QT/ Researchers show an old law still holds for quirky quantum materials
Quantum news biweekly vol.64, 24th November — 8th December
TL;DR
- In 1853, two scientists showed that the two admirable properties of metals were somehow related: At any given temperature, the ratio of electronic conductivity to thermal conductivity was roughly the same in any metal they tested. This so-called Wiedemann-Franz law has held ever since — except in quantum materials. Now, a theoretical argument put forth by physicists suggests that the law should, in fact, approximately hold for one type of quantum material, the cuprate superconductors.
- Researchers discover a new type of ultrafast magnetic switching by investigating fluctuations that normally tend to interfere with experiments as noise.
- Scientists have demonstrated a compact particle accelerator less than 20 meters long that produces an electron beam with an energy of 10 billion electron volts (10 GeV). There are only two other accelerators currently operating in the U.S. that can reach such high electron energies, but both are approximately 3 kilometers long. This type of accelerator is called a wakefield laser accelerator.
- A new kind of ‘wire’ for moving excitons could help enable a new class of devices, perhaps including room temperature quantum computers.
- Hopfions, magnetic spin structures predicted decades ago, have become a hot and challenging research topic in recent years. New findings open up new fields in experimental physics: identifying other crystals in which hopfions are stable, studying how hopfions interact with electric and spin currents, hopfion dynamics, and more.
- Experiments have provided the first direct evidence that electricity seems to flow through ‘strange metals’ in an unusual liquid-like form.
- Wondering whether whether Dark Matter particles actually are produced inside a jet of standard model particles, led researchers to explore a new detector signature known as semi-visible jets, which scientists never looked at before.
- Quantum physicists show that imperfect timekeeping places a fundamental limit to quantum computers and their applications. The team claims that even tiny timing errors add up to place a significant impact on any large-scale algorithm, posing another problem that must eventually be solved if quantum computers are to fulfill the lofty aspirations that society has for them.
- Researchers report a significant advance in quantum computing. They have prolonged the coherence time of their single-electron qubit to an impressive 0.1 milliseconds, nearly a thousand-fold improvement
- In physics, quasiparticles are used to describe complex processes in solids. In ultracold quantum gases, these quasiparticles can be reproduced and studied. Now scientists have been able to observe in experiments how Fermi polarons — a special type of quasiparticle — can interact with each other.
- 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
The Wiedemann-Franz law in doped Mott insulators without quasiparticles
by Wen O. Wang, Jixun K. Ding, Yoni Schattner, Edwin W. Huang, Brian Moritz, Thomas P. Devereaux in Science
Long before researchers discovered the electron and its role in generating electrical current, they knew about electricity and were exploring its potential. One thing they learned early on was that metals were great conductors of both electricity and heat.
And in 1853, two scientists showed that those two admirable properties of metals were somehow related: At any given temperature, the ratio of electronic conductivity to thermal conductivity was roughly the same in any metal they tested. This so-called Wiedemann-Franz law has held ever since — except in quantum materials, where electrons stop behaving as individual particles and glom together into a sort of electron soup. Experimental measurements have indicated that the 170-year-old law breaks down in these quantum materials, and by quite a bit.
Now, a theoretical argument put forth by physicists at the Department of Energy’s SLAC National Accelerator Laboratory, Stanford University and the University of Illinois suggests that the law should, in fact, approximately hold for one type of quantum material — the copper oxide superconductors, or cuprates, which conduct electricity with no loss at relatively high temperatures.
In a paper, they propose that the Wiedemann-Franz law should still roughly hold if one considers only the electrons in cuprates. They suggest that other factors, such as vibrations in the material’s atomic latticework, must account for experimental results that make it look like the law does not apply. This surprising result is important to understanding unconventional superconductors and other quantum materials, said Wen Wang, lead author of the paper and a PhD student with the Stanford Institute for Materials and Energy Sciences (SIMES) at SLAC.
“The original law was developed for materials where electrons interact with each other weakly and behave like little balls that bounce off defects in the material’s lattice,” Wang said. “We wanted to test the law theoretically in systems where neither of these things was true.”
Superconducting materials, which carry electric current without resistance, were discovered in 1911. But they operated at such extremely low temperatures that their usefulness was quite limited. That changed in 1986, when the first family of so-called high-temperature or unconventional superconductors — the cuprates — was discovered. Although cuprates still require extremely cold conditions to work their magic, their discovery raised hopes that superconductors could someday work at much closer to room temperature — making revolutionary technologies like no-loss power lines possible.
After nearly four decades of research, that goal is still elusive, although a lot of progress has been made in understanding the conditions in which superconducting states flip in and out of existence. Theoretical studies, performed with the help of powerful supercomputers, have been essential for interpreting the results of experiments on these materials and for understanding and predicting phenomena that are out of experimental reach.
For this study, the SIMES team ran simulations based on what’s known as the Hubbard model, which has become an essential tool for simulating and describing systems where electrons stop acting independently and join forces to produce unexpected phenomena. The results show that when you only take electron transport into account, the ratio of electronic conductivity to thermal conductivity approaches what the Wiedemann-Franz law predicts, Wang said. “So, the discrepancies that have been seen in experiments should be coming from other things like phonons, or lattice vibrations, that are not in the Hubbard model,” she said.
SIMES staff scientist and paper co-author Brian Moritz said that although the study did not investigate how vibrations cause the discrepancies, “somehow the system still knows that there is this correspondence between charge and heat transport amongst the electrons. That was the most surprising result.” From here, he added, “maybe we can peel the onion to understand a little bit more.”
Discovery of ultrafast spontaneous spin switching in an antiferromagnet by femtosecond noise correlation spectroscopy
by M. A. Weiss, A. Herbst, J. Schlegel, T. Dannegger, M. Evers, A. Donges, M. Nakajima, A. Leitenstorfer, S. T. B. Goennenwein, U. Nowak, T. Kurihara in Nature Communications
Noise on the radio when reception is poor is a typical example of how fluctuations mask a physical signal. In fact, such interference or noise occurs in every physical measurement in addition to the actual signal. “Even in the loneliest place in the universe, where there should be nothing at all, there are still fluctuations of the electromagnetic field,” says physicist Ulrich Nowak. In the Collaborative Research Centre (CRC) 1432 “Fluctuations and Nonlinearities in Classical and Quantum Matter beyond Equilibrium” at the University of Konstanz, researchers do not see this omnipresent noise as a disturbing factor that needs to be eliminated as far as possible, but as a source of information that tells us something about the signal.
This approach has now proved successful when investigating antiferromagnets. Antiferromagnets are magnetic materials in which the magnetizations of several sub-lattices cancel out each other. Nevertheless, antiferromagnetic insulators are considered promising for energy-efficient components in the field of information technology. As they have hardly any magnetic fields on the outside, they are very difficult to characterize physically. Yet, antiferromagnets are surrounded by magnetic fluctuations, which can tell us a lot about this weakly magnetic material.
In this spirit, the groups of the two materials scientists Ulrich Nowak and Sebastian Gönnenwein analysed the fluctuations of antiferromagnetic materials in the context of the CRC. The decisive factor in their theoretical as well as experimental study was the specific frequency range. “We measure very fast fluctuations and have developed a method with which fluctuations can still be detected on the ultrashort time scale of femtoseconds,” says experimental physicist Sebastian Gönnenwein. A femtosecond is one millionth of a billionth of a second.
On slower time scales, one could use electronics that are fast enough to measure these fluctuations. On ultrafast time scales, this no longer works, which is why a new experimental approach had to be developed. It is based on an idea from the research group of Alfred Leitenstorfer, who is also a member of the Collaborative Research Centre. Employing laser technology, the researchers use pulse sequences or pulse pairs in order to obtain information about fluctuations. Initially, this measurement approach was developed to investigate quantum fluctuations, and has now been extended to fluctuations in magnetic systems. Takayuki Kurihara from the University of Tokyo played a key role in this development as the third cooperation partner. He was a member of the Leitenstorfer research group and the Zukunftskolleg at the University of Konstanz from 2018 to 2020.
In the experiment, two ultrashort light pulses are transmitted through the magnet with a time delay, testing the magnetic properties during the transit time of each pulse, respectively. The light pulses are then checked for similarity using sophisticated electronics. The first pulse serves as a reference, the second contains information about how much the antiferromagnet has changed in the time between the first and second pulse. Different measurement results at the two points of time confirm the fluctuations. Ulrich Nowak’s research group also modelled the experiment in elaborate computer simulations in order to better understand its results.
One unexpected result was the discovery of what is known as telegraph noise on ultrashort time scales. This means that there is not only unsorted noise, but also fluctuations in which the system switches back and forth between two well-defined states.Such fast, purely random switching has never been observed before and could be interesting for applications such as random number generators. In any case, the new methodological possibilities for analyzing fluctuations on ultrashort time scales offer great potential for further discoveries in the field of functional materials.
The acceleration of a high-charge electron bunch to 10 GeV in a 10-cm nanoparticle-assisted wakefield accelerator
by Constantin Aniculaesei, Thanh Ha, Samuel Yoffe, Lance Labun, in Matter and Radiation at Extremes
Particle accelerators hold great potential for semiconductor applications, medical imaging and therapy, and research in materials, energy and medicine. But conventional accelerators require plenty of elbow room — kilometers — making them expensive and limiting their presence to a handful of national labs and universities.
Researchers from The University of Texas at Austin, several national laboratories, European universities and the Texas-based company TAU Systems Inc. have demonstrated a compact particle accelerator less than 20 meters long that produces an electron beam with an energy of 10 billion electron volts (10 GeV). There are only two other accelerators currently operating in the U.S. that can reach such high electron energies, but both are approximately 3 kilometers long.
“We can now reach those energies in 10 centimeters,” said Bjorn “Manuel” Hegelich, associate professor of physics at UT and CEO of TAU Systems, referring to the size of the chamber where the beam was produced. He is the senior author on a recent paper describing their achievement.
Hegelich and his team are currently exploring the use of their accelerator, called an advanced wakefield laser accelerator, for a variety of purposes. They hope to use it to test how well space-bound electronics can withstand radiation, to image the 3D internal structures of new semiconductor chip designs, and even to develop novel cancer therapies and advanced medical-imaging techniques.
This kind of accelerator could also be used to drive another device called an X-ray free electron laser, which could take slow-motion movies of processes on the atomic or molecular scale. Examples of such processes include drug interactions with cells, changes inside batteries that might cause them to catch fire, chemical reactions inside solar panels, and viral proteins changing shape when infecting cells.
The concept for wakefield laser accelerators was first described in 1979. An extremely powerful laser strikes helium gas, heats it into a plasma and creates waves that kick electrons from the gas out in a high-energy electron beam. During the past couple of decades, various research groups have developed more powerful versions. Hegelich and his team’s key advance relies on nanoparticles. An auxiliary laser strikes a metal plate inside the gas cell, which injects a stream of metal nanoparticles that boost the energy delivered to electrons from the waves. The laser is like a boat skimming across a lake, leaving behind a wake, and electrons ride this plasma wave like surfers.
“It’s hard to get into a big wave without getting overpowered, so wake surfers get dragged in by Jet Skis,” Hegelich said. “In our accelerator, the equivalent of Jet Skis are nanoparticles that release electrons at just the right point and just the right time, so they are all sitting there in the wave. We get a lot more electrons into the wave when and where we want them to be, rather than statistically distributed over the whole interaction, and that’s our secret sauce.”
For this experiment, the researchers used one of the world’s most powerful pulsed lasers, the Texas Petawatt Laser, which is housed at UT and fires one ultra-intense pulse of light every hour. A single petawatt laser pulse contains about 1,000 times the installed electrical power in the U.S. but lasts only 150 femtoseconds, less than a billionth as long as a lightning discharge. The team’s long-term goal is to drive their system with a laser they’re currently developing that fits on a tabletop and can fire repeatedly at thousands of times per second, making the whole accelerator far more compact and usable in much wider settings than conventional accelerators.
Enhanced Exciton Drift Transport through Suppressed Diffusion in One-Dimensional Guides
by Zidong Li, Matthias Florian, Kanak Datta, Zhaohan Jiang, Markus Borsch, Qiannan Wen, Mack Kira, Parag B. Deotare in ACS Nano
A new kind of “wire” for moving excitons, developed at the University of Michigan, could help enable a new class of devices, perhaps including room temperature quantum computers.
What’s more, the team observed a dramatic violation of Einstein’s relation, used to describe how particles spread out in space, and leveraged it to move excitons in much smaller packages than previously possible.
“Nature uses excitons in photosynthesis. We use excitons in OLED displays and some LEDs and solar cells,” said Parag Deotare, co-corresponding author of the study supervising the experimental work, and an associate professor of electrical and computer engineering. “The ability to move excitons where we want will help us improve the efficiency of devices that already use excitons and expand excitonics into computing.”
An exciton can be thought of as a particle (hence quasiparticle), but it’s really an electron linked with a positively-charged empty space in the lattice of the material (a “hole”). Because an exciton has no net electrical charge, moving excitons are not affected by parasitic capacitances, an electrical interaction between neighboring components in a device that causes energy losses. Excitons are also easy to convert to and from light, so they open the way for extremely fast and efficient computers that use a combination of optics and excitonics, rather than electronics.
This combination could help enable room temperature quantum computing, said Mackillo Kira, co-corresponding author of the study supervising the theory, and a professor of electrical and computer engineering. Excitons can encode quantum information, and they can hang onto it longer than electrons can inside a semiconductor. But that time is still measured in picoseconds (10–12 seconds) at best, so Kira and others are figuring out how to use femtosecond laser pulses (10–15 seconds) to process information.
“Full quantum-information applications remain challenging because degradation of quantum information is too fast for ordinary electronics,” he said. “We are currently exploring lightwave electronics as a means to supercharge excitonics with extremely fast processing capabilities.”
However, the lack of net charge also makes excitons very difficult to move. Previously, Deotare had led a study that pushed excitons through semiconductors with acoustic waves. Now, a pyramid structure enables more precise transport for smaller numbers of excitons, confined to one dimension like a wire.
The team used a laser to create a cloud of excitons at a corner of the pyramid’s base, bouncing electrons out of the valence band of a semiconductor into the conduction band — but the negatively charged electrons are still attracted to the positively charged holes left behind in the valence band. The semiconductor is a single layer of tungsten diselenide semiconductor, just three atoms thick, draped over the pyramid like a stretchy cloth. And the stretch in the semiconductor changes the energy landscape that the excitons experience.
It seems counterintuitive that the excitons should ride up the pyramid’s edge and settle at the peak when we imagine an energy landscape chiefly governed by gravity. But instead, the landscape is governed by how far apart the valence and conduction bands of the semiconductor are. The energy gap between the two, also known as the semiconductor’s band gap, shrinks where the semiconductor is stretched. The excitons migrate to the lowest energy state, funneled onto the pyramid’s edge where they then rise to its peak.
Usually, an equation penned by Einstein is good at describing how a bunch of particles diffuses outward and drifts. However, the semiconductor was imperfect, and those defects acted as traps that would nab some of the excitons as they tried to drift by. Because the defects at the trailing side of the exciton cloud were filled in, that side of the distribution diffused outward as predicted. The leading edge, however, did not extend so far. Einstein’s relation was off by more than a factor of 10.
“We’re not saying Einstein was wrong, but we have shown that in complicated cases like this, we shouldn’t be using his relation to predict the mobility of excitons from the diffusion,” said Matthias Florian, co-first-author of the study and a research investigator in electrical and computer engineering, working under Kira.
To directly measure both, the team needed to detect single photons, emitted when the bound electrons and holes spontaneously recombined. Using time-of-flight measurements, they also figured out where the photons came from precisely enough to measure the distribution of excitons within the cloud.
Hopfion rings in a cubic chiral magnet
by Fengshan Zheng, Nikolai S. Kiselev, Filipp N. Rybakov, Luyan Yang, Wen Shi, Stefan Blügel, Rafal E. Dunin-Borkowski in Nature
Hopfions, magnetic spin structures predicted decades ago, have become a hot and challenging research topic in recent years. In a study, the first experimental evidence is presented by a Swedish-German-Chinese research collaboration.
“Our results are important from both a fundamental and applied point of view, as a new bridge has emerged between experimental physics and abstract mathematical theory, potentially leading to hopfions finding an application in spintronics,” says Philipp Rybakov, researcher at the Department of Physics and Astronomy at Uppsala University, Sweden.
A deeper understanding of how different components of materials function is important for the development of innovative materials and future technology. The research field of spintronics, for example, which studies the spin of electrons, has opened up promising possibilities to combine the electrons electricity and magnetism for applications such as new electronics, etc.
Magnetic skyrmions and hopfions are topological structures — well-localized field configurations that have been a hot research topic over the past decade owing to their unique particle-like properties, which make them promising objects for spintronic applications. Skyrmions are two-dimensional, resembling vortex-like strings, while hopfions are three-dimensional structures within a magnetic sample volume resembling closed, twisted skyrmion strings in the shape of a donut-shaped ring in the simplest case.
Despite extensive research in recent years, direct observation of magnetic hopfions has only been reported in synthetic material. This current work is the first experimental evidence of such states stabilised in a crystal of B20-type FeGe plates using transmission electron microscopy and holography. The results are highly reproducible and in full agreement with micromagnetic simulations. The researchers provide a unified skyrmion-hopfion homotopy classification and offer an insight into the diversity of topological solitons in three-dimensional chiral magnets.
The findings open up new fields in experimental physics: identifying other crystals in which hopfions are stable, studying how hopfions interact with electric and spin currents, hopfion dynamics, and more.
“Since the object is new and many of its interesting properties remain to be discovered, it is difficult to make predictions about specific spintronic applications. However, we can speculate that hopfions may be of greatest interest when upgrading to the third dimension of almost any technology being developed with magnetic skyrmions: racetrack memory, neuromorphic computing, and qubits (basic unit of quantum information). Compared to skyrmions, hopfions have an additional degree of freedom due to three-dimensionality and thus can move in three rather than two dimensions,” explains Rybakov.
Shot noise in a strange metal
by Liyang Chen, Dale T. Lowder, Emine Bakali, Aaron Maxwell Andrews, Werner Schrenk, Monika Waas, Robert Svagera, Gaku Eguchi, Lukas Prochaska, Yiming Wang, Chandan Setty, Shouvik Sur, Qimiao Si, Silke Paschen, Douglas Natelson in Science
True to form, a “strange metal” quantum material proved strangely quiet in recent quantum noise experiments at Rice University. The measurements of quantum charge fluctuations known as “shot noise” provide the first direct evidence that electricity seems to flow through strange metals in an unusual liquidlike form that cannot be readily explained in terms of quantized packets of charge known as quasiparticles.
“The noise is greatly suppressed compared to ordinary wires,” said Rice’s Douglas Natelson, the study’s corresponding author. “Maybe this is evidence that quasiparticles are not well-defined things or that they’re just not there and charge moves in more complicated ways. We have to find the right vocabulary to talk about how charge can move collectively.”
The experiments were performed on nanoscale wires of a quantum critical material with a precise 1–2–2 ratio of ytterbium, rhodium and silicon (YbRh2Si2), which has been studied in great depth during the past two decades by Silke Paschen, a solid-state physicist at the Vienna University of Technology (TU Wien). The material contains a high degree of quantum entanglement that produces a very unusual (“strange”) temperature-dependent behavior that is very different from the one in normal metals such as silver or gold.
In such normal metals, each quasiparticle, or discrete unit, of charge is the product of incalculable tiny interactions between countless electrons. First put forward 67 years ago, the quasiparticle is a concept physicists use to represent the combined effect of those interactions as a single quantum object for the purposes of quantum mechanical calculations.
Some prior theoretical studies have suggested that the charge in a strange metal might not be carried by such quasiparticles, and shot noise experiments allowed Natelson, study lead author Liyang Chen, a former student in Natelson’s lab, and other Rice and TU Wien co-authors to gather the first direct empirical evidence to test the idea.
“The shot noise measurement is basically a way of seeing how granular the charge is as it goes through something,” Natelson said. “The idea is that if I’m driving a current, it consists of a bunch of discrete charge carriers. Those arrive at an average rate, but sometimes they happen to be closer together in time, and sometimes they’re farther apart.”
Applying the technique in YbRh2Si2 crystals presented significant technical challenges. Shot noise experiments cannot be performed on single macroscopic crystals but, rather, require samples of nanoscopic dimensions. Thus, the growth of extremely thin but nevertheless perfectly crystalline films had to be achieved, something that Paschen, Maxwell Andrews and their collaborators at TU Wien managed after almost a decade of hard work. Next, Chen had to find a way to maintain that level of perfection while fashioning wires from these thin films that were about 5,000 times narrower than a human hair.
Rice co-author Qimiao Si, the lead theorist on the study and the Harry C. and Olga K. Wiess Professor of Physics and Astronomy, said he, Natelson and Paschen first discussed the idea for the experiments while Paschen was a visiting scholar at Rice in 2016. Si said the results are consistent with a theory of quantum criticality he published in 2001 that he has continued to explore in a nearly two-decade collaboration with Paschen.
“The low shot noise brought about fresh new insights into how the charge-current carriers entwine with the other agents of the quantum criticality that underlies the strange metallicity,” said Si, whose group performed calculations that ruled out the quasiparticle picture. “In this theory of quantum criticality, the electrons are pushed to the verge of localization, and the quasiparticles are lost everywhere on the Fermi surface.”
Natelson said the larger question is whether similar behavior might arise in any or all of the dozens of other compounds that exhibit strange metal behavior.
“Sometimes you kind of feel like nature is telling you something,” Natelson said. “This ‘strange metallicity’ shows up in many different physical systems, despite the fact that the microscopic, underlying physics is very different. In copper-oxide superconductors, for example, the microscopic physics is very, very different than in the heavy-fermion system we’re looking at. They all seem to have this linear-in-temperature resistivity that’s characteristic of strange metals, and you have to wonder is there something generic going on that is independent of whatever the microscopic building blocks are inside them.”
Search for non-resonant production of semi-visible jets using Run 2 data in ATLAS
by G. Aad et al. in Physics Letters B
The existence of Dark Matter is a long-standing puzzle in our universe. Dark Matter makes up about a quarter of our universe, yet it does not interact significantly with ordinary matter. The existence of Dark Matter has been confirmed by a series of astrophysical and cosmological observations, including in the stunning recent pictures from James Webb Space Telescope. However, up to date, no experimental observation of dark matter has been reported. The existence of Dark Matter has been a question that high energy and astrophysicists around the world has been investigating for decades.
“This is the reason we do research in basic science, probing the deepest mysteries of the universe. The Large Hadron Collider at CERN is the largest experiment ever built, and particle collisions creating big-bang like condition can be exploited to look for hints of dark matter,” says Professor Deepak Kar, from the School of Physics at the University of the Witwatersrand in Johannesburg, South Africa.
Working at the ATLAS experiment at CERN, Kar and his former PhD student, Sukanya Sinha (now a postdoctoral researcher at the University of Manchester), has pioneered a new way of searching for Dark Matter.
“There have been plethora of collider searches for Dark Matter over the past few decades so far have focused on weakly interacting massive particles, termed WIMPs,” says Kar. “WIMPS is one class of particles that are hypothesised to explain Dark Matter as they do not absorb or emit light and don’t interact strongly with other particles. However, as no evidence of WIMPS’ has been found so far, we realised that the search for Dark Matter needed a paradigm shift.”
“What we were wondering, was whether Dark Matter particles actually are produced inside a jet of standard model particles,” said Kar. This led to the exploration of a new detector signature known as semi-visible jets, which scientists never looked at before.
High energy collisions of protons often result in production of collimated spray of particles, collected in what is termed as jets, from decay of ordinary quarks or gluons. Semi-visible jets would arise when hypothetical dark quarks decay partially to Standard-Model quarks (known particles) and partially to stable dark hadrons (the “invisible fraction”). Since they are produced in pairs, typically along with additional Standard-Model jets, the imbalance of energy or the missing energy in the detector arises when all the jets are not fully balanced. The direction of the missing energy is often aligned with one of the semi-visible jets.
This makes searches for semi-visible jets very challenging, as this event signature can also arise due to mis-measured jets in the detector. Kar and Sinha’s new way of looking for Dark Matter opens up new directions into looking for the existence of Dark Matter.
“Even though my PhD thesis does not contain a discovery of Dark Matter, it sets the first and rather stringent upper bounds on this production mode, and already inspiring further studies,” says Sinha.
Impact of Imperfect Timekeeping on Quantum Control
by Jake Xuereb, Paul Erker, Florian Meier, Mark T. Mitchison, Marcus Huber in Physical Review Letters
New research from a consortium of quantum physicists, led by Trinity College Dublin’s Dr Mark Mitchison, shows that imperfect timekeeping places a fundamental limit to quantum computers and their applications. The team claims that even tiny timing errors add up to place a significant impact on any large-scale algorithm, posing another problem that must eventually be solved if quantum computers are to fulfil the lofty aspirations that society has for them.
It is difficult to imagine modern life without clocks to help organise our daily schedules; with a digital clock in every person’s smartphone or watch, we take precise timekeeping for granted — although that doesn’t stop people from being late!
And for quantum computers, precise timing is even more essential, as they exploit the bizarre behaviour of tiny particles — such as atoms, electrons, and photons — to process information. While this technology is still at an early stage, it promises to dramatically speed up the solution of important problems, like the discovery of new pharmaceuticals or materials. This potential has driven significant investment across the private and public sector, such as the establishment of the Trinity Quantum Alliance academic-industrial partnership launched earlier this year.
Currently, however, quantum computers are still too small to be useful. A major challenge to scaling them up is the extreme fragility of the quantum states that are used to encode information. In the macroscopic world, this is not a problem. For example, you can add numbers perfectly using an abacus, in which wooden beads are pushed back and forth to represent arithmetic operations. The wooden beads have very stable states: each one sits in a specific place and it will stay in place unless intentionally moved. Importantly, whether you move the bead quickly or slowly does not affect the result. But in quantum physics, it is more complicated.
“Mathematically speaking, changing a quantum state in a quantum computer corresponds to a rotation in an abstract high-dimensional space,” says Jake Xuereb from the Atomic Institute at the Vienna University of Technology, the first author of the paper. “In order to achieve the desired state in the end, the rotation must be applied for a very specific period of time — otherwise you turn the state either too little or too far.”
Given that real clocks are never perfect, the team investigated the impact of imperfect timing on quantum algorithms.
“A quantum algorithm is like an app that runs on a quantum computer,” explains Trinity’s Dr Mitchison. “It was already known that timing errors could disrupt individual quantum logic gates, which are the building blocks of quantum algorithms. Our work extends this to full quantum algorithms, showing exactly how precise the clock must be to achieve a given computational accuracy.”
Since the error gets worse for more complex algorithms, it will ultimately pose a challenge for quantum computers.
“It’s not a problem at the moment,” clarifies Prof. Marcus Huber who leads the research team in Vienna. “Currently, the accuracy of quantum computers is still limited by other factors, for example the precision of the hardware components or the effect of stray electromagnetic fields. But our calculations also show that today we are not far from the regime in which the fundamental limits of time measurement will play the decisive role.”
The team is quick to emphasise that the message is not entirely pessimistic, because the problem could be mitigated in the future by designing clever error correction protocols.
Electron charge qubit with 0.1 millisecond coherence time
by Xianjing Zhou, Xinhao Li, Qianfan Chen, Gerwin Koolstra, Ge Yang, Brennan Dizdar, Yizhong Huang, Christopher S. Wang, Xu Han, Xufeng Zhang, David I. Schuster, Dafei Jin in Nature Physics
Coherence stands as a pillar of effective communication, whether it is in writing, speaking or information processing. This principle extends to quantum bits, or qubits, the building blocks of quantum computing. A quantum computer could one day tackle previously insurmountable challenges in climate prediction, material design, drug discovery and more.
A team led by the U.S. Department of Energy’s (DOE) Argonne National Laboratory has achieved a major milestone toward future quantum computing. They have extended the coherence time for their novel type of qubit to an impressive 0.1 milliseconds — nearly a thousand times better than the previous record.
In everyday life, 0.1 milliseconds is as fleeting as a blink of an eye. However, in the quantum world, it represents a long enough window for a qubit to perform many thousands of operations. Unlike classical bits, qubits seemingly can exist in both states, 0 and 1. For any working qubit, maintaining this mixed state for a sufficiently long coherence time is imperative. The challenge is to safeguard the qubit against the constant barrage of disruptive noise from the surrounding environment. The team’s qubits encode quantum information in the electron’s motional (charge) states. Because of that, they are called charge qubits.
“Among various existing qubits, electron charge qubits are especially attractive because of their simplicity in fabrication and operation, as well as compatibility with existing infrastructures for classical computers,” said Dafei Jin, a professor at the University of Notre Dame with a joint appointment at Argonne and the lead investigator of the project. “This simplicity should translate into low cost in building and running large-scale quantum computers.”
Jin is a former staff scientist at the Center for Nanoscale Materials (CNM), a DOE Office of Science user facility at Argonne. While there, he led the discovery of their new type of qubit, reported last year. The team’s qubit is a single electron trapped on an ultraclean solid-neon surface in a vacuum. The neon is important because it resists disturbance from the surrounding environment. Neon is one of a handful of elements that do not react with other elements. The neon platform keeps the electron qubit protected and inherently guarantees a long coherence time.
“Thanks to the small footprint of single electrons on solid neon, qubits made with them are more compact and promising for scaling up to multiple linked qubits,” said Xu Han, an assistant scientist in CNM with a joint appointment at the Pritzker School of Molecular Engineering at the University of Chicago. “These attributes, along with coherence time, make our electron qubit exceptionally compelling.”
Following continued experimental optimization, the team not only improved the quality of the neon surface but also significantly reduced disruptive signals. As reported in Nature Physics, their work paid off with a coherence time of 0.1 milliseconds. That is about a thousand-fold increase from the initial 0.1 microseconds.
“The long lifetime of our electron qubit allows us to control and read out the single qubit states with very high fidelity,” said Xinhao Li, a postdoctoral appointee at Argonne and the co-first author of the paper. This time is well above the requirements for quantum computing.
“Rather than 10 to 100 operations over the coherence times of conventional electron charge qubits, our qubits can perform 10,000 with very high precision and speed,” Jin said.
Yet another important attribute of a qubit is its scalability to link with many other qubits. The team achieved a significant milestone by showing that two-electron qubits can couple to the same superconducting circuit such that information can be transferred between them through the circuit. This marks a pivotal stride toward two-qubit entanglement, a critical aspect of quantum computing. The team has not yet fully optimized their electron qubit and will continue to work on extending the coherence time even further as well as entangling two or more qubits.
Mediated interactions between Fermi polarons and the role of impurity quantum statistics
by Cosetta Baroni, Bo Huang, Isabella Fritsche, Erich Dobler, Gregor Anich, Emil Kirilov, Rudolf Grimm, Miguel A. Bastarrachea-Magnani, Pietro Massignan, Georg M. Bruun in Nature Physics
In physics, quasiparticles are used to describe complex processes in solids. In ultracold quantum gases, these quasiparticles can be reproduced and studied. Now, for the first time, Austrian scientists led by Rudolf Grimm have been able to observe in experiments how Fermi polarons — a special type of quasiparticle — can interact with each other.
An electron moving through a solid generates a polarization in its environment due to its electric charge. In his theoretical considerations, the Russian physicist Lev Landau extended the description of such particles by their interaction with the environment and spoke of quasiparticles.
More than ten years ago, the team led by Rudolf Grimm at the Institute of Quantum Optics and Quantum Information (IQQOI) of the Austrian Academy of Sciences (ÖAW) and the Department of Experimental Physics of the University of Innsbruck succeeded in generating such quasiparticles for both attractive and repulsive interactions with the environment.
For this purpose, the scientists use an ultracold quantum gas consisting of lithium and potassium atoms in a vacuum chamber. With the help of magnetic fields, they control the interactions between the particles, and by means of radio-frequency pulses push the potassium atoms into a state in which they attract or repel the lithium atoms surrounding them. In this way, the researchers simulate a complex state similar to the one produced in the solid state by a free electron.
Now, the scientists led by Rudolf Grimm have been able to generate several such quasiparticles simultaneously in the quantum gas and observe their interactions with each other. “In a naive notion, one would assume that polarons always attract each other, regardless of whether their interaction with the environment is attractive or repulsive,” says the experimental physicist. “However, this is not the case. We see attractive interaction in bosonic polarons, repulsive interaction in fermionic polarons. Here, quantum statistics plays a crucial role.”
The researchers have now been able to demonstrate this behavior, which in principle already follows as a consequence of Landau’s theory, in an experiment for the first time. The theoretical calculations for this were done by colleagues from Mexico, Spain and Denmark.
“High experimental skills were required to implement this in the laboratory,” explains Cosetta Baroni, first author of the study, “because even the smallest deviations could have skewed the measurements.”
“Such investigations provide us with insights into very fundamental mechanisms of nature and offer us excellent opportunities to study them in detail,” says Rudolf Grimm excitedly.
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