Chuanhong (Vincent) Liu named to Fall 2020 cohort of the Quantum Information Science and Engineering Network (QISE-NET)
Graduate student Chuanhong (Vincent) Liu (McDermott Group) has had his project awarded funding through QISE-NET, the Quantum Information Science and Engineering Network. Run through the University of Chicago, QISE-NET is open to any student pursuing an advanced degree in any field of quantum science. Liu and other students in his cohort earn up to three years of support, including funding, mentoring and training at annual workshops. All awardees are paired with a mentoring QISE company or national lab, at which they will complete part of their projects. Liu describes his project, below. Cecilia Vollbrecht, a grad student in Chemistry, also earned this honor. Both Liu and Volbrecht are students in the Wisconsin Quantum Institute.
The Single Flux Quantum (SFQ) digital logic family has been proposed as a scalable approach for the control of next-generation multiqubit arrays. With NIST’s strong track record in the field of SFQ digital logic and the expertise of McDermott’s lab in the superconducting qubit area, we expect to achieve high fidelity SFQ-based qubit control. The successful completion of this research program will represent a major step forward in the development of a scalable quantum-classical interface, a critical component of a fully error-corrected fault-tolerant quantum computer.
New study provides understanding of astrophysical plasma dynamics
Stars, solar systems, and even entire galaxies form when astrophysical plasma — the flowing, molten mix of ions and electrons that makes up 99% of the universe — orbits around a dense object and attaches, or accretes, on to it. Physicists have developed models to explain the dynamics of this process, but in the absence of sending probes to developing stars, the experimental confirmation has been hard to come by.
In a study published in Physical Review Letters September 25, University of Wisconsin–Madison physicists recreated an astrophysical plasma in the lab, allowing them to investigate the plasma dynamics that explain the accretion disk formation. They found that electrons, not the momentum-carrying ions, dominate the magnetic field dynamics in less dense plasmas, a broad category that includes nearly all laboratory astrophysical plasma experiments.
Like water swirling around and down an open drain, plasma in an accretion disk spins faster nearer the heavy object in the center than further away. As the plasma falls inward, it loses angular momentum. A basic physics principle says that angular momentum needs to be conserved, so the faster rotating plasma must be transferring its momentum away from the center.
“This is an outstanding problem in astrophysics — how does that angular momentum get transported in an accretion disk?” says Ken Flanagan, a postdoctoral researcher with the department of physics at UW–Madison and lead author of the study.
The simplest explanation is friction, but it was ruled out when the corresponding accretion times, in some cases, would be longer than the age of the universe. A model developed by theoretical physicists posits that turbulence, or the chaotic changes in plasma flow speeds, can explain the phenomenon on a more realistic time scale.
“So ad hoc, astrophysicists say, ‘Okay, there’s this much turbulence and that explains it,’” Flanagan says. “Which is good, but you need to call in the plasma physicists to piece together where that turbulence comes from.”
Flanagan and colleagues, including UW–Madison physics professor Cary Forest, wanted to build off an idea that the turbulence was coming from an intrinsic property of some plasmas known as magnetorotational instability. This instability is seen in plasmas that are flowing fastest near the center and are in the presence of a weak magnetic field.
“And it’s lucky because there are weak magnetic fields all around the universe, and the flow profile in the accretion disks is set by the gravitational force,” Flanagan says. “So, we thought this plasma instability could be responsible for turbulence, and it explains how accretion disks work.”
To investigate if this intrinsic plasma instability explained the observation, the researchers turned to the Big Red Ball (BRB), a three-meter-wide hollow sphere with a 3000 magnets at its inner surface and various probes inside. They activate a plasma by ionizing gas inside the BRB, then applying a current to drive its movement.
Because they had previously been encountering problems in driving very fast flows, they tried a new technique to drive the flow across the entire volume of plasma, as opposed to just the edges. Fortuitously, the BRB had magnetic field probes from a previous experiment still attached, and when they activated the plasma under these conditions, they found that this new flow setup amplified the magnetic field strength with a peak at the center nearly twenty times the baseline strength.
“We didn’t expect to see that at all, because usually in plasma physics the simplest model is to think of plasmas as one fluid with the heavier ions dominating momentum,” Flanagan says. “The results suggested that the plasma is in the Hall regime, which means the electrons and their motion are entirely responsible for the plasma moving around magnetic fields.”
If they were correct in assuming it was the Hall effect that was driving magnetic field amplification, the equations governing magnetic fields and electric currents say that if you drive the current in the opposite direction, the strength of the magnetic field would be canceled out. So, they switched the current and measured the magnetic field strength: it was zero, supporting the Hall regime explanation.
While the results are not directly applicable to the plasma accretion disks around, say, a very dense black hole, they do directly impact the earth-bound experiments that attempt to recreate and study them.
“Nearly all plasma astrophysical experiments operate in the Hall regime, and so this sort of large qualitative effect is something you’re going to have to pay attention to when you make these sorts of flows in laboratory astrophysical plasmas,” Flanagan says. “In that sense, this work has a pretty broad impact for lots of different research areas.”
This research was supported in part by the National Science Foundation (#1518115) and by the U.S. Department of Energy (#DE-SC0018266).
Physics grad students share hands-on physics, art lessons with local fifth graders
UW–Madison physics grad student Aedan Gardill has been illustrating physics concepts with art for years, such as through his Instagram account, where he shares ink drawings. Earlier this year, he applied for a grant from the Madison Arts Commission to create hidden portraits of women in the physical sciences that could only be revealed by using polarized lenses. He also planned to visit local schools to explain the concept behind his art and help students make their own images based on his technique.
By the time Gardill learned he had been awarded the grant, the pandemic was in full force, and his plans had to change. While he could still present his portraits at the Wisconsin Science Festival, school visits were no longer in the cards.
“With the realization this summer that school was going to most likely be online in the fall, I had to rethink how I was going to use the funding from the grant,” Gardill explains. “And that has morphed into providing at-home, hands-on learning experiences that we’ll lead virtually.”
Funding for Gardill’s work is provided by a grant from the Madison Arts Commission, with additional funds from the Wisconsin Arts Board, the Optical Society of America, the International Society for Optics and Photonics, and the UW–Madison Department of Physics, with special thanks to Arts + Literature Laboratory. UW–Madison physics graduate student volunteers include Abby Bishop, Praful Gagrani, Jimena Gonzalez, Ben Harpt, Preston Huft, Brent Mode, Bryan Rubio Perez, Susan Sorensen, and Jessie Thwaites.
Massive halo finally explains stream of gas swirling around the Milky Way
The Large and Small Magellanic Clouds are satellite galaxies of the Milky Way. They are surrounded by a high-velocity gaseous structure called the Magellanic Stream, which consists of gas stripped from both clouds. So far, simulations have been unable to reconcile observations with a complete picture of how the stream was formed. In this Nature week’s issue, numerical simulations carried out at by Scott Lucchini, graduate student at the Physics Department working with Elena D’Onghia, present a model that potentially resolves this conundrum. By embedding the Large Magellanic Cloud in a corona of ionized gas, the researchers were able to simulate the Magellanic Stream accurately and explain its structure. Ellen Zweibel and Chad Bustard are also co-authors of the article.
First-year physics grad student uses her disrupted summer – and her science training – to research N95 safety
Shortly after incoming physics graduate student Winnie Wang attended a UW–Madison campus visit weekend in February, her plans took an abrupt change due to COVID-19. The University of Massachusetts, where she was studying, closed right before spring break, and she decided to go to Taipei to be with extended family. But first, she needed to follow the regulations in Taiwan and self-isolate for 14 days.
“I chose to be quarantined in a hotel, so I was by myself for two weeks. It was honestly kind of brutal, and for the first five days I was feeling pretty miserable,” Wang recalls. “I’m putting it bluntly, because that misery was what inspired me to do something about it. I was like, ‘Okay, well, why don’t I proactively use some of my free time.’”
Wang, who is from Canada and attended school in the U.S., watched what was happening to the case numbers in those two countries, especially compared to the relatively lower numbers in Taiwan, and started looking for ways to get involved. She posted on Facebook asking if anyone knew groups she could volunteer with, and eventually landed on a group called N95DECON.
According to the group’s website, N95DECON is a volunteer collective of scientists, engineers, clinicians, and students from universities across the U.S. as well as other professionals in the private sector. N95DECON seeks to review, collate, publish, and disseminate scientific information about N95 decontamination to help inform decisions about N95 decontamination and reuse.
“Hospitals use a lot of N95s, and you’ve probably heard of things where people have put masks in microwaves or rice cookers to decontaminate them. And basically, you don’t want to do that,” Wang says. “We looked at the research that’s already out there, looked at what the CDC recommends, and we culminated our findings into papers and seminars for hospitals to use around the world.”
Wang serves as a communications volunteer for the group, meaning she responds to emails and proofreads and edits the group’s publications. She says that when she first started, N95DECON did not have much in the way of formal documentation, so much of her early efforts were spent answering emails from the public asking about reuse procedures. But knowing that N95s were in short supply and time was of the essence, N95DECON worked quickly to put together online seminars that could be viewed by anyone.
“After we organized and recorded the seminars in May and put them on our website so that anyone can watch them, the email team received less email from the general public,” Wang says. “And I’ve moved on now to more literature review.”
N95DECON shared their work largely through the hospital networks of the health professionals that volunteer with the group, as well as through social media and other word-of-mouth. The group will continue to monitor research on best practices for decontaminating and reusing N95 masks and update their recommendations accordingly. Much of their current efforts are focused on translating their papers and seminars.
“We’d have people from all over the world join our seminars and talk about their experiences,” Wang says. “So, another aspect of our outreach is that we do translations. Our goal is to disperse this information around the world, and we’ve translated it into seven languages now.”
Wang plans to continue volunteering with N95DECON after the UW–Madison academic year begins. She is interested in studying experimental high energy physics for her doctorate.
Dark Energy Survey census of the smallest galaxies hones the search for dark matter
In particular, the new results constrain the minimum mass of the dark matter particles, as well as the strength of interactions between dark matter and normal matter.
According to these new results, a dark matter particle must be heavier than a zeptoelectronvolt, which is 10-21 electronvolts. That’s one trillionth of a trillionth of the mass of an electron. This study also shows that dark matter’s interactions with normal matter must be roughly 1,000 times weaker than the weak nuclear force. Of the known forces, only gravity is weaker.
These novel measurements used data from the Dark Energy Survey, a cosmological survey designed to study dark energy, the mysterious force driving the accelerated expansion of the universe. In contrast, dark matter is gravitationally attractive, resisting the expansion of the universe and gravitationally binding astronomical systems such as galaxies. The smallest “dwarf” galaxies can have hundreds to thousands of times more dark matter than normal matter. Over the past five years, the Dark Energy Survey has combined with other surveys to more than double the known population of these tiny galaxies. The current total is now over 50.
“The large number of dwarf galaxies that we found orbiting the Milky Way is consistent with expectations from the simplest picture of dark matter — that is, comprising slow-moving particles that interact only through gravity,” Bechtol explained. “In this new paper, we rule out several alternative possibilities for the nature of dark matter.”
Dark matter makes up 85% of the matter in the universe, but we have yet to detect it directly in the laboratory. The gravitational effects of dark matter are clearly visible in the motions of stars in galaxies, the clumpy distribution of galaxies in the universe, and even in the amount of lightweight elements. The robust astronomical evidence for the existence of dark matter has motivated many experimental searches here on Earth, using instruments ranging from cryogenic detectors buried deep underground to energetic particle colliders.
“The faintest galaxies are among the most valuable tools we have to learn about dark matter because they are sensitive to several of its fundamental properties all at once,” said Ethan Nadler, the study’s lead author and graduate student at Stanford University and SLAC.
In these multi-year, multi-telescope sky surveys, the raw data comes in the form of tens of thousands high-resolution digital images. But identifying these ultrafaint galaxies, as their description implies, is not as simple as looking at an image and seeing a faint smudge of light. Bechtol and his group, including physics grad student Mitch McNanna, designed the search algorithms needed to identify, with some statistical assurance, which individual stars are part of a dwarf galaxy.
“We worked closely with experts in galaxy formation and particle physics theory to compare the Dark Energy Survey observations with predictions,” Bechtol said. “Part of our job was to determine the sensitivity of our search — how far away from the Earth could we spot a galaxy with only a few hundred stars?”
By combining the observed census of dwarf galaxies with advanced cosmological simulations of the distribution of dark matter around the Milky Way, scientists were able to predict how the physical properties of dark matter would affect the number of small galaxies. Small galaxies form in regions where the dark matter density in the early universe is very slightly above average. Physical processes that smooth out these regions of higher density (if dark matter moves too quickly or gains energy due to interactions with normal matter) or prevent density variations from collapsing to form galaxies (thanks to quantum interference effects) would reduce the number of galaxies observed by the Dark Energy Survey.
“Astrophysical observations provide unique information about the fundamental nature of dark matter, and are complementary to searches for dark matter particles in terrestrial experiments.” Bechtol said. “With the Dark Energy Survey, we continue to learn about the deep connection between particle physics and the growth of cosmic structure, ranging from the vast network of galaxies in the cosmic web, down to smallest individual galaxies.”
The Dark Energy Survey is a collaboration of more than 300 scientists from 25 institutions in six countries. For more information about the survey, please visit the experiment’s website.
Funding for the DES Projects has been provided by the U.S. Department of Energy, the U.S. National Science Foundation, the Ministry of Science and Education of Spain, the Science and Technology Facilities Council of the United Kingdom, the Higher Education Funding Council for England, the National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign, the Kavli Institute of Cosmological Physics at the University of ChicagoFunding Authority for Funding and Projects in Brazil, Carlos Chagas Filho Foundation for Research Support of the State of Rio de Janeiro, Brazilian National Council for Scientific and Technological Development and the Ministry of Science and Technology, the German Research Foundation and the collaborating institutions in the Dark Energy Survey.
Vandenbroucke group plays instrumental role in proving viability of innovative gamma-ray telescope
Scientists in the Cherenkov Telescope Array (CTA) consortium have detected gamma rays from the Crab Nebula using the prototype Schwarzschild-Couder Telescope (pSCT), proving the viability of the novel telescope design for use in gamma-ray astrophysics. The announcement was made today by Justin Vandenbroucke, associate professor at the University of Wisconsin–Madison, on behalf of the CTA Consortium at the virtual 236th meeting of the American Astronomical Society (AAS).
“The Crab Nebula is the brightest steady source of TeV, or very high-energy, gamma rays in the sky, so detecting it is an excellent way of proving the pSCT technology,” says Vandenbroucke, who is also affiliated with the Wisconsin IceCube Particle Astrophysics Center (WIPAC) at UW–Madison. “Very high-energy gamma rays are the highest energy photons in the universe and can unveil the physics of extreme objects, including black holes and possibly dark matter.”
Vandenbroucke is coleader of a team made up of WIPAC scientists and other collaborators that developed and operate a critical part of the telescope: its high-speed camera. Vandenbroucke has worked on the design, construction, and integration of the camera since 2009.
Keith Bechtol, Rob Morgan win UW’s Cool Science Image contest
Congrats to Prof. Keith Bechtol and graduate student Rob Morgan for their winning entry in the UW–Madison Cool Science Images contest! Their winning entry — one of 12 selected out of 101 entries — earns them a large-format print which initially will be displayed in a gallery at the McPherson Eye Research Institute’s gallery in the WIMR building.
This snapshot of the sky contains thousands of distant galaxies, each containing billions of stars. Bechtol and Morgan were looking for the flash of the explosion of a single star, the potential source of a sub-atomic particle called a neutrino, spotted zipping through the Earth by the IceCube Neutrino Observatory at the South Pole. The distant galaxies, swirling billions of light years away, are all the harder to see because of nearby objects, like the pictured Helix Nebula. The image was captured with a Dark Energy Camera and Victor M. Blanco telescope.
Manipulating the magnetic response to light in natural materials
When light moves from one material into another, it bends — like how a partially submerged object appears distorted under water when viewed from above. What if, instead of bending, a material could change the light so much that the material was no longer visible at all?
In a study published in Physical Review A, University of Wisconsin–Madison researchers have shown for the first time that a similar response can be obtained and manipulated in naturally-occurring materials. The findings have implications from the development of “perfect” lenses for improved microscopy to Harry Potter-esque invisibility cloaks.
Visible light is made of both magnetic and electric fields, and the refractive index of a material — how much it bends the light — is determined by how the material interacts with those two fields. Nearly all materials we encounter in everyday life, though, interact entirely with light’s electric field.
Researchers have spent the past two decades developing artificial materials that more strongly interact with light’s magnetic field by manipulating the refractive index. With a strong enough response, the material could eventually have a negative refractive index, leading to unique optical properties. However, the response in synthetic materials is limited by the size of their repeating units. A naturally-occurring crystal that has much smaller unit cells is likely a better choice.
“Part of producing a negative refractive index is that the material needs to have a strong response to both electric and magnetic fields, so the big challenge is getting that magnetic response in natural materials,” explains Zach Buckholtz, a graduate student in UW–Madison physics professor Deniz Yavuz’s group and lead author of the study. “A few years ago, we published a paper showing that the crystal we’re working with has a magnetic response, and in this study, we were able to manipulate the response.”
The natural material Buckholtz is working with is a silicon-based crystal, which in general is optically ordinary, except that it has been “doped” with the rare earth metal Europium. Rare earth metals are unique in that they contain an abundance of electrons in the atoms’ outer energy shells. Those electrons can then work together to create a bigger magnetic response, but only if they are all in tune with each other.
“If you have some magnetic response and a much larger electric response to light, you can connect those two responses,” Buckholtz says. “To get to a negative refractive index from there, you have to set up coherences between the energy levels, meaning you have to make sure all those energy levels are oscillating together.”
To show they can manipulate the magnetic response, Buckholtz and Yavuz did two things. First, because the crystal is a mix of ions with slightly different electron responses, they needed to set up their experimental system to select for one class of ion. This uniformity allows for a cleaner interpretation of the results.
“We send a laser into the crystal, and then measure how much of the light is transmitted. But because the crystal isn’t perfect, instead of seeing a narrow peak for the transitions, you’ll see a really broad transition,” Buckholtz explains. “So, we do this procedure known as spectral hole burning to clear out the ions we don’t want and then we’ll be left with just one transition, which is necessary to move on to experiments that involve coherence.”
Next, they wanted to show if they could increase the magnetic response. To do so, they needed to take those selected ions, put them in coherence, and then measure the response compared to ions not in coherence. In these experiments, they shined one (a probe beam) or two (probe and coupling beam) wavelengths of laser at the ions. Both lasers excite electrons in the ions to a higher-energy state, and the scientists can again measure how might light is transmitted through the crystal as a readout of the electron transitions.
“With just the probe beam, we see just the normal transition, and that’s what we did in our previous study. But with the coupling beam added in, it connects and adds another transition state in there,” Buckholtz says. “If those states are in coherence, they cancel each other out, and we see that effect as a peak in transmitted light, which means the index of refraction is going toward zero.”
Buckholtz notes that the magnetic response they see is not yet large enough to produce the materials with interesting new optical properties they are hoping for. Still, he says, this work provides a path forward to continue manipulations to improve the response, such as investigating different rare earth metals.
“We have a magnetic response, we can set up coherence, and we can manipulate the response,” Buckholtz says. “Now, we want to increase the scale of the response to with a goal of eventually making the refractive index below zero.”