Research News

arXiv

Transistors are tiny switches that form the bedrock of modern computing—billions of them route electrical signals around inside a smartphone, for instance.

Quantum computers will need analogous hardware to manipulate quantum information. But the design constraints for this new technology are stringent, and today’s most advanced processors can’t be repurposed as quantum devices. That’s because quantum information carriers, dubbed qubits, have to follow different rules laid out by quantum physics. 

Magnets, whether in the form of a bar, horseshoe or electromagnet, always have two poles. If you break a magnet in half, you’ll end up with two new magnets, each with its own magnetic north and south.But some physics theories predict the existence of single-pole magnets—a situation akin to electric charges, which come in either positive or negative chunks. One particular incarnation—called the Yang monopole after its discoverer—was originally predicted in the context of high-energy physics, but it has never been observed. Now, a team at JQI led by postdoctoral researcher Seiji Sugawa and JQI Fellow Ian Spielman have succeeded in emulating a Yang monopole with an ultracold gas of rubidium atoms. The result, which provides another example of using cold quantum gases to simulate other areas of physics, was reported in the June 29 issue of Science.

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."

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.

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.

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.

arXiv

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.

arXiv