Thursday 31 August 2023

IBM makes major leap in quantum computing error-detection

 



                      
Quantum computing is on the verge of catapulting the digital revolution to new heights.

Turbocharged processing holds the promise of instantaneously diagnosing health ailments and providing rapid development of new medicines; greatly speeding up response time in AI systems for such time-sensitive operations as autonomous driving and space travel; optimizing traffic control in congested cities; helping aircraft better navigate extreme turbulence; speeding up weather forecasting that better prepares localities facing potential disaster, and optimizing supply chain systems for more efficient delivery times and cost savings.

But we're not there yet. One of the greatest obstacles facing quantum operations is error-correction.

The price for speedier operations in quantum systems is a higher error rate. Quantum computers are highly susceptible to noise such as electromagnetic signals, temperature change and disturbances in the Earth's magnetic field. Such noise triggers errors.

Qubits, the components particular to quantum computing, themselves are prone to error. Faults in frequencies, energy levels and coupling strength can cause miscalculations.

Unlike standard computer bits that are copied reliably most of the time, qubits, by their very nature, cannot be cloned without errors being introduced. Bits store easily replicated binary digit states while qubits store data in a complex mathematical quantum state that can be disrupted during copying. Additionally, qubits age quickly and deterioration can introduce errors.

Researchers at IBM Quantum say they have developed a system that dramatically improves error-detection in quantum computing. In an online post Aug. 28, they explained the challenge: "Standard classical error-correction only needs to correct bit flip errors," said IBM researcher Sergey Bravyi.

"Quantum computers must correct more kinds of errors, like phase errors which can corrupt the extra quantum information that qubits carry … Techniques must [also] correct errors without the ability to copy unknown quantum states, and without destroying the underlying quantum state."

In their research paper, IBM researchers described a process they say greatly trims the required arsenal currently used in quantum computing to catch errors.

Standard computer surface codes have long been successfully used for error-corrections. These are two-dimensional grids resembling a checkerboard. Efficient error-correction for qubits is more challenging.

Bravyi says many experts estimate fault-tolerant quantum computing would require millions of qubits, "a number we believe is too large to be practical at this stage of development."

IBM's solution, improved code and a redesign of qubit placement, achieves results requiring one-tenth the number of physical qubits currently used in error-correction.

"Practical error correction is far from a solved problem," the researchers acknowledged in a paper titled "High-threshold and low-overhead fault-tolerant quantum memory" published Aug. 15 in the preprint server arXiv.

"However, these new codes and other advances across the field are increasing our confidence that fault tolerant quantum computing isn't just possible, but is possible without having to build an unreasonably large quantum computer."

Their approach currently only works on quantum memory and not computational power.

"These techniques are a stepping stone towards a world of fault-tolerant computing," Bravyi says, "and this new … code is bringing that world closer. It's a promising result pointing us where we should look next for even better error correcting codes."


#QuantumComputingRevolution #QubitsAndBeyond #QuantumLeap #ComputingInParallel #EntanglementEra #QuantumBits #SuperpositionComputing #DecoherenceChallenges #QuantumAlgorithm

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Sunday 27 August 2023

Visualizing the microscopic phases of magic-angle twisted bilayer graphene

 




For the first time, the researchers were able to specifically capture unprecedentedly precise visualizations of the microscopic behavior of interacting electrons that give rise to the insulating quantum phase of MATBG. Additionally, through the use of novel and innovative theoretical techniques, they were able to interpret and understand these behaviors. Their study is published in the journal Nature.

The amazing properties of twisted bilayer graphene were first discovered in 2018 by Pablo Jarillo-Herrero and his team at the Massachusetts Institute of Technology (MIT). They showed that this material can be superconducting, a state in which electrons flow freely without any resistance. This state is vital to many of our everyday electronics, including magnets for MRIs and particle accelerators as well as in the making of quantum bits (called qubits) that are being used to build quantum computers.

Since that discovery, twisted bilayer graphene has demonstrated many novel quantum physical states, such as insulating, magnetic, and superconducting states, all of which are created by complex interactions of electrons. How and why electrons form insulating states in MATBG has been one of the key unsolved puzzles in the field.

The solution to this puzzle would not only unlock our understanding of both the insulator and the proximate superconductor, but also such behavior shared by many unusual superconductors that scientists seek to understand, including the high-temperature cuprate superconductors.

"MATBG shows a lot of interesting physics in a single material platform－much of which remains to be understood," said Kevin Nuckolls, the co-lead author of the paper, who earned his Ph.D. in 2023 in Princeton's physics department and is now a postdoctoral fellow at MIT. "This insulating phase, in which electrons are completely blocked from flowing, has been a real mystery."

To create the desired quantum effects, researchers place two sheets of graphene on top of each other with the top layer angled slightly. This off-kilter position creates a moiré pattern, which resembles and is named after a common French textile design. Importantly, however, the angle at which the top layer of graphene must be positioned is precisely 1.1 degrees. This is the "magic" angle that produces the quantum effect; that is, this angle induces strange, strongly correlated interactions between the electrons in the graphene sheets.

While physicists have been able to demonstrate different quantum phases in this material, such as the zero-resistance superconducting phase and the insulating phase, there has been very little understanding of why these phases occur in MATBG. Indeed, all previous experiments involving MATBG give good demonstrations of what the system is capable of producing, but not why the system is producing these states.

And that "why" became the basis for the current experiment.

"The general idea of this experiment is that we wanted to ask questions about the origins of these quantum phases—to really understand what exactly are the electrons doing on the graphene atomic scale," said Nuckolls. "Being able to probe the material microscopically, and to take images of its correlated states—to fingerprint them, effectively—gives us the ability to discern very distinctly and precisely the microscopic origins of some of these phases. Our experiment also helps guide theorists in the search for phases that were not predicted."

The study is the culmination of two years of work and was achieved by a team from Princeton University and the University of California, Berkeley. The scientists harnessed the power of the scanning tunneling microscope (STM) to probe this very minute realm. This tool relies on a technique called "quantum tunneling," where electrons are funneled between the sharp metallic tip of the microscope and the sample. The microscope uses this tunneling current rather than light to view the world of electrons on the atomic scale. Measurements of these quantum tunneling events are then translated into high resolution, highly sensitive images of materials.

However, the first step—and perhaps the most crucial step in the experiment's success—was the creation of what the researchers refer to as a "pristine" sample. The surface of carbon atoms that constituted the twisted bilayer graphene sample had to have no flaws or imperfections.
Visualizing the microscopic phases of magic-angle twisted bilayer graphene

"The technical breakthrough that made this paper happen was our group's ability to make the samples so pristine in terms of their cleanliness such that these high-resolution images that you see in the paper were possible," said Ali Yazdani, the Class of 1909 Professor of Physics and Director of the Center for Complex Materials at Princeton University. "In other words, you have to make one hundred thousand atoms without a single flaw or disorder."


The actual experiment involved placing the graphene sheets in the correct "magic angle," at 1.1 degrees. The researchers then positioned the sharp, metallic tip of the STM over the graphene sample and measured the quantum mechanical tunneling current as they moved the tip across the sample.

"Electrons at this quantum scale are not only particles, but they are also waves," said Ryan Lee, a graduate student in the Department of Physics at Princeton and one of the paper's co-lead authors. "And essentially, we're imaging wave-like patterns of electrons, where the exact way that they interfere (with each other) is telling us some very specific information about what is giving rise to the underlying electronic states."

This information allowed the researchers to make some very incisive interpretations about the quantum phases that were produced by the twisted bilayer graphene. Importantly, the researchers used this information to focus on and solve the long-standing puzzle that for many years has challenged researchers working in this field, namely, the quantum insulating phase that occurs when graphene is tuned to its magic angle.

To help understand this from a theoretical viewpoint, the Princeton researchers collaborated with a team from the University of California-Berkeley, led by physicists B. Andrei Bernevig at Princeton and Michael Zaletel at Berkeley. This team developed a novel and innovative theoretical framework called "local order parameter" analysis to interpret the STM images and understand what the electrons were doing—in other words, how they were interacting—in the insulating phase. What they discovered was that the insulating state occurs because of the strong repulsion between the electrons, on the microscopic level.

"In magic-angle twisted bilayer graphene, the challenge was to model the system," said Tomohiro Soejima, a graduate student and theorist at U.C. Berkeley and one of the paper's co-lead authors. "There were many competing theories, and no one knew which one was correct. Our experiment of 'finger-printing' was really crucial because that way we could pinpoint the actual electronic interactions that give rise to the insulating phase."

By using this theoretical framework, the researchers were able, for the first time, to make a measurement of the observed wave functions of the electrons. "The experiment introduces a new way of analyzing quantum microscopy," said Yazdani.

The researchers suggest the technology—both the imagery and the theoretical framework—can be used in the future to analyze and understand many other quantum phases in MATBG, and ultimately, to help comprehend new and unusual material properties that may be useful for next-generation quantum technological applications.

"Our experiment was a wonderful example of how Mother Nature can be so complicated—can be really confusing—until you have the right framework to look at it, and then you say, 'oh, that's what's happening,'" said Yazdani.



#HomogeneousPhase #MoirePatternPhase #MottInsulatorPhase #SuperconductingPhase
#FerromagneticPhase #AntiferromagneticPhase #TopologicalPhase)




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Friday 25 August 2023

Long-lived quantum state points the way to solving a mystery in radioactive nuclei

 




Timothy Gray of the Department of Energy's Oak Ridge National Laboratory led a study that may have revealed an unexpected change in the shape of an atomic nucleus. The surprise finding could affect our understanding of what holds nuclei together, how protons and neutrons interact and how elements form.

"We used radioactive beams of excited sodium-32 nuclei to test our understanding of nuclear shapes far from stability and found an unexpected result that raises questions about how nuclear shapes evolve," said Gray, a nuclear physicist. The results are published in Physical Review Letters.

The shapes and energies of atomic nuclei can shift over time between different configurations. Typically, nuclei live as quantum entities that have either spherical or deformed shapes. The former look like basketballs, and the latter resemble American footballs.

How shapes and energy levels relate is a major open question for the scientific community. Nuclear structure models have trouble extrapolating to regions with little experimental data.

For some exotic radioactive nuclei, the shapes predicted by traditional models are the opposite of those observed. Radioactive nuclei that were expected to be spherical in their ground states, or lowest-energy configurations, turned out to be deformed.
What can turn a quantum state on its head?

In principle, the energy of an excited deformed state can drop below that of a spherical ground state, making the spherical shape the high-energy one. Unexpectedly, this role reversal appears to be happening for some exotic nuclei when the natural ratio of neutrons to protons becomes unbalanced. Yet, the post-reversal excited spherical states have never been found. It is as though once the ground state becomes deformed, all the excited states do, too.

Many examples exist of nuclei with spherical ground states and deformed excited states. Similarly, plenty of nuclei have deformed ground states and subsequent excited states that are also deformed—sometimes with different amounts or kinds of deformation. However, nuclei with both deformed ground states and spherical excited states are much more elusive.

Using data collected in 2022 from the first experiment at the Facility for Rare Isotope Beams, or FRIB, a DOE Office of Science user facility at Michigan State University, Gray's team discovered a long-lived excited state of radioactive sodium-32. The newly observed excited state has an unusually long lifetime of 24 microseconds—about a million times longer than a typical nuclear excited state.

Long-lived excited states are called isomers. A long lifetime indicates that something unanticipated is going on. For example, if the excited state is spherical, a difficulty in returning to a deformed ground state could account for its long life.

The study involved 66 participants from 20 universities and national laboratories. Co-principal investigators came from Lawrence Berkeley National Laboratory, Florida State University, Mississippi State University, the University of Tennessee, Knoxville, and ORNL.

The 2022 experiment that generated the data used for the 2023 result employed the FRIB Decay Station initiator, or FDSi, a modular multidetector system that is extremely sensitive to rare isotope decay signatures.

"FDSi's versatile combination of detectors shows that the long-lived excited state of sodium-32 is delivered within the FRIB beam and that it then decays internally by emitting gamma rays to the ground state of the same nucleus," said ORNL's Mitch Allmond, a co-author of the paper who manages the FDSi project.

To stop FRIB's highly energetic radioactive beam, which travels at about 50% of the speed of light, an implantation detector built by UT Knoxville was positioned at FDSi's center. North of the beam line was a gamma-ray detector array called DEGAi, comprising 11 germanium clover-style detectors and 15 fast-timing lanthanum bromide detectors. South of the beam line were 88 modules of a detector called NEXTi to measure time of flight of neutrons emitted in radioactive decay.

A beam of excited sodium-32 nuclei stopped in the detector and decayed to the deformed ground state by emitting gamma rays. Analysis of gamma-ray spectra to discern the time difference between beam implantation and gamma-ray emission revealed how long the excited state existed. The new isomer's 24-microsecond existence was the longest lifetime seen among isomers with 20 to 28 neutrons that decay by gamma-ray emission. Approximately 1.8% of the sodium-32 nuclei were observed to be the new isomer.

"We can come up with two different models that equally well explain the energies and lifetime that we've observed in the experiment," Gray said.

An experiment with higher beam power is needed to determine whether the excited state in sodium-32 is spherical. If it is, then the state would have six quantized units of angular momentum, which is a quality of a nucleus related to its whole-body rotation or the orbital motion of its individual protons and/or neutrons about the center of mass. However, if the excited state in sodium-32 is deformed, then the state would have zero quantized units of angular momentum.

A planned upgrade to FRIB will provide more power, increasing the number of nuclei in the beam. Data from the more intense beam will enable an experiment that distinguishes between the two possibilities.

"We'd characterize correlations between the angles of two gamma rays that are emitted in a cascade," Gray said. "The two possibilities have very different angular correlations between the gamma rays. If we have enough statistics, we could disentangle the pattern that reveals a clear answer."


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Thursday 24 August 2023

Research group detects a quantum entanglement wave for the first time using real-space measurements

 


Triplons are tricky little things. Experimentally, they're exceedingly difficult to observe. And even then, researchers usually conduct the tests on macroscopic materials, in which measurements are expressed as an average across the whole sample.

That's where designer quantum materials offer a unique advantage, says Academy Research Fellow Robert Drost, the first author of a paper published in Physical Review Letters. These designer quantum materials let researchers create phenomena not found in natural compounds, ultimately enabling the realization of exotic quantum excitations.

"These materials are very complex. They give you very exciting physics, but the most exotic ones are also challenging to find and study. So, we are trying a different approach here by building an artificial material using individual components," says Professor Peter Liljeroth, head of the Atomic Scale physics research group at Aalto University.

Quantum materials are governed by the interactions between electrons at the microscopic level. These electronic correlations lead to unusual phenomena like high-temperature superconductivity or complex magnetic states, and quantum correlations give rise to new electronic states.

In the case of two electrons, there are two entangled states known as singlet and triplet states. Supplying energy to the electron system can excite it from the singlet to the triplet state. In some cases, this excitation can propagate through a material in an entanglement wave known as a triplon. These excitations are not present in conventional magnetic materials, and measuring them has remained an open challenge in quantum materials.
The team's triplon experiments

In the new study, the team used small organic molecules to create an artificial quantum material with unusual magnetic properties. Each of the cobalt-phthalocyanine molecules used in the experiment contains two frontier electrons.

"Using very simple molecular building blocks, we are able to engineer and probe this complex quantum magnet in a way that has never been done before, revealing phenomena not found in its independent parts," Drost says. "While magnetic excitations in isolated atoms have long been observed using scanning tunneling spectroscopy, it has never been accomplished with propagating triplons."

"We use these molecules to bundle electrons together, we pack them into a tight space and force them to interact," continues Drost. "Looking into such a molecule from the outside, we will see the joint physics of both electrons. Because our fundamental building block now contains two electrons, rather than one, we see a very different kind of physics."

The team monitored magnetic excitations first in individual cobalt-phthalocyanine molecules and later in larger structures like molecular chains and islands. By starting with the very simple and working towards increasing complexity, the researchers hope to understand emergent behavior in quantum materials. In the present study, the team could demonstrate that the singlet-triplet excitations of their building blocks can traverse molecular networks as exotic magnetic quasiparticles known as triplons.

"We show that we can create an exotic quantum magnetic excitation in an artificial material. This strategy shows that we can rationally design material platforms that open up new possibilities in quantum technologies," says Assistant Professor Jose Lado, one of the study's co-authors, who heads the Correlated Quantum Materials research group at Aalto University.

The team plans to extend their approach towards more complex building blocks to design other exotic magnetic excitations and ordering in quantum materials. Rational design from simple ingredients will not only help understand the complex physics of correlated electron systems but also establish new platforms for designer quantum materials.


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Physicists use a 350-year-old theorem to reveal new properties of light waves

 




Since the 17th century, when Isaac Newton and Christiaan Huygens first debated the nature of light, scientists have been puzzling over whether light is best viewed as a wave or a particle—or perhaps, at the quantum level, even both at once. Now, researchers at Stevens Institute of Technology have revealed a new connection between the two perspectives, using a 350-year-old mechanical theorem—ordinarily used to describe the movement of large, physical objects like pendulums and planets—to explain some of the most complex behaviors of light waves.

The work, led by Xiaofeng Qian, assistant professor of physics at Stevens and reported in the August 17 online issue of Physical Review Research, also proves for the first time that a light wave's degree of non-quantum entanglement exists in a direct and complementary relationship with its degree of polarization. As one rises, the other falls, enabling the level of entanglement to be inferred directly from the level of polarization, and vice versa. This means that hard-to-measure optical properties such as amplitudes, phases and correlations—perhaps even these of quantum wave systems—can be deduced from something a lot easier to measure: light intensity.

"We've known for over a century that light sometimes behaves like a wave, and sometimes like a particle, but reconciling those two frameworks has proven extremely difficult," said Qian "Our work doesn't solve that problem—but it does show that there are profound connections between wave and particle concepts not just at the quantum level, but at the level of classical light-waves and point-mass systems."

Qian's team used a mechanical theorem, originally developed by Huygens in a 1673 book on pendulums, that explains how the energy required to rotate an object varies depending on the object's mass and the axis around which it turns. "This is a well-established mechanical theorem that explains the workings of physical systems like clocks or prosthetic limbs," Qian explained. "But we were able to show that it can offer new insights into how light works, too."

This 350-year-old theorem describes relationships between masses and their rotational momentum, so how could it be applied to light where there is no mass to measure? Qian's team interpreted the intensity of a light as the equivalent of a physical object's mass, then mapped those measurements onto a coordinate system that could be interpreted using Huygens' mechanical theorem. "Essentially, we found a way to translate an optical system so we could visualize it as a mechanical system, then describe it using well-established physical equations," explained Qian.

Once the team visualized a light wave as part of a mechanical system, new connections between the wave's properties immediately became apparent—including the fact that entanglement and polarization stood in a clear relationship with one another.

"This was something that hadn't been shown before, but that becomes very clear once you map light's properties onto a mechanical system," said Qian. "What was once abstract becomes concrete: using mechanical equations, you can literally measure the distance between 'center of mass' and other mechanical points to show how different properties of light relate to one another."

Clarifying these relationships could have important practical implications, allowing subtle and hard-to-measure properties of optical systems—or even quantum systems—to be deduced from simpler and more robust measurements of light intensity, Qian explained. More speculatively, the team's findings suggest the possibility of using mechanical systems to simulate and better-understand the strange and complex behaviors of quantum wave systems.

"That still lies ahead of us, but with this first study we've shown clearly that by applying mechanical concepts, it's possible to understand optical systems in an entirely new way," Qian said. "Ultimately, this research is helping to simplify the way we understand the world, by allowing us to recognize the intrinsic underlying connections between apparently unrelated physical laws."

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Tuesday 22 August 2023

A new 'spin' on ergodicity breaking

 




In a recent Science paper, researchers led by JILA and NIST Fellow Jun Ye, along with collaborators JILA and NIST Fellow David Nesbitt, scientists from the University of Nevada, Reno, and Harvard University, observed novel ergodicity-breaking in C60, a highly symmetric molecule composed of 60 carbon atoms arranged on the vertices of a "soccer ball" pattern (with 20 hexagon faces and 12 pentagon faces).Their results revealed ergodicity breaking in the rotations of C60. Remarkably, they found that this ergodicity breaking occurs without symmetry breaking and can even turn on and off as the molecule spins faster and faster. Understanding ergodicity breaking can help scientists design better-optimized materials for energy and heat transfer.

Many everyday systems exhibit "ergodicity" such as heat spreading across a frying pan and smoke filling a room. In other words, matter or energy spreads evenly over time to all system parts as energy conservation allows. On the other hand, understanding how systems can violate (or "break") ergodicity, such as magnets or superconductors, helps scientists understand and engineer other exotic states of matter.

In many cases, ergodicity breaking is tied to what physicists call "symmetry breaking." For example, the internal magnetic moments of atoms in a magnet all point in one direction, either "up" or "down." Despite possessing the same energy, these two distinct configurations are separated by an energy barrier.

The "symmetry breaking" refers to the system assuming a configuration with lower symmetry than the physical laws governing its behavior would allow, such as all magnetic moments pointing "down" as the default state. At the same time, since the magnet has permanently settled into just one of two equal-energy configurations, it has also broken ergodicity.
Symmetry breaking: magnets and footballs

To understand rotational ergodicity breaking, postdoctoral researcher and lead author, Lee Liu explained, "Consider a football thrown in a tight clockwise spiral. You would never see the football spontaneously flip 180 degrees end-over-end in mid-flight, going from a low-energy 90-degree configuration to a 180-degree one. This is shown in figures 1B and 1C. This would require overcoming an energy barrier. So a spiraling football maintains its end-to-end orientation in free flight, breaking ergodicity and symmetry like a magnet does."

However, unlike footballs, isolated molecules must obey the rules of quantum mechanics. Specifically, the two ends of an ethylene molecule (a quantum analog of a football) are indistinguishable. Thus, reorienting a spinning ethylene molecule 180 degrees end-over-end also entails overcoming an energy barrier; the initial and final states are indistinguishable. The molecule does not have two distinct end-to-end orientations to choose from, and symmetry and ergodicity are restored, meaning that the molecule's ground state is a combination, or the superposition, of both the final and initial states.

Infrared spectroscopy of C60

To probe the rotational dynamics of the C60 molecule, the researchers turned to a technique pioneered by the Ye group in 2016: combining buffer gas cooling with sensitive cavity-enhanced infrared spectroscopy. Using this technique, the researchers measured the infrared spectrum of C60 with 1000-fold higher sensitivity than previously achieved. It involved shining laser light on C60 molecules and "listening" to the frequencies of light they absorb.

"Just like the sound of an instrument can tell you about its physical properties, molecular resonant frequencies, encoded in its infrared spectrum, can tell us about the structure and rotation dynamics of the molecule," said Liu. Rather than physically rotating the molecule faster and faster, the researchers probed a gas-phase sample of many C60 molecules in which some rotated rapidly and some slowly. The resulting infrared spectrum contained snapshots of the molecule at various rotation speeds.

"Stitching of these traces together generated the complete spectrum, unraveling the full picture of the ergodicity evolution (or breaking) of the molecule," elaborated Dina Rosenberg, a fellow postdoctoral researcher in Ye's group.

Through this process, the researchers uncovered an astonishing behavior of C60: spinning it at 2.3 GHz (billion rotations per second) makes it ergodic. This ergodic phase persists until 3.2 GHz when the molecule breaks ergodicity. As the molecule spins faster, it reverts back to being ergodic at 4.5 GHz. This peculiar switching behavior surprised the researchers, as ergodicity transitions typically occur only once the energy increases and in one direction. Curious, the team dove further into the spectrum to understand where this behavior originated.
Ergodicity breaking—quantum football, frisbee, and soccer

By analyzing the infrared spectrum, the researchers could infer deformations of the molecule induced by its rotation. "Just like drag race car's tires bulge more when rotated at a faster rate, the rotation rate of C60 dictates its structural deformation. The infrared spectra imply that two possibilities occur when the C60 rotation rate hits 2.3 GHz: It can flatten out into a frisbee shape or elongate into a football shape," said Liu.

"The former occurs if it is rotating about a pentagon, and the latter if it is rotating about a hexagon. When C60 reaches 3.2 GHz, hexagonal and pentagonal rotations result in football-like deformation. At 4.5 GHz, hexagonal rotation generates a frisbee-like deformation while pentagonal rotation generates a football-like deformation."

As it turns out, the peculiar ergodicity transitions of C60 could be attributed entirely to this sequence of deformations induced by the molecule's rotation.
Breaking ergodicity but not symmetry

In the gas phase, C60 molecules collide so infrequently that they behave as if they were isolated, meaning that the indistinguishability of each carbon atom in C60 becomes important. Therefore, spinning the molecule about any pentagon is equivalent to spinning it about any other pentagon. Likewise, spinning the molecule about any hexagon is equivalent to spinning it about any other hexagon.

Just as in ethylene, the quantum indistinguishability of C60's carbon atoms restores the symmetry of the pentagonal and hexagonal rotational sectors. Nevertheless, the researchers' data showed that the molecule's rotation axis never switched between sectors.

The data showed two reasons for this rotational isolation around a single axis. At rotation rates below 3.2 and above 4.5 GHz, the pentagonal and hexagonal rotational sectors are isolated due to energy conservation. "It takes more energy to spin a football than a frisbee [due to its mass]," said Liu. In this range, the C60 molecules are ergodic as the pentagonal and hexagonal sectors explore all possible states in distinct energy ranges, just as in the case of ethylene.

At rotation rates between 3.2 and 4.5 GHz, pentagonal and hexagonal sectors exist in the same energy range. "This is because spinning a hexagonal and a pentagonal football can take the same amount of energy," said Liu.

"Nevertheless, C60 still fails to switch between the two rotational sectors because of an energy barrier—the same barrier that prevents a football from flipping end-over-end mid-flight. In this regime, therefore, C60 has broken ergodicity without breaking symmetry. This mechanism of ergodicity breaking without symmetry breaking, which can be understood simply in terms of deformations of a spinning molecule, was a total surprise to us."

These results reveal a rare example of ergodicity breaking without symmetry breaking, giving further insight into the quantum dynamics of the system.

As the researchers surmise, many other molecular species await detailed investigation using the team's new technique. "Molecules will likely harbor many more surprises, and we're excited to discover them."


#ErgodicityTwist: Unraveling the Enigma of Nonconforming Probability Paths #BreakingBounds: Exploring Uncharted Territories of Ergodicity #ErgodicityReimagined: Challenging Assumptions and Redefining Probability Dynamics #BeyondAverages: Navigating the Labyrinthine Landscape of Ergodicity #ErgodicityUnleashed: Liberation from Conventional Probability Trajectories #DisruptingErgodicFlows: Pioneering New Perspectives on Probability Behavior


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Friday 18 August 2023

Study discovers pairing of electrons in artificial atoms, a quantum state predicted more than 50 years ago

 



Researchers from the Department of Physics at Universität Hamburg, observed a quantum state that was theoretically predicted more than 50 years ago by Japanese theoreticians but so far eluded detection. By tailoring an artificial atom on the surface of a superconductor, the researchers succeeded in pairing the electrons of the so-called quantum dot, thereby inducing the smallest possible version of a superconductor. The work appears in the journal Nature.

Usually, electrons repel each other due to their negative charge. This phenomenon has a huge impact on many materials properties such as the electrical resistance. The situation changes drastically if the electrons are "glued" together to pairs thereby becoming bosons. Bosonic pairs do not avoid each other like single electrons, but many of them can reside at the very same location or do the very same motion.

One of the most intriguing properties of a material with such electron pairs is superconductivity, the possibility to let an electrical current flow through the material without any electrical resistance. For many years, superconductivity has found many important technological applications, including magnetic resonance imaging or highly sensitive detectors for magnetic fields.

Today, the continuous downscaling of electronic devices heavily guides investigations on how superconductivity can be induced into much smaller structures at the nanoscale.

Researchers from the Department of Physics and The Cluster of Excellence "CUI: Advanced Imaging of Matter" at Universität Hamburg, have now realized the pairing of electrons in an artificial atom called quantum dot, which is the smallest building block for nanostructured electronic devices.

To that end, researchers led by PD Dr. Jens Wiebe from the Institute for Nanostructure and Solid State Physics locked the electrons into tiny cages that they built from silver, atom-by-atom. By coupling the locked electrons to an elemental superconductor, the electrons inherited the tendency towards pairing from the superconductor.  

Together with a team of theoretical physicists of the Cluster, led by Dr. Thore Posske, the researchers related the experimental signature, a spectroscopic peak at very low energy, to the quantum state predicted in the early 1970s by Kazushige Machida and Fumiaki Shibata.

While the state has so far eluded direct detection by experimental methods, recent work by researchers from the Netherlands and Denmark show it is beneficial for suppressing unwanted noise in transmon qubits, an essential building block of modern quantum computers.
Kazushige Machida wrote to the first author of the publication, Dr. Lucas Schneider: "I thank you for 'discovering' my old paper a half century ago. I thought for [a] long time that transition metal non-magnetic impurities produce the in-gap state, but the location of it is so near the superconducting gap edge [that] it is impossible to prove its existence. But by your ingenious method you have finally checked it to be true experimentally."



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Quantum sensors paving the way for new technologies

 




Increased cooperation between Norwegian industry and universities on quantum physics sensors is a win–win situation for society. Such sensors can provide new opportunities in areas as diverse as mineral extraction and agriculture.

There has been a lot of talk about the potential of quantum computers. Fewer people are aware that there are much more well-developed practical applications of quantum physics that are of direct relevance to Norwegian companies and industry.


Quantum physics effects can be used to create ultra-sensitive sensors to measure things like magnetic fields and changes to the structure, movement and gravitational fields of the Earth. There are clear applications for precision readings of these magnitudes in Norwegian industry, such as in the surveying of land and the seabed for the extraction of minerals and other resources.


Even though quantum computers have the potential to radically advance information technology in the future, the time is now ripe for putting what quantum physics can offer in high-precision readings to work.

Highly sensitive measuring devices

Studies conducted into quantum sensors aim to develop highly sensitive measuring devices using basic principles from quantum mechanics.


Traditional sensors are limited by the sensitivity of the detection methods, but the new techniques could potentially far exceed such limitations.


These measuring devices fully use the wavy nature of matter to measure physical magnitudes such as magnetic and electric fields, temperature, pressure and even gravitational waves.

One example of such a sensor is the atomic clock, which uses the vibrational frequency of atoms to measure time with extreme precision. Other examples of quantum sensors include magnetometers that can detect small magnetic fields.
Many fields of application for quantum sensors

Quantum sensors could revolutionize fields such as navigation and medical imaging.

The same can be said for areas as down-to-earth as mineral exploration and agriculture. In mineral exploration, quantum sensors can be used to detect minerals that are hard to find using traditional exploration methods.

This could allow us to explore for mineral deposits at completely different depths to those we can access today.

It would allow farmers to obtain detailed information about soil fertility, crop health and water use. This information can then be used to optimize crops and reduce waste.

By providing detailed information about soil and mineral properties, this new type of sensor can help reduce the environmental impact from these industries and make them more sustainable, contributing to the green transition.
Quantum sensors central to global community

These are important issues the global community needs to consider in order to sustainably feed an ever-growing population.

There is no doubt that basic research in this field will be essential. Quantum sensors are only one of many examples for which basic research in natural sciences will be crucial in creating important technological breakthroughs.
Norway has world-class professionals

Norway, one of the world's richest nations per capita, has researchers that excel among world leaders in their fields of research and we need to contribute here.

In Norway, major industry players such as Equinor and Yara have the financial strength it will take to support the free basic research that is required. The Research Council of Norway also needs to obtain the necessary funding to finance free basic research to a sufficient extent.
Investment in basic research needed

Basic research in physics is essential to the discovery of new technologies—just look at what happened during the 19th and 20th centuries.

Experiments on electromagnetic waves in the late 1800s laid the foundations for communicating across long distances using radio waves.

Ideas that emerged in the early 1900s on stimulated emission in atoms resulted in laser technology.

Experimental readings of electrical resistance in magnets in the late 1980s, without any intention for practical use, revolutionized magnetic storage technology ten years later. This is the basis of the enormous storage clouds from Google, Apple, Microsoft and Facebook.

Meticulous basic research on the transmission of electromagnetic signals through ultra-pure fiber-optic cables laid the foundation for the world's most important piece of infrastructure, the internet.

There are also many other examples we could mention. A lot of the essential technology we use today has its origins in basic research conducted many decades ago.

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Tuesday 8 August 2023

Who's afraid of quantum computing?




                                                                                       

The road to a quantum future may be longer and more winding than some expect, but the potential it holds is profound, writes UTS Associate Professor Chris Ferrie.

If the Sydney Harbor Bridge was rebuilt today engineers would design, build and test the new bridge in virtual worlds before a sod of dirt was turned.

Digital simulation has revolutionized science and technology, improving efficiency, reducing costs and significantly mitigating risks.

It could also do the same for medicine.

Today, drugs are not designed so much as 'discovered' because digital computers can't simulate the molecular interactions within the human body, and hence can't provide invaluable insights that would propel the development of novel treatments and cures.

Herein lies the promise of quantum computers.

In the future, chemistry will be simulated on quantum computers to design and test new drugs, materials and exotic new forms of matter.

Only time will tell if this will be a utopia, a technological doomsday scenario or just the mundane steady march of progress.

A quantum algorithm is a step-by-step set of instructions that changes quantum information, much like a conventional algorithm is a step-by-step set of instructions that changes digital information (the bits and bytes of your mobile phone and other computers).

Quantum information is encoded by the fine details of energy and matter, revealed over the last century by quantum physics and controllable by precision engineering at the microscopic scale.

Rather than the 0s and 1s of digital technology, quantum bits (or qubits) can be represented by long lists of numbers.

In the 1990s, it was discovered some problems could be solved with far fewer steps if they were encoded in qubits rather than bits.

This shortcut was so enticing, an international scientific effort began to build machines to do this in a fast and reliable way.

These machines are called quantum computers, and 30 years later, proof of principle prototype devices have been successfully built.

Quantum computer science researchers have compiled a list, called the Quantum Algorithm Zoo, of over 60 quantum algorithms that are believed to run in fewer steps than the best classical algorithm for the same problem.

The first on the list is also the most famous—Shor's factoring algorithm. Factoring is the process of breaking a large number (like 21) into the smaller numbers which produce it through multiplication (21 = 7 × 3).

For very large numbers, this is such a difficult problem for digital computers that the vast majority of communication systems (like the internet) use it for security.

However, Shor's algorithm requires far fewer steps to solve the problem, which is a big deal for privacy and security.

Many problems can be thought of as a search for the best solution among a large list of possible solutions.

Grover's search algorithm is another famous quantum algorithm that takes fewer steps to reach an answer than a classical search algorithm for especially difficult problems.

It's not yet known which real world problems will yield a significant practical advantage, but difficult problems of this type abound in critical areas including climate modeling, financial portfolio optimization and artificial intelligence.

More recently, researchers have suggested and provided proof of principle examples of training quantum devices to learn through examples, potentially ushering in a new paradigm of artificial intelligence.

Accurate simulation of chemical interactions require calculations arising from the theory of quantum physics. These are required to design new drugs, fertilizers, batteries and other materials.

The details of how practical a quantum computer might be in any particular instance are yet to be worked out, but a programmable quantum computer could virtually mimic the real world at this fundamental level in principle.

Often, the real transformative power of a technology lies not in its immediate applications, but in the ones that can't be foreseen.

Reflecting on the early days of the internet, few could have predicted the advent of online shopping, social media, or streaming services.

Similarly, while it is anticipated quantum technology will revolutionize fields like cryptography, drug discovery and climate modeling, its ultimate impact could be something that can't yet be conceived.

With all of this potential comes a lot of hype. But that must be tempered with a dose of reality.

In the past decade, quantum computers have slowly moved out of university physics departments into the engineering laboratories of large multinational corporations and start-up companies.

Research has transitioned from pure scientific discovery to being in service of specific engineering challenges. Indeed, these are some of the greatest challenges humanity has ever faced.

Quantum computers currently require extremely low temperatures or ultra-high vacuum to operate.

The degrees of freedom that encode quantum information are fragile—every stray particle they come in contact with is likely to cause an irreparable error.

Whereas the lifetime of a bit currently encoding your digital information might be billions of years, the lifetime of today's qubits is a thousandth of a second.

Still, there has been a steady march of improvement in quantum technology over the past few decades.

History teaches us that technology transitions tend to be slower than initial hype predicts. The transition to quantum technology won't be like flipping a switch—it will continue to be a gradual process.

To bring this all into perspective, it must be remembered that fear often arises from the unknown.

The complexities of quantum technology can be daunting, but that does not mean they are insurmountable.

The road to a quantum future may be longer and more winding than some expect, but the potential it holds is profound. And so, it is with a realistic yet optimistic lens that humanity should approach this emerging technology.

Who's afraid of quantum technology? Perhaps those who fear change, the unknown, or the challenges that inevitably accompany technological breakthroughs.

Yet, embracing quantum technology might be less about overcoming fear and more about fostering understanding, encouraging patience, and maintaining an open mind to the unlimited possibilities this technology promises to bring.

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Sunday 6 August 2023

Scientists discover unusual ultrafast motion in layered magnetic materials

                              

A common metal paper clip will stick to a magnet. Scientists classify such iron-containing materials as ferromagnets. A little over a century ago, physicists Albert Einstein and Wander de Haas reported a surprising effect with a ferromagnet. If you suspend an iron cylinder from a wire and expose it to a magnetic field, it will start rotating if you simply reverse the direction of the magnetic field.

"Einstein and de Haas's experiment is almost like a magic show," said Haidan Wen, a physicist in the Materials Science and X-ray Science divisions of the U.S. Department of Energy's (DOE) Argonne National Laboratory. "You can cause a cylinder to rotate without ever touching it."

In Nature, a team of researchers from Argonne and other U.S. national laboratories and universities now report an analogous yet different effect in an "anti"-ferromagnet. This could have important applications in devices requiring ultra-precise and ultrafast motion control. One example is high-speed nanomotors for biomedical applications, such as use in nanorobots for minimally invasive diagnosis and surgery.

The difference between a ferromagnet and antiferromagnet has to do with a property called electron spin. This spin has a direction. Scientists represent the direction with an arrow, which can point up or down or any direction in between. In the magnetized ferromagnet mentioned above, the arrows associated with all the electrons in the iron atoms can point in the same direction, say, up. Reversing the magnetic field reverses the direction of the electron spins. So, all arrows are pointing down. This reversal leads to the cylinder's rotation.

"In this experiment, a microscopic property, electron spin, is exploited to elicit a mechanical response in a cylinder, a macroscopic object," said Alfred Zong, a Miller Research Fellow at the University of California, Berkeley.

In antiferromagnets, instead of the electron spins all pointing up, for example, they alternate from up to down between adjacent electrons. These opposite spins cancel each other out, and antiferromagnets thus do not respond to changes in a magnetic field as ferromagnets do.

"The question we asked ourselves is, can electron spin elicit a response in an antiferromagnet that is different but similar in spirit to that from the cylinder rotation in the Einstein-de Hass experiment?" Wen said.

To answer that question, the team prepared a sample of iron phosphorus trisulfide (FePS3), an antiferromagnet. The sample consisted of multiple layers of FePS3, with each layer being only a few atoms thick.

"Unlike a traditional magnet, FePS3 is special because it is formed in a layered structure, in which the interaction between the layers is extremely weak," said Xiaodong Xu, professor of physics and materials science at the University of Washington"We designed a set of corroborative experiments in which we shot ultrafast laser pulses at this layered material and measured the resultant changes in material properties with optical, X-ray, and electron pulses," Wen added.

The team found that the pulses change the magnetic property of the material by scrambling the ordered orientation of electron spins. The arrows for electron spin no longer alternate between up and down in an orderly fashion, but are disordered.

"This scrambling in electron spin leads to a mechanical response across the entire sample. Because the interaction between layers is weak, one layer of the sample is able to slide back and forth with respect to an adjacent layer," explained Nuh Gedik, professor of physics at the Massachusetts Institute of Technology (MIT).

This motion is ultrafast, 10 to 100 picoseconds per oscillation. One picosecond equals one trillionth of a second. This is so fast that in one picosecond, light travels a mere third of a millimeter.

Measurements on samples with spatial resolution on the atomic scale and temporal resolution measured in picoseconds require world-class scientific facilities. To that end, the team relied on cutting-edge ultrafast probes that use electron and X-ray beams for analyses of atomic structures.

Motivated by optical measurements at the University of Washington, the initial studies employed the mega-electronvolt ultrafast electron diffraction facility at SLAC National Accelerator Laboratory. Further studies were performed at an ultrafast electron diffraction setup at MIT. These results were complemented by work at the ultrafast electron microscope facility in the Center for Nanoscale Materials (CNM) and the 11-BM and 7-ID beamlines at the Advanced Photon Source (APS). Both CNM and APS are DOE Office of Science user facilities at Argonne.

The electron spin in a layered antiferromagnet also has an effect at longer times than picoseconds. In an earlier study using APS and CNM facilities, members of the team observed that fluctuating motions of the layers slowed down dramatically near the transition from disordered to ordered behavior for the electron spins.

"The pivotal discovery in our current research was finding a link between electron spin and atomic motion that is special to the layered structure of this antiferromagnet," Zong said. "And because this link manifests at such short time and tiny length scales, we envision that the ability to control this motion by changing the magnetic field or, alternatively, by applying a tiny strain will have important implications for nanoscale devices."




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