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April 19, 2018 | PFC | Research News

Atoms may hum a tune from grand cosmic symphony

Researchers playing with a cloud of ultracold atoms uncovered behavior that bears a striking resemblance to the universe in microcosm. Their work, which forges new connections between atomic physics and the sudden expansion of the early universe, was published April 19 in Physical Review X and featured in Physics."From the atomic physics perspective, the experiment is beautifully described by existing theory," says Stephen Eckel, an atomic physicist at the National Institute of Standards and Technology (NIST) and the lead author of the new paper. "But even more striking is how that theory connects with cosmology."In several sets of experiments, Eckel and his colleagues rapidly expanded the size of a doughnut-shaped cloud of atoms, taking snapshots during the process. The growth happens so fast that the cloud is left humming, and a related hum may have appeared on cosmic scales during the rapid expansion of the early universe—an epoch that cosmologists refer to as the period of inflation.The work brought together experts in atomic physics and gravity, and the authors say it is a testament to the versatility of the Bose-Einstein condensate (BEC)—an ultracold cloud of atoms that can be described as a single quantum object—as a platform for testing ideas from other areas of physics."Maybe this will one day inform future models of cosmology," Eckel says. "Or vice versa. Maybe there will be a model of cosmology that’s difficult to solve but that you could simulate using a cold atomic gas."
March 28, 2018 | PFC | Research News

Latest nanowire experiment boosts confidence in Majorana sighting

In the latest experiment of its kind, researchers have captured the most compelling evidence to date that unusual particles lurk inside a special kind of superconductor. The result, which confirms theoretical predictions first made nearly a decade ago at the Joint Quantum Institute (JQI) and the University of Maryland (UMD), will be published in the April 5 issue of Nature. The stowaways, dubbed Majorana quasiparticles, are different from ordinary matter like electrons or quarks—the stuff that makes up the elements of the periodic table. Unlike those particles, which as far as physicists know can’t be broken down into more basic pieces, Majorana quasiparticles arise from coordinated patterns of many atoms and electrons and only appear under special conditions. They are endowed with unique features that may allow them to form the backbone of one type of quantum computer, and researchers have been chasing after them for years.The latest result is the most tantalizing yet for Majorana hunters, confirming many theoretical predictions and laying the groundwork for more refined experiments in the future. In the new work, researchers measured the electrical current passing through an ultra-thin semiconductor connected to a strip of superconducting aluminum—a recipe that transforms the whole combination into a special kind of superconductor.Experiments of this type expose the nanowire to a strong magnet, which unlocks an extra way for electrons in the wire to organize themselves at low temperatures. With this additional arrangement the wire is predicted to host a Majorana quasiparticle, and experimenters can look for its presence by carefully measuring the wire’s electrical response. The new experiment was conducted by researchers from QuTech at the Technical University of Delft in the Netherlands and Microsoft Research, with samples of the hybrid material prepared at the University of California, Santa Barbara and Eindhoven University of Technology in the Netherlands. Experimenters compared their results to theoretical calculations by JQI Fellow Sankar Das Sarma and JQI graduate student Chun-Xiao Liu.
March 13, 2018 | PFC | Research News

Two-toned light pattern creates steep quantum walls for atoms

Exotic physics can happen when quantum particles come together and talk to each other. Understanding such processes is challenging for scientists, because the particle interactions can be hard to glimpse and even harder to control. Moreover, modern computer simulations struggle to make sense of all the intricate dynamics going on in a large group of particles. Luckily, atoms cooled to near zero temperatures can provide insight into this problem.Lasers can make cold atoms mimic the physics seen in other systems—an approach that is familiar terrain for atomic physicists. They regularly use intersecting laser beams to capture atoms in a landscape of rolling hills and valleys called an optical lattice. Atoms, when cooled, don’t have enough energy to walk up the hills, and they get stuck in the valleys. In this environment, the atoms behave similarly to the electrons in the crystal structure of many solids, so this approach provides a straightforward way to learn about interactions inside real materials.But the conventional way to make optical lattices has some limitations. The wavelength of the laser light determines the location of the hills and valleys, and so the distance between neighboring valleys—and with that the spacing between atoms—can only be shrunk to half of the light’s wavelength. Bringing atoms closer than this limit could activate much stronger interactions between them and reveal effects that otherwise remain in the dark.Now, a team of scientists from the Joint Quantum Institute (JQI), in collaboration with researchers from the Institute for Quantum Optics and Quantum Information in Innsbruck, Austria, has circumvented the wavelength limit by leveraging the atoms’ inherent quantum features, which should allow atomic lattice neighbors to get closer than ever before. The new technique manages to squeeze the gentle lattice hills into steep walls separated by only one-fiftieth of the laser’s wavelength—25 times narrower than possible with conventional methods. The work, which is based on two prior theoretical proposals, was recently published in Physical Review Letters.
March 9, 2018 | Podcast

Physics at the edge of the world

Deep within the ice covering the South Pole, thousands of sensitive cameras strain their digital eyes in search of a faint blue glow—light that betrays the presence of high-energy neutrinos. For this episode, Chris sat down with UMD graduate student Liz Friedman and physics professor Kara Hoffman to learn more about IceCube, the massive underground neutrino observatory located in one of the most desolate spots on Earth. It turns out that IceCube is blind to the highest-energy neutrinos, and Friedman is heading down to the South Pole to help install stations for a new observatory—the Askaryan Radio Array—which uses radio waves instead of blue light to tune into the whispers of these ghostly visitors.
February 15, 2018 | People News

JQI Fellow Barkeshli receives 2018 Sloan Research Fellowship

Maissam Barkeshli, an assistant professor of physics at the University of Maryland and fellow of the Joint Quantum Institute, has been awarded a 2018 Sloan Research Fellowship. Granted by the Alfred P. Sloan Foundation, this award identifies 126 early-career scientists based on their potential to contribute fundamentally significant research to a wider academic community. Barkeshli, a theoretical condensed matter physicist interested in complex quantum many-body phenomena, will use the fellowship to further his research into the collective behavior that emerges in systems of strongly interacting particles governed by the laws of quantum mechanics.“I am honored to receive this prestigious fellowship,” said Barkeshli. “It represents an affirmation of my work by distinguished members of the physics community, and it encourages me to continue my efforts in understanding the complexities of quantum matter.”Barkeshli’s research mixes physics with mathematics and draws motivation from the ongoing pursuit to build next-generation computing devices ruled by quantum physics. Beyond the applications, his research explores the many ways that atoms and electrons—prototypical quantum particles—can combine in large numbers to produce a range of novel behaviors. For example, interesting things seem to happen at the interface between two different quantum materials. In 2014, Barkeshli and several colleagues showed that, at least theoretically, electrons can lose their electric charge or shed a quantum property called spin when they hop between two quantum materials. With the Sloan Research Fellowship, Barkeshli hopes to continue studying the novel ways that electrons and other, more exotic particles behave at these interfaces. This research could uncover new ways of building quantum computers that are virtually immune to noise, and has led to experimental proposals that could soon be tested in the lab.Barkeshli has authored more than 35 peer-reviewed journal articles. Before joining the UMD faculty in 2016, Barkeshli worked as a postdoctoral researcher at Microsoft Research’s Station Q (2013-2016) and at Stanford University (2010-2013). He earned a bachelor’s degree in physics and a second bachelor’s degree in electrical engineering and computer science from the University of California, Berkeley, in 2004. He received his doctoral degree in physics from the Massachusetts Institute of Technology in 2010.Barkeshli joins the list of 39 current UMD College of Computer, Mathematical, and Natural Sciences faculty members who have received Sloan Research Fellowships.The two-year $65,000 Sloan Research Fellowships are awarded to U.S. and Canadian researchers in the fields of chemistry, computer science, economics, mathematics, computational and evolutionary molecular biology, neuroscience, ocean sciences, and physics. Candidates must be nominated by their fellow scientists and winning fellows are selected by independent panels of senior scholars on the basis of each candidate’s independent research accomplishments, creativity and potential to become a leader in his or her field. “The Sloan Research Fellows represent the very best science has to offer,” said Adam Falk, president of the Alfred P. Sloan Foundation. “The brightest minds, tackling the hardest problems, and succeeding brilliantly—Fellows are quite literally the future of twenty-first century science.”
February 13, 2018 | People News

JQI Fellow Vladimir Manucharyan receives DARPA 2017 Young Faculty Award

JQI Fellow Vladimir Manucharyan has recently received the 2017 Young Faculty Award (YFA) from the Defence Advanced Research Projects Agency (DARPA) to support his research on topological superconductivity. According to DARPA, the YFA program seeks to “identify and engage rising research stars in junior faculty positions at U.S. academic institutions”. During the 2-year support period, DARPA grants awardees with mentoring and financial support.Manucharyan plans to use the award to implement superconducting semiconductors, novel devices that could become the building blocks of topological quantum computers. If the project is successful, DARPA will provide continuing support. Superconductivity arises when certain materials—usually at very low temperatures—lose all of their electric resistivity. This phenomenon occurs because electrons pair up to freely flow within the material due to an attractive interaction between them. Usually, semiconductors—substances that partially conduct electricity—don’t exhibit superconductivity, because their electrons are not close enough to each other to pair up. However, semiconductors are beneficial in other ways, and one of their biggest advantages is the ability to externally control their resistivity with an electric field. Manucharyan plans to create multi-terminal Josephson junctions, a novel device which incorporates the features of both superconductors and semiconductors. “It's a device where a central part consists of a small semiconducting region and more than two superconducting leads are connected to it,” he explains. Theory predicts that the nearby superconducting leads should induce superconductivity in the semiconducting region.Such devices could serve as superconducting transistors with reduced power loss and less heat dissipation, a major advance for the semiconductor chip technology. The multi-terminal Josephson junctions could also be used to study topological effects in exotic materials and to even model physics in more than three dimensions. Because of its topological properties, the novel device could form the basic units of topological quantum computers, paving the way towards a practical quantum computer.
February 12, 2018 | PFC | Research News

New hole-punched crystal clears a path for quantum light

Optical highways for light are at the heart of modern communications. But when it comes to guiding individual blips of light called photons, reliable transit is far less common. Now, a collaboration of researchers from the Joint Quantum Institute (JQI), led by JQI Fellows Mohammad Hafezi and Edo Waks, has created a photonic chip that both generates single photons, and steers them around. The device, described in the Feb. 9 issue of Science, features a way for the quantum light to seamlessly move, unaffected by certain obstacles."This design incorporates well-known ideas that protect the flow of current in certain electrical devices," says Hafezi. "Here, we create an analogous environment for photons, one that protects the integrity of quantum light, even in the presence of certain defects."The chip starts with a photonic crystal, which is an established, versatile technology used to create roadways for light. They are made by punching holes through a sheet of semiconductor. For photons, the repeated hole pattern looks very much like a real crystal made from a grid of atoms. Researchers use different hole patterns to change the way that light bends and bounces through the crystal. For instance, they can modify the hole sizes and separations to make restricted lanes of travel that allow certain light colors to pass, while prohibiting others.
January 12, 2018 | PFC | Research News

Light may unlock a new quantum dance for electrons in graphene

A team of researchers has devised a simple way to tune a hallmark quantum effect in graphene—the material formed from a single layer of carbon atoms—by bathing it in light. Their theoretical work, which was published recently in Physical Review Letters, suggests a way to realize novel quantum behavior that was previously predicted but has so far remained inaccessible in experiments."Our idea is to use light to engineer these materials in place," says Tobias Grass, a postdoctoral researcher at the Joint Quantum Institute (JQI) and a co-author of the paper. "The big advantage of light is its flexibility. It’s like having a knob that can change the physics in your sample."

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