Quantum Physics and Gravity Meet in New Assistant Professor's Research

Two landmark achievements of 20th century physics remain stubbornly isolated, despite decades of attempts by scientists to bring them together.

On their own, they’ve been wildly successful. General relativity—Einstein’s grand theory of gravity—fused space and time into a single entity. It birthed the global positioning system and forever changed our conception of the cosmos. Quantum physics, the theory that governs the microscopic realm, powered the engines of the digital revolution, and it’s lighting the way to a new paradigm in computing.

But the two fields have largely been marooned on parallel tracks, rarely intersecting because they seem to describe such disparate domains of reality. “When electricity flows through a circuit, gravity is there,” says Brian Swingle, an assistant professor of physics at the University of Maryland and the newest fellow of the Joint Center for Quantum Information and Computer Science (QuICS). “We just don’t usually need it to describe the physics, so we ignore it.”

That might be fine for the physics of electrical circuits, Swingle says, but scientists expect a unified picture to emerge for the highest-energy, densest material in the universe—a picture in which quantum physics and gravity contribute on equal footing. Swingle, who arrived at UMD this past summer, is part of a vanguard of physicists exploring the connections between these two fields. His work blends quantum information and condensed matter physics with a pinch of gravity, and it’s unearthing curious connections between some of the most eye-catching phenomena that modern physics has on offer—things like quantum entanglement and black holes.

He hopes to expand on this work at Maryland and foster more collaboration between experts in all of these fields. “I think UMD has a lot of great resources, and there’s an opportunity now for new connections to be forged,” Swingle says. “That’s definitely one of the things that really excites me.”

Swingle’s interest in physics was kindled as a teenager, when a set of mysterious symbols captured his imagination. “Senior year of high school I somehow got interested in Maxwell’s equations,” he says, recounting his first contact with the four equations that are the distillation all of the 19th century’s knowledge about electricity and magnetism. “There was this mysterious picture with upside down triangles, which looked like gobbledygook, and it seemed really interesting to me. I’ve sort of been hooked ever since.”

He pursued a physics degree as an undergraduate at Georgia Tech in Atlanta, where he got an early start in research, writing software to simulate the behavior of many interacting particles and spending a summer working as a research assistant at the University of Washington.

He arrived at MIT for graduate school in 2005 with a loose plan to study condensed matter physics. But some early projects didn’t work out. “I thought about switching to neuroscience,” Swingle says. “I was thinking of working on birdsong. I had a general interest in information networks and higher organization.”

But his neuroscience career also stalled, and eventually he ended up back in condensed matter physics working with Xiao-Gang Wen, a luminary in the field. Wen asked Swingle to look into a problem involving quantum entanglement, the curiously strong connection that two quantum objects can share, and something clicked. Swingle became fascinated with entanglement, calculating the entanglement properties of a variety of quantum systems. “I was looking for some kind of picture,” he says, “but I didn’t really know what it was.”

That all changed when he took a class in string theory, the theoretical effort to recast all of modern physics in the language of tiny vibrating strings. String theory made one of the earliest attempts to shoehorn gravity into quantum physics, and over the course of several decades string theorists discovered some interesting relationships between the two. One such connection was a duality between quantum physics playing out in a particular universe and a theory of gravity in a universe with one extra dimension.

Swingle explains the gist of that relationship using a round table. Imagine that the table’s edge—a circle that wraps its perimeter—is a one-dimensional universe with a bunch of interacting quantum systems. It turns out that picking a particular quantum state for this encompassing ring limits what can happen in the interior—the tabletop itself—and vice versa. The truly strange thing is that the tabletop ends up endowed with gravity, but it’s now a theory of gravity in two dimensions. “The question you can kind of ask is ‘Where does the extra dimension come from?’” Swingle says.

It’s a lot of abstract math that need not have much to do with our universe, but Swingle discovered a particular way in which this abstraction becomes real. “I started to see these connections,” Swingle says. “It was starting to gel in an interesting way.”

Tensor networks—graphical webs that can represent the complicated states of interacting quantum systems—provided the key ingredient. Drawing the tensor network of a quantum system living on the edge of a table turned out to be a picture of a theory of gravity inside, a discrete snapshot of the fabric of spacetime on the tabletop. It was a simple theory of quantum gravity, albeit one that does not describe our universe. “Quantum gravity in any spacetime is sufficiently mysterious and sufficiently poorly understood that it's worth understanding even a toy case,” Swingle says.

Since his pioneering result, Swingle has continued to develop the relationship between quantum entanglement—depicted via hierarchical webs of tensor networks—and geometry, and has recently been thinking about the role complexity and computation play in all of it. A recent paper with several collaborators demonstrated a precise mathematical connection between the complexity of a quantum state and the geometry of its dual theory of gravity. It’s part of a body of work that is continuing to shift physicists’ perspectives on quantum gravity. “The old slogan used to be that entanglement was the fabric of spacetime,” Swingle says. “Now maybe it’s more general. Maybe now we think of spacetime itself as a computational history of some process—the picture of the quantum circuit that prepares the quantum state, something like that.”

Now, as an assistant professor at UMD, Swingle hopes to continue research along these lines, perhaps bringing more quantum information tools into the fray. He is also a co-principal investigator for the “It from Qubit” collaboration launched by the Simons Foundation. The name is a play on “it from bit,” a phrase coined by physicist John Archibald Wheeler that underscored the significance of information to the bedrock upon which reality sits. “Of course, information is quantum mechanical, since the world is quantum mechanical,” Swingle says. “Somehow that’s an important ingredient in the story—the quantum-ness is really important.”

Swingle encourages any students interested in learning more about his research to contact him directly or stop by his weekly group meeting, which is typically held on Thursdays at 5 p.m. in PSC 3150.

—Story by Chris Cesare

Quantum Simulators Wield Control Over More Than 50 Qubits, Setting New Record

Two independent teams of scientists, including one from the Joint Quantum Institute, have used more than 50 interacting atomic qubits to mimic magnetic quantum matter, blowing past the complexity of previous demonstrations. The results appear in this week’s issue of Nature.

As the basis for its quantum simulation, the JQI team deploys up to 53 individual ytterbium ions—charged atoms trapped in place by gold-coated and razor-sharp electrodes. A complementary design by Harvard and MIT researchers uses 51 uncharged rubidium atoms confined by an array of laser beams. With so many qubits these quantum simulators are on the cusp of exploring physics that is unreachable by even the fastest modern supercomputers. And adding even more qubits is just a matter of lassoing more atoms into the mix.

Read more.

High-altitude Observatory Sheds Light on Origin of Excess Anti-matter

UMD-led HAWC collaboration suggests dark matter as possible culprit

Goodman hawcThe HAWC Observatory, perched next to a volcano at an altitude of 13,500 feet, uses its 300 massive water tanks to scoop up the products of high-energy particle collisions happening in the upper atmosphere. Image credit: Jordan Goodman

A mountaintop observatory in Mexico, built and operated by an international team of scientists, has captured the first wide-angle view of gamma rays emanating from two rapidly spinning stars. The High-Altitude Water Cherenkov (HAWC) Gamma-Ray Observatory provided the fresh perspective on high-energy light streaming from these stellar neighbors, casting serious doubt on one possible explanation for a mysterious excess of anti-matter particles near Earth.

In 2008, astronomers observed an unexpectedly high number of positrons—the anti-matter cousins of electrons—in orbit a few hundred miles above Earth’s atmosphere. Ever since, scientists have debated the cause of the anomaly, split over two competing theories of its origin. Some suggested a simple explanation: The extra particles might come from nearby collapsed stars called pulsars, which spin around several times a second and throw off electrons, positrons and other matter with violent force. Others speculated that the extra positrons might come from processes involving dark matter—the invisible but pervasive substance seen so far only through its gravitational pull.

Using new data from the HAWC observatory, researchers made the first detailed measurements of two pulsars previously identified as possible sources of the positron excess. By catching and counting particles of light streaming from these nearby stellar engines, HAWC collaboration researchers found that the two pulsars are unlikely to be the origin of the positron excess. Despite being the right age and the right distance from Earth, the pulsars are surrounded by an extended murky cloud that prevents most positrons from escaping, according to results published in the November 17, 2017 issue of the journal Science.

“This new measurement is tantalizing because it strongly disfavors the idea that these extra positrons are coming to Earth from two nearby pulsars, at least when you assume a relatively simple model for how positrons diffuse away from these spinning stars,” said Jordan Goodman, professor of physics at the University of Maryland and the lead investigator and U.S. spokesperson for the HAWC collaboration. “Our measurement doesn’t decide the question in favor of dark matter, but any new theory that attempts to explain the excess using pulsars will need to account for what we’ve found.”

Francisco Salesa Greus, the lead corresponding author of the new paper and a scientist at the Institute of Nuclear Physics of the Polish Academy of Sciences in Krakow, Poland, added that “we are closer to understanding the origin of the positron excess after excluding two of the main source candidates.”

An Eye in the Sky

As with an ordinary camera, collecting lots of light allows HAWC to build sharp images of individual gamma-ray sources. The most energetic gamma rays originate in the graveyards of big stars, around stellar remains like the spinning pulsar remnants of supernovae. But that light doesn’t come from the stars themselves. Instead, it's created when the spinning pulsar accelerates particles to extremely high energies, causing them to smash into lower-energy photons left over from the early universe.

The size of the debris field around powerful pulsars, measured by the patch of sky that glows bright in gamma rays, tells researchers how quickly matter moves relative to the spinning stars. This enables researchers to estimate how quickly positrons are moving and how many positrons could have reached Earth from a given source.

Using a recently published HAWC catalog of the high-energy sky, scientists have absolved the nearby pulsar Geminga and its sister—the pulsar PSR B0656+14—as sources of the positron excess. Even though the two are old enough and close enough to account for the excess, matter isn’t drifting away from the pulsars fast enough to have reached the Earth.

“The gamma rays HAWC measures demonstrate that there are high-energy positrons escaping from these sources,” said Rubén López-Coto, a scientist at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany and a corresponding author. “But according to our measurement, they could not be significantly contributing to the extra positrons seen at the Earth.”

This measurement wouldn’t have been possible without HAWC’s wide view. It continuously scans about one-third of the sky overhead, which provided researchers with a broad view of the space around the pulsars. Other observatories watching for high-energy gamma rays with a much narrower field of view missed the extended nature of the pulsars.

The HAWC Observatory sits at an elevation of 13,500 feet, flanking the Sierra Negra volcano inside Pico de Orizaba National Park in the Mexican state of Puebla. It consists of more than 300 massive water tanks that sit waiting for cascades of particles initiated by high-energy packets of light called gamma rays—many of which have more than 10 million times the energy of a dental X-ray.

When these gamma rays smash into the upper atmosphere, they blast apart atoms in the air, producing a shower of particles that moves at nearly the speed of light toward the ground. When this shower reaches HAWC’s tanks, it produces coordinated flashes of blue light in the water, allowing researchers to reconstruct the energy and cosmic origin of the gamma ray that kicked off the cascade.

“Thanks to its wide field of view, HAWC provides unique measurements on the very-high-energy gamma ray profiles caused by the particle diffusion around nearby pulsars, which allows us to determine how fast the particles diffuse more directly than previous measurements,” says Hao Zhou, a scientist at the Los Alamos National Laboratory in New Mexico and a corresponding author of the new paper.

It’s possible that a new insight about the astrophysics of these pulsars and their local environments could account for the positron excess at Earth, but it would require a more complicated theory of positron diffusion than physicists in the collaboration think is likely.  On the other hand, dark matter may provide the right explanation, but more evidence will ultimately be needed to decide.

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For more information about the HAWC Observatory:
https://www.hawc-observatory.org/
http://jqi.umd.edu/news/podcast/jqi-podcast- episode-12

In addition to Goodman, UMD co-authors of the paper include graduate students Kristy Engle and Israel Martinez-Castellanos; postdoctoral researchers Colas Rivière and Daniel Fiorino; research scientist Andrew Smith; and visiting faculty Robert Ellsworth and David Berley.

The research paper, “Extended gamma-ray sources around pulsars constrain the origin of the positron flux at Earth,” A.U. Abeysekara et al., was published in the journal Science on November 17, 2017.

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.

Writer: Chris Cesare

Congressional Hearing Highlights Need for Quantum Technology Initiative

Credit: E. Edwards/JQICredit: E. Edwards/JQI

On October 24, 2017, two Fellows of the Joint Quantum Institute and the Joint Center for Quantum Information and Computer Science were among those that testified during a joint congressional committee hearing on the topic of American Leadership in Quantum Technology.

Carl Williams and Christopher Monroe attended as expert panelists, reading prepared statements and answering questions from committee members. Williams, who is also the deputy director of the Physical Measurement Laboratory at the National Institute of Standards and Technology (NIST), provided testimony about quantum research at NIST. Monroe—a Distinguished University Professor of Physics at the University of Maryland (UMD) and a co-founder and chief scientist at UMD-based startup IonQ, Inc—advocated for a National Quantum Initiative in his testimony. Both shared their perspectives on the path toward industry’s adoption of this emerging new technology.

The hearing focused on the status of quantum research in the US. Two panels with a total of six experts from government, industry, academia, and national laboratories testified. The witnesses emphasized that quantum information science will play a critical role in future advanced computing and secure communications. They also noted potential applications related to chemistry, medicine, artificial intelligence, and even space exploration.

In answering questions about the maturity of quantum information research, participants cited both Monroe’s and IBM’s small-scale quantum devices. According to panelists, commercialization of quantum technology is an imminent reality, rather than a futuristic goal. Participants discussed the global impact that industrial quantum science will have, noting that governments worldwide are investing in large-scale quantum research. China, Australia and Europe, in particular, are beginning to pour massive resources into funding quantum research.

Quantum at Maryland

UMD’s flagship College Park campus is home to a thriving quantum enterprise that is actively producing a competitive workforce, delivering innovative research, and attracting a network of strategic partners. With more than 175 scientists on-site and countless collaborations within a vast global research network, quantum programs at Maryland are leading the charge toward a quantum future.

  • The Joint Quantum Institute (JQI), founded in 2006, is a physics research partnership with NIST and the Laboratory for Physical Sciences dedicated to intensely studying quantum science.

  • A quantum-focused NSF Physics Frontier Center was first awarded to UMD in 2008 and renewed in 2014. This is a prestigious designation that promotes collaborative exploration of challenging but highly promising research areas.
  • UMD enjoys vital relationships with industrial and government-laboratory efforts in quantum computing, such as Microsoft, Northrop-Grumman, Sandia National Laboratories, the Army Research Laboratory, Booz-Allen-Hamilton, and the startup IonQ, Inc. Many UMD graduates have taken positions at these places. 

MEDIA CONTACT

E. Edwards | This email address is being protected from spambots. You need JavaScript enabled to view it. | (301) 405-2291

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