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.

Researchers Develop First Fabric to Automatically Cool or Insulate Depending on Conditions

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Published: Thursday, February 07 2019 14:57

Physics Professor Min Ouyang and Biochemistry Professor YuHuang Wang have created a fabric that dynamically regulates heat passing through it.

Despite decades of innovation in fabrics with high-tech thermal properties that keep marathon runners cool or alpine hikers warm, there has never been a material that changes its insulating properties in response to the environment. Until now.

University of Maryland Chemistry and Biochemistry Professor YuHuang Wang (left) and Physics Professor Min Ouyang hold a swatch of their new fabric that can automatically adjust its insulating properties to warm or cool a human body. Photo: Faye Levine, University of Maryland 

University of Maryland researchers have created a fabric that can automatically regulate the amount of heat that passes through it. When conditions are warm and moist, such as those near a sweating body, the fabric allows infrared radiation (heat) to pass through. When conditions become cooler and drier, the fabric reduces the heat that escapes. The development was reported in the February 8, 2019 issue of the journal Science.

The researchers created the fabric from specially engineered yarn coated with a conductive metal. Under hot, humid conditions, the strands of yarn compact and activate the coating, which changes the way the fabric interacts with infrared radiation. They refer to the action as “gating” of infrared radiation, which acts as a tunable blind to transmit or block heat.

“This is the first technology that allows us to dynamically gate infrared radiation,” said YuHuang Wang, a professor of chemistry and biochemistry at UMD and one of the paper’s corresponding authors who directed the studies.

This new fabric being developed by University of Maryland scientists YuHuang Wang and Ouyang Min is the first textile to automatically change properties to trap or release heat depending on conditions. Photo: Faye Levine, University of Maryland

The base yarn for this new textile is created with fibers made of two different synthetic materials—one absorbs water and the other repels it. The strands are coated with carbon nanotubes, a special class of lightweight, carbon-based, conductive metal. Because materials in the fibers both resist and absorb water, the fibers warp when exposed to humidity such as that surrounding a sweating body. That distortion brings the strands of yarn closer together, which does two things. First, it opens the pores in the fabric. This has a small cooling effect because it allows heat to escape. Second, and most importantly, it modifies the electromagnetic coupling between the carbon nanotubes in the coating.

“You can think of this coupling effect like the bending of a radio antenna to change the wavelength or frequency it resonates with,” Wang said. “It’s a very simplified way to think of it, but imagine bringing two antennae close together to regulate the kind of electromagnetic wave they pick up. When the fibers are brought closer together, the radiation they interact with changes. In clothing, that means the fabric interacts with the heat radiating from the human body.”

Depending on the tuning, the fabric either blocks infrared radiation or allows it to pass through. The reaction is almost instant, so before people realize they’re getting hot, the garment could already be cooling them down. On the flip side, as a body cools down, the dynamic gating mechanism works in reverse to trap in heat.

“The human body is a perfect radiator. It gives off heat quickly,” said Min Ouyang, a professor of physics at UMD and the paper’s other corresponding author. “For all of history, the only way to regulate the radiator has been to take clothes off or put clothes on. But this fabric is a true bidirectional regulator.”

This new fabric being developed by University of Maryland scientists YuHuang Wang and Ouyang Min is the first textile to automatically change properties to trap or release heat depending on conditions. Photo: Faye Levine, University of Maryland

According to the Science paper, this is first textile shown to be able to regulate heat exchange with the environment.

“This pioneering work provides an exciting new switchable characteristic for comfort-adjusting clothing,” said Ray Baughman, a professor of chemistry at the University of Texas who was not involved in the study. “Textiles were known that increase porosity in response to sweat or increasing temperature, as well as textiles that transmit the infrared radiation associated with body temperatures. However, no one before had found a way to switch both the porosity and infrared transparency of a textile so as to provide increased comfort in response to environmental conditions.”

More work is needed before the fabric can be commercialized, but according to the researchers, materials used for the base fiber are readily available and the carbon coating can be easily added during standard dyeing process.

“I think it’s very exciting to be able to apply this gating phenomenon to the development of a textile that has the ability to improve the functionality of clothing and other fabrics,” Ouyang said.

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Additional co-authors of the research paper from UMD’s Department of Chemistry and Biochemistry include visiting research scientist Xu A. Zhang; postdoctoral researchers Xiaojian Wu, Beibei Xu, Min Li and Yongxin Wang; associate research professor Zhiwei Peng; postdoctoral associate  Shunliu Deng; and graduate student Zupeng Wu. UMD Department of Physics graduate research associate Shangjie Yu is also a co-author. In addition, Wen-An Chiou of the Maryland NanoCenter performed the microtome and Transmission Electron Microscopy (TEM) analysis.

This work was supported by the Advanced Research Projects Agency-Energy (ARPA-E), U.S. Department of Energy, as part of its “Delivering Efficient Local Thermal Amenities (DELTA)” program (Award No. DE-AR0000527). The content of this article does not necessarily reflect the views of this organization.

The research paper, “Dynamic gating of infrared radiation in a textile,” Xu A. Zhang, Shangjie Yu, Beibei Xu, Min Li, Zhiwei Peng, Yongxin Wang, Shunliu Deng, Xiaojian Wu, Zupeng Wu, Min Ouyang, YuHuang Wang, was published in the journal Science February 8, 2019.

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Glass Fibers and Light Offer New Control Over Atomic Fluorescence

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Published: Friday, February 01 2019 14:47

Electrons inside an atom whip around the nucleus like satellites around the Earth, occupying orbits determined by quantum physics. Light can boost an electron to a different, more energetic orbit, but that high doesn’t last forever. At some point the excited electron will relax back to its original orbit, causing the atom to spontaneously emit light that scientists call fluorescence.   

Scientists can play tricks with an atom’s surroundings to tweak the relaxation time for high-flying electrons, which then dictates the rate of fluorescence. In a new study, researchers at the Joint Quantum Institute observed that a tiny thread of glass, called an optical nanofiber, had a significant impact on how fast a rubidium atom releases light. The research, which appeared as an Editor’s Suggestion in Physical Review A, showed that the fluorescence depended on the shape of light used to excite the atoms when they were near the nanofiber.

“Atoms are kind of like antennas, absorbing light and emitting it back out into space, and anything sitting nearby can potentially affect this radiative process,” says Pablo Solano, the lead author on the study and a University of Maryland graduate student at the time this research was performed.  

To probe how the environment affects these atomic antennas, Solano and his collaborators surround a nanofiber with a cloud of rubidium atoms. Nanofibers are custom-made conduits that allow much of the light to travel on the outside of the fiber, enhancing its interactions with atoms. The atoms closest to the nanofiber—within 200 nanometers—felt its presence the most. Some of the fluorescence from atoms in this region hit the fiber and bounced back to the atoms in an exchange that ultimately modified how long a rubidium atom’s electron stayed excited.   

The researchers found that the electron lifetime and subsequent atomic emissions depended on the wave characteristics of the light. Light waves oscillate as they travel, sometimes slithering like a sidewinder snake and other times corkscrewing like a strand of DNA. The researchers saw that for certain light shapes the electron lingered in the excited state, and for others, it made a more abrupt exit.

“We were able to use the oscillation properties of light as a kind of knob to control how atomic fluorescence near the nanofiber turned on,” Solano says.  

The team originally set out to measure the effects the nanofiber had on atoms, and compare the results to theoretical predictions for this system. They found disagreements between their measurements and existing models that incorporate many of the complex details of rubidium’s internal structure. This new research paints a simpler picture of the atom-fiber interactions, and the team says more research is needed to understand the discrepancies. 

"We believe this work is an important step in the on-going quest for a better understanding of the interaction between light and atoms near a nanoscale light-guiding structure, such as the optical nanofiber we used here," says JQI Fellow and NIST scientist William Phillips, who is also one of the lead investigators on the study.   

Written by Emily Edwards

Solano is currently a postdoctoral researcher at the MIT-Harvard University Center for Ultra Cold Atoms.  In addition, the following researchers were authors on this study.  

Read more information on this and the Joint Quantum Institute.