Perfect Quantum Portal Emerges at Exotic Interface

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Published: Wednesday, June 19 2019 14:08

In Klein tunneling, an electron can transit perfectly through a barrier. In a new experiment, researchers observed the Klein tunneling of electrons into a special kind of superconductor. (Credit: E. Edwards/JQI)

A junction between an ordinary metal and a special kind of superconductor has provided a robust platform to observe Klein tunneling.

Researchers at the University of Maryland have captured the most direct evidence to date of a quantum quirk that allows particles to tunnel through a barrier like it’s not even there. The result, featured on the cover of the June 20, 2019 issue of the journal Nature, may enable engineers to design more uniform components for future quantum computers, quantum sensors and other devices.

The new experiment is an observation of Klein tunneling, a special case of a more ordinary quantum phenomenon. In the quantum world, tunneling allows particles like electrons to pass through a barrier even if they don’t have enough energy to actually climb over it. A taller barrier usually makes this harder and lets fewer particles through.

Klein tunneling occurs when the barrier becomes completely transparent, opening up a portal that particles can traverse regardless of the barrier’s height. Scientists and engineers from UMD’s Center for Nanophysics and Advanced Materials (CNAM), the Joint Quantum Institute (JQI) and the Condensed Matter Theory Center (CMTC), with appointments in UMD’s Department of Materials Science and Engineering and Department of Physics, have made the most compelling measurements yet of the effect.

“Klein tunneling was originally a relativistic effect, first predicted almost a hundred years ago,” says Ichiro Takeuchi, a professor of materials science and engineering (MSE) at UMD and the senior author of the new study. “Until recently, though, you could not observe it.”

It was nearly impossible to collect evidence for Klein tunneling where it was first predicted—the world of high-energy quantum particles moving close to the speed of light. But in the past several decades, scientists have discovered that some of the rules governing fast-moving quantum particles also apply to the comparatively sluggish particles traveling near the surface of some unusual materials.

One such material—which researchers used in the new study—is samarium hexaboride (SmB6), a substance that becomes a topological insulator at low temperatures. In a normal insulator like wood, rubber or air, electrons are trapped, unable to move even when voltage is applied. Thus, unlike their free-roaming comrades in a metal wire, electrons in an insulator can’t conduct a current.

Topological insulators such as SmB6 behave like hybrid materials. At low enough temperatures, the interior of SmB6 is an insulator, but the surface is metallic and allows electrons some freedom to move around. Additionally, the direction that the electrons move becomes locked to an intrinsic quantum property called spin that can be oriented up or down. Electrons moving to the right will always have their spin pointing up, for example, and electrons moving left will have their spin pointing down.

The metallic surface of SmB6 would not have been enough to spot Klein tunneling, though. It turned out that Takeuchi and colleagues needed to transform the surface of SmB6 into a superconductor—a material that can conduct electrical current without any resistance.

To turn SmB6 into a superconductor, they put a thin film of it atop a layer of yttrium hexaboride (YB6). When the whole assembly was cooled to just a few degrees above absolute zero, the YB6 became a superconductor and, due to its proximity, the metallic surface of SmB6 became a superconductor, too.

It was a “piece of serendipity” that SmB6 and its yttrium-swapped relative shared the same crystal structure, says Johnpierre Paglione, a professor of physics at UMD, the director of CNAM and a co-author of the research paper. “However, the multidisciplinary team we have was one of the keys to this success. Having experts on topological physics, thin-film synthesis, spectroscopy and theoretical understanding really got us to this point,” Paglione adds.

The combination proved the right mix to observe Klein tunneling. By bringing a tiny metal tip into contact with the top of the SmB6, the team measured the transport of electrons from the tip into the superconductor. They observed a perfectly doubled conductance—a measure of how the current through a material changes as the voltage across it is varied.

“When we first observed the doubling, I didn’t believe it,” Takeuchi says. “After all, it is an unusual observation, so I asked my postdoc Seunghun Lee and research scientist Xiaohang Zhang to go back and do the experiment again.”

When Takeuchi and his experimental colleagues convinced themselves that the measurements were accurate, they didn’t initially understand the source of the doubled conductance. So they started searching for an explanation. UMD’s Victor Galitski, a JQI Fellow, a professor of physics and a member of CMTC, suggested that Klein tunneling might be involved.

“At first, it was just a hunch,” Galitski says. “But over time we grew more convinced that the Klein scenario may actually be the underlying cause of the observations.”

Valentin Stanev, an associate research scientist in MSE and a research scientist at JQI, took Galitski’s hunch and worked out a careful theory of how Klein tunneling could emerge in the SmB6 system—ultimately making predictions that matched the experimental data well.

The theory suggested that Klein tunneling manifests itself in this system as a perfect form of Andreev reflection, an effect present at every boundary between a metal and a superconductor. Andreev reflection can occur whenever an electron from the metal hops onto a superconductor. Inside the superconductor, electrons are forced to live in pairs, so when an electron hops on, it picks up a buddy.

In order to balance the electric charge before and after the hop, a particle with the opposite charge—which scientists call a hole—must reflect back into the metal. This is the hallmark of Andreev reflection: an electron goes in, a hole comes back out. And since a hole moving in one direction carries the same current as an electron moving in the opposite direction, this whole process doubles the overall conductance—the signature of Klein tunneling through a junction of a metal and a topological superconductor.

In conventional junctions between a metal and a superconductor, there are always some electrons that don’t make the hop. They scatter off the boundary, reducing the amount of Andreev reflection and preventing an exact doubling of the conductance.

But because the electrons in the surface of SmB6 have their direction of motion tied to their spin, electrons near the boundary can’t bounce back—meaning that they will always transit straight into the superconductor.

“Klein tunneling had been seen in graphene as well,” Takeuchi says. “But here, because it’s a superconductor, I would say the effect is more spectacular. You get this exact doubling and a complete cancellation of the scattering, and there is no analog of that in the graphene experiment.”

Junctions between superconductors and other materials are ingredients in some proposed quantum computer architectures, as well as in precision sensing devices. The bane of these components has always been that each junction is slightly different, Takeuchi says, requiring endless tuning and calibration to reach the best performance. But with Klein tunneling in SmB6, researchers might finally have an antidote to that irregularity.

“In electronics, device-to-device spread is the number one enemy,” Takeuchi says. “Here is a phenomenon that gets rid of the variability.”

Story by Chris Cesare

In addition to Takeuchi, Paglione, Lee, Zhang, Galitski and Stanev, co-authors of the research paper include Drew Stasak, a former research assistant in MSE; Jack Flowers, a former graduate student in MSE; Joshua S. Higgins, a research scientist in CNAM and the Department of Physics; Sheng Dai, a research fellow in the department of chemical engineering and materials science at the University of California, Irvine (UCI); Thomas Blum, a graduate student in physics and astronomy at UCI; Xiaoqing Pan, a professor of chemical engineering and materials science and of physics and astronomy at UCI; Victor M. Yakovenko, a JQI Fellow, professor of physics at UMD and a member of CMTC; and Richard L. Greene, a professor of physics at UMD and a member of CNAM.

Ring Resonators Corner Light

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Published: Monday, June 17 2019 15:46

Researchers at the Joint Quantum Institute (JQI) have created the first silicon chip that can reliably constrain light to its four corners. The effect, which arises from interfering optical pathways, isn't altered by small defects during fabrication and could eventually enable the creation of robust sources of quantum light.

That robustness is due to topological physics, which describes the properties of materials that are insensitive to small changes in geometry. The cornering of light, which was reported June 17 in Nature Photonics, is a realization of a new topological effect, first predicted in 2017.

A new, grooved silicon chip keeps light in the corners using the physics of quadrupoles and topology. (Credit: E. Edwards/JQI)

In particular, the new work is a demonstration of quadrupole topological physics. A quadrupole is an arrangement of four poles—sinks and sources of force fields such as electrical charges or the poles of a magnet. You can visualize an electric quadrupole by imagining charges on each corner of a square that alternate positive-negative-positive-negative as you go along the perimeter.

The fact that the cornering arises from quadrupole physics instead of the physics of dipoles—that is, arrangements of just two poles—means it a higher-order topological effect.

Although the cornering effect has been observed in acoustic and microwave systems before, the new work is the first time it’s been observed in an optical system, says Associate Professor and JQI Fellow Mohammad Hafezi, the paper’s senior author. "We have been developing integrated silicon photonic systems to realize ideas derived from topology in a physical system," Hafezi says. "The fact that we use components compatible with current technology means that, if these systems are robust, they could possibly be translated into immediate applications."

In the new work, laser light is injected into a grid of resonators—grooved loops in the silicon that confine the light to rings. By placing the resonators at carefully measured distances, it's possible to adjust the interaction between neighboring resonators and alter the path that light takes through the grid.

The cumulative effect is that the light in the middle of the chip interferes with itself, causing most of the light injected into the chip to spend its time at the four corners.

Light doesn’t have an electric charge, but the presence or absence of light in a given resonator provides a kind of polar behavior. In this way, the pattern of resonators on the chip corresponds to a collection of interacting quadrupoles—precisely the conditions required by the first prediction of higher-order topological states of matter.

To test their fabricated pattern, Hafezi and his colleagues injected light into each corner of the chip and then captured an image of the chip with a microscope. In the collected light, they saw four bright peaks, one at each corner of the chip.

To show that the cornered light was trapped by topology, and not merely a result of where they injected the lasers, they tested a chip with the bottom two rows of resonators shifted. This changed their interactions with the resonators above, and, at least theoretically, changed where the bright spots should appear. They again injected the light at the corners, and this time—just as theory predicted—the lower two bright spots showed up above the rows of shifted resonators and not at the physical corners.

Despite the protection from small changes in resonator placement offered by topology, a second, more destructive fabrication defect remains in these chips. Since each resonator isn't exactly the same, the four points of light at the corners all shine with slightly different frequencies. This means that, for the moment, the chip may be no better than a single resonator if used as a source of photons—the quantum particles of light that many hope to harness as carriers of quantum information in future devices and networks.

"If you have many sources that are forced by topology to spit out identical photons, then you could interfere them, and that would be a game-changer," says Sunil Mittal, the lead author of the paper and a postdoctoral researcher at JQI. "I hope this work actually excites theorists to think about maybe looking for models that are insensitive to this lingering disorder in resonator frequencies."

Story by Chris Cesare

Hafezi and Mittal also have affiliations in the Department of Electrical and Computer Engineering, as well as the Institue for Research in Electronics and Applied Physics. Hafezi is also an associate professor in the Department of Physics.

High-resolution Imaging Technique Maps out an Atomic Wave Function

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Published: Monday, May 20 2019 09:41

Overlapping Laser Beams Offer a New Way to Extract A Quantum System's Essential Information

The team has used laser light to construct an image of an atomic wave function (shown in purple). The graphic is an artistic depiction of this process, showing a microscope objective trained on atoms (spheres) suspended in an optical lattice (tall white waves). The team's technique reveals information about an atomic wave function in unprecedented detail.

From NIST News

JQI researchers have demonstrated a new way to obtain the essential details that describe an isolated quantum system, such as a gas of atoms, through direct observation. The new method gives information about the likelihood of finding atoms at specific locations in the system with unprecedented spatial resolution. With this technique, scientists can obtain details on a scale of tens of nanometers—smaller than the width of a virus.

The new experiments use an optical lattice—a web of laser light that suspends thousands of individual atoms—to determine the probability that an atom might be at any given location. Because each individual atom in the lattice behaves like all the others, a measurement on the entire group of atoms reveals the likelihood of an individual atom to be in a particular point in space.  

Published in the journal Physical Review X, the technique (similar work was published simultaneously by a group at the University of Chicago) can yield the likelihood of the atoms’ locations at well below the wavelength of the light used to illuminate the atoms—50 times better than the limit of what optical microscopy can normally resolve. 

“It’s a demonstration of our ability to observe quantum mechanics,” says JQI Fellow and NIST physicist Trey Porto, one of the researchers behind the effort. “It hasn’t been done with atoms with anywhere near this precision.”

To understand a quantum system, physicists talk frequently about its “wave function.” It is not just an important detail; it’s the whole story. It contains all the information you need to describe the system.   

“It’s the description of the system,” says JQI Fellow and UMD physics professor Steve Rolston, another of the paper’s authors. “If you have the wave function information, you can calculate everything else about it—such as the object’s magnetism, its conductivity and its likelihood to emit or absorb light.”

While the wave function is a mathematical expression and not a physical object, the team’s method can reveal the behavior that the wave function describes: the probabilities that a quantum system will behave in one way versus another. In the world of quantum mechanics, probability is everything. 

Among the many strange principles of quantum mechanics is the idea that before we measure their positions, objects may not have a pinpointable location. The electrons surrounding the nucleus of an atom, for example, do not travel in regular planetlike orbits, contrary to the image some of us were taught in school. Instead, they act like rippling waves, so that an electron itself cannot be said to have a definite location. Rather, the electrons reside within fuzzy regions of space.

All objects can have this wavelike behavior, but for anything large enough for unaided eyes to see, the effect is imperceptible and the rules of classical physics are in force—we don’t notice buildings, buckets or breadcrumbs spreading out like waves. But isolate a tiny object such as an atom, and the situation is different because the atom exists in a size realm where the effects of quantum mechanics reign supreme. It’s not possible to say with certainty where it’s located, only that it will be found somewhere. The wave function provides the set of probabilities that the atom will be found in any given place. 

Quantum mechanics is well-enough understood—by physicists, anyway—that for a simple-enough system, experts can calculate the wave function from first principles without needing to observe it. Many interesting systems are complicated, though.

“There are quantum systems that can’t be calculated because they are too difficult,” Rolston says—such as molecules made of several large atoms. “This approach could help us understand those situations.”

As the wave function describes only a set of probabilities, how can physicists get a complete picture of its effects in short order? The team’s approach involves measuring a large number of identical quantum systems at the same time and combining the results into one overall picture. It’s sort of like rolling 100,000 pairs of dice at the same time—each roll gives a single result, and contributes a single point on the probability curve showing the values of all the dice. 

What the team observed were the positions of the roughly 100,000 atoms of ytterbium the optical lattice suspends in its lasers. The ytterbium atoms are isolated from their neighbors and restricted to moving back and forth along a one-dimensional line segment. To get a high-resolution picture, the team found a way to observe narrow slices of these line segments, and how often each atom showed up in its respective slice. After observing one region, the team measured another, until it had the whole picture.

Rolston says that while he hasn’t yet thought of a “killer app” that would take advantage of the technique, the mere fact that the team has directly imaged something central to quantum research fascinates him. 

“It’s not totally obvious where it will be used, but it’s a new technique that offers new opportunities,” he said. “We’ve been using an optical lattice to capture atoms for years, and now it’s become a new kind of measurement tool.” 

The original story was written by C. Boutin/NIST. Minor modifications were made for posting to this website.

Radioactive Material Detected Remotely Using Laser-induced Electron Avalanche Breakdown

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Published: Friday, March 22 2019 16:17

New method developed by UMD researchers could be scaled up to improve security at ports of entry

Physicists at the University of Maryland have developed a powerful new method to detect radioactive material. By using an infrared laser beam to induce a phenomenon known as an electron avalanche breakdown near the material, the new technique is able to detect shielded material from a distance. The method improves upon current technologies that require close proximity to the radioactive material.

With additional engineering, a new method to detect radioactive material, developed by physicists at the University of Maryland, could be scaled up to scan shipping containers at ports of entry—providing a powerful new tool for security applications. Image credit: USDA/APHIS (Click image to download hi-res version.)

With additional engineering advancements, the method could be scaled up and used to scan trucks and shipping containers at ports of entry, providing a powerful new tool to detect concealed, dangerous radioactive material. The researchers described their proof-of-concept experiments in a research paper published March 22, 2019 in the journal Science Advances.

“Traditional detection methods rely on a radioactive decay particle interacting directly with a detector. All of these methods decline in sensitivity with distance,” said Robert Schwartz, a physics graduate student at UMD and the lead author of the research paper. “The benefit of our method is that it is inherently a remote process. With further development, it could detect radioactive material inside a box from the length of a football field.”

As radioactive material emits decay particles, the particles strip electrons from—or ionize—nearby atoms in the air, creating a small number of free electrons that quickly attach to oxygen molecules. By focusing an infrared laser beam into this area, Schwartz and his colleagues easily detached these electrons from their oxygen molecules, seeding an avalanche-like rapid increase in free electrons that is relatively easy to detect.

“An electron avalanche can start with a single seed electron. Because the air near a radioactive source has some charged oxygen molecules—even outside a shielded container—it provides an opportunity to seed an avalanche by applying an intense laser field,” said Howard Milchberg, a professor of physics and electrical and computer engineering at UMD and senior author of the research paper. “Electron avalanches were among the first demonstrations after the laser was invented. This is not a new phenomenon, but we are the first to use an infrared laser to seed an avalanche breakdown for radiation detection. The laser’s infrared wavelength is important, because it can easily and specifically detach electrons from oxygen ions.”

This short animation illustrates a new method, developed by physicists at the University of Maryland, to detect concealed radioactive material by using an infrared laser beam to induce an electron avalanche breakdown near the material. Credit: R. Schwartz/H. Milchberg/U. of Maryland (Click image to download hi-res version.)

Applying an intense, infrared laser field causes the free electrons caught in the beam to oscillate and collide with atoms nearby. When these collisions become energetic enough, they can rip more electrons away from the atoms.

“A simple view of avalanche is that after one collision, you have two electrons. Then, this happens again and you have four. Then the whole thing cascades until you have full ionization, where all atoms in the system have at least one electron removed,” explained Milchberg, who also has an appointment at UMD’s Institute for Research in Electronics and Applied Physics (IREAP).

As the air in the laser’s path begins to ionize, it has a measurable effect on the infrared light reflected, or backscattered, toward a detector. By tracking these changes, Schwartz, Milchberg and their colleagues were able to determine when the air began to ionize and how long it took to reach full ionization.

The timing of the ionization process, or the electron avalanche breakdown, gives the researchers an indication of how many seed electrons were available to begin the avalanche. This estimate, in turn, can indicate how much radioactive material is present in the target.

“Timing of ionization is one of the most sensitive ways to detect initial electron density,” said Daniel Woodbury, a physics graduate student at UMD and a co-author of the research paper. “We’re using a relatively weak probe laser pulse, but it’s ‘chirped,’ meaning that shorter wavelengths pass though the avalanching air first, then longer ones. By measuring the spectral components of the infrared light that passes through versus what is reflected, we can determine when ionization starts and reaches its endpoint.”

The researchers note that their method is highly specific and sensitive to the detection of radioactive material. Without a laser pulse, radioactive material alone will not induce an electron avalanche. Similarly, a laser pulse alone will not induce an avalanche, without the seed electrons created by the radioactive material.

While the method remains a proof-of-concept exercise for now, the researchers envision further engineering developments that they hope will enable practical applications to enhance security at ports of entry across the globe.

“Right now we’re working with a lab-sized laser, but in 10 years or so, engineers may be able to fit a system like this inside a van,” Schwartz said. “Anywhere you can park a truck, you can deploy such a system. This would provide a very powerful tool to monitor activity at ports.”

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In addition to Milchberg, Schwartz, and Woodbury, UMD-affiliated co-authors of the research paper include Phillip Sprangle, professor of physics and electrical and computer engineering with an appointment at IREAP, and Joshua Isaacs, a physics graduate student.

The research paper, “Remote detection of radioactive material using mid-IR laser-driven avalanche breakdown,” Robert Schwartz, Daniel Woodbury, Joshua Isaacs, Phillip Sprangle and Howard Milchberg, was published in the journal Science Advances on March 22, 2019.

This work was supported by the Defense Threat Reduction Agency (Award No. HDTRA11510002), the Air Force Office of Scientific Research (Award Nos. FA9550-16-10121 and FA9550-16-10259), the Office of Naval Research (Award No. N00014-17-1-2705) and the Department of Energy (Award No. DE-NA0003864). The content of this article does not necessarily reflect the views of these organizations.

Ion experiment aces quantum scrambling test

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Published: Thursday, March 07 2019 12:38

Researchers at the Joint Quantum Institute have implemented an experimental test for quantum scrambling, a chaotic shuffling of the information stored among a collection of quantum particles. Their experiments on a group of seven atomic ions, reported in the March 7 issue of Nature, demonstrate a new way to distinguish between scrambling—which maintains the amount of information in a quantum system but mixes it up—and true information loss. The protocol may one day help verify the calculations of quantum computers, which harness the rules of quantum physics to process information in novel ways.

“In terms of the difficulty of quantum algorithms that have been run, we’re toward the top of that list,” says Kevin Landsman, a graduate student at JQI and the lead author of the new paper. “This is a very complicated experiment to run, and it takes a very high level of control.”

The research team, which includes JQI Fellow and UMD Distinguished University Professor Christopher Monroe and JQI Fellow Norbert Linke, performed their scrambling tests by carefully manipulating the quantum behavior of seven charged atomic ions using well-timed sequences of laser pulses. They found that they could correctly diagnose whether information had been scrambled throughout a system of seven atoms with about 80% accuracy.

“With scrambling, one particle’s information gets blended or spread out into the entire system,” Landsman says. “It seems lost, but it’s actually still hidden in the correlations between the different particles.”

Quantum scrambling is a bit like shuffling a fresh deck of cards. The cards are initially ordered in a sequence, ace through king, and the suits come one after another. Once it’s sufficiently shuffled, the deck looks mixed up, but—crucially—there’s a way to reverse that process. If you kept meticulous track of how each shuffle exchanged the cards, it would be simple (though tedious) to “unshuffle” the deck by repeating all those exchanges and swaps in reverse.

Quantum scrambling is similar in that it mixes up the information stored inside a set of atoms and can also be reversed, which is a key difference between scrambling and true, irreversible information loss. Landsman and colleagues used this fact to their advantage in the new test by scrambling up one set of atoms and performing a related scrambling operation on a second set. A mismatch between the two operations would indicate that the process was not scrambling, causing the final step of the method to fail.

That final step relied on quantum teleportation—a method for transferring information between two quantum particles that are potentially very far apart. In the case of the new experiment, the teleportation is over modest distances—just 35 microns separates the first atom from the seventh—but it is the signature by which the team detects scrambling: If information is successfully teleported from one atom to another, it means that the state of the first atom is spread out across all of the atoms—something that only happens if the information is scrambled. If the information was lost, successful teleportation would not be possible. Thus, for an arbitrary process whose scrambling properties might not be known, this method could be used to test whether—or even how much—it scrambles.

The authors say that prior tests for scrambling couldn’t quite capture the difference between information being hidden and lost, largely because individual atoms tend to look similar in both cases. The new protocol, first proposed by theorists Beni Yoshida of the Perimeter Institute in Canada, and Norman Yao at the University of California, Berkeley, distinguishes the two cases by taking correlations between particular particles into account in the form of teleportation.

“When our colleague Norm Yao told us about this teleportation litmus test for scrambling and how it needed at least seven qubits capable of running many quantum operations in a sequence, we knew that our quantum computer was uniquely-suited for the job,” says Linke.

The experiment was originally inspired by the physics of black holes. Scientists have long pondered what happens when something falls into a black hole, especially if that something is a quantum particle. The fundamental rules of quantum physics suggest that regardless of what a black hole does to a quantum particle, it should be reversible—a prediction that seems at odds with a black hole’s penchant for crushing things into an infinitely small point and spewing out radiation. But without a real black hole to throw things into, researchers have been stuck speculating.

Quantum scrambling is one suggestion for how information can fall into a black hole and come out as random-looking radiation. Perhaps, the argument goes, it’s not random at all, and black holes are just excellent scramblers. The paper discusses this motivation, as well as an interpretation of the experiment that compares quantum teleportation to information going through a wormhole.

“Regardless of whether real black holes are very good scramblers, studying quantum scrambling in the lab could provide useful insights for the future development of quantum computing or quantum simulation,” Monroe says.

By Chris Cesare

In addition to Landsman, Monroe and Linke, the new paper had four other coauthors: Caroline Figgatt, now at Honeywell in Colorado; Thomas Schuster at UC Berkeley; Beni Yoshida at the Perimeter Institute for Theoretical Physics; and Norman Yao at UC Berkeley and Lawrence Berkeley National Laboratory.