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.”

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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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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 particles 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. 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 LHCb UMD 2019UMD LHCb researchers Phoebe Hamilton, Will Parker, Hassan Jawahery, Yipeng Sun, Manuel Franco Sevilla and Zishuo Yang. (see https://arxiv.org/abs/1903.08726 and https://home.cern/news/press-release/physics/lhcb-sees-new-flavour-matter-antimatter-asymmetry.) 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.

Hybrid Device among First to Meld Quantum and Conventional Computing

Researchers at the University of Maryland (UMD) have trained a small hybrid quantum computer to reproduce the features in a particular set of images.

The result, which was published Oct. 18, 2019 in the journal Science Advances, is among the first demonstrations of quantum hardware teaming up with conventional computing power—in this case to do generative modeling, a machine learning task in which a computer learns to mimic the structure of a given dataset and generate examples that capture the essential character of the data.

“We combined one of the highest performance quantum computers with one of the most powerful AI programs—over the internet—to form a unique kind of hybrid machine,” says Joint Quantum Institute (JQI) Fellow Norbert Linke, an assistant professor of physics at UMD and a co-author of the new paper.

The researchers used four trapped atomic ions for the quantum half of their hybrid computer, with each ion representing a quantum bit, or qubit—the basic unit of information in a quantum computer. To manipulate the qubits, researchers punch commands into an ordinary computer, which interprets them and orchestrates a sequence of laser pulses that zap the qubits.Close-up photo of an ion trap. Credit: S. Debnath and E. Edwards/JQIClose-up photo of an ion trap. Credit: S. Debnath and E. Edwards/JQI

The UMD quantum computer is fully programmable, with connections between every pair of qubits. “We can implement any quantum function by executing a standard set of gates between the qubits,” says JQI and Joint Center for Quantum Information and Computer Science (QuICS) Fellow Christopher Monroe, a physics professor at UMD who was also a co-author of the new paper. “We just needed to optimize the parameters of each gate to train our machine learning algorithm. This is how quantum optimization works.”

Monroe, Linke and their colleagues trained their computer to produce an output that matched the “bars-and-stripes” set, a collection of images with blocks of color arranged vertically or horizontally to look like bars or stripes—a standard dataset in generative modeling because of its simplicity.

“Machine learning is generally categorized into two types,” says Daiwei Zhu, the lead author of the paper and a graduate student in physics at JQI. “One enables you to tell whether something is a cat or dog, and the other lets you generate an image of a cat or dog. We’re performing a scaled-back version of the latter task.”

Turning the hybrid system into a properly trained generative model meant finding the laser sequence that would turn a simple input state into an output capable of capturing the patterns in the bars-and-stripes set—something that qubits could do more efficiently than regular bits. “In essence, the power of this lies in the nature of quantum superposition,” says Zhu, referring to the ability of qubits to store multiple states—in this case, the entire set of bars-and-stripes images with four pixels—simultaneously.

Through a series of iterative steps, the researchers attempted to nudge the output of their hybrid computer closer and closer to the quantum bars-and-stripes state. They began by preparing the input qubits, subjecting them to a random sequence of laser pulses and measuring the resulting output. Those measurement results were then fed to a conventional, or “classical,” computer, which crunched the numbers and suggested adjustments to the laser pulses to make the output look more like the bars-and-stripes state.

By adjusting the laser parameters and repeating the procedure, the team could test whether the output eventually converged on the desired quantum state. They found that in some cases it did, and in some cases it didn’t.

The researchers studied the convergence using two different patterns of connectivity between qubits. In one, each qubit was able to interact with all the others, a situation that the team called all-to-all connectivity. In a second, a central qubit interacted with the other three, none of which interacted directly with one another.  They called this star connectivity. (This was an artificial constraint, as the four ions are naturally able to interact in the all-to-all fashion. But it could be relevant to experiments with a larger number of ions.)

The all-to-all interactions produced states closer to bars-and-stripes after training short sequences of pulses. But the experimenters had another setting to play with: They also studied the performance of two different number crunching methods used on the conventional half of the hybrid computer.

One method, called particle swarm optimization, worked well when all-to-all interactions were available, but it failed to converge on the bars-and-stripes output for star connectivity. A second method, which was suggested by three researchers at the Oxford, UK AI company Mind Foundry Limited, proved much more successful across the board.

The second method, called Bayesian optimization, was made available over the internet, which enabled the researchers to train sequences of laser pulses that could produce the bars-and-stripes state for both all-to-all and star connectivity. Not only that, but it significantly reduced the number of steps in the iterative training process, effectively cutting in half the time it took to converge on the correct output.

“What our experiment shows is that a quantum-classical hybrid machine, while in principle more powerful than either of the components individually, still needs the right classical piece to work,” says Linke.  “Using these schemes to solve problems in chemistry or logistics will require both a boost in quantum computer performance and tailored classical optimization strategies.”

Story by Chris Cesare 

In addition to Linke, Monroe and Zhu, co-authors of the research paper include University College London computer science student Marcello Benedetti; JQI physics graduate students Nhung Hong Nguyen, Cinthia Huerta Alderete and Laird Egan and recent JQI Ph.D. graduate Kevin Landsman; Zapata Computing scientist Alejandro Perdomo-Ortiz; Mind Foundry Limited scientists Nathan Korda, Alistair Garfoot and Charles Brecque; and Central Connecticut State University Mathematical Sciences Professor Oscar Perdomo.

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Stretched Photons Recover Lost Interference

The smallest pieces of nature—individual particles like electrons, for instance—are pretty much interchangeable. An electron is an electron is an electron, regardless of whether it’s stuck in a lab on Earth, bound to an atom in some chalky moon dust or shot out of an extragalactic black hole in a superheated jet. In practice, though, differences in energy, motion or location can make it easy to tell two electrons apart.

One way to test for the similarity of particles like electrons is to bring them together at the same time and place and look for interference—a quantum effect that arises when particles (which can also behave like waves) meet. This interference is important for everything from fundamental tests of quantum physics to the speedy calculations of quantum computers, but creating it requires exquisite control over particles that are indistinguishable.

Researchers recorded these patterns of quantum interference between three photons that started out as separate, distinguishable particles.Researchers recorded these patterns of quantum interference between three photons that started out as separate, distinguishable particles.

With an eye toward easing these requirements, researchers at the Joint Quantum Institute (JQI) and the Joint Center for Quantum Information and Computer Science (QuICS) have stretched out multiple photons—the quantum particles of light—and turned three distinct pulses into overlapping quantum waves. The work, which was published recently in the journal Physical Review Letters, restores the interference between photons and may eventually enable a demonstration of a particular kind of quantum supremacy—a clear speed advantage for computers that run on the rules of quantum physics.

“While photons do not directly interact with each other, when they meet they can exhibit a purely quantum feature absent from classical, non-quantum waves,” says JQI Fellow Mohammad Hafezi, a co-author of the paper and an associate professor of physics and electrical and computer engineering at the University of Maryland.

These days, testing the similarity of photons is routine. It involves bringing them together at a device called a beam splitter and measuring the light coming out the other side.

When a single photon hits a balanced beam splitter, there’s a 50 percent chance that it will travel straight through and a 50 percent chance that it will reflect off at an angle. By placing detectors in these two possible paths, scientists can measure which way individual photons end up going.

If two identical photons meet at the beam splitter, with one traveling to the east and the other to the north, it’s tempting to apply the same treatment to each particle individually. It’s true that both photons have an equal chance to travel through or reflect, but because the photons are indistinguishable, it’s impossible to tell which one goes where.

The upshot of this identity confusion is that two of the possible combinations—those in which both photons travel straight through the beam splitter and both photons reflect—cancel each other out, leaving behind a distinctly quantum result: The photons team up and travel as a pair, always ending up at one of the two detectors together.

Now Hafezi and his colleagues from UMD and the University of Portsmouth have observed a similar interference effect with distinguishable photons—pulses of light just two picoseconds long (a picosecond is a trillionth of a second) that are separated by tens of picoseconds. The essential trick was finding a way to make the pulses less distinguishable so that they could interfere.

“We used a single optical element that’s basically a fiber,” says Sunil Mittal, a postdoctoral researcher at JQI and a co-author of the new paper. “It emulates the equivalent of about 150 kilometers of fiber, which stretches the photons. It acts a bit like a lens in reverse, causing different frequencies in the pulses to disperse and defocus.”

By lengthening each photon by a factor of about 1000, the researchers could effectively erase the time delay between pulses and create large sections of overlap. That overlap made it more likely that photons would arrive to detectors at the same time and interfere with one another.

Prior experiments (including by JQI and QuICS Fellow Christopher Monroe and collaborators) have successfully interfered distinguishable photons, but those results required multiple channels for the incoming light—one for each photon. The new work uses just a single channel that carries light at standard telecom frequencies, which the authors say allows their system to easily scale to include many more photons.

Having more photons would allow researchers to study boson sampling, a computational problem that’s thought to be too hard for ordinary computers (similar to the problem Google is rumored to have solved). In its standard form, boson sampling concerns photons—which are members of a family of particles called bosons—making their way through a big network of beam splitters. The photons enter the network through different channels and exit to detectors, with one detector per channel.

The boson sampling “problem” amounts to doing a complicated coin flip, since each experiment samples from the underlying chance that (say) three photons entering the network at ports 1, 2 and 5 will end up at outputs 2, 3 and 7. The interference inside the network is complex and impossible to track with a regular computer—even for modest numbers of photons—and it gets harder the more photons you add. But with real photons in a real network, the problem would solve itself.

“The connection of this experiment to boson sampling is a great example of how the growing synergy between quantum many-body physics and computational complexity theory can lead to great progress in both fields,” says JQI and QuICS Fellow Alexey Gorshkov, an adjunct associate professor of physics at UMD and another co-author of the paper.

But up until now, boson sampling experiments have suffered from the problem of scalability: Solving the problem for more photons meant adding more channels, which meant taking up more space and timing the arrival of yet more photons to ensure their interference. Mittal says that their technique potentially solves both of these problems.

“In our system, the inputs don’t need to be in different fibers,” Mittal says. “All the photons can travel in a single fiber and the time differences can be erased by the same method we’ve already demonstrated.” Another off-the-shelf device could mimic the network of beam splitters, with the added benefit of allowing for easy reconfiguration, Mittal says. “We’re not doing boson sampling now, but it would be relatively easy to go in that direction.”

Story by Chris Cesare  This email address is being protected from spambots. You need JavaScript enabled to view it.

In addition to Hafezi, Mittal and Gorshkov, co-authors of the research paper include electrical and computer engineering graduate student Venkata Vikram Orre; JQI Research Scientist Elizabeth Goldschmidt, who is now an assistant professor of physics at the University of Illinois at Urbana-Champaign; physics graduate student Abhinav Deshpande; and Vincenzo Tamma, a physicist at the University of Portsmouth.