Catching New Patterns of Swirling Light Mid-flight

In many situations, it’s fair to say that light travels in a straight line without much happening along the way. But light can also hide complex patterns and behaviors that only a careful observer can uncover.

This is possible because light behaves like a wave, with properties that play a role in several interesting phenomena. One such property is phase, which measures where you are on an undulating wave—whether you sit at a peak, a trough or somewhere in between. When two (otherwise identical) light waves meet and are out of phase, they can interfere with one another, combining to create intricate patterns. Phase is integral to how light waves interact with each other and how energy flows in a beam or pulse of light.

Researchers at the University of Maryland, led by UMD Physics Professor Howard Milchberg, have discovered novel ways that the phase of light can form optical whorls—patterns known as spatiotemporal optical vortices (STOVs). In a paper published in the journal Optica on Dec. 18, 2019, the researchers captured the first view into these phase vortices situated in space and time, developing a new method to observe ultra-fast pulses of light.

Each STOV is a pulse of light with a particular pattern of intensity—a measure of where the energy is concentrated—and phase. In the STOVs prepared by Milchberg and his collaborators, the intensity forms a loop in space and time that the researchers describe as an edge-first flying donut: If you could see the pulse flying toward you, you would see only the edge of the donut and not the hole. (See leftmost image below, where negative times are earlier.) In the same region of space and time, the phase of the light pulse forms a swirling pattern, creating a vortex centered on the donut hole (rightmost image).Processed data showing the intensity forming a ring (left) and the phase forming the vortex (right) in a spatiotemporal optical vortex. The green arrow indicates the increase of the phase around the vortex. (Credit: Scott Hancock/University of Maryland)Processed data showing the intensity forming a ring (left) and the phase forming the vortex (right) in a spatiotemporal optical vortex. The green arrow indicates the increase of the phase around the vortex. (Credit: Scott Hancock/University of Maryland)

Milchberg and colleagues discovered STOVs in 2016 when they found structures akin to “optical smoke rings” forming around intense laser beams. These rings have a phase that varies around their edge, like the air currents swirling around a smoke ring. The vortices made in the new study are a similar but simpler structure: If you think of the original smoke ring as a bracelet made of beads, the new STOVs are like the individual beads.

The prior work showed that STOVs provide an elegant framework for understanding a well-known high-intensity laser effect—self-guiding. At high intensity, this effect occurs when a laser pulse, interacting with the medium it’s traveling through, compresses itself into a tight beam. The researchers showed that in this process, STOVs are responsible for directing the flow of energy and reshaping the laser, pushing energy together at its front and apart at its back.

That initial discovery looked at how these rings formed around a beam of light in two dimensions. But the researchers couldn’t explore the internal working of the vortices because each pulse is too short and fast for previously established techniques to capture. Each pulse passes by in just femtoseconds—about a 100 trillion times quicker than the blink of an eye.

“These are not microsecond or even nanosecond pulses that you just use electronics to capture,” says Sina Zahedpour, a co-author of the paper and UMD physics postdoctoral associate. “These are extremely short pulses that you need to use optical tricks to image.”

To capture both the intensity and phase of the new STOVs, researchers needed to prepare three additional pulses. The first pulse met with the STOV inside a thin glass window, producing an interference pattern encoded with the STOV intensity and phase. That pattern was read out using two longer pulses, producing data like that shown in the image above.

“The tools we had previously only looked at the amplitude of the light,” says Scott Hancock, a UMD physics graduate student and first author of the paper. “Now, we can get the full picture with phase, and this is proof that the principle works for studying ultrafast phenomena.”

STOVs may have a resilience that is useful for practical applications because their twisting, screw-like phase makes them robust against small obstacles. For example, as a STOV travels through the air, parts of the pulse might be blocked by water droplets and other small particles. But as they continue on, the STOVs tend to fill in the small sections that got knocked out, repairing minor damage in a way that could help preserve any information recorded in the pulse. Also, because a STOV pulse is so short and fast, it is indifferent to normal fluctuations in the air that are comparatively slow.

“Controlled generation of spatiotemporal optical vortices may lead to applications such as the resilient propagation of information or beam power through turbulence or fog,” says Milchberg. “These are important for applications such as free-space optical communications using lasers or for supplying power from ground stations to aerial vehicles.”


In addition to Milchberg, Zahedpour and Hancock, graduate student Andrew Goffin was a co-author.

This research was supported by the National Science Foundation (NSF) (Award No. 1619582), the Air Force Office of Scientific Research (Award Nos. FA9550-16-10121 and FA9550-16-10284) and the Office of Naval Research (Award Nos. N00014-17-1-2705 and N00014-17-12778). The content of this article does not necessarily reflect the views of the NSF, the Air Force Office of Scientific Research or the Office of Naval Research.

The paper “Free-space propagation of spatiotemporal optical vortices,” S. W. Hancock, S. Zahedpour, A. Goffin, and H. M. Milchberg was published in Optica on December 18, 2019.

Media Relations Contact: Bailey Bedford, 301-405-9401, This email address is being protected from spambots. You need JavaScript enabled to view it.


Johnpierre Paglione Receives $1.55M from the Moore Foundation

Physics Professor Johnpierre Paglione has been awarded more than $1.5 million by the Gordon and Betty Moore Foundation to study the complex behavior of electrons in quantum materials.paglione jpJohnpierre Paglione

“The Moore Foundation has played a pivotal role in supporting and promoting quantum materials research over the last five years, and I am extremely excited to continue to be part of this effort,” said Paglione, who also directs UMD’s Quantum Materials Center (formerly the Center for Nanophysics and Advanced Materials).

The new grant was awarded by the Moore Foundation’s Emergent Phenomena in Quantum Systems (EPiQS) initiative, a quantum materials research program that funds work on materials synthesis, experiments, and theory, with an interdisciplinary approach that includes physicists, chemists, and materials scientists. EPiQS focuses on exploratory research that develop deep questions about the organizing principles of complex quantum matter, and it also supports progress toward new applications, like quantum computing and precision measurement. 

Paglione’s award for materials synthesis was one of only 13 in the U.S. and renews an earlier grant he received from EPiQS, which has provided more than $120 million to researchers since 2013.

“Fundamental studies of quantum materials play a critical role in not only supporting current development of quantum technologies, but also the discovery of new phenomena that hold promise for future applications,” Paglione said.

In recognition of that critical role, UMD’s Center for Nanophysics and Advanced Materials was renamed to the Quantum Materials Center (QMC) in October. The change emphasized the evolving interests of the Center’s members, and it was announced at a one-day symposium in September organized by Paglione and several colleagues.

“Our center’s purpose will remain focused on the fundamental exploration and development of advanced materials and devices using multidisciplinary expertise drawn from the physics, chemistry, engineering and materials science departments,” Paglione said. “But we will place strong emphasis on the pursuit of optimized and novel quantum phenomena with potential to nucleate future computing, information and energy technologies.”

The symposium brought together many local scientists who study quantum materials, including researchers from the university’s Departments of Physics, Chemistry and Biochemistry, Electrical and Computer Engineering, and Materials Science and Engineering, in addition to researchers from the National Institute of Standards and Technology (NIST) and the Laboratory for Physical Sciences. Amitabh Varshney, dean of UMD’s College of Computer, Mathematical, and Natural Sciences, and Robert Briber, associate dean of UMD’s A. James Clark School of Engineering, attended and shared their perspectives on campus initiatives in quantum science, including the newly formed Quantum Technology Center.

That meeting was bookended by several exciting research results from Paglione and his colleagues in the QMC. In June, they reported capturing the best evidence yet of Klein tunneling, a quantum quirk that allows electrons to burrow through a barrier like it’s not even there. The result, which was featured on the cover of the journal Nature, arises from a duo of quantum effects at the junction of two materials. One is superconductivity, which keeps electrons paired off in highly correlated ways. The other has to do with the precise kind of superconductivity present—in this case, topological superconductivity that further constrains the way that electrons interact with the interface between the two materials. In a nutshell, electrons heading toward the junction aren’t allowed to reflect back, which leads to their perfect transmission.

In August, Paglione and his collaborators published a paper in the journal Science about a new, unconventional superconductor. That material—uranium ditelluride—may also exhibit some effects expected of a topological superconductor, including a demonstrated resilience to magnetic fields that typically destroy superconductivity. One of the paper’s co-authors, NIST scientist and Adjunct Associate Professor of Physics Nicholas Butch, called the material a potential “silicon of the quantum information age,” due to its stability and potential use as a storage medium for the basic units of information in quantum computers.

In a follow-up paper published in the journal Nature Physics in October, many of the same researchers teamed up with scientists from the National High Magnetic Field Laboratory to test the properties of uranium ditelluride under extreme magnetic fields. They observed a rare phenomenon called re-entrant superconductivity, furthering the case that uranium ditelluride is not only a profoundly exotic superconductor, but also a promising material for technological applications. Nicknamed “Lazarus superconductivity” after the biblical figure who rose from the dead, the phenomenon occurs when a superconducting state arises, breaks down, then re-emerges in a material due to a change in a specific parameter—in this case, the application of a very strong magnetic field.

“This is indeed a remarkable material and it’s keeping us very busy,” Paglione said. “Uranium ditelluride may very well become the ‘textbook’ spin-triplet superconductor that people have been seeking for dozens of years and, more importantly, may be the first manifestation of a true intrinsic topological superconductor with potential for all sorts of technologies to come!”

Written by Chris Cesare with contributions from Matthew Wright

Galactic Gamma-ray Source Map Reveals Birthplaces of High-energy Particles

Nine sources of extremely high-energy gamma rays have been identified in a new catalog compiled by researchers with the High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory, including nine University of Maryland physicists. All nine sources produce gamma rays with energies over 56 trillion electron volts (TeV)—more than eight times the energy of the most powerful proton beams produced at particle accelerators on Earth—and three emit gamma rays extending to 100 TeV and beyond, making these the highest-energy sources ever observed in our galaxy. The catalog helps to explain where the particles originate and how they are produced with such extreme energies.hawc 2020The High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory was used to create a map of the galactic plane indicating the highest energy gamma ray sources yet discovered. (Credit: Jordan Goodman/University of Maryland)

“The very high-energy gamma rays we detect are produced by interactions of even higher energy charged particles near their source,” said Jordan Goodman, a Distinguished University Professor of Physics at UMD and U.S. lead investigator and spokesperson for the HAWC collaboration.  “Charged particles are bent in the magnetic fields of our galaxy and don’t point back to their origin. Gamma rays, like light, travel in straight lines allowing us to use them to map the sources of the high-energy emission. HAWC, which is a wide field-of-view instrument, views the overhead sky 24/7 giving us a deep exposure to look for the rare high energy gamma ray events.”

The catalog of high-energy sources was published online in the journal Physical Review Letters on Jan. 15, 2020.  Higher-energy astrophysical particles have previously been detected, but this is the first time specific galactic sources have been pinpointed for such high-energy particles. All of the sources have extremely energetic pulsars nearby. The number of sources detected may indicate that ultra-high-energy emission is a generic feature of powerful particle winds coming from pulsars embedded in interstellar gas clouds known as nebulae, and that more detections will be forthcoming.

The HAWC Gamma-Ray Observatory consists of an array of water-filled tanks sitting high on the slopes of the Sierra Negra volcano in Puebla, Mexico, where the atmosphere is thin and offers better conditions for observing gamma rays. When gamma rays strike molecules in the atmosphere they produce showers of energetic particles. Nothing can travel faster than the speed of light in a vacuum, but in water light moves a little slower. As a result, some particles in cosmic ray showers travel faster than light in the water inside the HAWC detector tanks. The faster-than-light particles, in turn, produce characteristic flashes of light called Cherenkov radiation. Using recordings of the Cherenkov flashes in the HAWC water tanks, researchers reconstruct the sources of particle showers and learn about the particles that caused them.

The HAWC collaborators plan to continue searching for the sources of high-energy cosmic rays. By combining their data with measurements from other types of observatories, such as neutrino, X-ray, radio and optical telescopes, they hope to elucidate the astrophysical mechanisms that produce the cosmic rays that continuously rain down on our planet.

“There are still many unanswered questions about cosmic-ray origins and acceleration,” said Kelly Malone, an astrophysicist in the Neutron Science and Technology group at Los Alamos National Laboratory and a member of the HAWC scientific collaboration. “High energy gamma rays are produced near cosmic-ray sites and can be used to probe cosmic-ray acceleration. However, there is some ambiguity in using gamma rays to study this, as high-energy gamma rays can also be produced via other mechanisms, such as lower-energy photons scattering off of electrons, which commonly occurs near pulsars.”


In addition to Goodman, other UMD co-authors from the Department of Physics included Visiting Professor Robert Ellsworth; Principal Engineer Michael Schneider; Research Scientist Andrew James Smith; Graduate Students Kristi Engel and Elijah Job Tabachnick; and Postdoctoral Associates Colas Rivière, Chad Brisbois and Israel Martinez-Castellanos.

Text for this news item was adapted with permission from a press release written by Los Alamos National Laboratory. 

The paper “Multiple Galactic Sources with Emission Above 56 TeV Detected by HAWC,” A.U. Abeysekara, et al. was published in Physical Review Letters on January 15, 2020.

The National Science Foundation, the U.S. Department of Energy and Los Alamos National Laboratory provided funding for the United States’ participation in the HAWC project. The Consejo Nacional de Ciencia y Tecnología (CONACyT) is the primary funder for Mexican participation. The content of this article does not necessarily reflect the views of these organizations.  

Media Relations Contact: Bailey Bedford, 301-405-9401, This email address is being protected from spambots. You need JavaScript enabled to view it.  


Remote Quantum Systems Produce Interfering Photons

UMD physicists have observed, for the first time, interference between particles of light created using a trapped ion and a collection of neutral atoms. Their results could be an essential step toward the realization of a distributed network of quantum computers capable of processing information in novel ways.

In the new experiment, atoms in neighboring buildings produced photons—the quantum particles of light—in two distinct ways. Several hundred feet of optical cables then brought the photons together, and the research team, which included scientists from the Joint Quantum Institute (JQI) as well as the Army Research Lab, measured a telltale interference pattern. It was the first time that photons from these two particular quantum systems were manipulated into having the same wavelength, energy and polarization—a feat that made the particles indistinguishable. The result, which may prove vital for communicating over quantum networks of the future, was published recently in the journal Physical Review Letters.

“If we want to build a quantum internet, we need to be able to connect nodes of different types and functions,” says JQI Fellow Steve Rolston, a co-author of the paper and a professor of physics at the University of Maryland. “Quantum interference between photons generated by the different systems is necessary to eventually entangle the nodes, making the network truly quantum.”photon interference diagram v5 hires 002A schematic showing the paths taken by photons from two different sources in neighboring buildings. (Credit: S. Kelley/NIST)

The first source of photons was a single trapped ion—an atom that is missing an electron—held in place by electric fields. Collections of these ions, trapped in a chain, are leading candidates for the construction of quantum computers due to their long lifetimes and ease of control. The second source of photons was a collection of very cold atoms, still in possession of all their electrons. These uncharged, or neutral, atomic ensembles are excellent interfaces between light and matter, as they easily convert photons into atomic excitations and vice versa. The photons produced by each of these two systems are typically different, limiting their ability to work together.

In one building, researchers used a laser to excite a trapped barium ion to a higher energy. When it transitioned back to a lower energy, it emitted a photon at a known wavelength but in a random direction. When scientists captured a photon, they stretched its wavelength to match photons from the other source.

In an adjacent building, a cloud of tens of thousands of neutral rubidium atoms generated the photons. Lasers were again used to pump up the energy of these atoms, and that procedure imprinted a single excitation across the whole cloud through a phenomenon called the Rydberg blockade. When the excitation shed its energy as photons, they traveled in a well-defined direction, making it easy for researchers to collect them.

The team used an interferometer to measure the degree to which two photons were identical. A single photon entering the interferometer is equally likely to take either of two possible exits. And two distinguishable photons entering the interferometer at the same time don’t notice each other, acting like two independent single photons.

But when researchers brought together the photons from their two sources, they almost always took the same exit—a result of quantum interference and an indication that they were nearly identical. This was precisely what the research team had hoped for: the first demonstration of interference between photons from these two very different quantum systems.

In this experiment, photons traveled from the first building to the second via hundreds of feet of optical fiber. Due to this distance, sending photons from both systems to meet at the interferometer simultaneously was a feat of precise timing. Detectors were placed at the exits of the interferometer to detect where the photons came out, but the team often had to wait—gathering all the data took 24 hours over a period of 3 days.

Further experimental upgrades could be used to generate a special quantum connection called entanglement between the ion and the neutral atoms. In entanglement, two quantum objects become so closely linked that the results from measuring one are correlated with the results from measuring the other, even if the objects are separated by a huge distance. Entanglement is necessary for the speedy algorithms that scientists hope to run on quantum computers in the future.

Generating entanglement between different quantum systems usually requires identical photons, which the researchers were able to create. Unfortunately, trapped ions emit photons in a random direction, making the probability of catching them low. This meant that only about eight photons from the trapped ion made it to the interferometer each second. If the researchers attempted to perform more intricate experiments with that rate, the data could take months to collect. However, future work may increase how frequently the ion emits photons and allow for a useful rate of entanglement production.  

“This is a stepping-stone on the way to being able to entangle these two systems,” says Alexander Craddock, a graduate student at JQI and the lead author of this study. “And that would be fantastic, because you can then take advantage of all the different weird and wonderful properties of both of them.”

Story by Jillian Kunze

In addition to Rolston and Craddock, co-authors of the paper include JQI graduate students John Hannegan, Dalia Ornelas-Huerta, and Andrew Hachtel, JQI postdoctoral researcher James Siverns, Army Research Laboratory scientists and JQI Affiliates Elizabeth Goldschmidt (now an Assistant Professor of Physics at the University of Illinois) and Qudsia Quraishi, and JQI Fellow Trey Porto.

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Steve Rolston
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LHCb Experiment Discovers CP Violation in the Charm Quark System

A major unsolved puzzle in particle physics and cosmology is the apparent asymmetry in the abundance of matter and antimatter in the universe. One condition for such an asymmetry to arise is the breaking of charge-parity (CP) symmetry in laws of interactions amongst particles, resulting in differences in reactions involving partiPhoebe Hamilton, Will Parker, Hassan Jawahery, Yipeng Sun, Manuel Franco Sevilla and Zishuo Yang. Phoebe Hamilton, Will Parker, Hassan Jawahery, Yipeng Sun, Manuel Franco Sevilla and Zishuo Yang. cles and antiparticles. CP violation has been observed in particles containing the strange and bottom quarks--kaons and B mesons--and the results conform to the predictions of the Standard Model at the current level of experimental accuracy. However, CP violation in the Standard Model is not sufficient to explain the matter-antimatter asymmetry in the universe. A key goal of the LHCb experiment is to search for additional sources of CP violation beyond the Standard Model. The Standard Model predicts negligible CP violation in decays of particles containing the charm quark, thus any significant effect in charm mesons could be due to new sources of CP violation. Using the data recorded in 2011-2018, the LHCb experiment has observed CP violation in decays of D mesons – mesons made of a charm quark and anti-up quark.
(see and This work was rated a Physics World Breakthrough of the Year finalist for 2019.

Theorists are hard at work to explain the observed CP violation, which is very small but larger than most of the existing predictions within the Standard Model. Failing any Standard Model explanation, this result could open the door to exploring new sources of CP violation beyond the Standard Model.

The LHCb collaboration is now preparing to complete and install the new upgraded detector elements that will significantly enhance the precision of the spectrometer, thus allowing more precise measurements of CP violation and other very rare effects that may arise from particles and interactions that are not present in the Standard Model. The new upgraded detector elements would enable the experiment to operate at higher intensities of the large hadron collider (LHC) and read out the entire detector--millions of sensor signals--at beam collisions that occur every 25 nanoseconds. Over the past six years, the Maryland team in LHCb has been deeply engaged in the design and construction of a charge particle detector, based on silicon sensors, focusing on the development of the electronics that would enable the readout and transfer of the sensor signals. The team is now preparing to complete the construction of the nearly 600 main electronics boards and 2000 auxiliary boards, which will be shipped to CERN for installation in the LHCb experiment starting in Spring of 2020. The experiment is expected to resume operation in 2021.