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      Physicists have coaxed ultracold atoms into an elusive form of quantum matter

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      An elusive form of matter called a quantum spin liquid isn’t a liquid, and it doesn’t spin — but it sure is quantum.

      Predicted nearly 50 years ago, quantum spin liquids have long evaded definitive detection in the laboratory. But now, a lattice of ultracold atoms held in place with lasers has shown hallmarks of the long-sought form of matter, researchers report in the Dec. 3 Science.

      Quantum entanglement goes into overdrive in the newly fashioned material. Even atoms on opposite sides of the lattice share entanglement, or quantum links, meaning that the properties of distant atoms are correlated with one another. “It’s very, very entangled,” says physicist Giulia Semeghini of Harvard University, a coauthor of the new study. “If you pick any two points of your system, they are connected to each other through this huge entanglement.” This strong, long-range entanglement could prove useful for building quantum computers, the researchers say.

      The new material matches predictions for a quantum spin liquid, although its makeup strays a bit from conventional expectations. While the traditional idea of a quantum spin liquid relies on the quantum property of spin, which gives atoms magnetic fields, the new material is based on different atomic quirks.

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      A standard quantum spin liquid should arise among atoms whose spins are in conflict. Spin causes atoms to act as tiny magnets. Normally, at low temperatures, those atoms would align their magnetic poles in a regular pattern. For example, if one atom points up, its neighbors point down. But if atoms are arranged in a triangle, for example, each atom has two neighbors that themselves point in opposite directions. That arrangement leaves the third one with nowhere to turn — it can’t oppose both of its neighbors at once.

      So atoms in quantum spin liquids refuse to choose (SN: 9/21/21). Instead, the atoms wind up in a superposition, a quantum combination of spin up and down, and each atom’s state is linked with those of its compatriots. The atoms are constantly fluctuating and never settle down into an orderly arrangement of spins, similarly to how atoms in a normal liquid are scattered about rather than arranged in a regularly repeating pattern, hence the name.

      Conclusive evidence of quantum spin liquids has been hard to come by in solid materials. In the new study, the researchers took a different tack: They created an artificial material composed of 219 trapped rubidium atoms cooled to a temperature of around 10 microkelvins (about –273.15° Celsius). The array of atoms, known as a programmable quantum simulator, allows scientists to fine-tune how atoms interact to investigate exotic forms of quantum matter.

      In the new experiment, rather than the atoms’ spins being in opposition, a different property created disagreement. The researchers used lasers to put the atoms into Rydberg states, meaning one of an atom’s electrons is bumped to a very high energy level (SN: 8/29/16). If one atom is in a Rydberg state, its neighbors prefer not to be. That setup begets a Rydberg-or-not discord, analogous to the spin-up and -down battle in a traditional quantum spin liquid.

      The scientists confirmed the quantum spin liquid effect by studying the properties of atoms that fell along loops traced through the material. According to quantum math, those atoms should have exhibited certain properties unique to quantum spin liquids. The results matched expectations for a quantum spin liquid and revealed that long-range entanglement was present.

      Notably, the material’s entanglement is topological. That means it is described by a branch of mathematics called topology, in which an object is defined by certain geometrical properties, for example, its number of holes (SN: 10/4/16). Topology can protect information from being destroyed: A bagel that falls off the counter will still have exactly one hole, for example. This information-preserving feature could be a boon to quantum computers, which must grapple with fragile, easily destroyed quantum information that makes calculations subject to mistakes (SN: 6/22/20).

      Whether the material truly qualifies as a quantum spin liquid, despite not being based on spin, depends on your choice of language, says theoretical physicist Christopher Laumann of Boston University, who was not involved with the study. Some physicists use the term “spin” to describe other systems with two possible options, because it has the same mathematics as atomic spins that can point either up or down. “Words have meaning, until they don’t,” he quips. It all depends how you spin them. More

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      Scientists finally detected a quantum effect that blocks atoms from scattering light

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      A cloud of ultracold atoms is like a motel with a neon “no vacancy” sign.

      If a guest at the motel wants to switch rooms, they’re out of luck. No vacant rooms means there’s no choice but to stay put. Likewise, in new experiments, atoms boxed in by crowded conditions have no way to switch up their quantum states. That constraint means the atoms don’t scatter light as they normally would, three teams of researchers report in the Nov. 19 Science. Predicted more than three decades ago, this effect has now been seen for the first time.

      Under normal circumstances, atoms interact readily with light. Shine a beam of light on a cloud of atoms, and they’ll scatter some of that light in all directions. This type of light scattering is a common phenomenon: It happens in Earth’s atmosphere. “We see the sky as blue because of scattered radiation from the sun,” says Yair Margalit, who was part of the team at MIT that performed one of the experiments.

      But quantum physics comes to the fore in ultracold, dense atom clouds. “The way they interact with light or scatter light is different,” says physicist Amita Deb of the University of Otago in Dunedin, New Zealand, a coauthor of another of the studies.

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      According to a rule called the Pauli exclusion principle, atoms in the experiments can’t take on the same quantum state — namely, they can’t have the same momentum as another atom in the experiment (SN: 5/19/20). If atoms are packed together in a dense cloud and cooled to near absolute zero, they’ll settle into the lowest-energy quantum states. Those low-energy states will be entirely filled, like a motel with no open rooms.

      When an atom scatters light, it gets a kick of momentum, changing its quantum state, as it sends light off in another direction. But if the atom can’t change its state due to the crowded conditions, it won’t scatter the light. The atom cloud becomes more transparent, letting light through instead of scattering it.  

      To observe the effect, Margalit and colleagues beamed light through a cloud of lithium atoms, measuring the amount of light it scattered. Then, the team decreased the temperature to make the atoms fill up the lowest energy states, suppressing the scattering of light. As the temperature dropped, the atoms scattered 37 percent less light, indicating that many atoms were prevented from scattering light. (Some atoms can still scatter light, for example if they get kicked into higher-energy quantum states that are unoccupied.)

      In another experiment, physicist Christian Sanner of the research institute JILA in Boulder, Colo., and colleagues studied a cloud of ultracold strontium atoms. The researchers measured how much light was scattered at small angles, for which the atoms are jostled less by the light and therefore are even less likely to be able to find an unoccupied quantum state. At lower temperatures, the atoms scattered half as much light as at higher temperatures.

      The third experiment, performed by Deb and physicist Niels Kjærgaard, also of the University of Otago, measured a similar scattering drop in an ultracold potassium atom cloud and a corresponding increase in how much light was transmitted through the cloud.

      Because the Pauli exclusion principle also governs how electrons, protons and neutrons behave, it is responsible for the structure of atoms and matter as we know it. These new results reveal the wide-ranging principle in a new context, says Sanner. “It’s fascinating because it shows a very fundamental principle in nature at work.”

      The work also suggests new ways to control light and atoms. “One could imagine a lot of interesting applications,” says theoretical physicist Peter Zoller of the University of Innsbruck in Austria, who was not involved with the research. In particular, light scattering is closely related to a process called spontaneous emission, in which an atom in a high-energy state decays to a lower energy by emitting light. The results suggest that decay could be blocked, increasing the lifetime of the energetic state. Such a technique might be useful for storing quantum information for a lengthier period of time than is normally possible, for example in a quantum computer.

      So far, these applications are still theoretical, Zoller says. “How realistic they are is something to be explored in the future.” More

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      Researchers have unlocked the secret to pearls’ incredible symmetry

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      For centuries, researchers have puzzled over how oysters grow stunningly symmetrical, perfectly round pearls around irregularly shaped grains of sand or bits of debris. Now a team has shown that oysters, mussels and other mollusks use a complex process to grow the gems that follows mathematical rules seen throughout nature.

      Pearls are formed when an irritant gets trapped inside a mollusk, and the animal protects itself by building smooth layers of mineral and protein — together called nacre — around it. Each new layer of nacre built over this asymmetrical center adapts precisely to the ones preceding it, smoothing out irregularities to result in a round pearl, according to an analysis published October 19 in the Proceedings of the National Academy of Sciences.

      “Nacre is this incredibly beautiful, iridescent, shiny material that we see in the insides of some seashells or on the outside of pearls,” says Laura Otter, a biogeochemist at the Australian National University in Canberra.

      A pearl’s symmetrical growth as it lays down layers of nacre relies on the mollusk balancing two basic capabilities, Otter and her colleagues discovered. It corrects growth aberrations that appear as the pearl forms, preventing those variations from propagating over the pearl’s many layers. Otherwise, the resulting gem would be lopsided.

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      Additionally, the mollusk modulates the thickness of nacre layers, so that if one layer is especially thick, subsequent layers will be thinner in response (SN: 3/24/14). This helps the pearl maintain a similar average thickness over its thousands of layers so that it looks perfectly round and uniform. Without that constant adjustment, a pearl might resemble stratified sedimentary rock, amplifying small imperfections that detract from its spherical shape.

      The researchers studied keshi pearls collected from Akoya pearl oysters (Pinctada imbricata fucata) at an eastern Australia coastal pearl farm. They used a diamond wire saw to cut the pearls into cross sections, then polished and examined the gems using Raman spectroscopy, a nondestructive technique that allowed them to characterize the pearls’ structure. For one of the pearls showcased in the paper, they counted 2,615 layers, which were deposited over 548 days.

      This cross section of a keshi pearl shows that the round gem grows around a misshapen lump of debris.Jiseok Gim

      The analysis revealed that fluctuations in the thicknesses of the pearls’ layers of nacre exhibit a phenomenon called 1/f noise, or pink noise, in which events that appear to be random are actually connected. In this case, the formation of nacre layers of different thicknesses may appear random, but is actually dependent on the thickness of previous layers. The same phenomenon is at work in seismic activity: The rumbling of the ground seems random, but is actually connected to previous recent seismic activity. Pink noise also crops up in classical music and even when monitoring heartbeats and brain activity, says coauthor Robert Hovden, a materials scientist and engineer at the University of Michigan in Ann Arbor.  These phenomena “belong to a universal class of behavior and physics,” Hovden says.

      This is the first time that researchers have reported “that nacre self-heals and when a defect arises, it heals itself within a few [layers], without using an external scaffolding or template,” says Pupa Gilbert, a physicist studying biomineralization at the University of Wisconsin–Madison who wasn’t involved with the study. “Nacre is an even more remarkable material than we had previously appreciated.”

      Notes Otter: “These humble creatures are making a super light and super tough material so much more easily and better than we do with all our technology.” Made of just calcium, carbonate and protein, nacre is “3,000 times tougher than the materials from which it’s made of.”

      This new understanding of pearls, Hovden adds, could inspire “the next generation of super materials,” such as more energy-efficient solar panels or tough and heat-resistant materials optimized for use in spacecraft. More

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      Lithium-ion batteries made with recycled materials can outlast newer counterparts

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      Lithium-ion batteries with recycled cathodes can outperform batteries with cathodes made from pristine materials, lasting for thousands of additional charging cycles, a study finds. Growing demand for these batteries — which power devices from smartphones to electric vehicles — may outstrip the world’s supply of some crucial ingredients, such as cobalt (SN: 5/7/19). Ramping up recycling could help avert a potential shortage. But some manufacturers worry that impurities in recycled materials may cause battery performance to falter.

      “Based on our study, recycled materials can perform as well as, or even better than, virgin materials,” says materials scientist Yan Wang of Worcester Polytechnic Institute in Massachusetts.

      Using shredded spent batteries, Wang and colleagues extracted the electrodes and dissolved the metals from those battery bits in an acidic solution. By tweaking the solution’s pH, the team removed impurities such as iron and copper and recovered over 90 percent of three key metals: nickel, manganese and cobalt. The recovered metals formed the basis for the team’s cathode material.

      In tests of how well batteries maintain their capacity to store energy after repeated use and recharging, batteries with recycled cathodes outperformed ones made with brand-new commercial materials of the same composition. It took 11,600 charging cycles for the batteries with recycled cathodes to lose 30 percent of their initial capacity. That’s about 50 percent better than the respectable 7,600 cycles for the batteries with new cathodes, the team reports October 15 in Joule. Those thousands of extra cycles could translate into years of better battery performance, Wang says. More

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      Scientists are one step closer to error-correcting quantum computers

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      Mistakes happen — especially in quantum computers. The fragile quantum bits, or qubits, that make up the machines are notoriously error-prone, but now scientists have shown that they can fix the flubs.

      Computers that harness the rules of quantum mechanics show promise for making calculations far out of reach for standard computers (SN: 6/29/17). But without a mechanism for fixing the computers’ mistakes, the answers that a quantum computer spits out could be gobbledygook (SN: 6/22/20).

      Combining the power of multiple qubits into one can solve the error woes, researchers report October 4 in Nature. Scientists used nine qubits to make a single, improved qubit called a logical qubit, which, unlike the individual qubits from which it was made, can be probed to check for mistakes.

      “This is a key demonstration on the path to build a large-scale quantum computer,” says quantum physicist Winfried Hensinger of the University of Sussex in Brighton, England, who was not involved in the new study.

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      Still, that path remains a long one, Hensinger says. To do complex calculations, scientists will have to dramatically scale up the number of qubits in the machines. But now that scientists have shown that they can keep errors under control, he says, “there’s nothing fundamentally stopping us to build a useful quantum computer.”

      In a logical qubit, information is stored redundantly. That allows researchers to check and fix mistakes in the data. “If a piece of it goes missing, you can reconstruct it from the other pieces, like Voldemort,” says quantum physicist David Schuster of the University of Chicago, who was not involved with the new research. (The Harry Potter villain kept his soul safe by concealing it in multiple objects called Horcruxes.)

      In the new study, four additional, auxiliary qubits interfaced with the logical qubit, in order to identify errors in its data. Future quantum computers could make calculations using logical qubits in place of the original, faulty qubits, repeatedly checking and fixing any errors that crop up.

      To make their logical qubit, the researchers used a technique called a Bacon-Shor code, applying it to qubits made of ytterbium ions hovering above an ion-trapping chip inside a vacuum, which are manipulated with lasers. The researchers also designed sequences of operations so that errors don’t multiply uncontrollably, what’s known as “fault tolerance.”

      Thanks to those efforts, the new logical qubit had a lower error rate than that of the most flawed components that made it up, says quantum physicist Christopher Monroe of the University of Maryland in College Park and Duke University.

      However, the team didn’t quite complete the full process envisioned for error correction. While the computer detected the errors that arose, the researchers didn’t correct the mistakes and continue on with computation. Instead, they fixed errors after the computer was finished. In a full-fledged example, scientists would detect and correct errors multiple times on the fly.

      Demonstrating quantum error correction is a necessity for building useful quantum computers. “It’s like achieving criticality with [nuclear] fission,” Schuster says. Once that nuclear science barrier was passed in 1942, it led to technologies like nuclear power and atomic bombs (SN: 11/29/17).

      As quantum computers gradually draw closer to practical usefulness, companies are investing in the devices. Technology companies such as IBM, Google and Intel host major quantum computing endeavors. On October 1, a quantum computing company cofounded by Monroe, called IonQ, went public; Monroe spoke to Science News while on a road trip to ring the opening bell at the New York Stock Exchange.

      The new result suggests that full-fledged quantum error correction is almost here, says coauthor Kenneth Brown, a quantum engineer also at Duke University. “It really shows that we can get all the pieces together and do all the steps.” More

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      These colorful butterflies were created using transparent ink

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      You’ve heard of disappearing ink. Now get ready for suddenly appearing ink. Using a clear liquid, researchers can print a full rainbow of colors on transparent surfaces. The trick is printing the liquid in precise, microscale patterns that create structural color.

      Structural colors arise from the way different wavelengths of light bounce off microscopic imperfections on surfaces (SN: 8/17/21; SN: 6/1/16). “In nature, there are many beautiful structure colors, such as the wings of butterflies, the feathers of peacocks, the skin of chameleons and so on,” says Yanlin Song, a materials chemist at the Chinese Academy of Sciences in Beijing.

      Song and colleagues printed structural colors on transparent silicone sheets using an ordinary ink-jet printer and clear polymer ink. The printer studded the silicone sheets with millions of microscopic ink domes, each of which served as a single pixel in the resulting image. Adjusting the size of a microdome changed the wavelengths of light that the dome reflected and therefore its color (SN: 3/8/19). Increasing the width of a single dome from 6.6 to 11 micrometers shifted its hue along the spectrum from blue to red and back again, the researchers report online September 22 in Science Advances.

      The denser the domes were packed, the brighter the image. And printing a medley of differently colored ink pixels across a single area created blended shades, such as brown and gray. Using the technique, Song’s team printed multicolor, photorealistic portraits of Isaac Newton, Marilyn Monroe and other famous figures.

      By printing tiny dollops of clear ink on transparent surfaces, researchers created structural color portraits of famous figures, such as Isaac Newton, Audrey Hepburn and Marilyn Monroe.K. Li et al/Science Advances 2021

      By printing tiny dollops of clear ink on transparent surfaces, researchers created structural color portraits of famous figures, such as Isaac Newton, Audrey Hepburn and Marilyn Monroe.K. Li et al/Science Advances 2021

      “I was excited to see that somebody had used [structural color] for this purpose,” says Lauren Zarzar, a materials chemist at Penn State who has studied similar structural colors cast by water and oil droplets. “They had some nice examples that I think illustrated the versatility of this mechanism.”

      Zarzar imagines using structural colors to create complex optical signatures for anti-counterfeiting features on ID cards or currency. Such shimmery, colorfast hues could also make useful materials for cosmetics, clothing or architecture, she says. More

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      One of nature’s key constants is much larger in a quantum material

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      A crucial number that rules the universe goes big in a strange quantum material.

      The fine-structure constant is about 10 times its normal value in a type of material called quantum spin ice, physicists calculate in the Sept. 10 Physical Review Letters. The new calculation hints that quantum spin ice could give a glimpse at physics within an alternate universe where the constant is much larger.

      With an influence that permeates physics and chemistry, the fine-structure constant sets the strength of interactions between electrically charged particles. Its value, about 1/137, consternates physicists because they can’t explain why it has that value, even though it is necessary for the complex chemistry that is the basis of life (SN: 11/2/16).

      If the fine-structure constant throughout the cosmos were as large as the one in quantum spin ices, “the periodic table would only have 10 elements,” says theoretical physicist Christopher Laumann of Boston University. “And it probably would be hard to make people; there wouldn’t be enough richness to chemistry.”

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      Quantum spin ices are a class of substances in which particles can’t agree. The materials are made up of particles with spin, a quantum version of angular momentum, which makes them magnetic. In a normal material, particles would come to a consensus below a certain temperature, with the magnetic poles lining up in either the same direction or in alternating directions. But in quantum spin ices, the particles are arranged in such a way that the magnetic poles, or equivalently the spins, can’t agree even at a temperature of absolute zero (SN: 2/13/11).

      The impasse occurs because of the materials’ geometry: The particles are located at the corners of an array of pyramids that are connected at the corners. Conflicts between multiple sets of neighbors mean that the closest these particles can get to harmony is arranging themselves so that two spins face out from each pyramid, and two face in.

      In quantum spin ices, particles (black dots) are located at the corners of an array of pyramids (red). Normally, the spins of the particles (green arrows) arrange so that two are pointing into the pyramid and two out. If that rule is broken, as illustrated, quasiparticles called spinons (orange and blue) form.S.D. Pace et al/PRL 2021

      This uneasy truce can give rise to disturbances that behave like particles within the material, or quasiparticles (SN: 10/3/14). Flip particles’ spins around and you can get what are called spinons, quasiparticles that can move through the material and interact with other spinons in a manner akin to electrons and other charged particles found in the world outside the material. The material re-creates the theory of quantum electrodynamics, the piece of particles physics’ standard model that hashes out how electrically charged particles do their thing. But the specifics, including the fine-structure constant, don’t necessarily match those in the wider universe.

      So Laumann and colleagues set out to calculate the fine-structure constant in quantum spin ices for the first time. The team pegged the number at about 1/10, instead of 1/137. What’s more, the researchers found that they could change the value of the fine-structure constant by tweaking the properties of the theoretical material. That could help scientists study the effects of altering the fine-structure constant — a test that’s well out of reach in our own universe, where the fine-structure constant is fixed.

      Unfortunately, scientists haven’t yet found a material that definitively qualifies as quantum spin ice. But one much-studied prospect is a group of minerals called pyrochlores, which have magnetic ions, or electrically charged atoms, arranged in the appropriate pyramid configuration. Scientists might also be able to study the materials using a quantum computer or another quantum device designed to simulate quantum spin ices (SN: 6/29/17).

      If scientists succeed in creating quantum spin ice, the materials could reveal how quantum electrodynamics and the standard model would work in a universe with a much larger fine-structure constant. “That would be the hope,” says condensed matter theorist Shivaji Sondhi of the University of Oxford, who was not involved with the research. “It’s interesting to be able to make a fake standard model … and ask what would happen.” More

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      New ‘vortex beams’ of atoms and molecules are the first of their kind

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      Like soft serve ice cream, beams of atoms and molecules now come with a swirl.

      Scientists already knew how to dish up spiraling beams of light or electrons, known as vortex beams (SN: 1/14/11). Now, the first vortex beams of atoms and molecules are on the menu, researchers report in the Sept. 3 Science.

      Vortex beams made of light or electrons have shown promise for making special types of microscope images and for transmitting information using quantum physics (SN: 8/5/15). But vortex beams of larger particles such as atoms or molecules are so new that the possible applications aren’t yet clear, says physicist Sonja Franke-Arnold of the University of Glasgow in Scotland, who was not involved with the research. “It’s maybe too early to really know what we can do with it.”

      In quantum physics, particles are described by a wave function, a wavelike pattern that allows scientists to calculate the probability of finding a particle in a particular place (SN: 6/8/11). But vortex beams’ waves don’t slosh up and down like ripples on water. Instead, the beams’ particles have wave functions that move in a corkscrewing motion as a beam travels through space. That means the beam carries a rotational oomph known as orbital angular momentum. “This is something really very strange, very nonintuitive,” says physicist Edvardas Narevicius of the Weizmann Institute of Science in Rehovot, Israel.

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      Narevicius and colleagues created the new beams by passing helium atoms through a grid of specially shaped slit patterns, each just 600 nanometers wide. The team detected a hallmark of vortex beams: a row of doughnut-shaped rings imprinted on a detector by the atoms, in which each doughnut corresponds to a beam with a different orbital angular momentum.

      Another set of doughnuts revealed the presence of vortex beams of helium excimers, molecules created when a helium atom in an excited, or energized, state pairs up with another helium atom.

      A pattern of rings reveals the presence of vortex beams of atoms and molecules. Each doughnut shape corresponds to a beam of helium atoms with a different angular momentum. Two hard-to-see circles from helium molecules sit in between the center dot and the first two doughnuts left and right of the center.A. Luski et al/Science 2021

      A pattern of rings reveals the presence of vortex beams of atoms and molecules. Each doughnut shape corresponds to a beam of helium atoms with a different angular momentum. Two hard-to-see circles from helium molecules sit in between the center dot and the first two doughnuts left and right of the center.A. Luski et al/Science 2021

      Next, scientists might investigate what happens when vortex beams of molecules or atoms collide with light, electrons or other atoms or molecules. Such collisions are well-understood for normal particle beams, but not for those with orbital angular momentum. Similar vortex beams made with protons might also serve as a method for probing the subatomic particle’s mysterious innards (SN: 4/18/17).

      In physics, “most important things are achieved when we are revisiting known phenomena with a fresh perspective,” says physicist Ivan Madan of EPFL, the Swiss Federal Institute of Technology in Lausanne, who was not involved with the research. “And, for sure, this experiment allows us to do that.” More