From Science News: “Physicists exploit a quantum rule to create a new kind of crystal”

From Science News

5.19.20
Emily Conover

Atoms can arrange themselves in regular configurations thanks to the Pauli exclusion principle.

Scientists created a new type of crystal, based only on a quantum rule called the Pauli exclusion principle, by using lasers to confine lithium atoms (illustrated at center) to a region within a vacuum chamber. Credit S. Jochim Group/Heidelberg Univ.

Physicists have harnessed the aloofness of quantum particles to create a new type of crystal.

Some particles shun one another because they are forbidden to take on the same quantum state as their neighbors. Atoms can be so reluctant to overlap that they form a crystal-like arrangement even when they aren’t exerting any forces on one another, physicists report May 8 at arXiv.org [ https://arxiv.org/abs/2005.03929 ]. Called a Pauli crystal, the configuration is the result of a quantum mechanical rule called the Pauli exclusion principle.

Scientists had previously predicted the existence of Pauli crystals, but no one had observed them until now. “It just teaches us how beautiful physics is,” says quantum physicist Tilman Esslinger of ETH Zürich. The experiment reveals there are still new phenomena to be observed from a foundational principle taught in introductory physics classes. “If I wrote a textbook,” Esslinger says, “I would put that [experiment] in.”

Although the Pauli crystals themselves are based on known physics, the technique used to observe them could help scientists better understand certain mysterious states of matter, such as superconductors, materials that conduct electricity without resistance, or superfluids, which flow without friction.

Discovered by Austrian physicist Wolfgang Pauli in 1925, the Pauli exclusion principle forbids electrons within an atom from acquiring matching sets of quantum properties, such as energy and angular momentum (SN: 4/10/99). Physicists soon realized that the rule governs not only electrons but an entire class of particles called fermions, which in addition to electrons includes protons, neutrons and many types of atoms. As a result, fermions can repel one another without directly interacting. Whereas typical crystals form their regular arrangements thanks to electromagnetic interactions, a Pauli crystal forms only due to this repulsion.

“It’s the most simple state of matter that you can imagine,” says Selim Jochim of Heidelberg University in Germany.

Jochim and colleagues created their Pauli crystal out of lithium atoms, corralled by lasers into a two-dimensional region about a micrometer in radius. The researchers put groups of three or six atoms in that trap at a time. The atoms were too close together to directly image their positions to reveal any crystal-like structure. Instead, the team measured the atoms’ momenta by watching where the particles traveled when released. After the experiment was repeated many times, the researchers found correlations, or patterns, in the atoms’ momenta.

Flower-shaped patterns appear in the momenta of atoms due to the Pauli exclusion principle. These structures differ depending on the number of atoms involved: three (plotted on the left) or six (right).Credit S. Jochim Group/Heidelberg Univ.

Because position and momentum are closely related properties for these trapped particles, the relationship between the momenta also means that the atoms formed a regular spatial configuration akin to a crystal. Different flower-shaped configurations of the particles’ momenta arose depending on the number of particles in the trap.

“You can really see this pattern,” says Magdalena Załuska-Kotur of the Institute of Physics of the Polish Academy of Sciences, part of a team of physicists that had previously predicted [Nature]that such structures could be observed in this type of experiment.

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From SLAC National Accelerator Lab: “In a step forward for orbitronics, scientists break the link between a quantum material’s spin and orbital states”

From SLAC National Accelerator Lab

May 15, 2020
Glennda Chui

Credit SLAC National Accelerator Laboratory

The advance opens a path toward a new generation of logic and memory devices that could be 10,000 times faster than today’s.

In designing electronic devices, scientists look for ways to manipulate and control three basic properties of electrons: their charge; their spin states, which give rise to magnetism; and the shapes of the fuzzy clouds they form around the nuclei of atoms, which are known as orbitals.

Until now, electron spins and orbitals were thought to go hand in hand in a class of materials that’s the cornerstone of modern information technology; you couldn’t quickly change one without changing the other. But a study at the Department of Energy’s SLAC National Accelerator Laboratory shows that a pulse of laser light can dramatically change the spin state of one important class of materials while leaving its orbital state intact.

The results suggest a new path for making a future generation of logic and memory devices based on “orbitronics,” said Lingjia Shen, a SLAC research associate and one of the lead researchers for the study.

“What we’re seeing in this system is the complete opposite of what people have seen in the past,” Shen said. “It raises the possibility that we could control a material’s spin and orbital states separately, and use variations in the shapes of orbitals as the 0s and 1s needed to make computations and store information in computer memories.”

The international research team, led by Joshua Turner, a SLAC staff scientist and investigator with the Stanford Institute for Materials and Energy Sciences (SIMES), reported their results this week in Physical Review B Rapid Communications.

These balloon-and-disk shapes represent an electron orbital – a fuzzy electron cloud around an atom’s nucleus – in two different orientations. Scientists hope to someday use variations in the orientations of orbitals as the 0s and 1s needed to make computations and store information in computer memories, a system known as orbitronics. A SLAC study shows it’s possible to separate these orbital orientations from electron spin patterns, a key step for independently controlling them in a class of materials that’s the cornerstone of modern information technology. (Greg Stewart/SLAC National Accelerator Laboratory)

An intriguing, complex material

The material the team studied was a manganese oxide-based quantum material known as NSMO, which comes in extremely thin crystalline layers. It’s been around for three decades and is used in devices where information is stored by using a magnetic field to switch from one electron spin state to another, a method known as spintronics. NSMO is also considered a promising candidate for making future computers and memory storage devices based on skyrmions, tiny particle-like vortexes created by the magnetic fields of spinning electrons.

But this material is also very complex, said Yoshinori Tokura, director of the RIKEN Center for Emergent Matter Science in Japan, who was also involved in the study.

“Unlike semiconductors and other familiar materials, NSMO is a quantum material whose electrons behave in a cooperative, or correlated, manner, rather than independently as they usually do,” he said. “This makes it hard to control one aspect of the electrons’ behavior without affecting all the others.”

One common way to investigate this type of material is to hit it with laser light to see how its electronic states respond to an injection of energy. That’s what the research team did here. They observed the material’s response with X-ray laser pulses from SLAC’s Linac Coherent Light Source (LCLS) [below].

One melts, the other doesn’t

What they expected to see was that orderly patterns of electron spins and orbitals in the material would be thrown into total disarray, or “melted,” as they absorbed pulses of near-infrared laser light.

But to their surprise, only the spin patterns melted, while the orbital patterns stayed intact, Turner said. The normal coupling between the spin and orbital states had been completely broken, he said, which is a challenging thing to do in this type of correlated material and had not been observed before.

In SLAC experiments, scientists hit a quantum material with pulses of laser light (top) to see how this would affect zigzag patterns (middle) in its atomic lattice made by the spin directions of electrons (black arrows) and the orientations of electron orbitals (red balloon shapes). They were surprised to discover that the pulses disrupted the spin patterns while leaving the orbital patterns intact (bottom). This raises the possibility that spin and orbital states could be independently controlled to make much faster electronic devices. (Greg Stewart/SLAC National Accelerator Laboratory)

Tokura said, “Usually only a tiny application of photoexcitation destroys everything. Here, they were able to keep the electron state that is most important for future devices – the orbital state – undamaged. This is a nice new addition to the science of orbitronics and correlated electrons.”

Much as electron spin states are switched in spintronics, electron orbital states could be switched to provide a similar function. These orbitronic devices could, in theory, operate 10,000 faster than spintronic devices, Shen said.

Switching between two orbital states could be made possible by using short bursts of terahertz radiation, rather than the magnetic fields used today, he said: “Combining the two could achieve much better device performance for future applications.” The team is working on ways to do that.

Shen is now a postdoctoral researcher at Lund University in Sweden with a joint position with SIMES at SLAC. Scientists from the Advanced Light Source at DOE’s Lawrence Berkeley National Laboratory; the Swiss Light Source at the Paul Scherrer Institute in Sweden; the University of Tokyo and University of Tsukuba in Japan; and the University of Chicago also contributed to this research. Both LCLS and the Advanced Light Source are DOE Office of Science user facilities, and major support for the study came from the DOE Office of Science. Turner’s research was supported through the DOE Office of Science Early Career Research Program.

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SLAC National Accelerator Lab

SLAC/LCLS

SLAC/LCLS II projected view

SLAC is a multi-program laboratory exploring frontier questions in photon science, astrophysics, particle physics and accelerator research. Located in Menlo Park, California, SLAC is operated by Stanford University for the DOE’s Office of Science.

SSRL and LCLS are DOE Office of Science user facilities.

From Quanta Magazine: “What Goes On in a Proton? Quark Math Still Conflicts With Experiments”

From Quanta Magazine

May 6, 2020
Charlie Wood

The quark structure of the proton 16 March 2006 Arpad Horvath

Objects are made of atoms, and atoms are likewise the sum of their parts — electrons, protons and neutrons. Dive into one of those protons or neutrons, however, and things get weird. Three particles called quarks ricochet back and forth at nearly the speed of light, snapped back by interconnected strings of particles called gluons. Bizarrely, the proton’s mass must somehow arise from the energy of the stretchy gluon strings, since quarks weigh very little and gluons nothing at all.

Physicists uncovered this odd quark-gluon picture in the 1960s and matched it to an equation in the ’70s, creating the theory of quantum chromodynamics (QCD). The problem is that while the theory seems accurate, it is extraordinarily complicated mathematically. Faced with a task like calculating how three wispy quarks produce the hulking proton, QCD simply fails to produce a meaningful answer.

“It’s tantalizing and frustrating,” said Mark Lancaster, a particle physicist based at the University of Manchester in the United Kingdom. “We know absolutely that quarks and gluons interact with each other, but we can’t calculate” the result.

A million-dollar math prize awaits anyone who can solve the type of equation used in QCD to show how massive entities like protons form. Lacking such a solution, particle physicists have developed arduous workarounds that deliver approximate answers. Some infer quark activity experimentally at particle colliders, while others harness the world’s most powerful supercomputers. But these approximation techniques have recently come into conflict, leaving physicists unsure exactly what their theory predicts and thus less able to interpret signs of new, unpredicted particles or effects.

To understand what makes quarks and gluons such mathematical scofflaws, consider how much mathematical machinery goes into describing even well-behaved particles.

A humble electron, for instance, can briefly emit and then absorb a photon. During that photon’s short life, it can split into a pair of matter-antimatter particles, each of which can engage in further acrobatics, ad infinitum. As long as each individual event ends quickly, quantum mechanics allows the combined flurry of “virtual” activity to continue indefinitely.

In the 1940s, after considerable struggle, physicists developed mathematical rules that could accommodate this bizarre feature of nature. Studying an electron involved breaking down its virtual entourage into a series of possible events, each corresponding to a squiggly drawing known as a Feynman diagram and a matching equation. A perfect analysis of the electron would require an infinite string of diagrams — and a calculation with infinitely many steps — but fortunately for the physicists, the more byzantine sketches of rarer events ended up being relatively inconsequential. Truncating the series gives good-enough answers.

The discovery of quarks in the 1960s broke everything. By pelting protons with electrons, researchers uncovered the proton’s internal parts, bound by a novel force. Physicists raced to find a description that could handle these new building blocks, and they managed to wrap all the details of quarks and the “strong interaction” that binds them into a compact equation in 1973. But their theory of the strong interaction, quantum chromodynamics, didn’t behave in the usual way, and neither did the particles.

Feynman diagrams treat particles as if they interact by approaching each other from a distance, like billiard balls. But quarks don’t act like this. The Feynman diagram representing three quarks coming together from a distance and binding to one another to form a proton is a mere “cartoon,” according to Flip Tanedo, a particle physicist at the University of California, Riverside, because quarks are bound so strongly that they have no separate existence. The strength of their connection also means that the infinite series of terms corresponding to the Feynman diagrams grows in an unruly fashion, rather than fading away quickly enough to permit an easy approximation. Feynman diagrams are simply the wrong tool.

The strong interaction is weird for two main reasons. First, whereas the electromagnetic interaction involves just one variety of charge (electric charge), the strong interaction involves three: “color” charges nicknamed red, green and blue. Weirder still, the carrier of the strong interaction, dubbed the gluon, itself bears color charge. So while the (electrically neutral) photons that comprise electromagnetic fields don’t interact with each other, collections of colorful gluons draw together into strings. “That really drives the differences we see,” Lancaster said. The ability of gluons to trip over themselves, together with the three charges, makes the strong interaction strong — so strong that quarks can’t escape each other’s company.

Evidence piled up over the decades that gluons exist and act as predicted in certain circumstances. But for most calculations, the QCD equation has proved intractable. Physicists need to know what QCD predicts, however — not just to understand quarks and gluons, but to pin down properties of other particles as well, since they’re all affected by the dance of quantum activity that includes virtual quarks.

One approach has been to infer incalculable values by watching how quarks behave in experiments. “You take electrons and positrons and slam them together,” said Chris Polly, a particle physicist at the Fermi National Accelerator Laboratory, “and ask how often you make quark [products] in the final state.” From those measurements, he said, you can extrapolate how often quark bundles should pop up in the hubbub of virtual activity that surrounds all particles.

Other researchers have continued to try to wring information from the canonical QCD equation by calculating approximate solutions using supercomputers. “You just keep throwing more computing cycles at it and your answer will keep getting better,” said Aaron Meyer, a particle physicist at Brookhaven National Laboratory.

This computational approach, known as lattice QCD, turns computers into laboratories that model the behavior of digital quarks and gluons. The technique gets its name from the way it slices space-time into a grid of points. Quarks sit on the lattice points, and the QCD equation lets them interact. The denser the grid, the more accurate the simulation. The Fermilab physicist Andreas Kronfeld remembers how, three decades ago, these simulations had just a handful of lattice points on a side. But computing power has increased, and lattice QCD can now successfully predict the proton’s mass to within a few percent of the experimentally determined value.

Kronfeld is a spokesperson for USQCD, a federation of lattice QCD groups in the United States that have banded together to negotiate for bulk supercomputer time. He serves as the principal investigator for the federation’s efforts on the Summit supercomputer, currently the world’s fastest, located at Oak Ridge National Laboratory. USQCD runs one of Summit’s largest programs, occupying nearly 4% of the machine’s annual computing capacity.

ORNL IBM AC922 SUMMIT supercomputer, No.1 on the TOP500. Credit: Carlos Jones, Oak Ridge National Laboratory/U.S. Dept. of Energy

Theorists thought these digital laboratories were still a year or two away from becoming competitive with the collider experiments in approximating the effects quarks have on other particles. But in February a European collaboration shocked the community with a preprint claiming to nail a magnetic property of a particle called the muon to within 1% of its true value, using novel noise reduction techniques. “You might think of it as throwing down the gauntlet,” said Aida El-Khadra, a high-energy theorist at the University of Illinois, Urbana-Champaign.

The team’s prediction for virtual quark activity around the muon clashed with the inferences from electron-positron collisions, however. Meyer, who recently co-authored a survey of the conflicting results, says that many technical details in lattice QCD remain poorly understood, such as how to hop from the gritty lattice back to smooth space. Efforts to determine what QCD predicts for the muon, which many researchers consider a bellwether for undiscovered particles, are ongoing.

Meanwhile, mathematically minded researchers haven’t entirely despaired of finding a pen-and-paper strategy for tackling the strong interaction — and reaping the million-dollar reward offered by the Clay Mathematics Institute for a rigorous prediction of the mass of the lightest possible collection of quarks or gluons.

One such Hail Mary pass in the theoretical world is a tool called the holographic principle. The general strategy is to translate the problem into an abstract mathematical space where some hologram of quarks can be separated from each other, allowing an analysis in terms of Feynman diagrams.

Simple attempts look promising, according to Tanedo, but none come close to the hard-won accuracy of lattice QCD. For now, theorists will continue to refine their imperfect tools and dream of new mathematical machinery capable of taming the fundamental but inseparable quarks.

“That would be the holy grail,” Tanedo says. QCD is “just begging for us to figure out how that actually works.”

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Formerly known as Simons Science News, Quanta Magazine is an editorially independent online publication launched by the Simons Foundation to enhance public understanding of science. Why Quanta? Albert Einstein called photons “quanta of light.” Our goal is to “illuminate science.” At Quanta Magazine, scientific accuracy is every bit as important as telling a good story. All of our articles are meticulously researched, reported, edited, copy-edited and fact-checked.

From Max Planck Gesellschaft Institute for Nuclear Physics via phys.org: “Successfully measuring infinitesimal change in mass of individual atoms for the first time”

From Max Planck Gesellschaft Institute for Nuclear Physics

via

phys.org

Measurements at space-like temperatures: Pentatrap is located in a large superconducting magnet. The inside of the vessel is cooled to a temperature near absolute zero so that disturbing heat movements of the atoms are frozen. Because individuals in the room would influence the measurements by their body temperature, among other things, nobody is allowed to enter the laboratory during the experiment. The system is remote controlled. Credit: MPI for Nuclear Physics.

A new door to the quantum world has been opened: When an atom absorbs or releases energy via the quantum leap of an electron, it becomes heavier or lighter. This can be explained by Einstein’s theory of relativity (E = mc2). However, the effect is minuscule for a single atom. Nevertheless, the team of Klaus Blaum and Sergey Eliseev at the Max Planck Institute for Nuclear Physics has successfully measured this infinitesimal change in the mass of individual atoms for the first time. In order to achieve this, they used the ultra-precise Pentatrap atomic balance at the Institute in Heidelberg. The team discovered a previously unobserved quantum state in rhenium, which could be interesting for future atomic clocks. Above all, this extremely sensitive atomic balance enables a better understanding of the complex quantum world of heavy atoms.

Astonishing, but true: If you wind a mechanical watch, it becomes heavier. The same thing happens when you charge your smartphone. This can be explained by the equivalence of energy (E) and mass (m), which Einstein expressed in the most famous formula in physics: E = mc2 (c: speed of light in vacuum). However, this effect is so small that it completely eludes our everyday experience. A conventional balance would not be able to detect it.

But at the Max Planck Institute for Nuclear Physics in Heidelberg, there is a balance that can: Pentatrap. It can measure the minuscule change in mass of a single atom when an electron absorbs or releases energy via a quantum jump, thus opening a new world for precision physics. Such quantum jumps in the electron shells of atoms shape our world—whether in life-giving photosynthesis and general chemical reactions or in the creation of colour and our vision.

An ant on top of an elephant

Rima Schüssler, now a postdoctoral fellow at the Max Planck Institute for Nuclear Physics, has helped build Pentatrap since completing her Master’s thesis in 2014. She is the lead author of a paper [Nature] on an unexpected discovery made in a collaboration at the Max Planck PTB Riken Centre: In rhenium, there is a previously undiscovered electronic quantum state with special properties. Schüssler uses the following analogy to describe the degree of sensitivity with which Pentatrap can detect the jump of an electron into this quantum state via the mass change of a rhenium atom: “By weighing a six-tonne elephant, we were able to determine whether a ten-milligram ant was crawling on it.”

Pentatrap consists of five Penning traps. In order for such a trap to be able to weigh an atom, it must be electrically charged (i.e. become an ion). Because rhenium was stripped of 29 of its 75 electrons, it is highly charged. This dramatically increases the accuracy of the measurement. The trap captures this highly charged rhenium ion in a combination of a magnetic field and a specially shaped electric field. Inside, it travels in a circular path, which is intricately twisted into itself. In principle, it can be thought of as a ball on a rope, which is allowed to rotate in the air. If this is done with constant force, a heavier ball rotates slower than a lighter one.

An extremely precise atomic balance: Pentatrap consists of five Penning traps arranged one above the other (yellow tower in the middle). In these identically constructed traps, ions in the excited quantum state and in the ground state can be measured in comparison. In order to minimize uncertainties, the ions are also moved back and forth between different traps for comparative measurements. Credit: MPI for Nuclear Physics.

In Pentatrap, two rhenium ions rotated alternately in the stacked traps. One ion was in the energetically lowest quantum state. When the second ion was generated, an electron was randomly excited into a higher state by supplying energy. In a sense, it was the wound watch. Because of the stored energy, it became marginally heavier and thus circulated slower than the first ion. Pentatrap precisely counts the number of revolutions per time unit. The difference in the number of revolutions yielded the increase in weight.

Using this method, the team discovered an extremely long-lived quantum state in rhenium. It is metastable (i.e. it decays after a certain lifetime). According to the calculations of theoreticians from the institute led by Zoltán Harman and Christoph H. Keitel, the University of Heidelberg, and the Kastler Brossel Laboratory in Paris, this is 130 days. The position of the quantum state also agrees quite well with model calculations using state-of-the-art quantum mechanical methods.

Possible application in future atomic clocks

Such excited electronic states in highly charged ions are interesting for basic research as well as for possible application in future atomic clocks as researched by the working group of José Crespo López-Urrutia at the Institute in cooperation with the Physikalisch-Technische Bundesanstalt (PTB). For them, the metastable state in rhenium is attractive for several reasons. First, because of its longevity, it corresponds to a sharp orbital frequency of the electron around the atomic nucleus. Second, the electron can be excited with soft X-ray light to jump into this quantum state. In principle, such a clock could tick faster and therefore even more accurately than the current generation of optical atomic clocks. However, according to Ekkehard Peik, who is in charge of the Time and Frequency Department at PTB and who was not involved in the work, it is still too early to speculate whether the discovery could be suitable for a new generation of atomic clocks.

“Nevertheless, this new method for discovering long-lived quantum states is spectacular,” says the physicist. He imagines that atomic clocks working with such new quantum states could initially offer a new test field for basic research. Because the rhenium ions lack many mutually shielding electrons, the remaining electrons feel the electric field of the atomic nucleus particularly strongly. The electrons therefore race around the nucleus at such high speeds that their motion must be described using Einstein’s theory of special relativity. With the new atomic balance, it would also be possible to test with high precision whether special relativity and quantum theory interact as described by this theory.

In general, the new atomic balance offers a novel access to the quantum-like inner life of heavier atoms. Because these consist of many particles—electrons, protons, and neutrons—they cannot be calculated exactly. The atomic models for theoretical calculations are therefore based on simplifications, and these can now be checked extremely accurately. It might be possible to use such atoms as probes in the search for unknown particles, which can be detected only by the extremely weak gravitational force. This dark matter is one of the greatest unsolved mysteries of physics.

On the path to new physics

An important step towards the access of new physics with atomic-physical methods was also achieved with Pentatrap [Phys. Rev. Lett. 124, 113001]. The Heidelberg researchers carried out mass measurements on a chain of five pairs of xenon isotopes. Using high-resolution laser spectroscopy on similar chains of other elements such as calcium and ytterbium, a linear relationship can be inferred from the small energy differences (isotope shift). Nonlinear deviations from this can, however, be an indication of new physics (further fundamental interactions, new particles, dark matter), which manifests itself under extremely precise observation—an alternative to high-energy experiments. Here too, close cooperation with theory (group of Zoltan Harman at MPIK) should be emphasized. The direct measurement of the binding energy of an electron in a highly charged ion shows a very good agreement with relativistic atomic structure calculations. This creates the basis e.g. for future high-precision tests of quantum electrodynamics.

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The Max-Planck-Institut für Kernphysik (“MPI for Nuclear Physics” or MPIK for short) is a research institute in Heidelberg, Germany.

The institute is one of the 80 institutes of the Max-Planck-Gesellschaft (Max Planck Society), an independent, non-profit research organization. The Max Planck Institute for Nuclear Physics was founded in 1958 under the leadership of Wolfgang Gentner. Its precursor was the Institute for Physics at the MPI for Medical Research.

The Max Planck Society is Germany’s most successful research organization. Since its establishment in 1948, no fewer than 18 Nobel laureates have emerged from the ranks of its scientists, putting it on a par with the best and most prestigious research institutions worldwide. The more than 15,000 publications each year in internationally renowned scientific journals are proof of the outstanding research work conducted at Max Planck Institutes – and many of those articles are among the most-cited publications in the relevant field.

What is the basis of this success? The scientific attractiveness of the Max Planck Society is based on its understanding of research: Max Planck Institutes are built up solely around the world’s leading researchers. They themselves define their research subjects and are given the best working conditions, as well as free reign in selecting their staff. This is the core of the Harnack principle, which dates back to Adolph von Harnack, the first president of the Kaiser Wilhelm Society, which was established in 1911. This principle has been successfully applied for nearly one hundred years. The Max Planck Society continues the tradition of its predecessor institution with this structural principle of the person-centered research organization.

The currently 83 Max Planck Institutes and facilities conduct basic research in the service of the general public in the natural sciences, life sciences, social sciences, and the humanities. Max Planck Institutes focus on research fields that are particularly innovative, or that are especially demanding in terms of funding or time requirements. And their research spectrum is continually evolving: new institutes are established to find answers to seminal, forward-looking scientific questions, while others are closed when, for example, their research field has been widely established at universities. This continuous renewal preserves the scope the Max Planck Society needs to react quickly to pioneering scientific developments.

From University of Innsbruck: “Protocol identifies fascinating quantum states”

From University of Innsbruck

Image: A particularly fascinating class of quantum states are topological states of matter. (Credit: IQOQI Innsbruck/Harald Ritsch)

Nowadays, modern quantum simulators offer a wide range of possibilities to prepare and investigate complex quantum states. They are realized with ultracold atoms in optical lattices, Rydberg atoms, trapped ions or superconducting quantum bits. A particularly fascinating class of quantum states are topological states of matter. David Thouless, Duncan Haldane and Michael Kosterlitz were awarded the Nobel Prize in Physics in 2016 for their theoretical discovery. These states of matter are characterized by nonlocal quantum correlations and are particularly robust against local distortions that inevitably occur in experiments. “Identifying and characterizing such topological phases in experiments is a great challenge,” say Benoît Vermersch, Jinlong Yu and Andreas Elben from the Center for Quantum Physics at the University of Innsbruck and the Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences. “Topological phases cannot be identified by local measurements because of their special properties. We are therefore developing new measurement protocols that will enable experimental physicists to characterize these states in the laboratory”. In recent years this has already been achieved for non-interacting systems. However, for interacting systems, which in the future could also be used as topological quantum computers, this has not been possible so far.

With random measurements to a definite result

In Science Advances, the physicists of Peter Zoller’s research group now propose measurement protocols that enable the measurement of so-called topological invariants. These mathematical expressions describe common properties of topological spaces and make it possible to fully identify interacting topological states with global symmetry in one-dimensional, bosonic systems. “The idea of our method is to first prepare such a topological state in a quantum simulator. Now so-called random measurements are performed, and topological invariants are extracted from statistical correlations of these random measurements,” explains Andreas Elben. The specific feature of this method is that although the topological invariants are highly complex, non-local correlation functions, they can still be extracted from statistical correlations of simple, local random measurements. As with a method recently presented by the research group for comparing quantum states in computers or simulators, such random measurements are possible in experiments today. “Our protocols for measuring the topological invariants can therefore be directly applied in the existing experimental platforms,” says Benoît Vermersch.

The measurement method was developed by Andreas Elben, Jinlong Yu, Peter Zoller and Benoit Vermersch in Innsbruck in close cooperation with Guanyu Zhu from the Joint Quantum Institute, Maryland, USA and IBM Research, Mohammad Hafezi (JQI Maryland) and Frank Pollmann from the Technical University of Munich. The research was financially supported by the European Research Council and the EU flagship for quantum technologies, among others.

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The University of Innsbruck is currently the largest education facility in the Austrian Bundesland of Tirol, the third largest in Austria behind Vienna University and the University of Graz and according to The Times Higher Education Supplement World Ranking 2010 Austria’s leading university. Significant contributions have been made in many branches, most of all in the physics department. Further, regarding the number of Web of Science-listed publications, it occupies the third rank worldwide in the area of mountain research. In the Handelsblatt Ranking 2015, the business administration faculty ranks among the 15 best business administration faculties in German-speaking countries.

From Science News: “Quantum mechanics means some black hole orbits are impossible to predict”

From Science News

April 6, 2020
Emily Conover

For a trio of orbiting black holes (illustrated), even if their locations are known to the quantum limit, it’s not always possible to predict their future paths. Credit: NASA, CXC, A. Hobart, T. Tibbitts

Even if you could measure three black holes’ locations as precisely as physically possible, you still might not know where the black holes would go. Such a trio’s complex dance can be so chaotic that the motions are fundamentally unpredictable, new computer simulations show.

The paths of three black holes orbiting each other can be calculated based on their positions and velocities at one point in time. But in some cases, the orbits depend so sensitively on the black holes’ exact positions that the uncertainty of quantum physics comes into play. Tiny quantum uncertainties in specifying the locations of objects can explode as the black holes’ gyrations continue over tens of millions of years, astrophysicist Tjarda Boekholt and colleagues report in the April Monthly Notices of the Royal Astronomical Society. So the distant future of the black holes’ orbits is impossible to foresee.

Such extreme sensitivity to initial conditions is known as chaos. The new study suggests, in the case of three black holes, “quantum mechanics imprints into the universe chaos at a fundamental level,” says astrophysicist Nathan Leigh of Universidad de Concepción in Chile, who was not involved with the research.

In chaotic systems, tiny changes can generate wildly different outcomes. The classic example is a butterfly flapping its wings, thereby altering weather patterns, possibly producing a distant tornado that otherwise wouldn’t have formed (SN: 9/16/13). This chaos also shows up in the orbits of three black holes and other collections of three or more objects, making such orbits difficult to calculate, a conundrum known as the three-body problem.

To test whether the black holes’ motions were predictable, Boekholt, of the University of Coimbra in Portugal, and colleagues checked if they could run computer simulations of the orbits both forward and backward and achieve the same result. Starting with a given set of locations for three initially stationary black holes, the researchers evolved those orbits forward in time to an end point tens of millions of years in the future. Then, they rewound the simulation, reversing the motions to see if the black holes ended up where they started from.

Computer simulations have a limited level of accuracy. In this case, for example, the locations of black holes were known only to a certain number of decimal places. That tiny imprecision can balloon over millions of years of the simulation.

According to quantum mechanics, it is impossible to determine the position of any object better than an utterly tiny distance called the Planck length, about 1.6 times 10-35 meters, or 16 billionths of a trillionth of a trillionth of a millimeter (SN: 4/8/11). Yet even with accuracy the size of the Planck length, the researchers found that about 5 percent of the time the three black holes wouldn’t return to the same spots when the simulation was reversed. That means, even if you measured where the black holes were to the quantum mechanical limit, you couldn’t rewind to find out where they had come from.

“These systems are fundamentally irreversible,” says Boekholt. “You can’t go forwards and backwards for these 5 percent of systems in nature. And that was quite a surprising result.”

The result is theoretical and can’t be applied to real black holes, says astrophysicist Nicholas Stone of the Hebrew University of Jerusalem. For example, measurement errors would swamp the importance of quantum physics. But that doesn’t detract from the study’s importance, he says: “It is still quite interesting from a conceptual perspective.”

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From Penn Engineering: “Penn Engineers Ensure Quantum Experiments Get Off to the Right Start”

From University of Pennsylvania Engineering

Feb 17, 2020

Quantum experiments that utilize a defect within diamond to store information have to contend with uncertainty, specifically, the number of electrons trapped at that defect when the experiment begins. Penn Engineers have now developed an initialization procedure that addresses this problem. (Illustration: Ann Sizemore Blevins)

Tzu-Yung Huang, Lee Bassett and David Hopper in the Quantum Engineering Laboratory. (Image: Penn Engineering)

The quantum mechanical properties of electrons are beginning to open the door to a new class of sensors and computers with abilities far beyond what their counterparts based in classical physics can accomplish. Quantum states are notoriously difficult to read or write, however, and to make things worse, uncertainty about those states’ starting conditions can make experiments more laborious or even impossible.

Now, Penn Engineers have devised a system to reset those starting conditions, test them to see whether they are correct, and automatically start the experiment if they are, all in a matter of microseconds.

This new “initialization procedure” will save quantum researchers the time and effort of re-running experiments to statistically account for uncertain starting states, and enable new kinds of measurements that require exact starting conditions to be run at all.

Lee Bassett, assistant professor in the Department of Electrical and Systems Engineering and director of the Quantum Engineering Laboratory, along with lab members David Hopper and Joseph Lauigan, led a recent study demonstrating this new initialization procedure. Lab member Tzu-Yung Huang also contributed to the study.

It was published in the journal Physical Review Applied.

“Initialization is one of the key, fundamental requirements for doing almost any kind of quantum-information processing,” Bassett says. “You need to be able to deterministically set your quantum state before you can do anything useful with it, but the dirty little secret is that, in almost all quantum architectures, that initialization is not perfect.”

“Some of the time,” Hopper says, “we can accept that uncertainty, and by running an experimental protocol many thousands of times, come up with a measurement we’re ultimately confident in. But there are other experiments we’d like to do where this type of averaging over multiple runs won’t work.”

The particular type of uncertainty the researchers investigated has to do with a commonly used quantum system known as a nitrogen-vacancy (NV) center in diamond. These NV centers are defects that naturally occur within diamond, where the regular lattice of carbon atoms is occasionally disrupted with a nitrogen atom and a vacant spot next to it. The electron clouds of neighboring atoms overlap at this empty space, creating a “trapped molecule” in the diamond that can be probed with a laser, allowing researchers to measure, or alter, the electrons’ quantum property known as “spin.”

The electrons trapped at an NV center form a “qubit” — the basic unit of quantum information — that can be used to sense local fields, store quantum superposition states, and even perform quantum computations.

“Electrons are excellent magnetic sensors,” Bassett says, “and they can even detect the tiny magnetic fields associated with carbon nuclei surrounding the defect. Those nuclei can serve as qubits themselves and be controlled using the central electron to build up the entangled quantum states that form the basis of quantum computers. They also couple to photons, which are used to transmit quantum information over long distances. So NV centers really merge the three main areas of quantum science: sensing, communication and computation.”

As promising as NV centers are, researchers still must contend with an uncertain variable: the number of electrons that are trapped at the NV center when an experiment starts, as electrons can hop in and out of the defect when it is illuminated with a laser. An initialization procedure that guarantees a predictable number of electrons every time would reduce the amount of time it takes to successfully run an experiment, or enable experiments where uncertain starting conditions can’t be statistically corrected for after the fact.

“The NV center is like a box with a coin inside,” Lauigan says. “If we want to do our experiment only when the coin is on heads, we have to shake the box, check the coin, and repeat until we find that it landed the right way up. That’s the initialization procedure.”

To execute this initialization, the researchers used a pair of lasers, photon detectors and specialized hardware that could handle the precise timing necessary.

“We shine a green laser at the NV center, which basically ‘flips the coin’ and mixes up the number of electrons that are trapped in the defect,” Hopper says. “Then we come in with a red laser, and depending on the number of electrons that are there, the defect will either emit a photon or remain dark.”

“Once we detect the photon that tells us the right number of electrons are in the defect, specialized circuitry automatically starts the experiment,” Huang says. “This all happens in about 500 nanoseconds; there isn’t time to have the signal analyzed by a normal computer, so it all has to happen on these specialized chips called field programmable gate arrays.”

The researchers leveraged the power of advanced classical electronics to better control a particular quantum sensing system. They showed that, thanks to ideal starting conditions, their device can detect a tiny oscillating magnetic field of only 1.3 nanoteslas in one second of measurements, which is a sensitivity record for room-temperature quantum sensors based on single NV centers.

The researchers’ initialization procedure may also help hasten progress on new quantum architectures for computation and communication. Diamond is typically composed of two stable isotopes of carbon, carbon-12 and carbon-13. The former is the most common, but every few tenths of a nanometer, there is an atom of the latter. And because carbon-13 has an extra neutron, it exhibits nuclear spin and can be used as a qubit.

An NV center can be a “handle” for controlling those nuclear-spin qubits in a quantum computer, but in this situation the ability to precisely initialize its state becomes crucial. The errors associated with poor initialization multiply, and it quickly becomes impossible to perform a complex calculation. The type of real-time measurement and control used by the team in this work is a major step towards implementing more sophisticated error-correcting protocols in these quantum devices.

In the near term, the improved sensing ability will be useful in determining the locations of carbon-13 atoms in the diamond lattice.

“Finding all of those special carbon atoms is a laborious process, since there are so many atoms and each measurement takes a very long time,” Hopper says. “When we started this project, our goal was to see what was making those measurements take so long and whether there was any way to shorten it.”

The research was supported by the National Science Foundation under awards ECCS-1553511 and ECCS-1842655.

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From MIT News: “Correcting the “jitters” in quantum devices”

From MIT News

February 18, 2020
David L. Chandler

In a diamond crystal, three carbon atom nuclei (shown in blue) surround an empty spot called a nitrogen vacancy center, which behaves much like a single electron (shown in red). The carbon nuclei act as quantum bits, or qubits, and it turns out the primary source of noise that disturbs them comes from the jittery “electron” in the middle. By understanding the single source of that noise, it becomes easier to compensate for it, the researchers found. Image: David Layden.

A new study suggests a path to more efficient error correction, which may help make quantum computers and sensors more practical.

Labs around the world are racing to develop new computing and sensing devices that operate on the principles of quantum mechanics and could offer dramatic advantages over their classical counterparts. But these technologies still face several challenges, and one of the most significant is how to deal with “noise” — random fluctuations that can eradicate the data stored in such devices.

A new approach developed by researchers at MIT could provide a significant step forward in quantum error correction. The method involves fine-tuning the system to address the kinds of noise that are the most likely, rather than casting a broad net to try to catch all possible sources of disturbance.

The analysis is described in the journal Physical Review Letters, in a paper by MIT graduate student David Layden, postdoc Mo Chen, and professor of nuclear science and engineering Paola Cappellaro.

“The main issues we now face in developing quantum technologies are that current systems are small and noisy,” says Layden. Noise, meaning unwanted disturbance of any kind, is especially vexing because many quantum systems are inherently highly sensitive, a feature underlying some of their potential applications.

And there’s another issue, Layden says, which is that quantum systems are affected by any observation. So, while one can detect that a classical system is drifting and apply a correction to nudge it back, things are more complicated in the quantum world. “What’s really tricky about quantum systems is that when you look at them, you tend to collapse them,” he says.

Classical error correction schemes are based on redundancy. For example, in a communication system subject to noise, instead of sending a single bit (1 or 0), one might send three copies of each (111 or 000). Then, if the three bits don’t match, that shows there was an error. The more copies of each bit get sent, the more effective the error correction can be.

The same essential principle could be applied to adding redundancy in quantum bits, or “qubits.” But, Layden says, “If I want to have a high degree of protection, I need to devote a large part of my system to doing these sorts of checks. And this is a nonstarter right now because we have fairly small systems; we just don’t have the resources to do particularly useful quantum error correction in the usual way.” So instead, the researchers found a way to target the error correction very narrowly at the specific kinds of noise that were most prevalent.

The quantum system they’re working with consists of carbon nuclei near a particular kind of defect in a diamond crystal called a nitrogen vacancy center. These defects behave like single, isolated electrons, and their presence enables the control of the nearby carbon nuclei.

But the team found that the overwhelming majority of the noise affecting these nuclei came from one single source: random fluctuations in the nearby defects themselves. This noise source can be accurately modeled, and suppressing its effects could have a major impact, as other sources of noise are relatively insignificant.

“We actually understand quite well the main source of noise in these systems,” Layden says. “So we don’t have to cast a wide net to catch every hypothetical type of noise.”

The team came up with a different error correction strategy, tailored to counter this particular, dominant source of noise. As Layden describes it, the noise comes from “this one central defect, or this one central ‘electron,’ which has a tendency to hop around at random. It jitters.”

That jitter, in turn, is felt by all those nearby nuclei, in a predictable way that can be corrected.

“The upshot of our approach is that we’re able to get a fixed level of protection using far fewer resources than would otherwise be needed,” he says. “We can use a much smaller system with this targeted approach.”

The work so far is theoretical, and the team is actively working on a lab demonstration of this principle in action. If it works as expected, this could make up an important component of future quantum-based technologies of various kinds, the researchers say, including quantum computers that could potentially solve previously unsolvable problems, or quantum communications systems that could be immune to snooping, or highly sensitive sensor systems.

“This is a component that could be used in a number of ways,” Layden says. “It’s as though we’re developing a key part of an engine. We’re still a ways from building a full car, but we’ve made progress on a critical part.”

“Quantum error correction is the next challenge for the field,” says Alexandre Blais, a professor of physics at the University of Sherbrooke, in Canada, who was not associated with this work. “The complexity of current quantum error correcting codes is, however, daunting as they require a very large number of qubits to robustly encode quantum information.”

Blais adds, “We have now come to realize that exploiting our understanding of the devices in which quantum error correction is to be implemented can be very advantageous. This work makes an important contribution in this direction by showing that a common type of error can be corrected for in a much more efficient manner than expected. For quantum computers to become practical we need more ideas like this.​”

The research was supported by the U.S. Army Research Office and the National Science Foundation.

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From Science News: “How a quantum technique highlights math’s mysterious link to physics”

From Science News

February 17, 2020
Tom Siegfried

Verifying proofs to very hard math problems is possible with infinite quantum entanglement.

A technique that relies on quantum entanglement (illustrated) expands the realm of mathematical problems for which the solution could (in theory) be verified. inkoly/iStock/Getty Images Plus.

It has long been a mystery why pure math can reveal so much about the nature of the physical world.

Antimatter was discovered in Paul Dirac’s equations before being detected in cosmic rays. Quarks appeared in symbols sketched out on a napkin by Murray Gell-Mann several years before they were confirmed experimentally. Einstein’s equations for gravity suggested the universe was expanding a decade before Edwin Hubble provided the proof. Einstein’s math also predicted gravitational waves a full century before behemoth apparatuses detected those waves (which were produced by collisions of black holes — also first inferred from Einstein’s math).

Nobel laureate physicist Eugene Wigner alluded to math’s mysterious power as the “unreasonable effectiveness of mathematics in the natural sciences.” Somehow, Wigner said, math devised to explain known phenomena contains clues to phenomena not yet experienced — the math gives more out than was put in. “The enormous usefulness of mathematics in the natural sciences is something bordering on the mysterious and … there is no rational explanation for it,” Wigner wrote in 1960.

But maybe there’s a new clue to what that explanation might be. Perhaps math’s peculiar power to describe the physical world has something to do with the fact that the physical world also has something to say about mathematics.

At least that’s a conceivable implication of a new paper that has startled the interrelated worlds of math, computer science and quantum physics.

In an enormously complicated 165-page paper, computer scientist Zhengfeng Ji and colleagues present a result that penetrates to the heart of deep questions about math, computing and their connection to reality. It’s about a procedure for verifying the solutions to very complex mathematical propositions, even some that are believed to be impossible to solve. In essence, the new finding boils down to demonstrating a vast gulf between infinite and almost infinite, with huge implications for certain high-profile math problems. Seeing into that gulf, it turns out, requires the mysterious power of quantum physics.

Everybody involved has long known that some math problems are too hard to solve (at least without unlimited time), but a proposed solution could be rather easily verified. Suppose someone claims to have the answer to such a very hard problem. Their proof is much too long to check line by line. Can you verify the answer merely by asking that person (the “prover”) some questions? Sometimes, yes. But for very complicated proofs, probably not. If there are two provers, though, both in possession of the proof, asking each of them some questions might allow you to verify that the proof is correct (at least with very high probability). There’s a catch, though — the provers must be kept separate, so they can’t communicate and therefore collude on how to answer your questions. (This approach is called MIP, for multiprover interactive proof.)

Verifying a proof without actually seeing it is not that strange a concept. Many examples exist for how a prover can convince you that they know the answer to a problem without actually telling you the answer. A standard method for coding secret messages, for example, relies on using a very large number (perhaps hundreds of digits long) to encode the message. It can be decoded only by someone who knows the prime factors that, when multiplied together, produce the very large number. It’s impossible to figure out those prime numbers (within the lifetime of the universe) even with an army of supercomputers. So if someone can decode your message, they’ve proved to you that they know the primes, without needing to tell you what they are.

Someday, though, calculating those primes might be feasible, with a future-generation quantum computer. Today’s quantum computers are relatively rudimentary, but in principle, an advanced model could crack codes by calculating the prime factors for enormously big numbers.

That power stems, at least in part, from the weird phenomenon known as quantum entanglement. And it turns out that, similarly, quantum entanglement boosts the power of MIP provers. By sharing an infinite amount of quantum entanglement, MIP provers can verify vastly more complicated proofs than nonquantum MIP provers.

It is obligatory to say that entanglement is what Einstein called “spooky action at a distance.” But it’s not action at a distance, and it just seems spooky. Quantum particles (say photons, particles of light) from a common origin (say, both spit out by a single atom) share a quantum connection that links the results of certain measurements made on the particles even if they are far apart. It may be mysterious, but it’s not magic. It’s physics.

Say two provers share a supply of entangled photon pairs. They can convince a verifier that they have a valid proof for some problems. But for a large category of extremely complicated problems, this method works only if the supply of such entangled particles is infinite. A large amount of entanglement is not enough. It has to be literally unlimited. A huge but finite amount of entanglement can’t even approximate the power of an infinite amount of entanglement.

As Emily Conover explains in her report for Science News, this discovery proves false a couple of widely believed mathematical conjectures. One, known as Tsirelson’s problem, specifically suggested that a sufficient amount of entanglement could approximate what you could do with an infinite amount. Tsirelson’s problem was mathematically equivalent to another open problem, known as Connes’ embedding conjecture, which has to do with the algebra of operators, the kinds of mathematical expressions that are used in quantum mechanics to represent quantities that can be observed.

Refuting the Connes conjecture, and showing that MIP plus entanglement could be used to verify immensely complicated proofs, stunned many in the mathematical community. (One expert, upon hearing the news, compared his feces to bricks.) But the new work isn’t likely to make any immediate impact in the everyday world. For one thing, all-knowing provers do not exist, and if they did they would probably have to be future super-AI quantum computers with unlimited computing capability (not to mention an unfathomable supply of energy). Nobody knows how to do that in even Star Trek’s century.

Still, pursuit of this discovery quite possibly will turn up deeper implications for math, computer science and quantum physics.

It probably won’t shed any light on controversies over the best way to interpret quantum mechanics, as computer science theorist Scott Aaronson notes in his blog about the new finding. But perhaps it could provide some sort of clues regarding the nature of infinity. That might be good for something, perhaps illuminating whether infinity plays a meaningful role in reality or is a mere mathematical idealization.

On another level, the new work raises an interesting point about the relationship between math and the physical world. The existence of quantum entanglement, a (surprising) physical phenomenon, somehow allows mathematicians to solve problems that seem to be strictly mathematical. Wondering why physics helps out math might be just as entertaining as contemplating math’s unreasonable effectiveness in helping out physics. Maybe even one will someday explain the other.

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From KTH Royal Institute of Technology via phys.org: “Exotic atomic nuclei reveal traces of new form of superfluidity”

From KTH Royal Institute of Technology

via

phys.org

Published Feb 17, 2020
David Callahan

The team behind the discovery of the new form of superfluidity: from left, Bo Cederwall, professor of physics at KTH Royal Institute of Technology, Xiaoyu Liu, Wei Zhang, Aysegül Ertoprak, Farnaz Ghazi Moradi and Özge Aktas.Published Feb 17, 2020

Recent observations of the internal structure of the rare isotope ruthenium-88 shed new light on the internal structure of atomic nuclei, a breakthrough which could also lead to further insights into how some chemical elements in nature and their isotopes are formed.

Led by Bo Cederwall, Professor of Experimental Nuclear Physics at KTH Royal Institute of Technology, an international research team identified new rotational states in the extremely neutron-deficient, deformed, atomic nucleus 88Ru. The results suggest that the structure of this exotic nuclear system is heavily influenced by the presence of strongly-coupled neutron-proton pairs.

“Such a structure is fundamentally different from the normal conditions observed in atomic nuclei, where neutrons and protons interact in pairs in separate systems, forming a near-superfluid state,” Cederwall says.

The results may also suggest alternative explanations for how the production of different chemical elements, and in particular their most neutron-poor isotopes, proceeds in the nucleosynthesis reactions in certain stellar environments such as neutron star-red giant binaries, he says.

The discovery, which was published February 12 in the journal, Physical Review Letters, results from an experiment at the Grand Accélérateur National d’Ions Lourds (GANIL), France, using the Advanced Gamma Tracking Array (AGATA) [below].

The researchers used nuclear collisions to create highly unstable atomic nuclei with equal numbers of neutrons and protons. Their structure was studied by using sensitive instruments, including AGATA, detecting the radiation they emit in the form of high-energy photons, neutrons, protons and other particles.

The Advanced Gamma Tracking Array (AGATA), which researchers from KTH used to study unstable atomic nuclei generated at the Grand Accélérateur National d’Ions Lourds.

According to the Standard Model of particle physics describing the elementary particles and their interactions, there are two general types of particles in nature; bosons and fermions, which have integer and half-integer spins, respectively. Examples of fermions are fundamental particles like the electron and the electron neutrino but also composite particles like the proton and the neutron and their fundamental building blocks, the quarks. Examples of bosons are the fundamental force carriers; the photon, the intermediate vector bosons, the gluons and the graviton.

The properties of a system of particles differ considerably depending on whether it is based on fermions or bosons. As a result of the Pauli principle of quantum mechanics, in a system of fermions (such as an atomic nucleus) only one particle can hold a certain quantum state at a certain point in space and time. For several fermions to appear together, at least one property of each fermion, such as its spin, must be different. At low temperature systems of many fermions can exhibit condensates of paired particles manifested as superfluidity for uncharged particles (for example, the superfluid 3He), and superconductivity for charged particles, such as electrons in a superconductor below the critical temperature. Bosons, on the other hand, can condense individually with an unlimited number of particles in the same state, so-called Bose-Einstein condensates.

In most atomic nuclei that are close to the line of beta stability and in their ground state, or excited to an energy not too high above it, the basic structure appears to be based on pair-correlated condensates of particles with the same isospin quantum number but with opposite spins. This means that neutrons and protons are paired separately from each other. These isovector pair correlations give rise to properties similar to superfluidity and superconductivity. In deformed nuclei, this structure is for example revealed as discontinuities in the rotational frequency when the rotational excitation energy of the nucleus is increased.

Such discontinuities, which were discovered already in the early 1970s by KTH Professor emeritus Arne Johnson, have been labeled “backbending”. The backbending frequency is a measure of the energy required to break a neutron or proton pair and therefore also reflects the energy released by the formation of a pair of nucleons in the nucleus. There are long-standing theoretical predictions that systems of neutron-proton pairs can be mixed with, or even replace, the standard isovector pair correlations in exotic atomic nuclei with equal numbers of protons and neutrons. The nuclear structure resulting from the isoscalar component of such pair correlations is different from that found in “ordinary” atomic nuclei close to stability. Among different possible experimental observables, the backbending frequency in deformed nuclei is predicted to increase significantly compared with nuclei with different numbers of neutrons and protons.

The KTH research group has previously observed evidence of strong neutron-proton correlations in the spherical nuclear nucleus 92Pd, which was published in the journal Nature (B. Cederwall et al., Nature, volume 469, p 68-71 (2011)). The ruthenium isotope 88Ru, with 44 neutrons and 44 protons, is deformed and exhibits a rotation-like structure that has now been observed up to higher spin, or rotational frequency, than previously possible. The new measurement provides a different angle on nuclear pair correlations compared with the previous work. By confirming the theoretical predictions of a shift towards higher backbending frequency it provides complementary evidence for the occurrence of strong isoscalar pair correlations in the heaviest nuclear systems with equal numbers of neutrons and protons.

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