lowered. At the boundary itself, the probability of the system being an insulator or a superconductor is essentially the same.
Prof Hussey said: “Such physical symmetry is an unusual state of affairs and to develop such symmetry in a metal as the temperature is lowered, hence the term ‘emergent symmetry’, would constitute a world-first.”
Physicists are well versed in the phenomenon of symmetry breaking: lowering the symmetry of an electron system upon cooling. The complex arrangement of water molecules in an ice crystal is an example of such broken symmetry. But the converse is an extremely rare, if not unique, occurrence. Returning to the water/ice analogy, it is as though upon cooling the ice further, the complexity of the ice crystals ‘melts’ once again into something as symmetric and smooth as the water droplet.
Dr Chudzinski, now a Research Fellow at Queen’s University Belfast, said: “Imagine a magic trick where a dull, distorted figure transforms into a beautiful, perfectly symmetric sphere. This is, in a nutshell, the essence of emergent symmetry. The figure in question is our material, purple bronze, while our magician is nature itself.”
To further test whether the theory held water, an additional 100 individual crystals, some insulating and others superconducting, were investigated by another PhD student, Maarten Berben, working at Radboud University.
Prof Hussey added: “After Maarten’s Herculean effort, the story was complete and the reason why different crystals exhibited such wildly different ground states became apparent. Looking ahead, it might be possible to exploit this ‘edginess’ to create switches in quantum circuits whereby tiny stimuli induce profound, orders-of-magnitude changes in the switch resistance.”
Proximate spin liquid and fractionalization in the triangular antiferromagnet KYbSe2
by A. O. Scheie, E. A. Ghioldi, J. Xing, J. A. M. Paddison, et al in Nature Physics
In 1973, physicist Phil Anderson hypothesized that the quantum spin liquid, or QSL, state existed on some triangular lattices, but he lacked the tools to delve deeper. Fifty years later, a team led by researchers associated with the Quantum Science Center headquartered at the Department of Energy’s Oak Ridge National Laboratory has confirmed the presence of QSL behavior in a new material with this structure, KYbSe2.
QSLs — an unusual state of matter controlled by interactions among entangled, or intrinsically linked, magnetic atoms called spins — excel at stabilizing quantum mechanical activity in KYbSe2 and other delafossites. These materials are prized for their layered triangular lattices and promising properties that could contribute to the construction of high-quality superconductors and quantum computing components.
“Researchers have studied the triangular lattice of various materials in search of QSL behavior,” said QSC member and lead author Allen Scheie, a staff scientist at Los Alamos. “One advantage of this one is that we can swap out atoms easily to modify the material’s properties without altering its structure, and this makes it pretty ideal from a scientific perspective.”
Using a combination of theoretical, experimental and computational techniques, the team observed multiple hallmarks of QSLs: quantum entanglement, exotic quasiparticles and the right balance of exchange interactions, which control how a spin influences its neighbors. Although efforts to identify these features have historically been hindered by the limitations of physical experiments, modern neutron scattering instruments can produce accurate measurements of complex materials at the atomic level.
By examining KYbSe2’s spin dynamics with the Cold Neutron Chopper Spectrometer at ORNL’s Spallation Neutron Source — a DOE Office of Science user facility — and comparing the results to trusted theoretical models, the researchers found evidence that the material was close to the quantum critical point at which QSL characteristics thrive. They then analyzed its single-ion magnetic state with SNS’s Wide-Angular-Range Chopper Spectrometer.
The witnesses in question are the one-tangle, two-tangle and quantum Fisher information, which has played a key role in previous QSC research focused on examining a 1D spin chain, or a single line of spins within a material. KYbSe2 is a 2D system, a quality that made these endeavors more complex.
“We are taking a co-design approach, which is hardwired into the QSC,” said Alan Tennant, a professor of physics and materials science and engineering at UTK who leads a quantum magnets project for the QSC. “Theorists within the center are calculating things they haven’t been able to calculate before, and this overlap between theory and experiment enabled this breakthrough in QSL research.”
This study aligns with the QSC’s priorities, which include connecting fundamental research to quantum electronics, quantum magnets and other current and future quantum devices.
“Gaining a better understanding of QSLs is really significant for the development of next-generation technologies,” Tennant said. “This field is still in the fundamental research state, but we can now identify which materials we can modify to potentially make small-scale devices from scratch.”
Although KYbSe2 is not a true QSL, the fact that about 85% of the magnetism fluctuates at low temperature means that it has the potential to become one. The researchers anticipate that slight alternations to its structure or exposure to external pressure could potentially help it reach 100%.
QSC experimentalists and computational scientists are planning parallel studies and simulations focused on delafossite materials, but the researchers’ findings established an unprecedented protocol that can also be applied to study other systems. By streamlining evidence-based evaluations of QSL candidates, they aim to accelerate the search for genuine QSLs.
“The important thing about this material is that we’ve found a way to orient ourselves on the map so to speak and show what we’ve gotten right,” Scheie said. “We’re pretty sure there’s a full QSL somewhere within this chemical space, and now we know how to find it.”
Quantum-secure fault-tolerant distributed cloud storage system
by Chun-Li Ma, Dong-Dong Li, Yalin Li, Yinghao Wu, Song-Yan Ding, Jun Wang, Pei-Yuan Li, Song Zhang, Junjie Chen, Xiaoxing Zhang, Jia-Yong Wang, Jin Li, Qiang Li, Zhi-Tong Chen, Lei Zhou, Mei-Sheng Zhao, Yong Zhao in AIP Advances
Distributed cloud storage is a hot topic for security researchers around the globe pursuing secure data storage, and a team in China is now merging quantum physics with mature cryptography and storage techniques to achieve a cost-effective cloud storage solution.
Shamir’s secret sharing, a known method, is a key distribution algorithm. It involves distributing private information to a group so that “the secret” can be revealed only when a majority pools their knowledge. It’s common to combine quantum key distribution (QKD) and Shamir’s secret sharing algorithm for secure storage — at an utmost security level. But utmost security solutions tend to bring substantial cost baggage, including significant cloud storage space requirements.
The team presents its method that uses quantum random numbers as encryption keys, disperses the keys via Sharmir’s secret sharing algorithm, applies erasure coding within ciphertext, and securely transmits the data through QKD-protected networks to distributed clouds. Their method not only provides quantum security to the entire system but also offers fault tolerance and efficient storage — and this may help speed the adoption of quantum technologies.
“In essence, our solution is quantum-secure and serves as a practical application of the fusion between quantum and cryptography technologies,” said corresponding author Yong Zhao, vice president of QuantumCTek Co. Ltd., a quantum information technology company. “QKD-generated keys secure both user data uploads to servers and data transmissions to dispersed cloud storage nodes.”
The team explored whether quantum security services could expand beyond secure data transmission to offer a richer spectrum of quantum security applications such as data storage and processing. They came up with a more secure and cost-effective fault-tolerant cloud storage solution. “It not only achieves quantum security but also saves storage space when compared to traditional mirroring methods or ones based on Shamir’s secret sharing, which is commonly used for distributed management of sensitive data,” said Zhao.
When the team ran the solution through experimental tests ranging from encryption/decryption, key preservation, and data storage, it proved to be effective. The solution is currently feasible from both technological and engineering perspectives: It meets the requirement for relevant quantum and cryptographic standards to ensure a secure storage solution capable of withstanding the challenges posed by quantum computing.
“In the future, we plan to drive the commercial implementation of this technology to offer practical services,” said Zhao. “We’ll explore various usage models in multiuser scenarios, and we’re also considering integrating more quantum technologies, such as quantum secret sharing, into cloud storage.”
Three-dimensional flat bands in pyrochlore metal CaNi2
by Joshua P. Wakefield, Mingu Kang, Paul M. Neves, et al in Nature
Electrons move through a conducting material like commuters at the height of Manhattan rush hour. The charged particles may jostle and bump against each other, but for the most part they’re unconcerned with other electrons as they hurtle forward, each with their own energy.
But when a material’s electrons are trapped together, they can settle into the exact same energy state and start to behave as one. This collective, zombie-like state is what’s known in physics as an electronic “flat band,” and scientists predict that when electrons are in this state they can start to feel the quantum effects of other electrons and act in coordinated, quantum ways. Then, exotic behavior such as superconductivity and unique forms of magnetism may emerge. Now, physicists at MIT have successfully trapped electrons in a pure crystal. It is the first time that scientists have achieved an electronic flat band in a three-dimensional material. With some chemical manipulation, the researchers also showed they could transform the crystal into a superconductor — a material that conducts electricity with zero resistance.
The electrons’ trapped state is possible thanks to the crystal’s atomic geometry. The crystal, which the physicists synthesized, has an arrangement of atoms that resembles the woven patterns in “kagome,” the Japanese art of basket-weaving. In this specific geometry, the researchers found that rather than jumping between atoms, electrons were “caged,” and settled into the same band of energy. The researchers say that this flat-band state can be realized with virtually any combination of atoms — as long as they are arranged in this kagome-inspired 3D geometry. The results provide a new way for scientists to explore rare electronic states in three-dimensional materials. These materials might someday be optimized to enable ultraefficient power lines, supercomputing quantum bits, and faster, smarter electronic devices.
“Now that we know we can make a flat band from this geometry, we have a big motivation to study other structures that might have other new physics that could be a platform for new technologies,” says study author Joseph Checkelsky, associate professor of physics.
Checkelsky’s MIT co-authors include graduate students Joshua Wakefield, Mingu Kang, and Paul Neves, and postdoc Dongjin Oh, who are co-lead authors; graduate students Tej Lamichhane and Alan Chen; postdocs Shiang Fang and Frank Zhao; undergraduate Ryan Tigue; associate professor of nuclear science and engineering Mingda Li; and associate professor of physics Riccardo Comin, who collaborated with Checkelsky to direct the study; along with collaborators at multiple other laboratories and institutions.
In recent years, physicists have successfully trapped electrons and confirmed their electronic flat-band state in two-dimensional materials. But scientists have found that electrons that are trapped in two dimensions can easily escape out the third, making flat-band states difficult to maintain in 2D.
In their new study, Checkelsky, Comin, and their colleagues looked to realize flat bands in 3D materials, such that electrons would be trapped in all three dimensions and any exotic electronic states could be more stably maintained. They had an idea that kagome patterns might play a role. In previous work, the team observed trapped electrons in a two-dimensional lattice of atoms that resembled some kagome designs. When the atoms were arranged in a pattern of interconnected, corner-sharing triangles, electrons were confined within the hexagonal space between triangles, rather than hopping across the lattice. But, like others, the researchers found that the electrons could escape up and out of the lattice, through the third dimension.
The team wondered: Could a 3D configuration of similar lattices work to box in the electrons? They looked for an answer in databases of material structures and came across a certain geometric configuration of atoms, classified generally as a pyrochlore — a type of mineral with a highly symmetric atomic geometry. The pychlore’s 3D structure of atoms formed a repeating pattern of cubes, the face of each cube resembling a kagome-like lattice. They found that, in theory, this geometry could effectively trap electrons within each cube. To test this hypothesis, the researchers synthesized a pyrochlore crystal in the lab.
“It’s not dissimilar to how nature makes crystals,” Checkelsky explains. “We put certain elements together — in this case, calcium and nickel — melt them at very high temperatures, cool them down, and the atoms on their own will arrange into this crystalline, kagome-like configuration.”
They then looked to measure the energy of individual electrons in the crystal, to see if they indeed fell into the same flat band of energy. To do so, researchers typically carry out photoemission experiments, in which they shine a single photon of light onto a sample, that in turn kicks out a single electron. A detector can then precisely measure the energy of that individual electron. Scientists have used photoemission to confirm flat-band states in various 2D materials. Because of their physically flat, two-dimensional nature, these materials are relatively straightforward to measure using standard laser light. But for 3D materials, the task is more challenging.
“For this experiment, you typically require a very flat surface,” Comin explains. “But if you look at the surface of these 3D materials, they are like the Rocky Mountains, with a very corrugated landscape. Experiments on these materials are very challenging, and that is part of the reason no one has demonstrated that they host trapped electrons.”
The team cleared this hurdle with angle-resolved photoemission spectroscopy (ARPES), an ultrafocused beam of light that is able to target specific locations across an uneven 3D surface and measure the individual electron energies at those locations.
“It’s like landing a helicopter on very small pads, all across this rocky landscape,” Comin says.
With ARPES, the team measured the energies of thousands of electrons across a synthesized crystal sample in about half an hour. They found that, overwhelmingly, the electrons in the crystal exhibited the exact same energy, confirming the 3D material’s flat-band state. To see whether they could manipulate the coordinated electrons into some exotic electronic state, the researchers synthesized the same crystal geometry, this time with atoms of rhodium and ruthenium instead of nickel. On paper, the researchers calculated that this chemical swap should shift the electrons’ flat band to zero energy — a state that automatically leads to superconductivity. And indeed, they found that when they synthesized a new crystal, with a slightly different combination of elements, in the same kagome-like 3D geometry, the crystal’s electrons exhibited a flat band, this time at superconducting states.
Room-temperature addressing of single rare-earth atoms in optical fiber
by Mikio Takezawa, Ryota Suzuki, Junichi Takahashi, Kaito Shimizu, Ayumu Naruki, Kazutaka Katsumata, Kae Nemoto, Mark Sadgrove, Kaoru Sanaka in Physical Review Applied
Quantum-based systems promise faster computing and stronger encryption for computation and communication systems. These systems can be built on fiber networks involving interconnected nodes which consist of qubits and single-photon generators that create entangled photon pairs.
In this regard, rare-earth (RE) atoms and ions in solid-state materials are highly promising as single-photon generators. These materials are compatible with fiber networks and emit photons across a broad range of wavelengths. Due to their wide spectral range, optical fibers doped with these RE elements could find use in various applications, such as free-space telecommunication, fiber-based telecommunications, quantum random number generation, and high-resolution image analysis. However, so far, single-photon light sources have been developed using RE-doped crystalline materials at cryogenic temperatures, which limits the practical applications of quantum networks based on them.
In a study, a team of researchers from Japan, led by Associate Professor Kaoru Sanaka from Tokyo University of Science (TUS) has successfully developed a single-photon light source consisting of doped ytterbium ions (Yb3+) in an amorphous silica optical fiber at room temperature. Associate Professor Mark Sadgrove and Mr. Kaito Shimizu from TUS and Professor Kae Nemoto from the Okinawa Institute of Science and Technology Graduate University were also a part of this study. This newly developed single-photon light source eliminates the need for expensive cooling systems and has the potential to make quantum networks more cost-effective and accessible.
“Single-photon light sources are devices that control the statistical properties of photons, which represent the smallest energy units of light,” explains Dr. Sanaka. “In this study, we have developed a single-photon light source using an optical fiber material doped with optically active RE elements. Our experiments also reveal that such a source can be generated directly from an optical fiber at room temperature.”
Ytterbium is an RE element with favorable optical and electronic properties, making it a suitable candidate for doping the fiber. It has a simple energy-level structure, and ytterbium ion in its excited state has a long fluorescence lifetime of around one millisecond. To fabricate the ytterbium-doped optical fiber, the researchers tapered a commercially available ytterbium-doped fiber using a heat-and-pull technique, where a section of the fiber is heated and then pulled with tension to gradually reduce its diameter.
Within the tapered fiber, individual RE atoms emit photons when excited with a laser. The separation between these RE atoms plays a crucial role in defining the fiber’s optical properties. For instance, if the average separation between the individual RE atoms exceeds the optical diffraction limit, which is determined by the wavelength of the emitted photons, the emitted light from these atoms appears as though it is coming from clusters rather than distinct individual sources.
To confirm the nature of these emitted photons, the researchers employed an analytical method known as auto-correlation, which assesses the similarity between a signal and its delayed version. By analyzing the emitted photon pattern using autocorrelation, the researchers observed non-resonant emissions and further obtained evidence of photon emission from the single ytterbium ion in the doped filter.
While quality and quantity of emitted photons can be enhanced further, the developed optical fiber with ytterbium atoms can be manufactured without the need for expensive cooling systems. This overcomes a significant hurdle and opens doors to various next-generation quantum information technologies.
“We have demonstrated a low-cost single-photon light source with selectable wavelength and without the need for a cooling system. Going ahead, it can enable various next-generation quantum information technologies such as true random number generators, quantum communication, quantum logic operations, and high-resolution image analysis beyond the diffraction limit,” concludes Dr. Sanaka.
Large effective magnetic fields from chiral phonons in rare-earth halides
by Jiaming Luo, Tong Lin, Junjie Zhang, Xiaotong Chen, Elizabeth R. Blackert, Rui Xu, Boris I. Yakobson, Hanyu Zhu in Science
Quantum materials hold the key to a future of lightning-speed, energy-efficient information systems. The problem with tapping their transformative potential is that, in solids, the vast number of atoms often drowns out the exotic quantum properties electrons carry.
Rice University researchers in the lab of quantum materials scientist Hanyu Zhu found that when they move in circles, atoms can also work wonders: When the atomic lattice in a rare-earth crystal becomes animated with a corkscrew-shaped vibration known as a chiral phonon, the crystal is transformed into a magnet.
According to a study, exposing cerium fluoride to ultrafast pulses of light sends its atoms into a dance that momentarily enlists the spins of electrons, causing them to align with the atomic rotation. This alignment would otherwise require a powerful magnetic field to activate, since cerium fluoride is naturally paramagnetic with randomly oriented spins even at zero temperature.
“Each electron possesses a magnetic spin that acts like a tiny compass needle embedded in the material, reacting to the local magnetic field,” said Rice materials scientist and co-author Boris Yakobson. “Chirality — also called handedness because of the way in which left and right hands mirror each other without being superimposable — should not affect the energies of the electrons’ spin. But in this instance, the chiral movement of the atomic lattice polarizes the spins inside the material as if a large magnetic field were applied.”
Though short-lived, the force that aligns the spins outlasts the duration of the light pulse by a significant margin. Since atoms only rotate in particular frequencies and move for a longer time at lower temperatures, additional frequency- and temperature-dependent measurements further confirm that magnetization occurs as a result of the atoms’ collective chiral dance.
“The effect of atomic motion on electrons is surprising because electrons are so much lighter and faster than atoms,” said Zhu, Rice’s William Marsh Rice Chair and an assistant professor of materials science and nanoengineering. “Electrons can usually adapt to a new atomic position immediately, forgetting their prior trajectory. Material properties would remain unchanged if atoms went clockwise or counterclockwise, i.e., traveled forward or backward in time — a phenomenon that physicists refer to as time-reversal symmetry.”
The idea that the collective motion of atoms breaks time-reversal symmetry is relatively recent. Chiral phonons have now been experimentally demonstrated in a few different materials, but exactly how they impact material properties is not well understood.
“We wanted to quantitatively measure the effect of chiral phonons on a material’s electrical, optical and magnetic properties,” Zhu said. “Because spin refers to electrons’ rotation while phonons describe atomic rotation, there is a naive expectation that the two might talk with each other. So we decided to focus on a fascinating phenomenon called spin-phonon coupling.”
Spin-phonon coupling plays an important part in real-world applications like writing data on a hard disk. Earlier this year, Zhu’s group demonstrated a new instance of spin-phonon coupling in single molecular layers with atoms moving linearly and shaking spins.
In their new experiments, Zhu and the team members had to find a way to drive a lattice of atoms to move in a chiral fashion. This required both that they pick the right material and that they create light at the right frequency to send its atomic lattice aswirl with the help of theoretical computation from the collaborators.
“There is no off-the-shelf light source for our phonon frequencies at about 10 terahertz,” explained Jiaming Luo, an applied physics graduate student and the lead author of the study. “We created our light pulses by mixing intense infrared lights and twisting the electric field to ‘talk’ to the chiral phonons. Furthermore, we took another two infrared light pulses to monitor the spin and atomic motion, respectively.”
In addition to the insights into spin-phonon coupling derived from the research findings, the experimental design and setup will help inform future research on magnetic and quantum materials.
“We hope that quantitatively measuring the magnetic field from chiral phonons can help us develop experiment protocols to study novel physics in dynamic materials,” Zhu said. “Our goal is to engineer materials that do not exist in nature through external fields ? such as light or quantum fluctuations.”
Transport of bound quasiparticle states in a two-dimensional boundary superfluid
by Samuli Autti, Richard P. Haley, Asher Jennings, George R. Pickett, Malcolm Poole, Roch Schanen, Arkady A. Soldatov, Viktor Tsepelin, Jakub Vonka, Vladislav V. Zavjalov, Dmitry E. Zmeev in Nature Communications
Researchers from Lancaster University in the UK have discovered how superfluid helium 3He would feel if you could put your hand into it.
The interface between the exotic world of quantum physics and classical physics of the human experience is one of the major open problems in modern physics. Dr Samuli Autti is the lead author of the research.
Dr Autti said: “In practical terms, we don’t know the answer to the question ‘how does it feel to touch quantum physics?’ “These experimental conditions are extreme and the techniques complicated, but I can now tell you how it would feel if you could put your hand into this quantum system.
“Nobody has been able to answer this question during the 100-year history of quantum physics. We now show that, at least in superfluid 3He, this question can be answered.”
The experiments were carried out at about a 10000th of a degree above absolute zero in a special refrigerator and made use of mechanical resonator the size of a finger to probe the very cold superfluid. When stirred with a rod, superfluid 3He carries the generated heat away along the surfaces of the container. The bulk of the superfluid behaves like a vacuum and remains entirely passive.
Dr Autti said: “This liquid would feel two-dimensional if you could stick your finger into it. The bulk of the superfluid feels empty, while heat flows in a two-dimensional subsystem along the edges of the bulk — in other words, along your finger.”
The researchers conclude that the bulk of superfluid 3He is wrapped by an independent two-dimensional superfluid that interacts with mechanical probes instead of the bulk superfluid, only providing access to the bulk superfluid if given a sudden burst of energy. That is, superfluid 3He at the lowest temperatures and applied energies is thermo-mechanically two dimensional.
“This also redefines our understanding of superfluid 3He. For the scientist, that may be even more influential than hands-in quantum physics.”
Superfluid 3He is one of the most versatile macroscopic quantum systems in the laboratory. It often influences seemingly distant fields such as particle physics (for example the Higgs mechanism), cosmology (Kibble mechanism), and quantum information processing (time crystals). A redefinition of its basic structure may therefore have far-reaching consequences.
Resonant X-ray excitation of the nuclear clock isomer 45Sc
by Yuri Shvyd’ko, Ralf Röhlsberger, Olga Kocharovskaya, Jörg Evers, et al in Nature
An international research team involving Dr. Olga Kocharovskaya, a distinguished professor in the Department of Physics and Astronomy at Texas A&M University, has taken a major step toward development of a new generation of atomic clocks with mind-blowing potential affecting fundamental science and various industries, from nuclear physics to satellite navigation and telecommunications.
The team’s work, led by Argonne National Laboratory senior physicist Dr. Yuri Shvyd’ko, for the first time resonantly excited the scandium-45 nuclear isomer with the world’s brightest X-ray pulses at the European XFEl (EuXFEL) X-ray laser facility and determined the position of this nuclear resonance with unprecedented accuracy.
“Atomic clocks, such as the caesium-133 clock or the strontium-87 clock, rely on oscillations of electrons in an atom, which can oscillate at highly reliable frequencies when excited by microwave or optical radiation,” explained Kocharovskaya, principal investigator of the National Science Foundation (NSF) project that initiated and supported this research.
Scandium, an element used in aerospace components and sports equipment, enables an accuracy of one second in 300 billion years, or roughly a thousand times more precision than the current standard atomic clock. The combination of scandium-45 and ultra-bright X-ray pulses brings scientists a decisive step closer to creation of the first-ever nuclear clock that could harness the oscillation of the atomic nucleus rather than its electron shell.
“For purposes that demand such precision, including the study of certain aspects of relativity, gravitational theory and other physical phenomena such as dark matter, the nuclear clock is the ultimate timepiece,” said Dr. Xiwen Zhang, a postdoctoral researcher in Kocharovskaya’s group who co-authored the paper.
With their accuracy of up to one part in 10,000,000,000,000,000,000, Texas A&M physicist Dr. Grigory V. Rogachev notes that nuclear clocks could usher in a new era of precision timekeeping and enable transformative applications in myriad areas, resulting in a host of applications and advances.
“Humanity has been on the lookout for the technology to make the most precise clocks since the dawn of the modern ages,” said Rogachev, head of Texas A&M Physics and Astronomy and a member of the Texas A&M Cyclotron Institute. “At present, atomic clocks are the best. Dr. Kocharovskaya and her collaborators are now making the first step toward a new, breakthrough technology. Her research opens a new pathway to utilize the unique properties of the scandium-45 isotope to create the most precise clock ever — the nuclear clock. This advancement may have exciting applications in extreme metrology, ultra-high spectroscopy and potentially numerous other fields.”
Kocharavskaya’s research interests during the past decade have been focused on extending the field of traditional quantum optics — which she describes as dealing with controllable resonant interactions between optical photons and atomic transitions — into the emerging field of nuclear/x-ray quantum optics focused on control of resonant interaction between x-ray photons and nuclear transitions. In the process, she identified scandium-45 with its long-lived first-excited energy state as the superior candidate both for quantum nuclear storage and the nuclear clock. The main question, she says, was whether it was feasible to reach this state with available x-ray sources.
Together with Shvyd’ko, who had envisioned the high potential of scandium-45 for super-resolution-coherent-forward nuclear spectroscopy along with a possibility of its resonant excitation by X-rays from an emerging new generation of accelerator-based facilities 30 years ago, Kocharovskaya wrote a proposal to the NSF aimed at resonant excitation of a scandium-45 nuclear isomer using X-ray pulses.
“Initially it received mixed reviews, as it was considered a high-risk/high payoff project, but eventually, it was funded, allowing us to plan the experiment at EuXFEL,” said Kocharovskaya, a member of the Texas A&M Institute for Quantum Science and Engineering.
Kocharovskaya credits Shvyd’ko as not only the leader of the group’s research but also an inspiration for the entire team. From coordinating the efforts of all the groups entering every detail of the project to running weekly Zoom meetings discussing the multiple challenges and progress in preparation for the experiment, she says his leadership and hard work provided a tangible example of precisely what it means to see a long-term scientific dream become a reality. In addition, she notes that the project would not be successful without the major contributions of their German colleagues: Dr. Ralf Röhlsberger at DESY and the Helmholtz Institute, Jena; Dr. Jörg Evers at the Max Planck Institute for Nuclear Physics, Heidelberg; and Drs. Anders Madsen and Gianlcuca Geloni at EuXFEL, along with the groups they each lead.
“As soon as the resonance was seen within the first several hours of the data collection, we all joyfully celebrated this success,” she added. “It was rewarding for all of us, but especially for Yuri, who realized a high scientific potential of scandium-45 for super-resolution nuclear spectroscopy and the possibility to excite it with modern accelerator-based X-ray sources 33 years ago.”
Never one to rest on their laurels, the team already is focused on next steps and goals, starting with determining the resonant transition energy with even higher accuracy and measuring the exact lifetime of an isomer state. In addition, there’s also observation of the coherent forward nuclear scattering and measuring the linewidth of the nuclear transition.
“The next two steps can be achieved in a relatively simple way,” Zhang acknowledged. “While the third step is extremely challenging, it’s absolutely critical in order to estimate a projected accuracy and stability of any future nuclear clock. As a group and as a broader research team, we all look forward to the challenge.”
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