Tag Archives: #quantumcomputing

Harvard-led Physicists Take Big Step in Race To Quantum Computing (Physics)

Team develops simulator with 256 qubits, largest of its kind ever created

A team of physicists from the Harvard-MIT Center for Ultracold Atoms and other universities has developed a special type of quantum computer known as a programmable quantum simulator capable of operating with 256 quantum bits, or “qubits.”

The system marks a major step toward building large-scale quantum machines that could be used to shed light on a host of complex quantum processes and eventually help bring about real-world breakthroughs in material science, communication technologies, finance, and many other fields, overcoming research hurdles that are beyond the capabilities of even the fastest supercomputers today. Qubits are the fundamental building blocks on which quantum computers run and the source of their massive processing power.

“This moves the field into a new domain where no one has ever been to thus far,” said Mikhail Lukin, the George Vasmer Leverett Professor of Physics, co-director of the Harvard Quantum Initiative, and one of the senior authors of the study published today in the journal Nature. “We are entering a completely new part of the quantum world.”

Gif of laser-cooled atoms.
By arranging them in sequential frames and taking images of single atoms, the researchers can even make fun atom videos. Courtesy of Lukin group

According to Sepehr Ebadi, a physics student in the Graduate School of Arts and Sciences and the study’s lead author, it is the combination of system’s unprecedented size and programmability that puts it at the cutting edge of the race for a quantum computer, which harnesses the mysterious properties of matter at extremely small scales to greatly advance processing power. Under the right circumstances, the increase in qubits means the system can store and process exponentially more information than the classical bits on which standard computers run.

“The number of quantum states that are possible with only 256 qubits exceeds the number of atoms in the solar system,” Ebadi said, explaining the system’s vast size.

Already, the simulator has allowed researchers to observe several exotic quantum states of matter that had never before been realized experimentally, and to perform a quantum phase transition study so precise that it serves as the textbook example of how magnetism works at the quantum level.

These experiments provide powerful insights on the quantum physics underlying material properties and can help show scientists how to design new materials with exotic properties.

The project uses a significantly upgraded version of a platform the researchers developed in 2017, which was capable of reaching a size of 51 qubits. That older system allowed the researchers to capture ultra-cold rubidium atoms and arrange them in a specific order using a one-dimensional array of individually focused laser beams called optical tweezers.Dolev Bluvstein looks at 420 mm laser that allows them to control and entangle Rydberg atoms.

Dolev Bluvstein looking at a 420 mm laser.
Dolev Bluvstein looks at 420 mm laser that allows them to control and entangle Rydberg atoms. © Harvard Gazette

This new system allows the atoms to be assembled in two-dimensional arrays of optical tweezers. This increases the achievable system size from 51 to 256 qubits. Using the tweezers, researchers can arrange the atoms in defect-free patterns and create programmable shapes like square, honeycomb, or triangular lattices to engineer different interactions between the qubits.

“The workhorse of this new platform is a device called the spatial light modulator, which is used to shape an optical wavefront to produce hundreds of individually focused optical tweezer beams,” said Ebadi. “These devices are essentially the same as what is used inside a computer projector to display images on a screen, but we have adapted them to be a critical component of our quantum simulator.”

The initial loading of the atoms into the optical tweezers is random, and the researchers must move the atoms around to arrange them into their target geometries. The researchers use a second set of moving optical tweezers to drag the atoms to their desired locations, eliminating the initial randomness. Lasers give the researchers complete control over the positioning of the atomic qubits and their coherent quantum manipulation.

Laser-cooled atoms make a Harvard veritas shield.
The researchers can use optical tweezers to position individually laser-cooled atoms in programmable geometries. Shown are fluorescence images of individual atoms after rearrangement into Harvard shields. Courtesy of Lukin group

Other senior authors of the study include Harvard Professors Subir Sachdev and Markus Greiner, who worked on the project along with Massachusetts Institute of Technology Professor Vladan Vuletić, and scientists from Stanford, the University of California Berkeley, the University of Innsbruck in Austria, the Austrian Academy of Sciences, and QuEra Computing Inc. in Boston.

“Our work is part of a really intense, high-visibility global race to build bigger and better quantum computers,” said Tout Wang, a research associate in physics at Harvard and one of the paper’s authors. “The overall effort [beyond our own] has top academic research institutions involved and major private-sector investment from Google, IBM, Amazon, and many others.”

The researchers are currently working to improve the system by improving laser control over qubits and making the system more programmable. They are also actively exploring how the system can be used for new applications, ranging from probing exotic forms of quantum matter to solving challenging real-world problems that can be naturally encoded on the qubits.

“This work enables a vast number of new scientific directions,” Ebadi said. “We are nowhere near the limits of what can be done with these systems.”

This work was supported by the Center for Ultracold Atoms, the National Science Foundation, the Vannevar Bush Faculty Fellowship, the U.S. Department of Energy, the Office of Naval Research, the Army Research Office MURI, and the DARPA ONISQ program.

Featured image: Dolev Bluvstein (from left), Mikhail Lukin, and Sepehr Ebadi developed a special type of quantum computer known as a programmable quantum simulator. Ebadi is aligning the device that allows them to create the programmable optical tweezers. Photos by Rose Lincoln/Harvard Staff Photographer


Reference: Ebadi, S., Wang, T.T., Levine, H. et al. Quantum phases of matter on a 256-atom programmable quantum simulator. Nature 595, 227–232 (2021). https://doi.org/10.1038/s41586-021-03582-4


Provided by Harvard Gazette

New Discovery Of A Rare Superconductor May be Vital For the Future of Quantum Computing (Quantum)

Research led by the University of Kent and the STFC Rutherford Appleton Laboratory has resulted in the discovery of a new rare topological superconductor, LaPt3P. This discovery may be of huge importance to the future operations of quantum computers.

Superconductors are vital materials able to conduct electricity without any resistance when cooled below a certain temperature, making them highly desirable in a society needing to reduce its energy consumption.

Superconductors manifest quantum properties on the scale of everyday objects, making them highly attractive candidates for building computers which use quantum physics to store data and perform computing operations, and can vastly outperform even the best supercomputers in certain tasks. As a result, there is an increasing demand from leading tech companies like Google, IBM and Microsoft to make quantum computers on an industrial scale using superconductors.

However, the elementary units of quantum computers (qubits) are extremely sensitive and lose their quantum properties due to electromagnetic fields, heat and collisions with air molecules. Protection from these can be achieved by making more resilient qubits using a special class of superconductors called topological superconductors which in addition to being superconductors also host protected metallic states on their boundaries or surfaces.

Topological superconductors, such as LaPt3P, newly discovered through muon spin relaxation experiments and extensive theoretical analysis, are exceptionally rare and are of tremendous value to the future industry of quantum computing.

To ensure its properties are sample and instrument independent, two different sets of samples were prepared in the University of Warwick and in ETH Zurich. Muon experiments were then performed in two different types of muon facilities: in the ISIS Pulsed Neutron and Muon Source in the STFC Rutherford Appleton Laboratory and in PSI, Switzerland.

Dr Sudeep Kumar Ghosh, Leverhulme Early Career Fellow at Kent and Principle Investigator said: ‘This discovery of the topological superconductor LaPt3P has tremendous potential in the field of quantum computing. Discovery of such a rare and desired component demonstrates the importance of muon research for the everyday world around us.’

The paper ‘Chiral singlet superconductivity in the weakly correlated metal LaPt3P’ is published in Nature Communications (University of Kent: Dr. Sudeep K. Ghosh; STFC Rutherford Appleton Laboratory: Dr. Pabitra K. Biswas, Dr. Adrian D. Hillier; University of Warwick – Dr. Geetha Balakrishnan, Dr. Martin R. Lees, Dr. Daniel A. Mayoh; Paul Scherrer Institute: Dr. Charles Baines; Zhejiang University of Technology: Dr. Xiaofeng Xu; ETH Zurich: Dr. Nikolai D. Zhigadlo; Southwest University of Science and Technology: Dr. Jianzhou Zhao). URL: https://www.nature.com/articles/s41467-021-22807-8 DOI: https://doi.org/10.1038/s41467-021-22807-8


Provided by University of Kent

Quantum Computing With Holes (Physics)

Scientists found a new and promising qubit at a place where there is nothing

Quantum computers with their promises of creating new materials and solving intractable mathematical problems are a dream of many physicists. Now, they are slowly approaching viable realizations in many laboratories all over the world. But there are still enormous challenges to master. A central one is the construction of stable quantum bits – the fundamental unit of quantum computation called qubit for short – that can be networked together.

In a study published in Nature Materials and led by Daniel Jirovec from the Katsaros group at IST Austria in close collaboration with researchers from the L-NESS Inter-university Centre in Como, Italy, scientists now have created a new and promising candidate system for reliable qubits.

Spinning Absence

The researchers created the qubit using the spin of so-called holes. Each hole is just the absence of an electron in a solid material. Amazingly, a missing negatively charged particle can physically be treated as if it were a positively charged particle. It can even move around in the solid when a neighboring electron fills the hole. Thus, effectively the hole described as positively charged particle is moving forward.

These holes even carry the quantum-mechanical property of spin and can interact if they come close to each other. “Our colleagues at L-NESS layered several different mixtures of silicon and germanium just a few nanometers thick on top of each other. That allows us to confine the holes to the germanium-rich layer in the middle,” Jirovec explains. “On top, we added tiny electrical wires – so-called gates – to control the movement of holes by applying voltage to them. The electrically positively charged holes react to the voltage and can be extremely precisely moved around within their layer.”

Daniel Jirovec, Institute of Science and Technology Austria© Daniel Jirovec

Using this nano-scale control, the scientists moved two holes close to each other to create a qubit out of their interacting spins. But to make this work, they needed to apply a magnetic field to the whole setup. Here, their innovative approach comes into play.

Linking Qubits

In their setup, Jirovec and his colleagues cannot only move holes around but also alter their properties. By engineering different hole properties, they created the qubit out of the two interacting hole spins using less than ten millitesla of magnetic field strength. This is a weak magnetic field compared to other similar qubit setups, which employ at least ten times stronger fields.

But why is that relevant? “By using our layered germanium setup we can reduce the required magnetic field strength and therefore allow the combination of our qubit with superconductors, usually inhibited by strong magnetic fields,” Jirovec says. Superconductors – materials without any electrical resistance – support the linking of several qubits due to their quantum-mechanical nature. This could enable scientists to build new kinds of quantum computers combining semiconductors and superconductors.

In addition to the new technical possibilities, these hole spin qubits look promising because of their processing speed. With up to one hundred million operations per second as well as their long lifetime of up to 150 microseconds they seem particularly viable for quantum computing. Usually, there is a tradeoff between these properties, but this new design brings both advantages together.

Featured image: The two holes are confined to the germanium-rich layer just a few nanometers thick. On top, the electrical gates are formed by individual wires with voltages applied. The positively charged holes feel the push and pull from the wires and can therefore be moved around within their layer. © Daniel Jirovec


Reference: Jirovec, D., Hofmann, A., Ballabio, A. et al. A singlet-triplet hole spin qubit in planar Ge. Nat. Mater. (2021). https://doi.org/10.1038/s41563-021-01022-2


Provided by IST Austria

Quantum Systems Learn Joint Computing (Quantum)

MPQ researchers realize the first quantum-logic computer operation between two separate quantum modules in different laboratories.

Today’s quantum computers contain up to several dozen memory and processing units, the so-called qubits. Severin Daiss, Stefan Langenfeld, and colleagues from the Max Planck Institute of Quantum Optics in Garching have successfully interconnected two such qubits located in different labs to a distributed quantum computer by linking the qubits with a 60-meter-long optical fiber. Over such a distance they realized a quantum-logic gate – the basic building block of a quantum computer. It makes the system the worldwide first prototype of a distributed quantum computer.

The limitations of previous qubit architectures

Quantum computers are considerably different from traditional “binary” computers: Future realizations of them are expected to easily perform specific calculations for which traditional computers would take months or even years – for example in the field of data encryption and decryption. While the performance of binary computers results from large memories and fast computing cycles, the success of the quantum computer rests on the fact that one single memory unit – a quantum bit, also called “qubit” – can contain superpositions of different possible values at the same time. Therefore, a quantum computer does not only calculate one result at a time, but instead many possible results in parallel. The more qubits there are interconnected in a quantum computer; the more complex calculations it can perform.

The basic computing operations of a quantum computer are quantum-logic gates between two qubits. Such an operation changes – depending on the initial state of the qubits – their quantum mechanical states. For a quantum computer to be superior to a normal computer for various calculations, it would have to reliably interconnect many dozens, or even thousands of qubits for equally thousands of quantum operations. Despite great successes, all current laboratories are still struggling to build such a large and reliable quantum computer, since every additionally required qubit makes it much harder to build a quantum computer in just one single set-up. The qubits are implemented, for instance, with single atoms, superconductive elements, or light particles, all of which need to be isolated perfectly from each other and the environment. The more qubits are arranged next to one another, the harder it is to both isolate and control them from outside at the same time.

Data line and processing unit combined

One way to overcome the technical difficulties in the construction of quantum computers is presented in a new study in the journal Science by Severin Daiss, Stefan Langenfeld and colleagues from the research group of Gerhard Rempe at the Max Planck Institute of Quantum Optics in Garching. In this work supported by the Institute of Photonic Sciences (Castelldefels, Spain), the team succeeded in connecting two qubit modules across a 60-meter distance in such a way that they effectively form a basic quantum computer with two qubits. “Across this distance, we perform a quantum computing operation between two independent qubit setups in different laboratories,” Daiss emphasizes. This enables the possibility to merge smaller quantum computers to a joint processing unit.

First author, Severin Daiss, in front of part one of their distributed quantum computer. © Max Planck Gesellschaft

Simply coupling distant qubits to generate entanglement between them has been achieved in the past, but now, the connection can additionally be used for quantum computations. For this purpose, the researchers employed modules consisting of a single atom as a qubit that is positioned amidst two mirrors. Between these modules, they send one single light quanta, a photon, that is transported in the optical fiber. This photon is then entangled with the quantum states of the qubits in the different modules. Subsequently, the state of one of the qubits is changed according to the measured state of the “ancilla photon”, realizing a quantum mechanical CNOT-operation with a fidelity of 80 percent. A next step would be to connect more than two modules and to host more qubits in the individual modules.

Higher performance quantum computers through distributed computing

Team leader and institute director Gerhard Rempe believes the result will allow to further advance the technology: “Our scheme opens up a new development path for distributed quantum computing”. It could enable, for instance, to build a distributed quantum computer consisting of many modules with few qubits that are interconnected with the newly introduced method. This approach could circumvent the limitation of existing quantum computers to integrate more qubits into a single setup and could therefore allow more powerful systems.

Featured image: This picture shows the two qubit modules (red atom between two blue mirrors) that have been interconnected to implement a basic quantum computation (depicted as light blue symbol) over a distance of 60 meters. The modules reside in different laboratories of the same building and are connected by an optical fiber. The computation operation is mediated by a single photon (flying red sphere) that interacts successively with the two modules. © Stephan Welte, Severin Daiss (MPQ)


Reference: Severin Daiss, Stephan Langenfeld, Stephan Welte, Emanuele Distante, Philip Thomas, Lukas Hartung, Olivier Morin, Gerhard Rempe, “A Quantum-Logic Gate between Distant Quantum-Network Modules”, Science 05 Feb 2021: Vol. 371, Issue 6529, pp. 614-617. https://dx.doi.org/10.1126/science.abe3150 https://science.sciencemag.org/content/371/6529/614


Provided by Max Planck Institute of Quantum Optics

Quantum Computing: When Ignorance is Wanted (Quantum)

Quantum technologies for computers open up new concepts of preserving the privacy of input and output data of a computation. Scientists from the University of Vienna, the Singapore University of Technology and Design and the Polytechnic University of Milan have shown that optical quantum systems are not only particularly suitable for some quantum computations, but can also effectively encrypt the associated input and output data. This demonstration of a so-called quantum homomorphic encryption of a quantum computation has now been published in “NPJ Quantum Information”.

Quantum computers promise not only to outperform classical machines in certain important tasks, but also to maintain the privacy of data processing. The secure delegation of computations has been an increasingly important issue since the possibility of utilizing cloud computing and cloud networks. Of particular interest is the ability to exploit quantum technology that allows for unconditional security, meaning that no assumptions about the computational power of a potential adversary need to be made.

Different quantum protocols have been proposed, all of which make trade-offs between computational performance, security, and resources. Classical protocols, for example, are either limited to trivial computations or are restricted in their security. In contrast, homomorphic quantum encryption is one of the most promising schemes for secure delegated computation. Here, the client’s data is encrypted in such a way that the server can process it even though he cannot decrypt it. Moreover, opposed to other protocols, the client and server do not need to communicate during the computation which dramatically boosts the protocol’s performance and practicality.

In an international collaboration led by Prof. Philip Walther from the University of Vienna scientists from Austria, Singapore and Italy teamed up to implement a new quantum computation protocol where the client has the option of encrypting his input data so that the computer cannot learn anything about them, yet can still perform the calculation. After the computation, the client can then decrypt the output data again to read out the result of the calculation. For the experimental demonstration, the team used quantum light, which consists of individual photons, to implement this so-called homomorphic quantum encryption in a quantum walk process. Quantum walks are interesting special-purpose examples of quantum computation because they are hard for classical computers, whereas being feasible for single photons.

By combining an integrated photonic platform built at the Polytechnic University of Milan, together with a novel theoretical proposal developed at the Singapore University of Technology and Design, scientist from the University of Vienna demonstrated the security of the encrypted data and investigated the behavior increasing the complexity of the computations.

The team was able to show that the security of the encrypted data improves the larger the dimension of the quantum walk calculation becomes. Furthermore, recent theoretical work indicates that future experiments taking advantage of various photonic degrees of freedom would also contribute to an improvement in data security; one can anticipate further optimizations in the future. “Our results indicate that the level of security improves even further, when increasing the number of photons that carry the data”, says Philip Walther and concludes “this is exciting and we anticipate further developments of secure quantum computing in the future”.

Publication in “NPJ Quantum Information”:
Jonas Zeuner, Ioannis Pitsios, Si-Hui Tan, Aditya Sharma, Joseph Fitzsimons, Roberto Osellame and Philip Walther, Experimental Quantum Homomorphic Encryption, npj Quantum Information 7, 25 (2021); DOI: 10.1038/s41534-020-00340-8

Featured image: Artistic image of a homomorphic-encrypted quantum computation using a photonic quantum computer. (© Equinox Graphics, Universität Wien)


Provided by Universität Wein

UMass Amherst Team Helps Demonstrate Spontaneous Quantum Error Correction (Quantum)

New research tackles a central challenge of powerful quantum computing

To build a universal quantum computer from fragile quantum components, effective implementation of quantum error correction (QEC) is an essential requirement and a central challenge. QEC is used in quantum computing, which has the potential to solve scientific problems beyond the scope of supercomputers, to protect quantum information from errors due to various noise.

Published by the journal Nature, research co-authored by University of Massachusetts Amherst physicist Chen Wang, graduate students Jeffrey Gertler and Shruti Shirol, and postdoctoral researcher Juliang Li takes a step toward building a fault-tolerant quantum computer. They have realized a novel type of QEC where the quantum errors are spontaneously corrected.

Today’s computers are built with transistors representing classical bits (0’s or 1’s). Quantum computing is an exciting new paradigm of computation using quantum bits (qubits) where quantum superposition can be exploited for exponential gains in processing power. Fault-tolerant quantum computing may immensely advance new materials discovery, artificial intelligence, biochemical engineering and many other disciplines.

Since qubits are intrinsically fragile, the most outstanding challenge of building such powerful quantum computers is efficient implementation of quantum error correction. Existing demonstrations of QEC are active, meaning that they require periodically checking for errors and immediately fixing them, which is very demanding in hardware resources and hence hinders the scaling of quantum computers.

In contrast, the researchers’ experiment achieves passive QEC by tailoring the friction (or dissipation) experienced by the qubit. Because friction is commonly considered the nemesis of quantum coherence, this result may appear quite surprising. The trick is that the dissipation has to be designed specifically in a quantum manner. This general strategy has been known in theory for about two decades, but a practical way to obtain such dissipation and put it in use for QEC has been a challenge.

“Although our experiment is still a rather rudimentary demonstration, we have finally fulfilled this counterintuitive theoretical possibility of dissipative QEC,” says Chen. “Looking forward, the implication is that there may be more avenues to protect our qubits from errors and do so less expensively. Therefore, this experiment raises the outlook of potentially building a useful fault-tolerant quantum computer in the mid to long run.”

Chen Wang

Chen describes in layman’s terms how strange the quantum world can be. “As in German physicist Erwin Schrödinger’s famous (or infamous) example, a cat packed in a closed box can be dead or alive at the same time. Each logical qubit in our quantum processor is very much like a mini-Schrödinger’s cat. In fact, we quite literally call it a ‘cat qubit.’ Having lots of such cats can help us solve some of the world’s most difficult problems.”

“Unfortunately, it is very difficult to keep a cat staying that way since any gas, light, or anything leaking into the box will destroy the magic: The cat will become either dead or just a regular live cat,” explains Chen. “The most straightforward strategy to protect a Schrodinger’s cat is to make the box as tight as possible, but that also makes it harder to use it for computation. What we just demonstrated was akin to painting the inside of the box in a special way and that somehow helps the cat better survive the inevitable harm of the outside world.”

Co-authors also include Brian Baker and Jens Koch from Northwestern University.


Reference: Gertler, J.M., Baker, B., Li, J. et al. Protecting a bosonic qubit with autonomous quantum error correction. Nature 590, 243–248 (2021). https://doi.org/10.1038/s41586-021-03257-0


Provided by UMass Amherst

Quantum Computing Enables Simulations to Unravel Mysteries Of Magnetic Materials (Quantum)

A multi-institutional team became the first to generate accurate results from materials science simulations on a quantum computer that can be verified with neutron scattering experiments and other practical techniques.

Researchers from the Department of Energy’s Oak Ridge National Laboratory; the University of Tennessee, Knoxville; Purdue University and D-Wave Systems harnessed the power of quantum annealing, a form of quantum computing, by embedding an existing model into a quantum computer.  

Characterizing materials has long been a hallmark of classical supercomputers, which encode information using a binary system of bits that are each assigned a value of either 0 or 1. But quantum computers — in this case, D-Wave’s 2000Q – rely on qubits, which can be valued at 0, 1 or both simultaneously because of a quantum mechanical capability known as superposition.

“The underlying method behind solving materials science problems on quantum computers had already been developed, but it was all theoretical,” said Paul Kairys, a student at UT Knoxville’s Bredesen Center for Interdisciplinary Research and Graduate Education who led ORNL’s contributions to the project. “We developed new solutions to enable materials simulations on real-world quantum devices.”

This unique approach proved that quantum resources are capable of studying the magnetic structure and properties of these materials, which could lead to a better understanding of spin liquids, spin ices and other novel phases of matter useful for data storage and spintronics applications. The researchers published the results of their simulations — which matched theoretical predictions and strongly resembled experimental data — in PRX Quantum.

Eventually, the power and robustness of quantum computers could enable these systems to outperform their classical counterparts in terms of both accuracy and complexity, providing precise answers to materials science questions instead of approximations. However, quantum hardware limitations previously made such studies difficult or impossible to complete.  

To overcome these limitations, the researchers programmed various parameters into the Shastry-Sutherland Ising model. Because it shares striking similarities with the rare earth tetraborides, a class of magnetic materials, subsequent simulations using this model could provide substantial insights into the behavior of these tangible substances.

“We are encouraged that the novel quantum annealing platform can directly help us understand materials with complicated magnetic phases, even those that have multiple defects,” said co-corresponding author Arnab Banerjee, an assistant professor at Purdue. “This capability will help us make sense of real material data from a variety of neutron scattering, magnetic susceptibility and heat capacity experiments, which can be very difficult otherwise.”

Using the D-Wave chip (foreground), the team simulated the experimental signature of a sample material (background), producing results that are directly comparable to the output from real-world experiments. Credit: Paul Kairys/UT Knoxville

Magnetic materials can be described in terms of magnetic particles called spins. Each spin has a preferred orientation based on the behavior of its neighboring spins, but rare earth tetraborides are frustrated, meaning these orientations are incompatible with each other. As a result, the spins are forced to compromise on a collective configuration, leading to exotic behavior such as fractional magnetization plateaus. This peculiar behavior occurs when an applied magnetic field, which normally causes all spins to point in one direction, affects only some spins in the usual way while others point in the opposite direction instead.

Using a Monte Carlo simulation technique powered by the quantum evolution of the Ising model, the team evaluated this phenomenon in microscopic detail.

“We came up with new ways to represent the boundaries, or edges, of the material to trick the quantum computer into thinking that the material was effectively infinite, and that turned out to be crucial for correctly answering materials science questions,” said co-corresponding author Travis Humble. Humble is an ORNL researcher and deputy director of the Quantum Science Center, or QSC, a DOE Quantum Information Science Research Center established at ORNL in 2020. The individuals and institutions involved in this research are QSC members.

Quantum resources have previously simulated small molecules to examine chemical or material systems. Yet, studying magnetic materials that contain thousands of atoms is possible because of the size and versatility of D-Wave’s quantum device.

“D-Wave processors are now being used to simulate magnetic systems of practical interest, resembling real compounds. This is a big deal and takes us from the notepad to the lab,” said Andrew King, director of performance research at D-Wave.”The ultimate goal is to study phenomena that are intractable for classical computing and outside the reach of known experimental methods.”

The researchers anticipate that their novel simulations will serve as a foundation to streamline future efforts on next-generation quantum computers. In the meantime, they plan to conduct related research through the QSC, from testing different models and materials to performing experimental measurements to validate the results.

“We completed the largest simulation possible for this model on the largest quantum computer available at the time, and the results demonstrated the significant promise of using these techniques for materials science studies going forward,” Kairys said.

This work was funded by the DOE Office of Science Early Career Research Program. Access to the D-Wave 2000Q system was provided through the Quantum Computing User Program managed by the Oak Ridge Leadership Computing Facility, a DOE Office of Science user facility located at ORNL. Research performed at ORNL’s Spallation Neutron Source, also a DOE Office of Science user facility located at ORNL, was supported by the DOE Office of Science.

Featured image: The researchers embedded a programmable model into a D-Wave quantum computer chip. Credit: D-Wave


Reference: Paul Kairys, Andrew D. King, Isil Ozfidan, Kelly Boothby, Jack Raymond, Arnab Banerjee, and Travis S. Humble, “Simulating the Shastry-Sutherland Ising Model Using Quantum Annealing”, PRX Quantum 1, 020320 – Published 14 December 2020. https://journals.aps.org/prxquantum/abstract/10.1103/PRXQuantum.1.020320


Provided by Oak Ridge National Laboratory

Light-induced Twisting of Weyl Nodes Switches on Giant Electron Current (Material Science)

Scientists at the U.S. Department of Energy’s Ames Laboratory and collaborators at Brookhaven National Laboratory and the University of Alabama at Birmingham have discovered a new light-induced switch that twists the crystal lattice of the material, switching on a giant electron current that appears to be nearly dissipationless. The discovery was made in a category of topological materials that holds great promise for spintronics, topological effect transistors, and quantum computing.

Schematic of light-induced formation of Weyl points in a Dirac material of ZrTe5. Jigang Wang and collaborators report how coherently twisted lattice motion by laser pulses, i.e., a phononic switch, can control the crystal inversion symmetry and photogenerate giant low dissipation current with an exceptional ballistic transport protected by induced Weyl band topology. © AMES Laboratory

Weyl and Dirac semimetals can host exotic, nearly dissipationless, electron conduction properties that take advantage of the unique state in the crystal lattice and electronic structure of the material that protects the electrons from doing so. These anomalous electron transport channels, protected by symmetry and topology, don’t normally occur in conventional metals such as copper. After decades of being described only in the context of theoretical physics, there is growing interest in fabricating, exploring, refining, and controlling their topologically protected electronic properties for device applications. For example, wide-scale adoption of quantum computing requires building devices in which fragile quantum states are protected from impurities and noisy environments. One approach to achieve this is through the development of topological quantum computation, in which qubits are based on “symmetry-protected” dissipationless electric currents that are immune to noise.

“Light-induced lattice twisting, or a phononic switch, can control the crystal inversion symmetry and photogenerate giant electric current with very small resistance,” said Jigang Wang, senior scientist at Ames Laboratory and professor of physics at Iowa State University. “This new control principle does not require static electric or magnetic fields, and has much faster speeds and lower energy cost.”

“This finding could be extended to a new quantum computing principle based on the chiral physics and dissipationless energy transport, which may run much faster speeds, lower energy cost and high operation temperature.” said Liang Luo, a scientist at Ames Laboratory and first author of the paper. 

Wang, Luo, and their colleagues accomplished just that, using terahertz (one trillion cycles per second) laser light spectroscopy to examine and nudge these materials into revealing the symmetry switching mechanisms of their properties.

In this experiment, the team altered the symmetry of the electronic structure of the material, using laser pulses to twist the lattice arrangement of the crystal. This light switch enables “Weyl points” in the material, causing electrons to behave as massless particles that can carry the protected, low dissipation current that is sought after.

“We achieved this giant dissipationless current by driving periodic motions of atoms around their equilibrium position in order to break crystal inversion symmetry,” says Ilias Perakis, professor of physics and chair at the University of Alabama at Birmingham. “This light-induced Weyl semimetal transport and topology control principle appears to be universal and will be very useful in the development of future quantum computing and electronics with high speed and low energy consumption.”

“What we’ve lacked until now is a low energy and fast switch to induce and control symmetry of these materials,” said Qiang Li, Group leader of the Brookhaven National Laboratory’s Advanced Energy Materials Group. “Our discovery of a light symmetry switch opens a fascinating opportunity to carry dissipationless electron current, a topologically protected state that doesn’t weaken or slow down when it bumps into imperfections and impurities in the material.”

The research is further discussed in the paper “A Light-induced Phononic Symmetry Switch and Giant Dissipationless Topological Photocurrent in ZrTe5,” authored by L. Luo, D. Cheng, B. Song, L.-L. Wang, C. Vaswani, P. M. Lozano, G. Gu, C. Huang, R. H. J. Kim, Z. Liu, J.-M. Park, Y. Yao, K.-M. Ho, I. E. Perakis, Q. Li and J. Wang; and published in Nature Materials.

Terahertz photocurrent and laser spectroscopy experiments and model building were performed at Ames Laboratory. Sample development and magneto-transport measurements were conducted by Brookhaven National Laboratory. Data analysis was conducted by the University of Alabama at Birmingham. First-principles calculations and topological analysis were conducted by the Center for the Advancement of Topological Semimetals, an Energy Frontier Research Center funded by the DOE Office of Science.

Reference: Luo, L., Cheng, D., Song, B. et al. A light-induced phononic symmetry switch and giant dissipationless topological photocurrent in ZrTe5. Nat. Mater. (2021). https://doi.org/10.1038/s41563-020-00882-4

Provided by AMES Laboratory

About AMES Laboratory

Ames Laboratory is a U.S. Department of Energy Office of Science National Laboratory operated by Iowa State University. Ames Laboratory creates innovative materials, technologies and energy solutions. We use our expertise, unique capabilities and interdisciplinary collaborations to solve global problems.

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Ultra-thin Designer Materials Unlock Quantum Phenomena (Material Science / Quantum)

New research, published in Nature, has measured highly sought-after Majorana quantum states.

A team of theoretical and experimental physicists have designed a new ultra-thin material that they have used to create elusive quantum states. Called one-dimensional Majorana zero energy modes, these quantum states could have a huge impact for quantum computing.

Majorana Zero Energy Modes are found at the edge of 2D topological superconductors © Alex Tokarev, Ella Maru Studio Aalto University

At the core of a quantum computer is a qubit, which is used to make high-speed calculations. The qubits that Google, for example, in its Sycamore processor unveiled last year, and others are currently using are very sensitive to noise and interference from the computer’s surroundings, which introduces errors into the calculations. A new type of qubit, called a topological qubit, could solve this issue, and 1D Majorana zero energy modes may be the key to making them.

‘A topological quantum computer is based on topological qubits, which are supposed to be much more noise tolerant than other qubits. However, topological qubits have not been produced in the lab yet,’ explains Professor Peter Liljeroth, the lead researcher on the project.

What are MZMs?

MZMs are groups of electrons bound together in a specific way so they behave like a particle called a Majorana fermion, a semi-mythical particle first proposed by semi-mythical physicist Ettore Majorana in the 1930s. If Majorana’s theoretical particles could be bound together, they would work as a topological qubit. One catch: no evidence for their existence has ever been seen, either in the lab or in astronomy. Instead of attempting to make a particle that no one has ever seen anywhere in the universe, researchers instead try to make regular electrons behave like them.

To make MZMs, researchers need incredibly small materials, an area in which Professor Liljeroth’s group at Aalto University specialises. MZMs are formed by giving a group of electrons a very specific amount of energy, and then trapping them together so they can’t escape. To achieve this, the materials need to be 2-dimensional, and as thin as physically possible. To create 1D MZMs, the team needed to make an entirely new type of 2D material: a topological superconductor.

Topological superconductivity is the property that occurs at the boundary of a magnetic electrical insulator and a superconductor. To create 1D MZMs, Professor Liljeroth’s team needed to be able to trap electrons together in a topological superconductor, however it’s not as simple as sticking any magnet to any superconductor.

‘If you put most magnets on top of a superconductor, you stop it from being a superconductor,’ explains Dr. Shawulienu Kezilebieke, the first author of the study. ‘The interactions between the materials disrupt their properties, but to make MZMs, you need the materials to interact just a little bit. The trick is to use 2D materials: they interact with each other just enough to make the properties you need for MZMs, but not so much that they disrupt each other.’

The property in question is the spin. In a magnetic material, the spin is aligned all in the same direction, whereas in a superconductor the spin is anti-aligned with alternating directions. Bringing a magnet and a superconductor together usually destroys the alignment and anti-alignment of the spins. However, in 2D layered materials the interactions between the materials are just enough to “tilt” the spins of the atoms enough that they create the specific spin state, called Rashba spin-orbit coupling, needed to make the MZMs.

Finding the MZMs

The topological superconductor in this study is made of a layer of chromium bromide, a material which is still magnetic when only one-atom-thick. Professor Liljeroth’s team grew one-atom-thick islands of chromium bromide on top of a superconducting crystal of niobium diselenide, and measured their electrical properties using a scanning tunneling microscope. At this point, they turned to the computer modelling expertise of Professor Adam Foster at Aalto University and Professor Teemu Ojanen, now at Tampere University, to understand what they had made.

‘There was a lot of simulation work needed to prove that the signal we’re seeing was caused by MZMs, and not other effects,’ says Professor Foster. ‘We needed to show that all the pieces fitted together to prove that we had produced MZMs.’

Now the team is sure that they can make 1D MZMs in 2-dimensional materials, the next step will be to attempt to make them into topological qubits. This step has so far eluded teams who have already made 0-dimensional MZMs, and the Aalto team are unwilling to speculate on if the process will be any easier with 1-dimensional MZMs, however they are optimistic about the future of 1D MZMs.

‘The cool part of this paper is that we’ve made MZMs in 2D materials,’ said Professor Liljeroth ‘In principle these are easier to make and easier to customise the properties of, and ultimately make into a usable device.’

References: Kezilebieke, S., Huda, M.N., Vaňo, V. et al. Topological superconductivity in a van der Waals heterostructure. Nature 588, 424–428 (2020). https://www.nature.com/articles/s41586-020-2989-y https://doi.org/10.1038/s41586-020-2989-y

Provided by AALTO University