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Shawhan Named a Distinguished Scholar-Teacher

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Category: Department News
Published: Monday, May 24 2021 11:07

Peter Shawhan has been named a University of Maryland Distinguished Scholar-Teacher. The award honors faculty of outstanding scholarly accomplishment and excellence in teaching. He will give his DST lecture, The Simple (and Not-So-Simple) Physics of Detecting Gravitational Waves, on Tuesday, December 7, 2021 at 4 p.m. in lecture hall 1412 of the John S. Toll Physics Building. Refreshments precede the event, starting at 3:30 p.m.

"Peter clearly deserves this recognition," said physics chair Steve Rolston. "He has been a key contributor to LIGO's celebrated successes, and we are just beginning to reap the rewards of his great contributions to multi-messenger astronomy," which integrates data from previously-disconnected satellites and observatories. "Peter is equally dedicated to our education mission. He was an excellent graduate director for five years, and has been a great teacher across the range of our course offerings. Last fall, he designed and launched PHYS 172, Succeeding in Physics, to help students who might otherwise struggle with the major's requirements to build better understanding."

Shawhan is also chair of the department's newly-established Climate Committee, which is working to ensure a welcoming and supportive environment for all.Peter ShawhanPeter Shawhan

“I’m fortunate to have an amazing group of colleagues who made LIGO a reality, after decades of careful preparations,” said Shawhan.  “It really works!  And now we are routinely detecting gravitational wave events from galaxies far, far away and getting important astrophysics insights from them.  But one of the great things about being a professor is that I can also talk about current research in my classes, connecting it with the course material and sharing some of the excitement of actually using physics to do revolutionary things.”

Shawhan received his Ph.D. in physics from the University of Chicago, and was appointed a Millikan Prize Postdoctoral Fellow at the California Institute of Technology. He continued at Caltech as a Senior Scientist before accepting a faculty appointment with UMD Physics in 2006.  Shawhan’s primary research for the past 20 years has been direct detection of gravitational waves with the LIGO and Virgo detectors, and he has held numerous leadership positions within the LIGO Scientific Collaboration, including Burst Analysis Working Group Co-Chair (2004-11) and LSC Data Analysis Coordinator (2017-present).  He was instrumental in establishing and nurturing a program of sharing prompt information about gravitational-wave event candidates with astronomers to allow them to look for corresponding signals in their instruments.  That groundwork enabled a remarkably rich campaign of astronomical follow-up observations and study, spanning the whole electromagnetic spectrum, when LIGO and Virgo detected the first binary neutron merger event, in August 2017.  That first event has provided scientific breakthroughs in fundamental physics, neutron star properties, high-energy astrophysics, and cosmology.  LIGO and Virgo are currently being upgraded and preparing to report more event candidates as they are identified.

Shawhan served as the Physics Associate Chair for Graduate Education from 2014-19 and is a member of the UMD-Goddard Joint Space-Science Institute and its Executive Committee. In addition, he is a past Chair of the Division of Gravitational Physics of the American Physical Society and was elected an APS fellow in 2019. Shawhan received the Richard A. Ferrell Distinguished Faculty Fellowship from the UMD Department of Physics in 2016. He was the recipient in 2018 of the Kirwan Faculty Research and Scholarship Prize and the USM Board of Regents Faculty Award for Excellence in Research.

Time Delay Acquires a New Dimension

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Category: Research News
Published: Wednesday, May 19 2021 12:33

Physicists love to do scattering experiments.  When they are trying to figure out a new force of nature, or discover a new particle, they fire up the accelerator and shoot tiny particles at their target, and measure what comes out.  Usually they carefully measure the energy and momentum of the incident and exiting particles, and try to learn about what took place in the target from that.  All of this information is summarized in an elegant quantity known as the Scattering Matrix. 

Schematic of time delay for scattering of a wave packet from a real-life ray-chaotic billiard. The symmetric and smooth wave packet goes in to the scattering region through one scattering channel and emerges later from another channel as a delayed and strongly distorted pulse. The complex time delay accounts for these changes.Schematic of time delay for scattering of a wave packet from a real-life ray-chaotic billiard. The symmetric and smooth wave packet goes in to the scattering region through one scattering channel and emerges later from another channel as a delayed and strongly distorted pulse. The complex time delay accounts for these changes.

Less studied is the question of how long the particle lingers in the interaction region before coming out.  This quantity is called time delay, and it has been studied since the early days of nuclear physics.  However, time delay never graduated from the confines of its original home, namely relatively simple quantum mechanical settings where the corrupting influences of “dissipation,” “de-coherence” and “dephasing” do not come into play.  Because of the rather restrictive conditions under which time delay was originally defined, it has always been taken to be a real, and generally positive, number.  In a paper published on May 18, 2021 in Physical Review E, Lei Chen, Steve Anlage,Illustration of complex time delay associated with a single resonant mode of a complex scattering system known as a quantum graph. Shown is the evolution of the complex time delay as a function of frequency near the resonance, illustrating how the real and imaginary parts of the time delay form a closed figure in the complex time-delay plane. The cases of two resonances, one with small loss and another with large loss, are shown for illustration. These results are from a simulation of the quantum graph.Illustration of complex time delay associated with a single resonant mode of a complex scattering system known as a quantum graph. Shown is the evolution of the complex time delay as a function of frequency near the resonance, illustrating how the real and imaginary parts of the time delay form a closed figure in the complex time-delay plane. The cases of two resonances, one with small loss and another with large loss, are shown for illustration. These results are from a simulation of the quantum graph. and Yan V. Fyodorov describe having generalized this time delay to real-world situations where there is dissipation and de-phasing, and created a very useful complex version of time delay.  This generalized time delay has a real part that can be positive or negative, and an imaginary part, which can also have either sign. 

The real part still tells something about the lingering time of the particle, but the imaginary part conveys how much the waves that describe the particle quantum mechanically are distorted by the lossy and disruptive scattering system.  This new quantity gives tremendous insights into the microscopic physics of the scattering system by cleverly encoding information about locations of the poles (infinities) and zeros of the scattering matrix.  Knowing all of those locations is equivalent to knowing essentially everything there is to know about the scattering process.  The exciting thing is that now complex time delay can be used to uncover fundamental properties of scattering systems that arise not just in quantum physics, but also in electromagnetic and acoustic reverberant systems, and the world of the small, but not too small, called mesoscopic physics.

More recently, in a paper published on November 12, 2021 in Physical Review Letters, the same authors have applied their generalized time delay concept to complex scattering systems with many overlapping resonances and zeros.  By looking at the statistical properties of the time delays, they utilized concepts from random matrix theory to reveal information about the distribution of poles in the complex frequency plane.  This deepens the understanding of a broad variety of complicated wave scattering systems, and provides a quantitative tool to characterize and classify them.

To read more, see the paper  "Generalization of Wigner time delay to subunitary scattering systems", in the 1 May 2021 issue of Physical Review E (Vol. 103, No. 5): https://link.aps.org/doi/10.1103/PhysRevE.103.L050203
DOI: 10.1103/PhysRevE.103.L050203

And the follow-up paper “Statistics of Complex Wigner Time Delays as a counter of S-matrix poles: Theory and Experiment,” in the 12 November 2021 issue of Physical Review Letters (Vol. 127, No. 20): https://doi.org/10.1103/PhysRevLett.127.204101
DOI: 10.1103/PhysRevLett.127.204101

Researchers Generate Tunable Twin Particles of Light

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Category: Research News
Published: Monday, May 10 2021 05:33

Identical twins might seem “indistinguishable,” but in the quantum world the word takes on a new level of meaning. While identical twins share many traits, the universe treats two indistinguishable quantum particles as intrinsically interchangeable. This opens the door for indistinguishable particles to interact in unique ways—such as in quantum interference—that are needed for quantum computers.A new technique sees two distinct particles of light enter a chip and two identical twin particles of light leave it. The image artistically combines the journey of twin particles of light along the outer edge of a checkerboard of rings with the abstract shape of its topological underpinnings. (Credit: Kaveh Haerian)A new technique sees two distinct particles of light enter a chip and two identical twin particles of light leave it. The image artistically combines the journey of twin particles of light along the outer edge of a checkerboard of rings with the abstract shape of its topological underpinnings. (Credit: Kaveh Haerian)

While generating a crowd of photons—particles of light—is as easy as flipping a light switch, it’s trickier to make a pair of indistinguishable photons. And it takes yet more work to endow that pair with a quantum mechanical link known as entanglement. In a paper published May 10, 2021 in the journal Nature Photonics(link is external), JQI researchers and their colleagues describe a new way to make entangled twin particles of light and to tune their properties using a method conveniently housed on a chip, a potential boon for quantum technologies that require a reliable source of well-tailored photon pairs.

The researchers, led by JQI fellow Mohammad Hafezi, designed the method to harness the advantages of topological physics. Topological physics explores previously untapped physical phenomena using the mathematical field of topology, which describes common traits shared by different shapes. (Where geometry concerns angles and sizes, topology is more about holes and punctures—all-encompassing characteristics that don’t depend on local details.) Researchers have made several major discoveries by applying this approach, which describes how quantum particles—like electrons or, in this case, photons—can move in a particular material or device by analyzing its broad characteristics through the lens of topological features that correspond to abstract shapes (such as the donut in the image above). The topological phenomena that have been revealed are directly tied to the general nature of the material; they must exist even in the presence of material impurities that would upset the smooth movement of photons or electrons in most other circumstances.

Their new method builds on previous work, including the generation of a series of distinguishable photon pairs. In both the new and old experiments, the team created a checkerboard of rings on a silicon chip. Each ring is a resonator that acts like a tiny race track designed to keep certain photons traveling round and round for a long time. But since individual photons in a resonator live by quantum rules, the racecars (photons) can sometimes just pass unchanged through an intervening wall and proceed to speed along a neighboring track.

The repeating grid of rings mimics the repeating grid of atoms that electrons travel through in a solid, allowing the researchers to design situations for light that mirror well known topological effects in electronics. By creating and exploring different topological environments, the team has developed new ways to manipulate photons.

“It's exactly the same mathematics that applies to electrons and photons,” says Sunil Mittal, a JQI postdoctoral researcher and the first author of the paper. “So you get more or less all the same topological features. All the mathematics that you do with electrons, you can simply carry to photonic systems.”

In the current work, they recreated an electronic phenomenon called the anomalous quantum Hall effect that opens up paths for electrons on the edge of a material. These edge paths, which are called topological edge states, exist because of topological effects, and they can reliably transport electrons while leaving routes through the interior easily disrupted and impassable. Achieving this particular topological effect requires that localized magnetic fields push on electrons and that the total magnetic field when averaged over larger sections of the material cancels out to zero.

But photons lack the electrical charge that makes electrons susceptible to magnetic shoves, so the team had to recreate the magnetic push in some other way. To achieve this, they laid out the tracks so that it is easier for the photons to quantum mechanically jump between rings in certain directions. This simulates the missing magnetic influence and creates an environment with a photonic version of the anomalous quantum Hall effect and its stable edge paths.

For this experiment, the team sent two laser beams of two different colors (frequencies) of light into this carefully designed environment. Inside a resonator, a photon from each of the beams spontaneously combine. The researchers then observed how the photons reformed into two indistinguishable photons, travelled through the topological edge paths and were eventually ejected from the chip.

Since the new photons formed inside a resonator ring, they took on the traits (polarization and spatial mode) of the photons that the resonators are designed to hold. The only trait left that the team needed to worry about was their frequencies.

The researchers matched the frequencies of the photons by selecting the appropriate input frequencies for the two lasers based on how they would combine inside the silicon resonators. With the appropriate theoretical understanding of the experiment, they can produce photons that are quantum mechanically indistinguishable.

The nature of the formation of the new photons provides the desired quantum characteristics. The photons are quantum mechanically entangled due to the way they were generated as pairs; their combined properties—like the total energy of the pair—are constrained by what the original photons brought into the mix, so observing the property of one instantly reveals the equivalent fact about the other. Until they are observed—that is, detected by the researchers—they don’t exist as two individual particles with distinct quantum states for their frequencies. Rather, they are identical mixtures of possible frequency states called a superposition. The two photons being indistinguishable means they can quantum mechanically interfere with each other

The resulting combination of being indistinguishable and entangled is essential for many potential uses of photons in quantum technologies. An additional lucky side effect of the researcher’s topological approach is that it gives them a greater ability to adjust the frequencies of the twin photons based on the frequencies they pump into the chip and how well the frequencies match with the topological states on the edge of the device.

“This is not the only way to generate entangled photon pairs—there are many other devices—but they are not tunable,” Mittal says. “So once you fabricate your device, it is what it is. If you want to change the bandwidth of the photons or do something else, it's not possible. But in our case, we don't have to design a new device. We showed that, just by tuning the pump frequencies, we could tune the interference properties. So, that was very exciting.”

The combination of the devices being tunable and robust against manufacturing imperfections make them an appealing option for practical applications, the authors say. The team plans to continue exploring the potential of this technique and related topological devices and possible ways to further improve the devices such as using other materials to make them.

Original story by Bailey Bedford: https://jqi.umd.edu/news/jqi-researchers-generate-tunable-twin-particles-light

In addition to Hafezi and Mittal, former JQI graduate student Venkata Vikram Orre and former JQI postdoctoral researcher and current assistant professor at the University of Illinois Urbana-Champaign Elizabeth Goldschmidt were also co-authors of the paper.

 
Research Contact: Mohammad Hafezi This email address is being protected from spambots. You need JavaScript enabled to view it.  

Buonanno Elected to National Academy of Sciences

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Category: Department News
Published: Tuesday, April 27 2021 12:46

Alessandra Buonanno has been elected to the National Academy of Sciences (NAS).

Buonanno is the director of the Astrophysical and Cosmological Relativity Department at the Max Planck Institute for Gravitational Physics in Potsdam and a Research Professor at the University of Maryland. She joined the UMD Department of Physics in 2005, and received an Alfred P. Sloan Foundation Fellowship and the Richard A. Ferrell Distinguished Faculty Fellowship. She is a Fellow of the American Physical Society and the International Society of General Relativity and Gravitation. In 2018, she received the Leibniz Prize, Germany's prestigious research award. Earlier in 2021, she was awarded the Galileo Galilei Medal of the National Institute for Nuclear Physics (INFN).

Buonanno was one of 59 women elected to the NAS this year, the most in the academy’s history. Also elected was UMD Professor of Psychology Michele Gelfand.Alessandra Buonanno © A. Klaer Alessandra Buonanno © A. Klaer

The new class brings the total number of active members to 2,461 and the total number of international members to 511.  Other UMD physics members of the NAS include Michael Fisher, Jim Gates, Chris Jarzynski, John Mather, Chris Monroe, Bill Phillips, Roald Sagdeev, Rob Tycko, John Weeks and Ellen Williams.

Buonanno was also recently elected to the German National Academy of Sciences Leopoldina, which originated in 1652 as a classical scholarly society.

Buonanno's research has spanned several topics in gravitational-wave theory, data-analysis and cosmology. She is a Principal Investigator of the LIGO Scientific Collaboration, and her waveform modeling of cosmological events has been crucial in the experiment’s many successes.

Buonanno, Charlie Misner, Peter Shawhan and others detailed UMD's contributions to gravitational studies in a 2016 forum, A Celebration of Gravitational Waves. 

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