QT/ Materials fusion: Tailoring superconductivity for quantum computing
Quantum news biweekly vol.68, 1st February — 22nd February
TL;DR
- New materials with unique electrical properties could revolutionize superconductivity, providing a foundation for more robust quantum computing.
- A breakthrough technique controls microscopic defects in diamonds, enhancing the sensitivity of qubits for quantum sensing applications.
- Researchers create a thin scintillator with perovskite nanocrystals for real-time tracking of single protons, attributing exceptional sensitivity to biexcitonic radiative emission.
- The impact of tantalum oxide on quantum information retention is explored using scanning transmission electron microscopy (STEM) and computational modeling.
- A magnesium layer is discovered to enhance tantalum’s properties for building qubits, potentially improving quantum information retention.
- A logical qubit with error-correction capabilities is generated from a single light pulse.
- Two-dimensional semiconductors are investigated for potential applications in future computer and photovoltaic technologies.
- Direct images of ‘second sound’ within a superfluid expand understanding of heat flow in superconductors and neutron stars.
- Researchers develop a method to transform everyday materials like glass into substances suitable for quantum computers.
- A highly durable time crystal is produced, validating a phenomenon postulated by Nobel laureate Frank Wilczek, with implications for quantum computing and science fiction.
- 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
Interface-induced superconductivity in magnetic topological insulators
by Hemian Yi, Yi-Fan Zhao, Ying-Ting Chan, Jiaqi Cai, Ruobing Mei, et al in Science
A new fusion of materials, each with special electrical properties, has all the components required for a unique type of superconductivity that could provide the basis for more robust quantum computing. The new combination of materials, created by a team led by researchers at Penn State, could also provide a platform to explore physical behaviors similar to those of mysterious, theoretical particles known as chiral Majoranas, which could be another promising component for quantum computing.
The work describes how the researchers combined the two magnetic materials in what they called a critical step toward realizing the emergent interfacial superconductivity, which they are currently working toward.
Superconductors — materials with no electrical resistance — are widely used in digital circuits, the powerful magnets in magnetic resonance imaging (MRI) and particle accelerators, and other technology where maximizing the flow of electricity is crucial. When superconductors are combined with materials called magnetic topological insulators — thin films only a few atoms thick that have been made magnetic and restrict the movement of electrons to their edges — the novel electrical properties of each component work together to produce “chiral topological superconductors.” The topology, or specialized geometries and symmetries of matter, generates unique electrical phenomena in the superconductor, which could facilitate the construction of topological quantum computers.
Quantum computers have the potential to perform complex calculations in a fraction of the time it takes traditional computers because, unlike traditional computers which store data as a one or a zero, the quantum bits of quantum computers store data simultaneously in a range of possible states. Topological quantum computers further improve upon quantum computing by taking advantage of how electrical properties are organized to make the computers robust to decoherence, or the loss of information that happens when a quantum system is not perfectly isolated.
“Creating chiral topological superconductors is an important step toward topological quantum computation that could be scaled up for broad use,” said Cui-Zu Chang, Henry W. Knerr Early Career Professor and associate professor of physics at Penn State and co-corresponding author of the paper. “Chiral topological superconductivity requires three ingredients: superconductivity, ferromagnetism and a property called topological order. In this study, we produced a system with all three of these properties.”
The researchers used a technique called molecular beam epitaxy to stack together a topological insulator that has been made magnetic and an iron chalcogenide (FeTe), a promising transition metal for harnessing superconductivity. The topological insulator is a ferromagnet — a type of magnet whose electrons spin the same way — while FeTe is an antiferromagnet, whose electrons spin in alternating directions. The researchers used a variety of imaging techniques and other methods to characterize the structure and electrical properties of the resulting combined material and confirmed the presence of all three critical components of chiral topological superconductivity at the interface between the materials.
Prior work in the field has focused on combining superconductors and nonmagnetic topological insulators. According to the researchers, adding in the ferromagnet has been particularly challenging.
“Normally, superconductivity and ferromagnetism compete with each other, so it is rare to find robust superconductivity in a ferromagnetic material system,” said Chao-Xing Liu, professor of physics at Penn State and co-corresponding author of the paper. “But the superconductivity in this system is actually very robust against the ferromagnetism. You would need a very strong magnetic field to remove the superconductivity.”
The research team is still exploring why superconductivity and ferromagnetism coexist in this system.
“It’s actually quite interesting because we have two magnetic materials that are non-superconducting, but we put them together and the interface between these two compounds produces very robust superconductivity,” Chang said. “Iron chalcogenide is antiferromagnetic, and we anticipate its antiferromagnetic property is weakened around the interface to give rise to the emergent superconductivity, but we need more experiments and theoretical work to verify if this is true and to clarify the superconducting mechanism.”
The researchers said they believe this system will be useful in the search for material systems that exhibit similar behaviors as Majorana particles — theoretical subatomic particles first hypothesized in 1937. Majorana particles act as their own antiparticle, a unique property that could potentially allow them to be used as quantum bits in quantum computers.
“Providing experimental evidence for the existence of chiral Majorana will be a critical step in the creation of a topological quantum computer,” Chang said. “Our field has had a rocky past in trying to find these elusive particles, but we think this is a promising platform for exploring Majorana physics.”
Control of an Environmental Spin Defect beyond the Coherence Limit of a Central Spin
by Alexander Ungar, Paola Cappellaro, Alexandre Cooper, Won Kyu Calvin Sun in PRX Quantum
In quantum sensing, atomic-scale quantum systems are used to measure electromagnetic fields, as well as properties like rotation, acceleration, and distance, far more precisely than classical sensors can. The technology could enable devices that image the brain with unprecedented detail, for example, or air traffic control systems with precise positioning accuracy.
As many real-world quantum sensing devices are emerging, one promising direction is the use of microscopic defects inside diamonds to create “qubits” that can be used for quantum sensing. Qubits are the building blocks of quantum devices.
Researchers at MIT and elsewhere have developed a technique that enables them to identify and control a greater number of these microscopic defects. This could help them build a larger system of qubits that can perform quantum sensing with greater sensitivity. Their method builds off a central defect inside a diamond, known as a nitrogen-vacancy (NV) center, which scientists can detect and excite using laser light and then control with microwave pulses. This new approach uses a specific protocol of microwave pulses to identify and extend that control to additional defects that can’t be seen with a laser, which are called dark spins.
The researchers seek to control larger numbers of dark spins by locating them through a network of connected spins. Starting from this central NV spin, the researchers build this chain by coupling the NV spin to a nearby dark spin, and then use this dark spin as a probe to find and control a more distant spin which can’t be sensed by the NV directly. The process can be repeated on these more distant spins to control longer chains.
“One lesson I learned from this work is that searching in the dark may be quite discouraging when you don’t see results, but we were able to take this risk. It is possible, with some courage, to search in places that people haven’t looked before and find potentially more advantageous qubits,” says Alex Ungar, a PhD student in electrical engineering and computer science and a member of the Quantum Engineering Group at MIT, who is lead author of a paper.
To create NV centers, scientists implant nitrogen into a sample of diamond. But introducing nitrogen into the diamond creates other types of atomic defects in the surrounding environment. Some of these defects, including the NV center, can host what are known as electronic spins, which originate from the valence electrons around the site of the defect. Valence electrons are those in the outermost shell of an atom. A defect’s interaction with an external magnetic field can be used to form a qubit.
Researchers can harness these electronic spins from neighboring defects to create more qubits around a single NV center. This larger collection of qubits is known as a quantum register. Having a larger quantum register boosts the performance of a quantum sensor.
Some of these electronic spin defects are connected to the NV center through magnetic interaction. In past work, researchers used this interaction to identify and control nearby spins. However, this approach is limited because the NV center is only stable for a short amount of time, a principle called coherence. It can only be used to control the few spins that can be reached within this coherence limit.
In this new paper, the researchers use an electronic spin defect that is near the NV center as a probe to find and control an additional spin, creating a chain of three qubits. They use a technique known as spin echo double resonance (SEDOR), which involves a series of microwave pulses that decouple an NV center from all electronic spins that are interacting with it. Then, they selectively apply another microwave pulse to pair the NV center with one nearby spin. Unlike the NV, these neighboring dark spins can’t be excited, or polarized, with laser light. This polarization is a required step to control them with microwaves.
Once the researchers find and characterize a first-layer spin, they can transfer the NV’s polarization to this first-layer spin through the magnetic interaction by applying microwaves to both spins simultaneously. Then once the first-layer spin is polarized, they repeat the SEDOR process on the first-layer spin, using it as a probe to identify a second-layer spin that is interacting with it.
This repeated SEDOR process allows the researchers to detect and characterize a new, distinct defect located outside the coherence limit of the NV center. To control this more distant spin, they carefully apply a specific series of microwave pulses that enable them to transfer the polarization from the NV center along the chain to this second-layer spin.
“This is setting the stage for building larger quantum registers to higher-layer spins or longer spin chains, and also showing that we can find these new defects that weren’t discovered before by scaling up this technique,” Ungar says.
To control a spin, the microwave pulses must be very close to the resonance frequency of that spin. Tiny drifts in the experimental setup, due to temperature or vibrations, can throw off the microwave pulses. The researchers were able to optimize their protocol for sending precise microwave pulses, which enabled them to effectively identify and control second-layer spins, Ungar says.
“We are searching for something in the unknown, but at the same time, the environment might not be stable, so you don’t know if what you are finding is just noise. Once you start seeing promising things, you can put all your best effort in that one direction. But before you arrive there, it is a leap of faith,” Cappellaro says.
While they were able to effectively demonstrate a three-spin chain, the researchers estimate they could scale their method to a fifth layer using their current protocol, which could provide access to hundreds of potential qubits. With further optimization, they may be able to scale up to more than 10 layers. In the future, they plan to continue enhancing their technique to efficiently characterize and probe other electronic spins in the environment and explore different types of defects that could be used to form qubits.
Real-time single-proton counting with transmissive perovskite nanocrystal scintillators
by Zhaohong Mi, Hongyu Bian, Chengyuan Yang, Yanxin Dou, Andrew A. Bettiol, Xiaogang Liu in Nature Materials
Researchers have developed a transmissive thin scintillator using perovskite nanocrystals, designed for real-time tracking and counting of single protons. The exceptional sensitivity is attributed to biexcitonic radiative emission generated through proton-induced upconversion and impact ionisation.
The detection of energetic particles plays an important role in advancing science and technology in various fields, ranging from fundamental physics to quantum technology, deep space exploration and proton cancer therapy. The increasing demand for precise dose control in proton therapy has fuelled extensive research into proton detectors. One promising approach to enable proton counting during radiotherapy involves the development of high-performance thin-film detectors that are transmissive to protons.
Despite advancements in silicon-based, chemical vapour deposition, diamond-based, and other types of proton detectors in recent years, a fundamental challenge remains unresolved: achieving real-time proton irradiation with single-proton counting accuracy. In single-proton detection, the detectable signal is fundamentally limited by the thickness of the detector. Therefore, a proton-transmissive detector must be fabricated at an ultrathin thickness while retaining sensitivity for single-proton detection.
Existing particle detectors, such as ionisation chambers, silicon-based detectors and single-crystal scintillators, are too bulky to allow the transmission of protons. Additionally, organic plastic scintillators suffer from low scintillation yields and low particle radiation tolerances due to their low electron density, which hampers their single-proton detection sensitivity.
A research team led by Professor LIU Xiaogang from the NUS Department of Chemistry and Associate Professor Andrew BETTIOL from the NUS Department of Physics demonstrated the real-time detection and counting of single protons using thin-film transmissive scintillators made of CsPbBr3 nanocrystals. This approach offers unparalleled sensitivity with a light yield approximately double that of commercially available BC-400 plastic thin-film scintillators and ten times greater than conventional bulk scintillators such as LYSO:Ce, BGO and YAG:Ce crystals.
The thin-film nanocrystal scintillators, with a thickness of approximately 5 µm, exhibit high sensitivity that allows for a detection limit of 7 protons per second. This sensitivity is about five orders of magnitude lower than clinically relevant counting rates, making it a significant advancement in single-proton detection technology. The research team has put forward and substantiated a novel theory regarding the scintillation mechanisms induced by protons in CsPbBr3 nanocrystals.
They have verified that proton-induced scintillation primarily arises from the population of the biexcitonic state in CsPbBr3 nanocrystals, facilitated by the process of proton-induced upconversion and impact ionisation. This finding represents a significant contribution to the understanding of proton scintillation in perovskite nanocrystals.
By utilising the enhanced sensitivity, together with the fast response (~336 ps) to proton beams and pronounced iono-stability (up to a fluence of 1014 protons per cm2), the reseachers demonstrated additional applications of the CsPbBr3 nanocrystal scintillators. These include single-proton tracing, real-time patterned irradiation and super-resolution proton imaging. Remarkably, their study has showcased a spatial resolution of sub-40 nm for proton imaging; this holds tremendous promise for advancing various fields, such as materials characterisation, medical imaging and scientific research.
Prof Liu said, “The breakthrough presented in this work would be of considerable interest to particle radiation detection communities, offering both fundamental insights into new mechanisms of proton scintillation and technical advances in groundbreaking single-ion detection sensitivity using ultrathin proton-transmissive scintillators. In particular, these CsPbBr3 nanocrystal scintillators hold overwhelming promise for advancing detection technology in proton therapy and proton radiography.”
Probing Oxidation-Driven Amorphized Surfaces in a Ta(110) Film for Superconducting Qubit
by Junsik Mun, Peter V. Sushko, Emma Brass, Chenyu Zhou, Kim Kisslinger, Xiaohui Qu, Mingzhao Liu, Yimei Zhu in ACS Nano
Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and DOE’s Pacific Northwest National Laboratory (PNNL) have used a combination of scanning transmission electron microscopy (STEM) and computational modeling to get a closer look and deeper understanding of tantalum oxide. When this amorphous oxide layer forms on the surface of tantalum — a superconductor that shows great promise for making the “qubit” building blocks of a quantum computer — it can impede the material’s ability to retain quantum information. Learning how the oxide forms may offer clues as to why this happens — and potentially point to ways to prevent quantum coherence loss.
The paper builds on earlier research by a team at Brookhaven’s Center for Functional Nanomaterials (CFN), Brookhaven’s National Synchrotron Light Source II (NSLS-II), and Princeton University that was conducted as part of the Co-design Center for Quantum Advantage (C2QA), a Brookhaven-led national quantum information science research center in which Princeton is a key partner.
“In that work, we used X-ray photoemission spectroscopy at NSLS-II to infer details about the type of oxide that forms on the surface of tantalum when it is exposed to oxygen in the air,” said Mingzhao Liu, a CFN scientist and one of the lead authors on the study. “But we wanted to understand more about the chemistry of this very thin layer of oxide by making direct measurements,” he explained.
So, in the new study, the team partnered with scientists in Brookhaven’s Condensed Matter Physics & Materials Science (CMPMS) Department to use advanced STEM techniques that enabled them to study the ultrathin oxide layer directly. They also worked with theorists at PNNL who performed computational modeling that revealed the most likely arrangements and interactions of atoms in the material as they underwent oxidation. Together, these methods helped the team build an atomic-level understanding of the ordered crystalline lattice of tantalum metal, the amorphous oxide that forms on its surface, and intriguing new details about the interface between these layers.
“The key is to understand the interface between the surface oxide layer and the tantalum film because this interface can profoundly impact qubit performance,” said study co-author Yimei Zhu, a physicist from CMPMS, echoing the wisdom of Nobel laureate Herbert Kroemer, who famously asserted, “The interface is the device.”
Emphasizing that “quantitatively probing a mere one-to-two-atomic-layer-thick interface poses a formidable challenge,” Zhu noted, “we were able to directly measure the atomic structures and bonding states of the oxide layer and tantalum film as well as identify those of the interface using the advanced electron microscopy techniques developed at Brookhaven.”
“The measurements reveal that the interface consists of a ‘suboxide’ layer nestled between the periodically ordered tantalum atoms and the fully disordered amorphous tantalum oxide. Within this suboxide layer, only a few oxygen atoms are integrated into the tantalum crystal lattice,” Zhu said.
The combined structural and chemical measurements offer a crucially detailed perspective on the material. Density functional theory calculations then helped the scientists validate and gain deeper insight into these observations.
“We simulated the effect of gradual surface oxidation by gradually increasing the number of oxygen species at the surface and in the subsurface region,” said Peter Sushko, one of the PNNL theorists.
By assessing the thermodynamic stability, structure, and electronic property changes of the tantalum films during oxidation, the scientists concluded that while the fully oxidized amorphous layer acts as an insulator, the suboxide layer retains features of a metal.
“We always thought if the tantalum is oxidized, it becomes completely amorphous, with no crystalline order at all,” said Liu. “But in the suboxide layer, the tantalum sites are still quite ordered.”
With the presence of both fully oxidized tantalum and a suboxide layer, the scientists wanted to understand which part is most responsible the loss of coherence in qubits made of this superconducting material.
“It’s likely the oxide has multiple roles,” Liu said.
First, he noted, the fully oxidized amorphous layer contains many lattice defects. That is, the locations of the atoms are not well defined. Some atoms can shift around to different configurations, each with a different energy level. Though these shifts are small, each one consumes a tiny bit of electrical energy, which contributes to loss of energy from the qubit.
“This so-called two-level system loss in an amorphous material brings parasitic and irreversible loss to the quantum coherence — the ability of the material to hold onto quantum information,” Liu said.
But because the suboxide layer is still crystalline, “it may not be as bad as people were thinking,” Liu said. Maybe the more-fixed atomic arrangements in this layer will minimize two-level system loss. Then again, he noted, because the suboxide layer has some metallic characteristics, it could cause other problems.
“When you put a normal metal next to a superconductor, that could contribute to breaking up the pairs of electrons that move through the material with no resistance,” he noted. “If the pair breaks into two electrons again, then you will have loss of superconductivity and coherence. And that is not what you want.”
Ultrathin Magnesium‐Based Coating as an Efficient Oxygen Barrier for Superconducting Circuit Materials
by Chenyu Zhou, Junsik Mun, Juntao Yao, Aswin kumar Anbalagan, Mohammad D. Hossain, Russell A. McLellan, Ruoshui Li, Kim Kisslinger, Gengnan Li, Xiao Tong, Ashley R. Head, Conan Weiland, Steven L. Hulbert, Andrew L. Walter, Qiang Li, Yimei Zhu, Peter V. Sushko, Mingzhao Liu in Advanced Materials
Scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory have discovered that adding a layer of magnesium improves the properties of tantalum, a superconducting material that shows great promise for building qubits, the basis of quantum computers. As described in a paper, a thin layer of magnesium keeps tantalum from oxidizing, improves its purity, and raises the temperature at which it operates as a superconductor. All three may increase tantalum’s ability to hold onto quantum information in qubits.
This work builds on earlier studies in which a team from Brookhaven’s Center for Functional Nanomaterials (CFN), Brookhaven’s National Synchrotron Light Source II (NSLS-II), and Princeton University sought to understand the tantalizing characteristics of tantalum, and then worked with scientists in Brookhaven’s Condensed Matter Physics & Materials Science (CMPMS) Department and theorists at DOE’s Pacific Northwest National Laboratory (PNNL) to reveal details about how the material oxidizes. Those studies showed why oxidation is an issue.
“When oxygen reacts with tantalum, it forms an amorphous insulating layer that saps tiny bits of energy from the current moving through the tantalum lattice. That energy loss disrupts quantum coherence — the material’s ability to hold onto quantum information in a coherent state,” explained CFN scientist Mingzhao Liu, a lead author on the earlier studies and the new work.
While the oxidation of tantalum is usually self-limiting — a key reason for its relatively long coherence time — the team wanted to explore strategies to further restrain oxidation to see if they could improve the material’s performance.
“The reason tantalum oxidizes is that you have to handle it in air and the oxygen in air will react with the surface,” Liu explained. “So, as chemists, can we do something to stop that process? One strategy is to find something to cover it up.”
All this work is being carried out as part of the Co-design Center for Quantum Advantage (C2QA), a Brookhaven-led national quantum information science research center. While ongoing studies explore different kinds of cover materials, the new paper describes a promising first approach: coating the tantalum with a thin layer of magnesium.
“When you make a tantalum film, it is always in a high-vacuum chamber, so there is not much oxygen to speak of,” said Liu. “The problem always happens when you take it out. So, we thought, without breaking the vacuum, after we put the tantalum layer down, maybe we can put another layer, like magnesium, on top to block the surface from interacting with the air.”
Studies using transmission electron microscopy to image structural and chemical properties of the material, atomic layer by atomic layer, showed that the strategy to coat tantalum with magnesium was remarkably successful. The magnesium formed a thin layer of magnesium oxide on the tantalum surface that appears to keep oxygen from getting through.
“Electron microscopy techniques developed at Brookhaven Lab enabled direct visualization not only of the chemical distribution and atomic arrangement within the thin magnesium coating layer and the tantalum film but also of the changes of their oxidation states,” said Yimei Zhu, a study co-author from CMPMS. “This information is extremely valuable in comprehending the material’s electronic behavior,” he noted.
X-ray photoelectron spectroscopy studies at NSLS-II revealed the impact of the magnesium coating on limiting the formation of tantalum oxide. The measurements indicated that an extremely thin layer of tantalum oxide — less than one nanometer thick — remains confined directly beneath the magnesium/tantalum interface without disrupting the rest of the tantalum lattice.
“This is in stark contrast to uncoated tantalum, where the tantalum oxide layer can be more than three nanometers thick — and significantly more disruptive to the electronic properties of tantalum,” said study co-author Andrew Walter, a lead beamline scientist in the Soft X-ray Scattering & Spectroscopy program at NSLS-II.
Collaborators at PNNL then used computational modeling at the atomic scale to identify the most likely arrangements and interactions of the atoms based on their binding energies and other characteristics. These simulations helped the team develop a mechanistic understanding of why magnesium works so well.
At the simplest level, the calculations revealed that magnesium has a higher affinity for oxygen than tantalum does.
“While oxygen has a high affinity to tantalum, it is ‘happier’ to stay with the magnesium than with the tantalum,” said Peter Sushko, one of the PNNL theorists. “So, the magnesium reacts with oxygen to form a protective magnesium oxide layer. You don’t even need that much magnesium to do the job. Just two nanometers of thickness of magnesium almost completely blocks the oxidation of tantalum.”
The scientists also demonstrated that the protection lasts a long time: “Even after one month, the tantalum is still in pretty good shape. Magnesium is a really good oxygen barrier,” Liu concluded. The magnesium had an unexpected beneficial effect: It “sponged out” inadvertent impurities in the tantalum and, as a result, raised the temperature at which it operates as a superconductor.
“Even though we are making these materials in a vacuum, there is always some residual gas — oxygen, nitrogen, water vapor, hydrogen. And tantalum is very good at sucking up these impurities,” Liu explained. “No matter how careful you are, you will always have these impurities in your tantalum.”
But when the scientists added the magnesium coating, they discovered that its strong affinity for the impurities pulled them out. The resulting purer tantalum had a higher superconducting transition temperature. That could be very important for applications because most superconductors must be kept very cold to operate. In these ultracold conditions, most of the conducting electrons pair up and move through the material with no resistance.
“Even a slight elevation in the transition temperature could reduce the number of remaining, unpaired electrons,” Liu said, potentially making the material a better superconductor and increasing its quantum coherence time.
“There will have to be follow-up studies to see if this material improves qubit performance,” Liu said. “But this work provides valuable insights and new materials design principles that could help pave the way to the realization of large-scale, high-performance quantum computing systems.”
Logical states for fault-tolerant quantum computation with propagating light
by Shunya Konno, Warit Asavanant, Fumiya Hanamura, Hironari Nagayoshi, Kosuke Fukui, Atsushi Sakaguchi, Ryuhoh Ide, Fumihiro China, Masahiro Yabuno, Shigehito Miki, Hirotaka Terai, Kan Takase, Mamoru Endo, Petr Marek, Radim Filip, Peter van Loock, Akira Furusawa in Science
Researchers at the universities of Mainz, Olomouc, and Tokyo succeeded in generating a logical qubit from a single light pulse that has the inherent capacity to correct errors.
There has been significant progress in the field of quantum computing. Big global players, such as Google and IBM, are already offering cloud-based quantum computing services. However, quantum computers cannot yet help with problems that occur when standard computers reach the limits of their capacities because the availability of qubits or quantum bits, i.e., the basic units of quantum information, is still insufficient.
One of the reasons for this is that bare qubits are not of immediate use for running a quantum algorithm. While the binary bits of customary computers store information in the form of fixed values of either 0 or 1, qubits can represent 0 and 1 at one and the same time, bringing probability as to their value into play. This is known as quantum superposition. This makes them very susceptible to external influences, which means that the information they store can readily be lost.
In order to ensure that quantum computers supply reliable results, it is necessary to generate a genuine entanglement to join together several physical qubits to form a logical qubit. Should one of these physical qubits fail, the other qubits will retain the information. However, one of the main difficulties preventing the development of functional quantum computers is the large number of physical qubits required.
Many different concepts are being employed to make quantum computing viable. Large corporations currently rely on superconducting solid-state systems, for example, but these have the disadvantage that they only function at temperatures close to absolute zero. Photonic concepts, on the other hand, work at room temperature.
Single photons usually serve as physical qubits here. These photons, which are, in a sense, tiny particles of light, inherently operate more rapidly than solid-state qubits but, at the same time, are more easily lost. To avoid qubit losses and other errors, it is necessary to couple several single-photon light pulses together to construct a logical qubit — as in the case of the superconductor-based approach.
Researchers of the University of Tokyo together with colleagues from Johannes Gutenberg University Mainz (JGU) in Germany and Palacký University Olomouc in the Czech Republic have recently demonstrated a new means of constructing a photonic quantum computer. Rather than using a single photon, the team employed a laser-generated light pulse that can consist of several photons.
“Our laser pulse was converted to a quantum optical state that gives us an inherent capacity to correct errors,” stated Professor Peter van Loock of Mainz University.
“Although the system consists only of a laser pulse and is thus very small, it can — in principle — eradicate errors immediately.” Thus, there is no need to generate individual photons as qubits via numerous light pulses and then have them interact as logical qubits.
“We need just a single light pulse to obtain a robust logical qubit,” added van Loock.
To put it in other words, a physical qubit is already equivalent to a logical qubit in this system — a remarkable and unique concept. However, the logical qubit experimentally produced at the University of Tokyo was not yet of a sufficient quality to provide the necessary level of error tolerance. Nonetheless, the researchers have clearly demonstrated that it is possible to transform non-universally correctable qubits into correctable qubits using the most innovative quantum optical methods.
Probing electron-hole Coulomb correlations in the exciton landscape of a twisted semiconductor heterostructure
by Jan Philipp Bange, David Schmitt, Wiebke Bennecke, Giuseppe Meneghini, AbdulAziz AlMutairi, Kenji Watanabe, Takashi Taniguchi, Daniel Steil, Sabine Steil, R. Thomas Weitz, G. S. Matthijs Jansen, Stephan Hofmann, Samuel Brem, Ermin Malic, Marcel Reutzel, Stefan Mathias in Science Advances
Semiconductors are ubiquitous in modern technology, working to either enable or prevent the flow of electricity. In order to understand the potential of two-dimensional semiconductors for future computer and photovoltaic technologies, researchers from the Universities of Göttingen, Marburg and Cambridge investigated the bond that builds between the electrons and holes contained in these materials. By using a special method to break up the bond between electrons and holes, they were able to gain a microscopic insight into charge transfer processes across a semiconductor interface.
When light shines on a semiconductor, its energy is absorbed. As a result, negatively charged electrons and positively charged holes combine in the semiconductor to form pairs, known as excitons. In the most modern two-dimensional semiconductors, these excitons have an extraordinarily high binding energy.
In their study, the researchers set themselves the challenge of investigating the hole of the exciton. As physicist and first author Jan Philipp Bange from the University of Göttingen explains: “In our laboratory, we use photoemission spectroscopy to investigate how the absorption of light in quantum materials leads to charge transfer processes. So far, we have concentrated on the electrons that are part of the electron-hole pair, which we can measure using an electron analyser. Up to now, we didn’t have any way to directly access the holes themselves. So, we were interested in the question of how we could characterise not just the electron of the exciton but also its hole.”
To answer this question, the researchers, led by Dr Marcel Reutzel and Professor Stefan Mathias at Göttingen University’s Faculty of Physics, used a special microscope for photoelectrons in combination with a high-intensity laser. In the process, the breaking up of an exciton leads to a loss of energy in the electron measured in the experiment.
Reutzel explains: “This energy loss is characteristic for different excitons, depending on the environment in which the electron and the hole interact with each other.” In the current study, the researchers used a structure consisting of two different atomically thin semiconductors to show that the hole of the exciton transfers from one semiconductor layer to the other, similar to a solar cell.
Professor Ermin Malic’s team at the University of Marburg was able to explain this charge transfer process with a model to describe what happens at a microscopic level.
Mathias summarises: “In the future, we want to use the spectroscopic signature of the interaction between electrons and holes to study novel phases in quantum materials at ultrashort time and length scales. Such studies can be the basis for the development of new technologies and we hope to contribute to this in the future.”
Thermography of the superfluid transition in a strongly interacting Fermi gas
by Zhenjie Yan, Parth B. Patel, Biswaroop Mukherjee, Chris J. Vale, Richard J. Fletcher, Martin W. Zwierlein in Science
In most materials, heat prefers to scatter. If left alone, a hotspot will gradually fade as it warms its surroundings. But in rare states of matter, heat can behave as a wave, moving back and forth somewhat like a sound wave that bounces from one end of a room to the other. In fact, this wave-like heat is what physicists call “second sound.”
Signs of second sound have been observed in only a handful of materials. Now MIT physicists have captured direct images of second sound for the first time. The new images reveal how heat can move like a wave, and “slosh” back and forth, even as a material’s physical matter may move in an entirely different way. The images capture the pure movement of heat, independent of a material’s particles.
“It’s as if you had a tank of water and made one half nearly boiling,” Assistant Professor Richard Fletcher offers as analogy. “If you then watched, the water itself might look totally calm, but suddenly the other side is hot, and then the other side is hot, and the heat goes back and forth, while the water looks totally still.”
Led by Martin Zwierlein, the Thomas A Frank Professor of Physics, the team visualized second sound in a superfluid — a special state of matter that is created when a cloud of atoms is cooled to extremely low temperatures, at which point the atoms begin to flow like a completely friction-free fluid. In this superfluid state, theorists have predicted that heat should also flow like a wave, though scientists had not been able to directly observe the phenomenon until now. The new results, will help physicists get a more complete picture of how heat moves through superfluids and other related materials, including superconductors and neutron stars.
“There are strong connections between our puff of gas, which is a million times thinner than air, and the behavior of electrons in high-temperature superconductors, and even neutrons in ultradense neutron stars,” Zwierlein says. “Now we can probe pristinely the temperature response of our system, which teaches us about things that are very difficult to understand or even reach.”
Zwierlein and Fletcher’s co-authors on the study are first author and former physics graduate student Zhenjie Yan and former physics graduate students Parth Patel and Biswaroop Mikherjee, along with Chris Vale at Swinburne University of Technology in Melbourne, Australia. The MIT researchers are part of the MIT-Harvard Center for Ultracold Atoms (CUA).
When clouds of atoms are brought down to temperatures close to absolute zero, they can transition into rare states of matter. Zwierlein’s group at MIT is exploring the exotic phenomena that emerge among ultracold atoms, and specifically fermions — particles, such as electrons, that normally avoid each other. Under certain conditions, however, fermions can be made to strongly interact and pair up. In this coupled state, fermions can flow in unconventional ways. For their latest experiments, the team employs fermionic lithium-6 atoms, which are trapped and cooled to nanokelvin temperatures.
In 1938, the physicist László Tisza proposed a two-fluid model for superfluidity — that a superfluid is actually a mixture of some normal, viscous fluid and a friction-free superfluid. This mixture of two fluids should allow for two types of sound, ordinary density waves and peculiar temperature waves, which physicist Lev Landau later named “second sound.” Since a fluid transitions into a superfluid at a certain critical, ultracold temperature, the MIT team reasoned that the two types of fluid should also transport heat differently: In normal fluids, heat should dissipate as usual, whereas in a superfluid, it could move as a wave, similarly to sound.
“Second sound is the hallmark of superfluidity, but in ultracold gases so far you could only see it in this faint reflection of the density ripples that go along with it,” Zwierlein says. “The character of the heat wave could not be proven before.”
Zwierlein and his team sought to isolate and observe second sound, the wave-like movement of heat, independent of the physical motion of fermions in their superfluid. They did so by developing a new method of thermography — a heat-mapping technique. In conventional materials one would use infrared sensors to image heat sources. But at ultracold temperatures, gases do not give off infrared radiation. Instead, the team developed a method to use radio frequencyto “see” how heat moves through the superfluid. They found that the lithium-6 fermions resonate at different radio frequencies depending on their temperature: When the cloud is at warmer temperatures, and carries more normal liquid, it resonates at a higher frequency. Regions in the cloud that are colder resonate at a lower frequency.
The researchers applied the higher resonant radio frequency, which prompted any normal, “hot” fermions in the liquid to ring in response. The researchers then were able to zero in on the resonating fermions and track them over time to create “movies” that revealed heat’s pure motion — a sloshing back and forth, similar to waves of sound.
“For the first time, we can take pictures of this substance as we cool it through the critical temperature of superfluidity, and directly see how it transitions from being a normal fluid, where heat equilibrates boringly, to a superfluid where heat sloshes back and forth,” Zwierlein says.
The experiments mark the first time that scientists have been able to directly image second sound, and the pure motion of heat in a superfluid quantum gas. The researchers plan to extend their work to more precisely map heat’s behavior in other ultracold gases. Then, they say their findings can be scaled up to predict how heat flows in other strongly interacting materials, such as in high-temperature superconductors, and in neutron stars.
“Now we will be able to measure precisely the thermal conductivity in these systems, and hope to understand and design better systems,” Zwierlein concludes.
Controllable strain-driven topological phase transition and dominant surface-state transport in HfTe5
by Jinyu Liu, Yinong Zhou, Sebastian Yepez Rodriguez, Matthew A. Delmont, Robert A. Welser, Triet Ho, Nicholas Sirica, Kaleb McClure, Paolo Vilmercati, Joseph W. Ziller, Norman Mannella, Javier D. Sanchez-Yamagishi, Michael T. Pettes, Ruqian Wu, Luis A. Jauregui in Nature Communications
Researchers at the University of California, Irvine and Los Alamos National Laboratory,, describe the discovery of a new method that transforms everyday materials like glass into materials scientists can use to make quantum computers.
“The materials we made are substances that exhibit unique electrical or quantum properties because of their specific atomic shapes or structures,” said Luis A. Jauregui, professor of physics & astronomy at UCI and lead author of the new paper. “Imagine if we could transform glass, typically considered an insulating material, and convert it into efficient conductors akin to copper. That’s what we’ve done.”
Conventional computers use silicon as a conductor, but silicon has limits. Quantum computers stand to help bypass these limits, and methods like those described in the new study will help quantum computers become an everyday reality.
“This experiment is based on the unique capabilities that we have at UCI for growing high-quality quantum materials. How can we transform these materials that are poor conductors into good conductors?” said Jauregui, who’s also a member of UCI’s Eddleman Quantum Institute. “That’s what we’ve done in this paper. We’ve been applying new techniques to these materials, and we’ve transformed them to being good conductors.”
The key, Jauregui explained, was applying the right kind of strain to materials at the atomic scale. To do this, the team designed a special apparatus called a “bending station” at the machine shop in the UCI School of Physical Sciences that allowed them to apply large strain to change the atomic structure of a material called hafnium pentatelluride from a “trivial” material into a material fit for a quantum computer.
“To create such materials, we need to ‘poke holes’ in the atomic structure,” said Jauregui. “Strain allows us to do that.”
“You can also turn the atomic structure change on or off by controlling the strain, which is useful if you want to create an on-off switch for the material in a quantum computer in the future,” said Jinyu Liu, who is the first author of the paper and a postdoctoral scholar working with Jauregui.
“I am pleased by the way theoretical simulations offer profound insights into experimental observations, thereby accelerating the discovery of methods for controlling the quantum states of novel materials,” said co-author Ruqian Wu, professor of physics and Associate Director of the UCI Center for Complex and Active Materials — a National Science Foundation Materials Research Science and Engineering Center (MRSEC). “This underscores the success of collaborative efforts involving diverse expertise in frontier research.”
“I’m excited that our team was able to show that these elusive and much-sought-after material states can be made,” said Michael Pettes, study co-author and scientist with the Center for Integrated Nanotechnologies at Los Alamos National Laboratory. “This is promising for the development of quantum devices, and the methodology we demonstrate is compatible for experimentation on other quantum materials as well.”
Right now, quantum computers only exist in a few places, such as in the offices of companies like IBM, Google and Rigetti.
“Google, IBM and many other companies are looking for effective quantum computers that we can use in our daily lives,” said Jauregui. “Our hope is that this new research helps make the promise of quantum computers more of a reality.”
Robust continuous time crystal in an electron–nuclear spin system
by A. Greilich, N. E. Kopteva, A. N. Kamenskii, P. S. Sokolov, V. L. Korenev, M. Bayer in Nature Physics
A team from TU Dortmund University recently succeeded in producing a highly durable time crystal that lived millions of times longer than could be shown in previous experiments. By doing so, they have corroborated an extremely interesting phenomenon that Nobel Prize laureate Frank Wilczek postulated around ten years ago and which had already found its way into science fiction movies.
Crystals or, to be more precise, crystals in space, are periodic arrangements of atoms over large length scales. This arrangement gives crystals their fascinating appearance, with smooth facets like in gemstones. As physics often treats space and time on one and the same level, for example in special relativity, Frank Wilczek, physicist at the Massachusetts Institute of Technology (MIT) and winner of the Nobel Prize in Physics, postulated in 2012 that, in addition to crystals in space, there must also be crystals in time.
For this to be the case, he said, one of their physical properties would have to spontaneously begin to change periodically in time, even though the system does not experience corresponding periodic interference. That such time crystals could be possible was the subject of controversial scientific debate for several years — but quick to arrive in the movie theater: For example, a time crystal played a central role in Marvel Studios’ movie Avengers: Endgame (2019). From 2017 onwards, scientists have indeed succeeded on a handful of occasions in demonstrating a potential time crystal. However, these were systems that — unlike Wilczek’s original idea — are subjected to a temporal excitation with a specific periodicity, but then react with another period twice as long.
A crystal that behaves periodically in time, although excitation is time-independent, i.e. constant, was only demonstrated in 2022 in a Bose-Einstein condensate. However, the crystal lived for just a few milliseconds.
The Dortmund physicists led by Dr. Alex Greilich have now designed a special crystal made of indium gallium arsenide, in which the nuclear spins act as a reservoir for the time crystal. The crystal is continuously illuminated so that a nuclear spin polarization forms through interaction with electron spins. And it is precisely this nuclear spin polarization that then spontaneously generates oscillations, equivalent to a time crystal.
The status of the experiments at the present time is that the crystal’s lifetime is at least 40 minutes, which is ten million times longer than has been demonstrated to date, and it could potentially live far longer.
It is possible to vary the crystal’s period over wide ranges by systematically changing the experimental conditions. However, it is also possible to move into areas where the crystal “melts,” i.e. loses its periodicity. These areas are also interesting, as chaotic behavior, which can be maintained over long periods of time, is then manifested. This is the first time that scientists have been able to use theoretical tools to analyze the chaotic behavior of such systems.
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