Arnold J. Glick, 1931- 2020

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Category: Department News

Published: Thursday, August 27 2020 00:01

Professor Emeritus Arnold J. Glick died on Aug. 16, after falling ill two days earlier.

Glick was the only child of immigrants who met and married in Brooklyn and operated a small clothing store. His early interest and acuity in math and science led to acceptance into New York’s renowned and specialized Stuyvesant High School.

After graduation, Glick moved to Israel to work at Kibbutz Gal On at a time of scarcity in the new nation. Undernourished and in receipt of a U.S. Army draft notice, he returned home and was sent to the 82^nd Airborne Division at Fort Bragg, North Carolina, where he worked on radio communications during the Korean War. Near the end of his service, he was charged in a McCarthy-era military court with being a communist. Eventually, it was revealed that the accusation stemmed from his father’s attendance at a lecture by the writer Howard Fast.  Once Glick was cleared and honorably discharged, he earned his bachelor’s degree in physics from Brooklyn College and entered the UMD physics graduate program under the tutelage of Richard A. Ferrell, studying how electrons in metals respond when heated or subjected to electric fields.

Glick received his doctorate in 1961 and accepted a postdoctoral position in nuclear physics at the Weizmann Institute of Science in Israel. There he did his most recognized work, on many-particle phenomena, including a paper with well over 1,000 citations. He was then invited to join the UMD physics faculty by then-chair John Sampson Toll.

Glick developed a new major focus on the properties of 1D (one-dimensional) polyacetylene, an intrinsically conducting polymer invented in 1958; interest flourished after a landmark 1979 paper showing that it supports solitons. Collaborating with him was postdoc Garnett W. Bryant, now Group Leader of the Atomic-Scale Device Group at NIST. “It was very fun at the time working with Arnie on these projects and having a chance, for the first time, to experience the excitement of working in a high profile, emerging area of physics,” said Bryant.

Collaborating with his student Shyamalendu M. Bose and Prof. Angelo Bardasis, Glick calculated quasiparticle damping in a free-electron gas. With George A Ausman, Jr., he studied many-body effects that occurred near the threshold for core excitations in metals caused by soft x-rays.  Other work included Auger emission spectra in metals with student Amy Liu Hagen, many-body effects in core-level spectroscopy with student Harvey Gotts, fluctuation-induced tunneling in a 1D tight-binding model with student Ronald J. Cohen and soliton contributions with student David M. Mackie.

Glick’s student William R. Bandy, the department’s 2012 Distinguished Alumnus, studied electron tunneling and diagonal disorder with Glick. Bandy recalled “sitting in his office talking through my latest technical challenge, hearing about the latest engine replacement for his aging VW bus camper that he drove to Aspen every summer…. he was always giving me recommendations for good hiking trails in the region.”

Glick's parents taught him to hike as a youngster. The days spent ascending New Hampshire's White Mountains and snacking on mulberries inspired a love of nature that lasted throughout his lifetime.    

Another keen interest was folk dancing, which he enjoyed for decades. Glick also availed himself of UMD’s breadth and studied modern dance, ballet, scuba diving, squash, movies and sculpture. He and his wife Rachel were frequent visitors at the department’s special lectures, retreats and retirement celebrations and enjoyed lectures and performances at the Clarice Smith Performing Arts Center.

In addition to Rachel, survivors include his first wife, Nevet Montgomery; daughters Jody Glick, Jeri Glick (Charles Anderson), and Ora DeMorrow (Shannon Lynch); and four grandchildren.

His obituary in the Greenbelt News Review is here: https://greenbeltnewsreview.com/issues/GNR20200827.pdf

Arnie Glick and Rick Greene at a holiday gathering.

Richard Prange, Ching-Ping Wang, Arnie Glick and Victor Yakovenko.

Physics faculty, 1970s.

Physics faculty, 1977.

Physics faculty, 1975.

UMD Researchers Included in DoE Quantum Project

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Category: Research News

Published: Wednesday, August 26 2020 05:33

Quantum Computers Do the (Instantaneous) Twist

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Category: Research News

Published: Wednesday, August 19 2020 05:33

Regardless of what makes up the innards of a quantum computer, its speedy calculations all boil down to sequences of simple instructions applied to qubits—the basic units of information inside a quantum computer.

Whether that computer is built from chains of ions, junctions of superconductors, or silicon chips, it turns out that a handful of simple operations, which affect only one or two qubits at a time, can mix and match to create any quantum computer program—a feature that makes a particular handful “universal.” Scientists call these simple operations quantum gates, and they have spent years optimizing the way that gates fit together. They’ve slashed the number of gates (and qubits) required for a given computation and discovered how to do it all while ensuring that errors don’t creep in and cause a failure.

Now, researchers at JQI have discovered ways to implement robust, error-resistant gates using just a constant number of simple building blocks—achieving essentially the best reduction possible in a parameter called circuit depth. Their findings, which apply to quantum computers based on topological quantum error correcting codes, were reported in two papers published recently in the journals Physical Review Letters(link is external) and Physical Review B(link is external), and expanded on in a third paper published earlier in the journal Quantum(link is external).Unlike other kinds of quantum computers, quantum computers built atop topological error correction smear a single qubit’s worth of information out among a network of many qubits. (Credit: Gerd Altmann/Pixabay)

Circuit depth counts the number of gates that affect each qubit, and a constant depth means that the number of gates needed for a given operation won’t increase as the computer grows—a necessity if errors are to be kept at bay. This is a promising feature for robust and universal quantum computers, says Associate Professor Maissam Barkeshli. 

“We have discovered that a huge class of operations in topological states of matter and topological error correcting codes can be implemented via constant depth unitary circuits,” says Barkeshli, who is a member of the Joint Quantum Institute and the Condensed Matter Theory Center at UMD.

Unlike other kinds of quantum computers, quantum computers built atop topological error correction—which so far have only been studied theoretically—don’t store information in individual physical qubits. Instead, they smear a single qubit’s worth of information out among a network of many qubits—or, more exotically, across special topological materials.

This information smearing provides resilience against stray bits of light or tiny vibrations—quantum disturbances that may cause errors—and it allows small errors to be detected and then actively corrected during a computation. It’s one of the main advantages that quantum computers based on topological error correction offer. But the advantage comes at a cost: If noise can’t get to the information easily, neither can you.

Until now it seemed that operating such a quantum computer required small, sequential changes to the network that stores the information—often depicted as a grid or lattice in two dimensions. In time, these small changes add up and effectively move one region of the lattice in a loop around another region, leaving the network looking the same as when it started.

These transformations of the network are known as braids because the patterns they trace out in space and time look like braided hair or a plaited loaf of bread. If you imagine stacking snapshots of the network up like pancakes, they will form—step by step—an abstract braid. Depending on the underlying physics of the network—including the kinds of particles, called anyons, that can hop around on it—these braids can be enough to run any quantum program.

In the new work, the authors showed that braiding can be accomplished almost instantaneously. Gone are the knotted diagrams, replaced by in-situ rearrangements of the network.

“It was kind of a textbook dogma that these braids can only be done adiabatically or very slowly so as to avoid creating errors in the process,” says Guanyu Zhu, a former JQI postdoctoral researcher who is currently a research staff member at the IBM Thomas J. Watson Research Center. “However, in this work, we realized that instead of slowly moving regions with anyons around each other, we could just stretch or squeeze the space between them in a constant number of steps.”

The new recipe requires two ingredients. One is the ability to make local modifications that reconfigure the interactions between the physical qubits that make up the network. This part isn’t too different from what ordinary braiding requires, but it is assumed to happen in parallel across the region being braided. The second ingredient is the ability to swap the information on physical qubits that are not close to each other—potentially even at opposite corners of the braiding region.

Networks of qubits (represented by black dots in the image on the right) are deformed in order to braid two regions (represented by red and blue dots) around each other. These images show two intermediate stages of the process. Images provided courtesy of the authors.

Networks of qubits (represented by black dots in the image on the right) are deformed in order to braid two regions (represented by red and blue dots) around each other. These images show two intermediate stages of the process. Images provided courtesy of the authors.

This second requirement is a big ask for some quantum computing hardware, but the authors say that there are systems that could naturally support it.

“A variety of experimental platforms with long-range connectivity could support our scheme, including ion traps, circuit QED systems with long transmission-line resonators, modular architectures with superconducting cavities, and silicon photonic devices,” says Zhu. “Or you could imagine using platforms with movable qubits. One can think of such platforms as fluid quantum computers, where qubits can freely flow around via classical motion.”

In the paper in Physical Review Letters, the authors provided explicit instructions for how to achieve their instantaneous braids in a particular class of topological quantum codes. In the Physical Review B and Quantum papers, they extended this result to a more general setting and even examined how it would apply to a topological code in hyperbolic space (where, additionally, adding a new smeared out qubit requires adding only a constant number of physical qubits to the network).

The authors haven’t yet worked out how their new braiding techniques will mesh with the additional goals of detecting and correcting errors; that remains an open problem for future research.

“We hope our results may ultimately be useful for establishing the possibility of fault-tolerant quantum computation with constant space-time overhead,” says Barkeshli.

Original story by Chris Cesare: https://jqi.umd.edu/news/quantum-computers-do-instantaneous-twist

In addition to Barkeshli and Zhu, the three papers had one additional co-author: Ali Lavasani, a JQI graduate fellow and physics graduate student at UMD.

Plasma Guides Maintain Laser Focus

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Category: Research News

Published: Friday, August 14 2020 08:17

In science fiction, firing powerful lasers looks easy—the Death Star can just send destructive power hurtling through space as a tight beam. But in reality, once a powerful laser has been fired, care must be taken to ensure it doesn’t get spread too thin.

If you’ve ever pointed a flashlight at a wall, then you’ve observed a more mundane example of this diffusion of light. The farther you are from the wall, the more the beam spreads, resulting in a larger and dimmer spot of light. Lasers generally expand much more slowly than a flashlight, but the effect is important when the laser travels a long way or must maintain a high intensity.

Whether your goal is blowing up a planet to achieve galactic domination or, more realistically, accelerating electrons to incredible speeds for physics research, you’ll want as tight and powerful a beam as possible to maximize the intensity. For terrestrial experiments, researchers can use devices called waveguides, like the optical fibers that might be carrying internet throughout your neighborhood, to transport a laser while keeping it contained to a narrow beam. The distinct core and outer shell—or cladding—of a waveguide keep the laser from spreading out. But if the laser pulse is too intense you run into a problem—it will destroy an optical fiber in a thousandth of a nanosecond.

Lasers are used to create an indestructible optical fiber out of plasma that helps researchers confine a separate laser pulse as it travels through the plasma. (Credit: Intense Laser-Matter Interactions Lab, University of Maryland)

Researchers at the University of Maryland, led by UMD Physics Professor Howard Milchberg, have developed an improved technique to make waveguides that can withstand the power of intense lasers. In a paper published on August 14, 2020 in Physical Review Letters, they demonstrated how powerful pulses can be transmitted along a waveguide that is created by firing weaker laser pulses into a cloud of hydrogen. They predict that the technique, developed with support from the US Department of Energy High Energy Physics program and the National Science Foundation, will be a powerful tool in high-energy particle acceleration experiments.

“A plasma waveguide can be a powerful tool for a variety of fields,” says Bo Miao, a co-author of the paper and UMD physics postdoctoral associate. “I’m excited that the experiment finally worked out after two years of hard work of alternating delight and frustration.”

Their technique relies on building a waveguide from a plasma—a gas where the electrons have been torn from the nuclei of the atoms.

“A plasma waveguide has all the structure of an optical fiber, the classic core, the classic cladding,” says Milchberg. “Although in this case, it's indestructible. The hydrogen plasma forming the waveguide is already ripped up into its protons and electrons, so there's not much more violence you can do to it.”

In the early 1990s, Milchberg and colleagues developed a related technique to use lasers to create plasma waveguides for other, more intense, lasers. In this earlier technique a laser beam is sent into a gas; as it travels, it rips electrons from their atoms along the beam, creating a plasma tunnel that is warmer than the surrounding gas. Due to its heat, the plasma expands, forming a low-density plasma core surrounded by a high-density wall formed by the shockwave from the plasma’s rush outward.

This structure is precisely what is needed for a waveguide, but the method has a pitfall—researchers can’t craft the core and wall independently. To get the wall to have the necessary thickness and density of electrical charges to function as a waveguide, required the core to be kept too dense for particle acceleration applications.

In the new paper, the team demonstrates an improved method that lets them craft the wall and core independently. Their insight was to use two specialized laser beams—called Bessel beams—to craft the waveguide. The first laser is a simple Bessel beam that forms the low-density core while causing less heating than the previous method.

Caption: On the left is a cross section of the intensity of the Bessel beam responsible for creating the low-density plasma core. On the right is a cross section of the intensity of the Bessel beam that creates the high-density plasma wall. The left image is 50 micrometers across and the right image is 100 micrometers across. (Credit: Intense Laser-Matter Interactions Lab, University of Maryland)But the second laser beam is more exotic. It is a hollow tube of light that allows them to build the wall of the waveguide by creating additional plasma from the gas surrounding the plasma core. Since the second laser pulse can match the shape of the high-density wall, they can tailor it without impacting the conditions of the core.

“Basically, the version of the technique that was used up until our paper is very constrained in the size of the guide, the length and other parameters,” says Linus Feder, a co-author of the paper and a UMD physics graduate student. “This new technique is highly adaptable and tunable. It just does away with a lot of the restrictions on the types of laser beams you can guide.”

The researchers demonstrated that the improvement allowed them to guide a laser for 30 centimeters in a tight beam—about 50% farther than previous experiments that used wider, 20-centimeter plasma waveguides created with a different technique.

Milchberg says their waveguide is like a long hypodermic needle and that the older method was more like a drinking straw. With the smaller guide, the laser’s energy is packed into a much smaller area, resulting in a much higher intensity.

“The only reason we were limited to 30 centimeters was lab geometry and not having enough laser energy,” says Milchberg. “But with more laser energy, there's no obstacle to us doing this for a couple of meters.

The new method may increase the practicality of using plasma waveguiding of intense laser pulses to accelerate charged particles for high energy physics experiments. The group is planning experiments to confirm their predictions of how the process will work with more powerful lasers.

The paper was selected as an editors’ suggestion and was highlighted in Physical Review Focus.

Story by Bailey Bedford: This email address is being protected from spambots. You need JavaScript enabled to view it.

In addition to Milchberg, Feder, and Miao, graduate students Andrew Goffin and Jaron Shrock were co-authors.
This research was supported by US Department of Energy (DESC0015516) and the National Science Foundation (PHY1619582).

RESEARCH CONTACTS
Bo Miao: This email address is being protected from spambots. You need JavaScript enabled to view it.
Linus Feder:This email address is being protected from spambots. You need JavaScript enabled to view it.