QT/ Quantum cryptography: Hacking is futile
Quantum news biweekly vol.32, 18th July — 31th July
TL;DR
- An international team has successfully implemented an advanced form of quantum cryptography for the first time. Moreover, encryption is independent of the quantum device used and therefore even more secure against hacking attempts.
- Researchers discover that nickel oxide superconductors contain a phase of quantum matter, known as charge density waves, that’s common in other unconventional superconductors. In other ways, though, they’re surprisingly unique. Unconventional superconductors contain a mix of weird quantum states. Researchers found one of them — frozen electron ripples known as charge density waves — in a nickelate superconductor they discovered three years ago.
- Physicists are claiming significant progress in using quantum computers to study and predict how the state of a large number of interacting quantum particles evolves over time. This was done by developing a quantum algorithm that they run on an IBM quantum computer.
- Quantum clocks are shrinking, thanks to new technologies. A team of quantum physicists have devised new approaches that not only reduce the size of their clock, but also make it robust enough to be transported out of the laboratory and employed in the ‘real world’.
- Scientists have used neutron scattering to determine whether a specific material’s atomic structure could host a novel state of matter called a spiral spin liquid. By tracking tiny magnetic moments known as ‘spins’ on the honeycomb lattice of a layered iron trichloride magnet, the team found the first 2D system to host a spiral spin liquid.
- A method known as quantum key distribution has long held the promise of communication security unattainable in conventional cryptography. An international team of scientists has now demonstrated experimentally, for the first time, an approach to quantum key distribution that is based on high-quality quantum entanglement — offering much broader security guarantees than previous schemes.
- For decades computers have been synonymous with binary information — zeros and ones. Now a team has realized a quantum computer that breaks out of this paradigm and unlocks additional computational resources, hidden in almost all of today’s quantum devices.
- Physicists have demonstrated how simulations using quantum computing can enable observation of a distinctive state of matter taken out of its normal equilibrium. Such novel states of matter could one day lead to developments in fast, powerful quantum information storage and precision measurement science.
- About three years ago, a team of astronomers went looking for the universe’s missing mass, better known as dark matter, in the heart of an atom. Their expedition didn’t lead them to dark matter, but they still found something that had never been seen before, something that defied explanation. Well, at least an explanation that everyone could agree on.
- Researchers have come up with a novel way to study the thermodynamic properties of molten salts, which are used in many nuclear and solar energy applications.
- 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
A device-independent quantum key distribution system for distant users
by Zhang, W., van Leent, T., Redeker, K. et al. in Nature
The Internet is teeming with highly sensitive information. Sophisticated encryption techniques generally ensure that such content cannot be intercepted and read. But in the future high-performance quantum computers could crack these keys in a matter of seconds. It is just as well, then, that quantum mechanical techniques not only enable new, much faster algorithms, but also exceedingly effective cryptography.
Quantum key distribution (QKD) — as the jargon has it — is secure against attacks on the communication channel, but not against attacks on or manipulations of the devices themselves. The devices could therefore output a key which the manufacturer had previously saved and might conceivably have forwarded to a hacker. With device- independent QKD (abbreviated to DIQKD), it is a different story. Here, the cryptographic protocol is independent of the device used. Theoretically known since the 1990s, this method has now been experimentally realized for the first time, by an international research group led by LMU physicist Harald Weinfurter and Charles Lim from the National University of Singapore (NUS).
For exchanging quantum mechanical keys, there are different approaches available. Either light signals are sent by the transmitter to the receiver, or entangled quantum systems are used. In the present experiment, the physicists used two quantum mechanically entangled rubidium atoms, situated in two laboratories located 400 meters from each other on the LMU campus. The two locations are connected via a fiber optic cable 700 meters in length, which runs beneath Geschwister Scholl Square in front of the main building.
To create an entanglement, first the scientists excite each of the atoms with a laser pulse. After this, the atoms spontaneously fall back into their ground state, each thereby emitting a photon. Due to the conservation of angular momentum, the spin of the atom is entangled with the polarization of its emitted photon. The two light particles travel along the fiber optic cable to a receiver station, where a joint measurement of the photons indicates an entanglement of the atomic quantum memories.
To exchange a key, Alice und Bob — as the two parties are usually dubbed by cryptographers — measure the quantum states of their respective atom. In each case, this is done randomly in two or four directions. If the directions correspond, the measurement results are identical on account of entanglement and can be used to generate a secret key. With the other measurement results, a so-called Bell inequality can be evaluated. Physicist John Stewart Bell originally developed these inequalities to test whether nature can be described with hidden variables. “It turned out that it cannot,” says Weinfurter. In DIQKD, the test is used “specifically to ensure that there are no manipulations at the devices — that is to say, for example, that hidden measurement results have not been saved in the devices beforehand,” explains Weinfurter.
In contrast to earlier approaches, the implemented protocol, which was developed by researchers at NUS, uses two measurement settings for key generation instead of one: “By introducing the additional setting for key generation, it becomes more difficult to intercept information, and therefore the protocol can tolerate more noise and generate secret keys even for lower-quality entangled states,” says Charles Lim.
With conventional QKD methods, by contrast, security is guaranteed only when the quantum devices used have been characterized sufficiently well. “And so, users of such protocols have to rely on the specifications furnished by the QKD providers and trust that the device will not switch into another operating mode during the key distribution,” explains Tim van Leent, one of the four lead authors of the paper alongside Wei Zhang and Kai Redeker. It has been known for at least a decade that older QKD devices could easily be hacked from outside, continues van Leent.
“With our method, we can now generate secret keys with uncharacterized and potentially untrustworthy devices,” explains Weinfurter. In fact, he had his doubts initially whether the experiment would work. But his team proved his misgivings were unfounded and significantly improved the quality of the experiment, as he happily admits. Alongside the cooperation project between LMU and NUS, another research group from the University of Oxford demonstrated the device-independent key distribution. To do this, the researchers used a system comprising two entangled ions in the same laboratory. “These two projects lay the foundation for future quantum networks, in which absolutely secure communication is possible between far distant locations,” says Charles Lim.
One of the next goals is to expand the system to incorporate several entangled atom pairs. “This would allow many more entanglement states to be generated, which increases the data rate and ultimately the key security,” says van Leent. In addition, the researchers would like to increase the range. In the present set-up, it was limited by the loss of around half the photons in the fiber between the laboratories. In other experiments, the researchers were able to transform the wavelength of the photons into a low-loss region suitable for telecommunications. In this way, for just a little extra noise, they managed to increase the range of the quantum network connection to 33 kilometers.
A broken translational symmetry state in an infinite-layer nickelate
by Matteo Rossi, Motoki Osada, Jaewon Choi, Stefano Agrestini, et al in Nature Physics
A new study shows that nickel oxide superconductors, which conduct electricity with no loss at higher temperatures than conventional superconductors do, contain a type of quantum matter called charge density waves, or CDWs, that can accompany superconductivity.
The presence of CDWs shows that these recently discovered materials, also known as nickelates, are capable of forming correlated states — “electron soups” that can host a variety of quantum phases, including superconductivity, researchers from the Department of Energy’s SLAC National Accelerator Laboratory and Stanford University reported.
“Unlike in any other superconductor we know about, CDWs appear even before we dope the material by replacing some atoms with others to change the number of electrons that are free to move around,” said Wei-Sheng Lee, a SLAC lead scientist and investigator with the Stanford Institute for Materials and Energy Science (SIMES) who led the study.
“This makes the nickelates a very interesting new system — a new playground for studying unconventional superconductors.”
In the 35 years since the first unconventional “high-temperature” superconductors were discovered, researchers have been racing to find one that could carry electricity with no loss at close to room temperature. This would be a revolutionary development, allowing things like perfectly efficient power lines, maglev trains and a host of other futuristic, energy-saving technologies. But while a vigorous global research effort has pinned down many aspects of their nature and behavior, people still don’t know exactly how these materials become superconducting. So the discovery of nickelate’s superconducting powers by SIMES investigators three years ago was exciting because it gave scientists a fresh perspective on the problem.
Since then, SIMES researchers have explored the nickelates’ electronic structure — basically the way their electrons behave — and magnetic behavior. These studies turned up important similarities and subtle differences between nickelates and the copper oxides or cuprates — the first high-temperature superconductors ever discovered and still the world record holders for high-temperature operation at everyday pressures. Since nickel and copper sit right next to each other on the periodic table of the elements, scientists were not surprised to see a kinship there, and in fact had suspected that nickelates might make good superconductors. But it turned out to be extraordinarily difficult to construct materials with just the right characteristics.
“This is still very new,” Lee said. “People are still struggling to synthesize thin films of these materials and understand how different conditions can affect the underlying microscopic mechanisms related to superconductivity.”
CDWs are just one of the weird states of matter that jostle for prominence in superconducting materials. You can think of them as a pattern of frozen electron ripples superimposed on the material’s atomic structure, with a higher density of electrons in the peaks of the ripples and a lower density of electrons in the troughs. As researchers adjust the material’s temperature and level of doping, various states emerge and fade away. When conditions are just right, the material’s electrons lose their individual identities and form an electron soup, and quantum states such as superconductivity and CDWs can emerge.
An earlier study by the SIMES group did not find CDWs in nickelates that contain the rare-earth element neodymium. But in this latest study, the SIMES team created and examined a different nickelate material where neodymium was replaced with another rare-earth element, lanthanum.
“The emergence of CDWs can be very sensitive to things like strain or disorder in their surroundings, which can be tuned by using different rare-earth elements,” explained Matteo Rossi, who led the experiments while a postdoctoral researcher at SLAC.
The team carried out experiments at three X-ray light sources — the Diamond Light Source in the UK, the Stanford Synchrotron Radiation Lightsource at SLAC and the Advanced Light Source at DOE’s Lawrence Berkeley National Laboratory. Each of these facilities offered specialized tools for probing and understanding the material at a fundamental level. All the experiments had to be carried out remotely because of pandemic restrictions.
The experiments showed that this nickelate could host both CDWs and superconducting states of matter — and that these states were present even before the material was doped. This was surprising, because doping is usually an essential part of getting materials to superconduct. Lee said the fact that this nickelate is essentially self-doping makes it significantly different from the cuprates.
“This makes nickelates a very interesting new system for studying how these quantum phases compete or intertwine with each other,” he said. “And it means a lot of tools that are used to study other unconventional superconductors may be relevant to this one, too.”
Field deployable atomics package for an optical lattice clock
by Yogeshwar B Kale, Alok Singh, Markus Gellesch, Jonathan M Jones, David Morris, Matthew Aldous, Kai Bongs, Yeshpal Singh in Quantum Science and Technology
Quantum clocks are shrinking, thanks to new technologies developed at the University of Birmingham-led UK Quantum Technology Hub Sensors and Timing.
Working in collaboration with and partly funded by the UK’s Defence Science and Technology Laboratory (Dstl), a team of quantum physicists have devised new approaches that not only reduce the size of their clock, but also make it robust enough to be transported out of the laboratory and employed in the ‘real world’.
Quantum — or atomic — clocks are widely seen as essential for increasingly precise approaches to areas such as online communications across the world, navigation systems, or global trading in stocks, where fractions of seconds could make a huge economic difference. Atomic clocks with optical clock frequencies can be 10,000 times more accurate than their microwave counterparts, opening up the possibility of redefining the standard (SI) unit of measurement.
Even more advanced optical clocks could one day make a significant difference both in everyday life and in fundamental science. By allowing longer periods between needing to resynchronise than other kinds of clock, they offer increased resilience for national timing infrastructure and unlock future positioning and navigation applications for autonomous vehicles. The unparalleled accuracy of these clocks can also help us see beyond standard models of physics and understand some of the most mysterious aspects of the universe, including dark matter and dark energy. Such clocks will also help to address fundamental physics questions such as whether the fundamental constants are really ‘constants’ or they are varying with time.
Lead researcher, Dr Yogeshwar Kale, said: “The stability and precision of optical clocks make them crucial to many future information networks and communications. Once we have a system that is ready for use outside the laboratory, we can use them, for example, on -ground navigation networks where all such clocks are connected via optical fibre and started talking with each other. Such networks will reduce our dependence on GPS systems, which can sometimes fail.
“These transportable optical clocks not only will help to improve geodetic measurements — the fundamental properties of the Earth’s shape and gravity variations — but will also serve as precursors to monitor and identify geodynamic signals like earthquakes and volcanoes at early stages.”
Although such quantum clocks are advancing rapidly, key barriers to deploying them are their size — current models come in a van or in a car trailer and are about 1500 litres — and their sensitivity to environmental conditions limiting their transport between different places.
The Birmingham team, based within the UK Quantum Technology Hub Sensors and Timing, have come up with a solution that addresses both of these challenges in a package that is a ‘box’ of about 120 litres that weighs less than 75 kg.
A spokesperson for Dstl added: “Dstl sees optical clock technology as a key enabler of future capabilities for the Ministry of Defence. These kinds of clocks have the potential to shape the future by giving national infrastructure increased resilience and changing the way communication and sensor networks are designed. With Dstl’s support, the University of Birmingham have made significant progress in miniaturising many of the subsystems of an optical lattice clock, and in doing so overcame many significant engineering challenges. We look forward to seeing what further progress they can make in this exciting and fast-moving field.”
The clocks work by using lasers to both produce and then measure quantum oscillations in atoms. These oscillations can be measured highly accurately and, from the frequency, it is possible to also measure the time. A challenge is minimising the outside influences on the measurements, such as mechanical vibrations and electromagnetic interference. To do that, the measurements must take place within a vacuum and with minimal external interference.
At the heart of the new design is an ultra-high vacuum chamber, smaller than any yet used in the field of quantum time-keeping. This chamber can be used to trap the atoms and then cool them down very close to the ‘absolute zero’ value so they reach a state where they can be used for precision quantum sensors.
The team demonstrated that they could capture nearly 160 thousand ultra-cold atoms within the chamber in less than a second. Furthermore, they showed they could transport the system over 200 km, before setting it up to be ready to take measurements in less than 90 minutes. The system was able to survive a rise in temperature of 8 degrees above room temperature during the journey.
Dr Kale, added: “We’ve been able to show a robust and resilient system, that can be transported and set up rapidly by a single trained technician. This brings us a step closer to seeing these highly precise quantum instruments being used in challenging settings outside a laboratory environment.”
Evidence of a Near-Threshold Resonance in B11 Relevant to the β-Delayed Proton Emission of Be11
by Y. Ayyad, W. Mittig, T. Tang, B. Olaizola, G. Potel, N. Rijal, et al in Physical Review Letters
About three years ago, Wolfgang “Wolfi” Mittig and Yassid Ayyad went looking for the universe’s missing mass, better known as dark matter, in the heart of an atom.
Their expedition didn’t lead them to dark matter, but they still found something that had never been seen before, something that defied explanation. Well, at least an explanation that everyone could agree on.
“It’s been something like a detective story,” said Mittig, a Hannah Distinguished Professor in Michigan State University’s Department of Physics and Astronomy and a faculty member at the Facility for Rare Isotope Beams, or FRIB.
“We started out looking for dark matter and we didn’t find it,” he said. “Instead, we found other things that have been challenging for theory to explain.”
So the team got back to work, doing more experiments, gathering more evidence to make their discovery make sense. Mittig, Ayyad and their colleagues bolstered their case at the National Superconducting Cyclotron Laboratory, or NSCL, at Michigan State University. Working at NSCL, the team found a new path to their unexpected destination. In doing so, they also revealed interesting physics that’s afoot in the ultra-small quantum realm of subatomic particles. In particular, the team confirmed that when an atom’s core, or nucleus, is overstuffed with neutrons, it can still find a way to a more stable configuration by spitting out a proton instead.
Dark matter is one of the most famous things in the universe that we know the least about. For decades, scientists have known that the cosmos contains more mass than we can see based on the trajectories of stars and galaxies. For gravity to keep the celestial objects tethered to their paths, there had to be unseen mass and a lot of it — six times the amount of regular matter that we can observe, measure and characterize. Although scientists are convinced dark matter is out there, they have yet to find where and devise how to detect it directly.
“Finding dark matter is one of the major goals of physics,” said Ayyad, a nuclear physics researcher at the Galician Institute of High Energy Physics, or IGFAE, of the University of Santiago de Compostela in Spain.
Speaking in round numbers, scientists have launched about 100 experiments to try to illuminate what exactly dark matter is, Mittig said.
“None of them has succeeded after 20, 30, 40 years of research,” he said. “But there was a theory, a very hypothetical idea, that you could observe dark matter with a very particular type of nucleus,” said Ayyad, who was previously a detector systems physicist at NSCL.
This theory centered on what it calls a dark decay. It posited that certain unstable nuclei, nuclei that naturally fall apart, could jettison dark matter as they crumbled. So Ayyad, Mittig and their team designed an experiment that could look for a dark decay, knowing the odds were against them. But the gamble wasn’t as big as it sounds because probing exotic decays also lets researchers better understand the rules and structures of the nuclear and quantum worlds. The researchers had a good chance of discovering something new. The question was what that would be.
When people imagine a nucleus, many may think of a lumpy ball made up of protons and neutrons, Ayyad said. But nuclei can take on strange shapes, including what are known as halo nuclei. Beryllium-11 is an example of a halo nuclei. It’s a form, or isotope, of the element beryllium that has four protons and seven neutrons in its nucleus. It keeps 10 of those 11 nuclear particles in a tight central cluster. But one neutron floats far away from that core, loosely bound to the rest of the nucleus, kind of like the moon ringing around the Earth, Ayyad said. Beryllium-11 is also unstable. After a lifetime of about 13.8 seconds, it falls apart by what’s known as beta decay. One of its neutrons ejects an electron and becomes a proton. This transforms the nucleus into a stable form of the element boron with five protons and six neutrons, boron-11. But according to that very hypothetical theory, if the neutron that decays is the one in the halo, beryllium-11 could go an entirely different route: It could undergo a dark decay.
In 2019, the researchers launched an experiment at Canada’s national particle accelerator facility, TRIUMF, looking for that very hypothetical decay. And they did find a decay with unexpectedly high probability, but it wasn’t a dark decay. It looked like the beryllium-11’s loosely bound neutron was ejecting an electron like normal beta decay, yet the beryllium wasn’t following the known decay path to boron. The team hypothesized that the high probability of the decay could be explained if a state in boron-11 existed as a doorway to another decay, to beryllium-10 and a proton. For anyone keeping score, that meant the nucleus had once again become beryllium. Only now it had six neutrons instead of seven.
“This happens just because of the halo nucleus,” Ayyad said. “It’s a very exotic type of radioactivity. It was actually the first direct evidence of proton radioactivity from a neutron-rich nucleus.”
But science welcomes scrutiny and skepticism, and the team’s 2019 report was met with a healthy dose of both. That “doorway” state in boron-11 did not seem compatible with most theoretical models. Without a solid theory that made sense of what the team saw, different experts interpreted the team’s data differently and offered up other potential conclusions.
“We had a lot of long discussions,” Mittig said. “It was a good thing.”
As beneficial as the discussions were — and continue to be — Mittig and Ayyad knew they’d have to generate more evidence to support their results and hypothesis. They’d have to design new experiments.
In the team’s 2019 experiment, TRIUMF generated a beam of beryllium-11 nuclei that the team directed into a detection chamber where researchers observed different possible decay routes. That included the beta decay to proton emission process that created beryllium-10. For the new experiments, which took place in August 2021, the team’s idea was to essentially run the time-reversed reaction. That is, the researchers would start with beryllium-10 nuclei and add a proton. Collaborators in Switzerland created a source of beryllium-10, which has a half-life of 1.4 million years, that NSCL could then use to produce radioactive beams with new reaccelerator technology. The technology evaporated and injected the beryllium into an accelerator and made it possible for researchers to make a highly sensitive measurement. When beryllium-10 absorbed a proton of the right energy, the nucleus entered the same excited state the researchers believed they discovered three years earlier. It would even spit the proton back out, which can be detected as signature of the process.
“The results of the two experiments are very compatible,” Ayyad said.
That wasn’t the only good news. Unbeknownst to the team, an independent group of scientists at Florida State University had devised another way to probe the 2019 result. Ayyad happened to attend a virtual conference where the Florida State team presented its preliminary results, and he was encouraged by what he saw.
“I took a screenshot of the Zoom meeting and immediately sent it to Wolfi,” he said. “Then we reached out to the Florida State team and worked out a way to support each other.”
The two teams were in touch as they developed their reports, and both scientific publications now appear in the same issue of Physical Review Letters. And the new results are already generating a buzz in the community.
“The work is getting a lot of attention. Wolfi will visit Spain in a few weeks to talk about this,” Ayyad said.
Part of the excitement is because the team’s work could provide a new case study for what are known as open quantum systems. It’s an intimidating name, but the concept can be thought of like the old adage, “nothing exists in a vacuum.” Quantum physics has provided a framework to understand the incredibly tiny components of nature: atoms, molecules and much, much more. This understanding has advanced virtually every realm of physical science, including energy, chemistry and materials science.
Much of that framework, however, was developed considering simplified scenarios. The super small system of interest would be isolated in some way from the ocean of input provided by the world around it. In studying open quantum systems, physicists are venturing away from idealized scenarios and into the complexity of reality. Open quantum systems are literally everywhere, but finding one that’s tractable enough to learn something from is challenging, especially in matters of the nucleus. Mittig and Ayyad saw potential in their loosely bound nuclei and they knew that NSCL, and now FRIB could help develop it.
Spiral Spin Liquid on a Honeycomb Lattice
by Shang Gao, Michael A. McGuire, Yaohua Liu, Douglas L. Abernathy, Clarina dela Cruz, Matthias Frontzek, Matthew B. Stone, Andrew D. Christianson in Physical Review Letters
Scientists at the Department of Energy’s Oak Ridge National Laboratory used neutron scattering to determine whether a specific material’s atomic structure could host a novel state of matter called a spiral spin liquid. By tracking tiny magnetic moments known as “spins” on the honeycomb lattice of a layered iron trichloride magnet, the team found the first 2D system to host a spiral spin liquid.
The discovery provides a test bed for future studies of physics phenomena that may drive next-generation information technologies. These include fractons, or collective quantized vibrations that may prove promising in quantum computing, and skyrmions, or novel magnetic spin textures that could advance high-density data storage.
“Materials hosting spiral spin liquids are particularly exciting due to their potential to be used to generate quantum spin liquids, spin textures and fracton excitations,” said ORNL’s Shang Gao, who led the study.
A long-held theory predicted that the honeycomb lattice can host a spiral spin liquid — a novel phase of matter in which spins form fluctuating corkscrew-like structures. Yet, until the present study, experimental evidence of this phase in a 2D system had been lacking. A 2D system comprises a layered crystalline material in which interactions are stronger in the planar than in the stacking direction.
Gao identified iron trichloride as a promising platform for testing the theory, which was proposed more than a decade ago. He and co-author Andrew Christianson of ORNL approached Michael McGuire, also of ORNL, who has worked extensively on growing and studying 2D materials, asking if he would synthesize and characterize a sample of iron trichloride for neutron diffraction measurements. Like 2D graphene layers exist in bulk graphite as honeycomb lattices of pure carbon, 2D iron layers exist in bulk iron trichloride as 2D honeycomb layers. “Previous reports hinted that this interesting honeycomb material could show complex magnetic behavior at low temperatures,” McGuire said.
“Each honeycomb layer of iron has chlorine atoms above and below it, making chlorine-iron-chlorine slabs,” McGuire said. “The chlorine atoms on top of one slab interact very weakly with the chlorine atoms on the bottom of the next slab through van der Waals bonding. This weak bonding makes materials like this easily peeled apart into very thin layers, often down to a single slab. This is useful for developing devices and understanding the evolution of quantum physics from three dimensions to two dimensions.”
In quantum materials, electron spins can behave collectively and exotically. If one spin moves, all react — an entangled state Einstein called “spooky action at a distance.” The system stays in a state of frustration — a liquid that preserves disorder because electron spins constantly change direction, forcing other entangled electrons to fluctuate in response.
The first neutron diffraction studies of ferric chloride crystals were performed at ORNL 60 years ago. Today, ORNL’s extensive expertise in materials synthesis, imaging, neutron scattering, theory, simulation and computation enables pioneering explorations of magnetic quantum materials that drive development of next-generation technologies for information security and storage.
Mapping spin movements in the spiral spin liquid was made possible by experts and tools at the Spallation Neutron Source and the High Flux Isotope Reactor, DOE Office of Science user facilities at ORNL. ORNL co-authors were essential for the success of the neutron scattering experiments: Clarina dela Cruz, who led experiments using HFIR’s POWDER diffractometer; Yaohua Liu, who led experiments employing SNS’s CORELLI spectrometer; Matthias Frontzek, who led experiments engaging HFIR’s WAND2 diffractometer; Matthew Stone, who led experiments operating SNS’s SEQUOIA spectrometer; and Douglas Abernathy, who led experiments working SNS’s ARCS spectrometer.
“The neutron scattering data from our measurements at SNS and HFIR provided compelling evidence of a spiral spin liquid phase,” Gao said.
“The neutron scattering experiments measured how the neutrons exchange energy and momentum with the sample, allowing the magnetic properties to be inferred,” said co-author Matthew Stone. He described the magnetic structure of a spiral spin liquid: “It looks like a topographic map of a group of mountains with a bunch of rings going outward. If you were to walk along a ring, all spins would point in the same direction. But if you walk outward and cross different rings, you’re going to see those spins begin to rotate about their axes. That’s the spiral.”
“Our study shows that the concept of a spiral spin liquid is viable for the broad class of honeycomb lattice materials,” said co-author Andrew Christianson. “It gives the community a new route to explore spin textures and novel excitations, such as fractons, that then may be used in future applications, such as quantum computing.”
Probing Geometric Excitations of Fractional Quantum Hall States on Quantum Computers
by Ammar Kirmani, Kieran Bull, Chang-Yu Hou, Vedika Saravanan, Samah Mohamed Saeed, Zlatko Papić, Armin Rahmani, Pouyan Ghaemi in Physical Review Letters
City College of New York physicist Pouyan Ghaemi and his research team are claiming significant progress in using quantum computers to study and predict how the state of a large number of interacting quantum particles evolves over time. This was done by developing a quantum algorithm that they run on an IBM quantum computer. “To the best of our knowledge, such particular quantum algorithm which can simulate how interacting quantum particles evolve over time has not been implemented before,” said Ghaemi, associate professor in CCNY’s Division of Science.
“Quantum mechanics is known to be the underlying mechanism governing the properties of elementary particles such as electrons,” said Ghaemi. “But unfortunately there is no easy way to use equations of quantum mechanics when we want to study the properties of large number of electrons that are also exerting force on each other due to their electric charge. His team’s discovery, however, changes this and raises other exciting possibilities.
“On the other front, recently, there has been extensive technological developments in building the so-called quantum computers. These new class of computers utilize the law of quantum mechanics to preform calculations which are not possible with classical computers.”
We know that when electrons in material interact with each other strongly, interesting properties such as high-temperature superconductivity could emerge,” Ghaemi noted. “Our quantum computing algorithm opens a new avenue to study the properties of materials resulting from strong electron-electron interactions. As a result it can potentially guide the search for useful materials such as high temperature superconductors.”
He added that based on their results, they can now potentially look at using quantum computers to study many other phenomena that result from strong interaction between electrons in solids. “There are many experimentally observed phenomena that could be potentially understood using the development of quantum algorithms similar to the one we developed.”
Experimental quantum key distribution certified by Bell’s theorem
by Nadlinger, D.P., Drmota, P., Nichol, B.C. et al. in Nature
A method known as quantum key distribution has long held the promise of communication security unattainable in conventional cryptography. An international team of scientists has now demonstrated experimentally, for the first time, an approach to quantum key distribution that is based on high-quality quantum entanglement — offering much broader security guarantees than previous schemes.
The art of cryptography is to skillfully transform messages so that they become meaningless to everyone but the intended recipients. Modern cryptographic schemes, such as those underpinning digital commerce, prevent adversaries from illegitimately deciphering messages — say, credit-card information — by requiring them to perform mathematical operations that consume a prohibitively large amount of computational power. Starting from the 1980s, however, ingenious theoretical concepts have been introduced in which security does not depend on the eavesdropper’s finite number-crunching capabilities. Instead, basic laws of quantum physics limit how much information, if any, an adversary can ultimately intercept. In one such concept, security can be guaranteed with only a few general assumptions about the physical apparatus used. Implementations of such ‘device-independent’ schemes have long been sought after, but remained out of reach. Until now, that is. An international team of researchers from the University of Oxford, EPFL, ETH Zurich, the University of Geneva and CEA report the first demonstration of this sort of protocol — taking a decisive step towards practical devices offering such exquisite security.
Secure communication is all about keeping information private. It might be surprising, therefore, that in real-world applications large parts of the transactions between legitimate users are played out in public. The key is that sender and receiver do not have to keep their entire communication hidden. In essence, they only have to share one ‘secret’; in practice, this secret is string of bits, known as a cryptographic key, that enables everyone in its possession to turn coded messages into meaningful information. Once the legitimate parties have ensured for a given round of communication that they, and only they, share such a key, pretty much all the other communication can happen in plain view, for everyone to see. The question, then, is how to ensure that only the legitimate parties share a secret key. The process of accomplishing this is known as ‘key distribution’.
In the cryptographic algorithms underlying, for instance, RSA — one of the most widely used cryptographic systems — key distribution is based on the (unproven) conjecture that certain mathematical functions are easy to compute but hard to revert. More specifically, RSA relies on the fact that for today’s computers it is hard to find the prime factors of a large number, whereas it is easy for them to multiply known prime factors to obtain that number. Secrecy is therefore ensured by mathematical difficulty. But what is impossibly difficult today might be easy tomorrow. Famously, quantum computers can find prime factors significantly more efficiently than classical computers. Once quantum computers with a sufficiently large number of qubits become available, RSA encoding is destined to become penetrable.
But quantum theory provides the basis not only for cracking the cryptosystems at the heart of digital commerce, but also for a potential solution to the problem: a way entirely different from RSA for distributing cryptographic keys — one that has nothing to do with the hardness of performing mathematical operations, but with fundamental physical laws. Enter quantum key distribution, or QKD for short.
In 1991, the Polish-British physicist Artur Ekert showed in a seminal paper that the security of the key-distribution process can be guaranteed by directly exploiting a property that is unique to quantum systems, with no equivalent in classical physics: quantum entanglement. Quantum entanglement refers to certain types of correlations in the outcomes of measurements performed on separate quantum systems. Importantly, quantum entanglement between two systems is exclusive, in that nothing else can be correlated to these systems. In the context of cryptography this means that sender and receiver can produce between them shared outcomes through entangled quantum systems, without a third party being able to secretly gain knowledge about these outcomes. Any eavesdropping leaves traces that clearly flag the intrusion. In short: the legitimate parties can interact with one another in ways that are — thanks to quantum theory — fundamentally beyond any adversary’s control. In classical cryptography, an equivalent security guarantee is provably impossible.
Over the years, it was realized that QKD schemes based on the ideas introduced by Ekert can have a further remarkable benefit: users have to make only very general assumptions regarding the devices employed in the process. By contrast, earlier forms of QKD based on other basic principles require detailed knowledge about the inner workings of the devices used. The novel form of QKD is now generally known as ‘device-independent QKD’ (DIQKD), and an experimental implementation thereof became a major goal in the field. Hence the excitement as such a breakthrough experiment has now finally been achieved.
The scale of the challenge is reflected in the breadth of the team, which combines leading experts in theory and experiment. The experiment involved two single ions — one for the sender and one for the receiver — confined in separate traps that were connected with an optical-fibre link. In this basic quantum network, entanglement between the ions was generated with record-high fidelity over millions of runs. Without such a sustained source of high-quality entanglement, the protocol could not have been run in a practically meaningful manner. Equally important was to certify that the entanglement is suitably exploited, which is done by showing that conditions known as Bell inequalities are violated. Moreover, for the analysis of the data and an efficient extraction of the cryptographic key, significant advances in the theory were needed.
In the experiment, the ‘legitimate parties’ — the ions — were located in one and the same laboratory. But there is a clear route to extending the distance between them to kilometres and beyond. With that perspective, together with further recent progress made in related experiments in Germany and China, there is now a real prospect of turning the theoretical concept of Ekert into practical technology.
Deep neural network based quantum simulations and quasichemical theory for accurate modeling of molten salt thermodynamics
by Yu Shi, Stephen T. Lam, Thomas L. Beck in Chemical Science
A chemist at the University of Cincinnati has come up with a novel way to study the thermodynamic properties of molten salts, which are used in many nuclear and solar energy applications.
UC College of Arts and Sciences research associate and computational chemist Yu Shi and his collaborators developed a new simulation method to calculate free energy using deep learning artificial intelligence.
Molten salt is salt heated to high temperatures where it becomes liquid. UC researchers studied sodium chloride, commonly known as table salt. Shi said molten salt has properties that make it a valuable medium for cooling systems in nuclear power plants. In solar towers, they can be used to transfer heat or store energy. Paradoxically, while salt is an insulator, molten salt conducts electricity.
“Molten salts are stable at high temperatures and can hold a lot of energy in a liquid state,” Shi said. “They have good thermodynamic properties. That makes them a good energy storage material for concentrated solar power plants. And they can be used as a coolant in nuclear reactors.”
The study could help researchers examine the corrosion that these salts can cause in metal containers like those found in the next generation of nuclear reactors. The study provides a reliable approach to study the conversion of dissolved gas to vapor in molten salts, helping engineers understand the effect of different impurities and solutes (the substance dissolved in a solution) on corrosion. Shi said it also will help researchers study the release of potentially toxic gas into the atmosphere, which will be extremely useful for fourth-generation molten salt nuclear reactors.
“We used our quasi-chemical theory and our deep neural network, which we trained using data generated by quantum simulations, to model the solvation thermodynamics of molten salt with chemical accuracy,” Shi said.
Study co-author Thomas Beck is former head of UC’s Department of Chemistry and now works as section head of science engagement for the Oak Ridge National Laboratory in Tennessee. Beck said molten salts do not expand when heated, unlike water which can create extreme pressure at high temperatures.
“The pressure inside a nuclear reactor goes up a lot. That’s the difficulty of reactor design — it leads to more risks and higher costs,” he said.
Researchers turned to UC’s Advanced Research Computing Center and the Ohio Supercomputer Center to run the simulations.
“At Oak Ridge, we have the world’s fastest supercomputer, so our experiment would take less time here,” Beck said. “But on typical supercomputers, it can take weeks or months to run these quantum simulations.”
The research team also included Stephen Lam at the University of Massachusetts Lowell.
“It’s important to have accurate models of these salts. We were the first group to calculate free energy of sodium chloride at high temperature in liquid and compare it to previous experiments,” Beck said. “So we proved it’s a useful technique.”
In 2020, Shi and Beck established a free-energy scale for single-ion hydration using quasi-chemical theory and quantum mechanical simulations of the sodium ion in water in a study . It was the first solvation free-energy calculation for the charged solute using quantum mechanics, Shi said.
Beck said molten salts will be important for developing new sources of energy — even perhaps one day fusion energy.
“They’re proposing using molten salts as a coating coolant for the high-temperature reactor,” he said. “But fusion is farther down the road.”
A universal qudit quantum processor with trapped ions
by Martin Ringbauer, Michael Meth, Lukas Postler, Roman Stricker, Rainer Blatt, Philipp Schindler, Thomas Monz in Nature Physics
For decades computers have been synonymous with binary information — zeros and ones. Now a team at the University of Innsbruck, Austria, realized a quantum computer that breaks out of this paradigm and unlocks additional computational resources, hidden in almost all of today’s quantum devices.
We all learn from early on that computers work with zeros and ones, also known as binary information. This approach has been so successful that computers now power everything from coffee machines to self-driving cars and it is hard to imagine a life without them. Building on this success, today’s quantum computers are also designed with binary information processing in mind. “The building blocks of quantum computers, however, are more than just zeros and ones,” explains Martin Ringbauer, an experimental physicist from Innsbruck, Austria. “Restricting them to binary systems prevents these devices from living up to their true potential.”
The team led by Thomas Monz at the Department of Experimental Physics at the University of Innsbruck, now succeeded in developing a quantum computer that can perform arbitrary calculations with so-called quantum digits (qudits), thereby unlocking more computational power with fewer quantum particles.
Although storing information in zeros and ones is not the most efficient way of doing calculations, it is the simplest way. Simple often also means reliable and robust to errors and so binary information has become the unchallenged standard for classical computers. In the quantum world, the situation is quite different. In the Innsbruck quantum computer, for example, information is stored in individual trapped Calcium atoms. Each of these atoms naturally has eight different states, of which typically only two are used to store information. Indeed, almost all existing quantum computers have access to more quantum states than they use for computation.
The physicists from Innsbruck now developed a quantum computer that can make use of the full potential of these atoms, by computing with qudits. Contrary to the classical case, using more states does not make the computer less reliable. “Quantum systems naturally have more than just two states and we showed that we can control them all equally well,” says Thomas Monz. On the flipside, many of the tasks that need quantum computers, such as problems in physics, chemistry, or material science, are also naturally expressed in the qudit language. Rewriting them for qubits can often make them too complicated for today’s quantum computers. “Working with more than zeros and ones is very natural, not only for the quantum computer but also for its applications, allowing us to unlock the true potential of quantum systems,” explains Martin Ringbauer.
Digital quantum simulation of Floquet symmetry-protected topological phases
by Xu Zhang, Wenjie Jiang, Jinfeng Deng, Ke Wang, Jiachen Chen, Pengfei Zhang, Wenhui Ren, Hang Dong, Shibo Xu, Yu Gao, Feitong Jin, Xuhao Zhu, Qiujiang Guo, Hekang Li, Chao Song, Alexey V. Gorshkov, Thomas Iadecola, Fangli Liu, Zhe-Xuan Gong, Zhen Wang, Dong-Ling Deng, H. Wang in Nature
Physicists have demonstrated how simulations using quantum computing can enable observation of a distinctive state of matter taken out of its normal equilibrium. Such novel states of matter could one day lead to developments in fast, powerful quantum information storage and precision measurement science.
Thomas Iadecola worked his way through the title of the latest research paper that includes his theoretical and analytical work, patiently explaining digital quantum simulation, Floquet systems and symmetry-protected topological phases. Then he offered explanations of nonequilibrium systems, time crystals, 2T periodicity and the 2016 Nobel Prize in Physics.
Iadecola’s corner of quantum condensed matter physics — the study of how states of matter emerge from collections of atoms and subatomic particles — can be counterintuitive and needs an explanation at most every turn and term. The bottom line, as explained by the Royal Swedish Academy of Sciences in announcing that 2016 physics prize to David Thouless, Duncan Haldane and Michael Kosterlitz, is that researchers are revealing more and more of the secrets of exotic matter, “an unknown world where matter can assume strange states.”
The new paper co-authored by Iadecola, an Iowa State University assistant professor of physics and astronomy and an Ames National Laboratory scientist, describes simulations using quantum computing that enabled observation of a distinctive state of matter taken out of its normal equilibrium. The paper’s corresponding author is Dong-Ling Deng of Tsinghua University in Beijing, China. Deng and Iadecola worked together in 2017 and ’18 as postdoctoral researchers at the University of Maryland.
“Our work paves the way to exploring novel non-equilibrium phases of matter,” the authors wrote in a summary of their paper.
For you and me, those novel states of matter could one day provide unique and useful properties for new technologies. Possible applications in quantum information processing include precision measurement science and information storage. For this project, Iadecola was a supporting scientist who contributed theoretical work and data analysis. For example, “In a collaborative project like this, my role is to help define the questions the experimentalists need to address,” he said. The major question they answered in this paper is how a quantum computing platform can be used to study and understand exotic states of matter.
“This paper demonstrates the researchers have a very nice digital quantum simulation platform,” Iadecola said. “This platform can also be applied to other interesting problems in quantum many-body physics.”
While the project is all about theory and education, a summary says it will be approached “with a view towards emerging quantum technologies.”
“We’re thinking about new phenomena,” Iadecola said. “Realizing these phenomena on present-day quantum hardware could set the stage for moving us toward these applications in quantum information processing.”
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