News Archives

Smooth sailing for electrons in graphene

Posted on February 16, 2023

two panels in heat-map style. both panels have circles in the middle. The panel on the left has more yellow and red to the left of the circle and a bright yellow ring around the circle; the right panel has a less sharp transition of colors from left to right and no bright ring around the circles. — A heatmap of electron location in graphene shows that at the lower temperature (left panel), the electrons are more likely to bump into impurities (circles), with relatively fewer making it through the channel between impurities. At higher temperatures (right panel), electron flow shifts to being fluid-like. Fewer are stuck at the impurities and more flow through the channels. UNIVERSITY OF WISCONSIN–MADISON

This story was originally published by University Communications

Physicists at the University of Wisconsin–Madison directly measured, for the first time at nanometer resolution, the fluid-like flow of electrons in graphene. The results, which will appear in the journal Science on Feb. 17, have applications in developing new, low-resistance materials, where electrical transport would be more efficient.

Graphene, an atom-thick sheet of carbon arranged in a honeycomb pattern, is an especially pure electrical conductor, making it an ideal material to study electron flow with very low resistance. Here, researchers intentionally added impurities at known distances and found that electron flow changes from gas-like to fluid-like as temperatures rise.

Zach Krebs

“All conductive materials contain impurities and imperfections that block electron flow, which causes resistance. Historically, people have taken a low-resolution approach to identifying where resistance comes from,” says Zach Krebs, a physics graduate student at UW–Madison and co-lead author of the study. “In this study, we image how charge flows around an impurity and actually see how that impurity blocks current and causes resistance, which is something that hasn’t been done before to distinguish gas-like and fluid-like electron flow.

The researchers intentionally introduced obstacles in the graphene, spaced at controlled distances and then applied a current across the sheet. Using a technique called scanning tunneling potentiomentry (STP), they measured the voltage with nanometer resolution at all points on the graphene, producing a 2D map of the electron flow pattern.

No matter the obstacle spacing, the drop in voltage through the channel was much lower at higher temp (77 kelvins) vs lower temp (4 K), indicating lower resistance with more electrons passing through.

At temperatures near absolute zero, electrons in graphene behave like a gas: they diffuse in all directions and are more likely to hit obstacles than they are to interact with each other. Resistance is higher, and electron flow is relatively inefficient. At higher temperatures — 77 K, or minus 196 C — the fluid-like behavior of electron flow means they are interacting with each other more than they are hitting obstacles, flowing like water between two rocks in the middle of a stream. It is as if the electrons are communicating information about the obstacle to each other and diverting around the rocks.

“We did a quantitative analysis [of the voltage map] and found that at the higher temperature, the resistance is much lower in the channel. The electrons were flowing more freely and fluid-like,” Krebs says. “Graphene is so clean that we’re forcing the electrons to interact with each other before they interact with anything else, and that is crucial in getting them to behave like a fluid.”

Former UW–Madison graduate student Wyatt Behn is a co-first author on this study conducted in physics professor Victor Brar’s group. Funding was provided by the U.S. Department of Energy (DE-SC00020313), the Office of Naval Research (N00014-20-1-2356) and the National Science Foundation (DMR-1653661).

Celebrating International Day of Women and Girls in Science!

Posted on February 11, 2023

February 11 is the International Day of Women and Girls in Science, and we’re more than happy to showcase some of our women physicists! We collected photos from women in the department, which you can see in a collage above. Some women also chose to share a bit about their research and/or what being a woman in science and woman in physics means to them. Those quotes are below.

Abby Warden, graduate student

My name is Abby Warden, a 5th year graduate student working in experimental high energy physics. My current work includes assembling Gas Electron Multiplier (GEM) chambers for electronics testing. GEMs are the newest muon sub detector that will be installed in the general particle detector, the Compact Muon Solenoid (CMS).

Dr. Camilla Galloni, Post-doc

Seen some muons? Working on the muon detector of the CMS experiment: GE11 installed in 2020 and successfully operated is the precursor of the GE21 and ME0 detectors now being constructed for the high luminosity LHC upgrade. This big “camera” takes “snapshots” of particles produced in high energy proton collisions and helps understand the fundamental interactions of nature.

Elise Chavez, graduate student

I’ve always been drawn to figuring out how the world works and it led me to my research and passion of learning how the universe works fundamentally at the subatomic level. I work with the Compact Muon Solenoid (CMS) that lies along the Large Hadron Colider (LHC) at CERN in Geneva. Being a woman in physics is a strange duality. There are times when I feel empowered and times I feel very small. It is strength, confidence, and understanding, but it is also alienating, discouraging, and conflicting. It has taught me a lot about people and myself. It gave me a passion to help and support women and minorities in physics because it is for everyone. Diversity is what helps discovery thrive and I hope one day that it can be solely an uplifting experience.

Dr. Charis Koraka, post-doc

Curiosity, along with kindness and compassion are some of the greatest human qualities and those that make societies prosper. The quest of understanding the laws and properties of the universe, has always been a driving source and what made me turn to physics. With perseverance, nothing is impossible!

Wren Vetens, graduate student

My experience as a woman in physics has been marked by perseverance, community, and solidarity. There is still much to be done to achieve equality within the field of physics but we can do our part by standing up for and supporting each other, especially supporting our juniors and those who are disabled, LGBTQ+, and/or POC. I chose physics because I am compelled to always look deeper when I have questions about the nature of life, the universe, and everything. The very same drive that led me to study Physics also led me to coming to terms with my own identity as a queer person, nonbinary person, and transgender woman. Suffice to say, I would not be who I am today without physics or without my gender, and really the two are simply manifestations of that drive. I am currently wrapping up my PhD in experimental particle physics as a part of the CMS collaboration and hoping to graduate this year. My research topic is a search for the unique signature of a long-lived composite particle made of six quarks, which could in principle be produced at the LHC and detected with the CMS detector.

Prof. Tulika Bose

I am an experimental particle physicist working on the CMS experiment at the Large Hadron Collider. I love being part of a large international physics collaboration looking to answer some of the most fundamental questions in physics today – what is responsible for dark matter ? What is the matter-antimatter asymmetry in our universe due to ? Are there new exotic particles out there ? We try to answer these questions using our detector, cutting-edge instrumentation, modern software (incorporating Artificial Intelligence/Machine Learning) and high-performance computing!

(for a video describing Prof. Bose’s work, please see: https://www.youtube.com/watch?v=E7Kzx2xZFdc)

Prof. Ellen Zweibel

I study plasma astrophysics: how electric and magnetic fields interact with charged particles in astrophysical systems. This is an incredibly broad field and I enjoy all of it – how sunspots and solar flares work, how a single proton can acquire the energy of a hard hit tennis ball, and what the blotchy rings imaged around supermassive black holes are really telling us, to give just a few examples.

Having the time and capacity to study these things has been an incredible privilege. I’m grateful to my parents, who thought my mind was worth developing, and to my many wonderful teachers, colleagues, and students – I hope I do as well by them as they did and do by me. I’m grateful to the social infrastructure that gave me food, water, and shelter, cured my illnesses, and allowed me reproductive freedom of choice so I could become a person who lives her dreams.

Haddie McLean, outreach specialist

I love that my job allows me to bring physics to children, our next generation of scientists. I want to show them that physics is fun and it’s for everyone. I hope to inspire them to pursue a career in science.

Prof. Jim Lawler has passed away

Posted on February 7, 2023

Professor Jim Lawler, the Arthur and Aurelia Schawlow Professor Emerit of Physics at UW–Madison, passed away January 29, 2023. He was 71.

Lawler was an atomic, molecular & optical physicist with a focus developing and applying laser spectroscopic techniques for determining accurate absolute atomic transition probabilities. He received his MS (’74) and PhD (’78) from this department, studying with now-professor emerit Wilmer Anderson. In the two years after earning his doctorate, he was a research associate at Stanford University, and returned to UW–Madison as an assistant professor in 1980. He remained on the faculty until his retirement in May 2022.

Lawler served as department chair from 1994-1997. He also accumulated numerous awards and honors over his distinguished career. He was a fellow of the American Physical Society, the Optical Society of America, the U.K. Institute of Physics, and in 2020 he was elected a Legacy Fellow of the inaugural class of American Astronomical Society Fellows. He won the 1992 W. P. Allis Prize of the American Physical Society and the 1995 Penning Award from the International Union of Pure and Applied Physics for research in plasma physics, the two highest National and International Awards in the field of Low Temperature Plasma Physics. In 2017, he won Laboratory Astrophysics Prize of the American Astronomical Society for research in spectroscopy.

At the time of Lawler’s retirement, longtime collaborator Blair Savage, UW–Madison professor emeritus of astronomy, said of Lawler’s contributions to the field:

“Jim’s work in laboratory astrophysics provided extremely important atomic ultraviolet transition probabilities in support of the Hubble Space Telescope programs to determine elemental abundances of gaseous matter in the interstellar medium from three different ultraviolet spectrographs over the 32-year history of the space observatory. They included the Goddard High Resolution Spectrograph, the Space Telescope Imaging Spectrograph and the Cosmic Origins Spectrograph.”

During his tenure, Lawler supervised 26 PhD students and 10 terminal MS students. Those students and postdocs have gone on to prestigious National Research Council Fellowships, group lead positions at major companies, and tenured professorships, amongst many others.

Please visit the department’s tribute page to Jim Lawler to submit and/or read stories from Jim’s colleagues.

Peter Weix remembered for his technical, mentoring, and outreach efforts in physics

Posted on January 27, 2023

The Department of Physics mourns the loss of Peter Weix, who passed away January 13, 2023.

Peter began his career as an electronics technician in the U.S. Navy in 1984, where he served until 1990. Following his Navy service, he worked as an electronics technician for several companies in California before joining the SLAC National Accelerator Laboratory at Stanford. At SLAC he was a Senior Technician with involvement on both the Stanford Synchrotron Radiation Lightsource and what is now the Linac Coherent Light Source. He also served as a Safety Officer with special emphasis on earthquake safety. Peter and his wife, Sheri, relocated to Wisconsin in 2001 so that Peter could join the Plasma Physics Group at UW–Madison where he worked for more than 20 years and advanced to Senior Instrumentation Specialist.

a man stands behind a lectern with physics gadgets behind him. he is wearing a costume that centers around the theme of time. — Peter Weix at the 2020 The Wonders of Physics annual shows | DEPARTMENT OF PHYSICS

Peter’s work responsibilities at UW spanned a diverse range of technical operations for both the Madison Symmetric Torus (MST) and the Big Red Plasma Ball (BRB), two intermediate-scale experimental facilities for plasma physics research. He oversaw the mechanical and electrical aspects of the MST facility and its high-voltage pulsed-power systems, making sure the facility functioned as required, both technically and safely. He also oversaw all aspects of the high vacuum systems for both MST and BRB. There are many researchers, both in the local group and visiting collaborators, who relied on Peter’s efforts to make sure research projects stayed on track. Additionally, Peter directed key parts of large construction projects, such as the new programmable power supplies that replace MST’s passive capacitor-inductor circuits.

Peter’s involvement with plasma physics research included supervision of around 4-6 undergraduate students at any given time; he mentored an estimated more than 50 students during his time here. The students came from many areas of study, not just science and engineering, and rarely joined the group with the specific skills required to support research activities. Peter welcomed them into the department and provided them all with on-the-job training, teaching them skills and tricks of the trade to allow them to grow and become valuable members of the team.

In addition to his dedicated service to the plasma group, Peter recognized the importance and value of physics outreach. He became a vital member of The Wonders of Physics program for over twenty years. His involvement started when one of the participants was suddenly unavailable at the start of one of the public shows. Peter saved the day by learning on the fly how to operate the complicated audiovisual system. His performance under pressure was impressive, and he was then asked to be the coordinator and main announcer for the approximately 200 shows that followed. Through the years, he provided ideas, elaborate props, personnel, wisdom, and a calming influence on the entire cast. He spent countless hours volunteering his time to the program.

In recognition of his many contributions to the department and university, Peter was awarded the 2022-23 George Ott award for staff excellence, the only department-level staff award given. He will be recognized at the annual Awards and Scholarship banquet in May.

Please visit the department’s tribute page to Peter Weix to submit and/or read stories from Peter’s colleagues.

Profs. John Sarff and Clint Sprott contributed to this piece

Finding some wiggle room in semiconductor quantum computers

Posted on January 25, 2023

a geometric pattern of lines in green, light gold, and black/dark purple, representing the qubit

Classical computers rarely make mistakes, thanks largely to the digital behavior of semiconductor transistors. They are either on or they’re off, corresponding to the ones and zeros of classical bits.

On the other hand, quantum bits, or qubits, can equal zero, one or an arbitrary mixture of the two, allowing quantum computers to solve certain calculations that exceed the capacity of any classical computer. One complication with qubits, however, is that they can occupy energy levels outside the computational one and zero. If those additional levels are too close to one or zero, errors are more likely to occur.

“In a classical computer, all the aspects of a transistor are super uniform,” says UW–Madison Distinguished Scientist Mark Friesen, an author on both papers. “Silicon qubits are in many ways like transistors, and we’ve gotten to the stage where we can control the qubit properties very well, except for one.”

That one property, known as the valley splitting, is the buffer between the computational one-zero energy levels and the additional energy levels, helping to reduce quantum computing errors.

In two papers published in Nature Communications in December, researchers from the University of Wisconsin–Madison, the University of New South Wales and TU-Delft showed that tweaking a qubit’s physical structure, known as a silicon quantum dot, creates sufficient valley splitting to reduce computing errors. The findings turn conventional wisdom on its head by showing that a less perfect silicon quantum dot can be beneficial.

Read the full story

Beating the diffraction limit in diamonds

Posted on January 17, 2023

by Daniel Heimsoth

Resolving very small objects that are close together is a frequent goal of scientists, making the microscope a crucial tool for research in many different fields from biology to materials science.

The resolution of even the best modern confocal microscopes — a common optical microscope popular in biology, medicine, and crystallography — is limited by an optical bound on how narrow a laser beam can be focused, known as the diffraction limit.

In a study recently published in the journal ACS Photonics, UW–Madison physics professor Shimon Kolkowitz and his group developed a method to image atomic-level defects in diamonds with super-resolution, reaching a spatial resolution fourteen times better than the diffraction limit achievable with their optics. And, because the technique uses a standard confocal microscope, this super-resolution should be available to any researchers that already have access to this common equipment.

Aedan Gardill

While methods to achieve super-resolution already exist, such as stimulated emission depletion microscopy (STED), nearly all of these methods either require the addition of special optics, which can be expensive and difficult to install, or specialized samples and extensive post processing of the data. The UW–Madison technique, which they call “super-resolution Airy disk microscopy” (SAM), avoids such barriers to entry.

“You can get this all for free with the existing setup that a lot of labs already have, and it performs almost just as well,” says Aedan Gardill, a graduate student in Kolkowitz’s group and lead author of the paper. “We were able to get resolution down to twenty nanometers, which is comparable with standard techniques using [STED].”

The ‘Airy disk’ in SAM refers to a key feature of light beams that gives rise to the diffraction limit but which the researchers turned to their advantage.

Confocal microscopes use laser beams of specific wavelengths to excite matter in a sample, causing that matter to emit light. On the microscopic scale, the laser beam does not create a solid circle of light on the sample in the same way a flashlight would. Rather, light hits the object in a series of light and dark rings called an Airy pattern. Within the dark rings, the matter receives no light, which means it cannot be detected by the microscope’s light sensors.

The novelty of the SAM technique is in its two laser beam pulses, one spatially offset from the other such that the overlapping Airy patterns can distinguish between two closely spaced objects.

In their paper, the research team studied nitrogen-vacancy (NV) centers in diamond crystal, which are regions in the crystal lattice where one of two neighboring carbon atoms is replaced by a nitrogen atom, and the other is left empty. NV centers are known to have two different charge states based on how many electrons are in the defect, one that fluoresces and one that remains dark when yellow light is applied to them.

To resolve two NV centers separated by a distance less than the diffraction limit of the microscope, the SAM procedure first shines green light on them, preparing both centers into their fluorescent charge state. Then, a red laser is applied, offset such that only one of the two NV centers is in the dark ring of the Airy pattern and thus is not affected by the beam. The NV center that does see the red light is switched to the dark state.

a cartoon-rendered image of a microscope objective, with a red cylinder (light) hitting a sample that shows concentric rings of red and blue, as described in the text — Super-resolution Airy disk Microscopy uses the Airy disk (red pattern) generated by diffraction from an objective lens aperture (gray cylinder) to localize and control an emitter (here a nitrogen vacancy center in diamond) below the diffraction limit. Emitter fluorescence is suppressed everywhere except in a very narrow ring (blue donut).

“It goes to another dark charge state where it does not interact with yellow light,” Gardill explains. “But the initial bright charge state does interact with yellow light and will emit light.”

Finally, when the yellow laser is applied, one NV center emits light while the other does not, effectively differentiating between the two neighboring sites. By repeating these steps iteratively over a grid, the researchers could reconstruct a full image of the two nearby NVs with spectacular resolution.

The idea for this technique came as a bit of a surprise while the team was studying charge properties of NV centers in 2020.

“We tried the combinations of red-green, green-red, red-red, green-green with those first two [laser] pulses, and the one that was green then red, we ended up seeing this ring,” Gardill recounts. “And Shimon was like, ‘The width of the ring is smaller than the size of [the confocal image of] the NV. That is super-resolution.’”

This method could find wide use in many different fields, including biology and chemistry where NV centers are used as nanoscale sensors of magnetic and electric fields and of temperature in compounds and organic material. NV centers have also been studied as candidates for quantum repeaters in quantum networks, and the research team has considered the feasibility of using the SAM technique to aid in this application. Currently, the SAM method has only been applied to NV centers in diamond crystal, and more research is needed to extend its use to different systems.

That all of this can be done with hardware that many labs across the world already have access to cannot be overstated. Gardill reiterates, “If they have a basic confocal microscope and don’t want to buy another super-resolution microscope, they can utilize this technique.”

This work was supported by the U.S. Department of Energy, Office of Science, Basic Energy Sciences under Award #DE-SC0020313.

Daniel Heimsoth is a second-year PhD student in Physics. This was his first news story for the department.

Welcome, assistant professor Ilya Esterlis

Posted on December 27, 2022

Ilya Esterlis

When Lake Mendota freezes over in the winter and thaws in the spring, those water/ice phase transitions might seem mundane. But, says new assistant professor of physics Ilya Esterlis, interesting things happen during phase transitions, and commonalities exist between phase transitions of any matter.

“That’s very surprising and strange sounding, but it turns out that there’s a very general framework in which to understand [these commonalities],” Esterlis says. “It’s this notion of universality, and by studying phase transitions you’re simultaneously studying a very broad class of materials.”

Esterlis, a condensed matter theorist whose research focuses on materials and phase transitions, joins the department January 1, 2023. He is currently a postdoctoral fellow at Harvard, and joined us for a virtual interview earlier this fall.

Can you please give an overview of your research?

I am a condensed matter theorist, so I study materials, and in particular I try to classify different phases of matter and the phase transitions between those phases of matter. I’m mostly interested in electronic systems, where you have a large macroscopic number of interacting electrons and are trying to understand the kind of phenomena that can emerge when you have that large number of degrees of freedom interacting with one another. And a lot of these things are motivated by experiments — not all of them. There are some more academic questions that I’m interested in investigating and they’re a bit more formal. But I’m also motivated by interesting things that are happening in the lab. Part of my work is not only trying to characterize and understand phases of matter, but also trying to propose ways that different phases could be detected experimentally, how they would manifest themselves in different experimental signatures.

I’m also interested in superconductivity. My PhD work focused a lot on trying to understand the optimal conditions for making superconductors — if you could have every knob at your disposal, what would you do to optimize them? Optimize in this case means: make superconductors that exist at as high of a temperature as possible. Superconductivity is typically a low temperature phenomenon, so there’s a holy grail in condensed matter physics trying to make higher temperature superconductors. Part of my work has been organized around trying to understand what would be even in principle the optimal route towards achieving higher temperature superconductors.

Once you’re in Madison, what are one or two research projects you and your group will focus on?

I will focus a good amount of my research efforts on studying superconductivity, continuing this line of investigation into what the optimal conditions for superconductors are. If you had all the freedom in the world, how would you build the best superconductor that exists to high temperatures and under normal laboratory conditions? Not under extreme, unrealistic conditions but in an everyday parameter regime. And that involves understanding the superconducting state itself. Superconductors are a phase of matter that is distinct from, say, a metal, which is also a good conductor but not a superconductor. But oftentimes to understand superconductors better, one has to understand the state from which they came. That is to say, you take a metal and you cool it down to low temperatures and it goes from being a good conductor to a superconductor. To understand that superconductor, it’s often helpful to understand the metal from which it came at higher temperature. And sometimes those metals can be conventional, like copper wires, but sometimes they can be very unconventional metals and strange for various reasons. One open question is: what is the interplay between superconductivity and unusual metals? If you take a high temperature unusual metal, what is the kind of superconductor that it turns into at lower temperature? And unusual in this context means that it has some properties that are not typical to conventional metals. For instance, there’s predictions for how resistance changes with temperature in a conventional metal but unusual metals have rather different resistance behaviors.

What is your favorite element and/or elementary particle?

Helium is remarkable in that it has a number of unusual properties. For instance, if you cool it down to zero temperature it does not crystallize, it remains a liquid. That’s solely due to quantum mechanics, which is kind of an incredible thing. If you do make it crystallize by applying pressure, then that solid itself also has very interesting properties.

And my favorite elementary particle is the anyon. It’s not elementary, say, in the sense of electrons or quarks. But it’s this really remarkable thing that happens in condensed matter systems where if you take a macroscopic number of electrons and you subject them to a very large magnetic field, then a remarkable thing happens where the behavior of the system, as viewed kind of on macroscopic scales, does not look like the behavior of electrons, it really looks like the behavior of particles called anyons that have fractional electric charge. So they are elementary in condensed matter physics.

What hobbies and interests do you have?

I really love to play music, guitar specifically. And I have two small kids, two daughters, and I just like hanging out with them.

Welcome, Roman Kuzmin, the Dunson Cheng Assistant Professor of Physics

Posted on December 27, 2022

Roman Kuzmin

In the modern, cutting-edge field of quantum computing, it can be a bit puzzling to hear a researcher relate their work to low-tech slide rules. Yet that is exactly the analogy that Roman Kuzmin uses to describe one of his research goals, creating quantum simulators to model various materials. He also studies superconducting qubits and ways to increase coherence in this class of quantum computer.

Kuzmin, a quantum information and condensed matter scientist, will join the department as an the Dunson Cheng Assistant Professor of Physics on January 1. He is currently a research scientist at the University of Maryland’s Joint Quantum Institute in College Park, Md, and recently joined us for an interview.

Can you please give an overview of your research?

My main fields are quantum information and condensed matter physics. For example, one of my interests is to solve complicated condensed matter problems using new techniques and materials which quantum information science developed. Also, it works in the other direction. I am also trying to improve materials which are used in quantum information. I work in the subfield of superconducting circuits. There are several different directions in quantum information, and the physics department at Wisconsin has many of them already, so I will complement work in the department.

Once you’re in Madison and your lab is up and running, what are the first big one or two big things you want to really focus your energy on?

One is in quantum information and quantum computing. So, qubits are artificial atoms or building blocks of a quantum computer. I’m simplifying it, of course, but there are environments which try to destroy coherence. In order to scale up those qubits and make quantum computers larger and larger — because that’s what you need eventually to solve anything, to do something useful with it — you need to mitigate decoherence processes which basically prevent qubits from working long enough. So, I will look at the sources of those decoherence processes and try to make qubits live longer and be longer coherent.

A second project is more on the condensed matter part. I will build very large circuits out of Josephson junctions, inductors and capacitors, and such large circuits behave like some many-body objects. It creates a problem which is very hard to solve because it contains many parts, and these parts interact with each other such that the problem is much more complicated than just the sum of those parts.

What are some applications of your work?

Of course this work is interesting for developing theory and understanding our world. But the application, for example for the many-body system I just described, it’s called the quantum impurity. One of my goals is to use this to create a simulator which can potentially model some useful material. It’s like if you have a quantum computer, you can write a program and it will solve something for you. A slide rule is a physical device that allows you to do complicated, logarithmic calculations, but it’s designed to do only this one calculation. I’m creating kind of a quantum slide rule.

What is your favorite element and/or elementary particle?

So, I have my favorite circuit element: Josephson junction. (editor’s note: the question did not specify atomic element, so we appreciate this clever answer!). And for elementary particle, the photon, especially microwave photons, because that’s what I use in these circuits to do simulations. They’re very versatile and they’re just cool.

What hobbies and interests do you have?

I like reading, travelling, and juggling.

New technique reveals changing shapes of magnetic noise in space and time

Posted on December 23, 2022

This article was originally published by Princeton Engineering

Electromagnetic noise poses a major problem for communications, prompting wireless carriers to invest heavily in technologies to overcome it. But for a team of scientists exploring the atomic realm, measuring tiny fluctuations in noise could hold the key to discovery.

“Noise is usually thought of as a nuisance, but physicists can learn many things by studying noise,” said Nathalie de Leon, an associate professor of electrical and computer engineering at Princeton University. “By measuring the noise in a material, they can learn its composition, its temperature, how electrons flow and interact with one another, and how spins order to form magnets. It is generally difficult to measure anything about how the noise changes in space or time.”

Using specially designed diamonds, a team of researchers at Princeton and the University of Wisconsin–Madison have developed a technique to measure noise in a material by studying correlations, and they can use this information to learn the spatial structure and time-varying nature of the noise. This technique, which relies on tracking tiny fluctuations in magnetic fields, represents a stark improvement over previous methods that averaged many separate measurements.

a small square chip sits on a metallic microscope stand with green laser light bouncing off of it in places — Using specially designed diamonds with nitrogen-vacancy centers, researchers at Princeton University and the University of Wisconsin-Madison have developed a technique to measure noise in a material by studying correlations, and they can use this information to learn the spatial structure and time-varying nature of the noise. In this image, a diamond with near-surface nitrogen-vacancy centers is illuminated by green laser light from a microscope objective lens | Photo by David Kelly Crow and provided by Princeton University

De Leon is a leader in the fabrication and use of highly controlled diamond structures called nitrogen-vacancy (NV) centers. These NV centers are modifications to a diamond’s lattice of carbon atoms in which a carbon is replaced by a nitrogen atom, and adjacent to it is an empty space, or vacancy, in the molecular structure. Diamonds with NV centers are one of the few tools that can measure changes in magnetic fields at the scale and speed needed for critical experiments in quantum technology and condensed matter physics.

While a single NV center allowed scientists to take detailed readings of magnetic fields, it was only when de Leon’s team worked out a method to harness multiple NV centers simultaneously that they were able to measure the spatial structure of noise in a material. This opens the door to understanding the properties of materials with bizarre quantum behaviors that until now have been analyzed only theoretically, said de Leon, the senior author of a paper describing the technique published online Dec. 22 in the journal Science.

“It’s a fundamentally new technique,” said de Leon. “It’s been clear from a theoretical perspective that it would be very powerful to be able to do this. The audience that I think is most excited about this work is condensed matter theorists, now that there’s this whole world of phenomena they might be able to characterize in a different way.”

One of these phenomena is a quantum spin liquid, a material first explored in theories nearly 50 years ago that has been difficult to characterize experimentally. In a quantum spin liquid, electrons are constantly in flux, in contrast to the solid-state stability that characterizes a typical magnetic material when cooled to a certain temperature.

Shimon Kolkowitz

“The challenging thing about a quantum spin liquid is that by definition there’s no static magnetic ordering, so you can’t just map out a magnetic field” the way you would with another type of material, said de Leon. “Until now there’s been essentially no way to directly measure these two-point magnetic field correlators, and what people have instead been doing is trying to find complicated proxies for that measurement.”

By simultaneously measuring magnetic fields at multiple points with diamond sensors, researchers can detect how electrons and their spins are moving across space and time in a material. In developing the new method, the team applied calibrated laser pulses to a diamond containing NV centers, and then detected two spikes of photon counts from a pair of NV centers — a readout of the electron spins at each center at the same point in time. Previous techniques would have taken an average of these measurements, discarding valuable information and making it impossible to distinguish the intrinsic noise of the diamond and its environment from the magnetic field signals generated by a material of interest.

“One of those two spikes is a signal we’re applying, the other is a spike from the local environment, and there’s no way to tell the difference,” said study coauthor Shimon Kolkowitz, an associate professor of physics at the University of Wisconsin–Madison. “But when we look at the correlations, the one that is correlated is from the signal we’re applying and the other is not. And we can measure that, which is something people couldn’t measure before.”

Kolkowitz and de Leon met as Ph.D. students at Harvard University, and have been in touch frequently since then. Their research collaboration arose early in the COVID-19 pandemic, when laboratory research slowed, but long-distance collaboration became more attractive as most interactions took place over Zoom, said de Leon.

Jared Rovny, the study’s lead author and a postdoctoral research associate in de Leon’s group, led both the theoretical and experimental work on the new method. Contributions by Kolkowitz and his team were critical to designing the experiments and understanding the data, said de Leon. The paper’s coauthors also included Ahmed Abdalla and Laura Futamura, who conducted summer research with de Leon’s team in 2021 and 2022, respectively, as interns in the Quantum Undergraduate Research at IBM and Princeton (QURIP) program, which de Leon cofounded in 2019.

The article, Nanoscale covariance magnetometry with diamond quantum sensors, was published online Dec. 22 in Science. Other coauthors were Zhiyang Yuan, a Ph.D. student at Princeton; Mattias Fitzpatrick, who earned a Ph.D. at Princeton in 2019 and was a postdoctoral research fellow in de Leon’s group (now an assistant professor at Dartmouth’s Thayer School of Engineering); and Carter Fox and Matthew Carl Cambria of the University of Wisconsin–Madison. Support for the research was provided in part by the U.S. National Science Foundation, the U.S. Department of Energy, the Princeton Catalysis Initiative and the Princeton Quantum Initiative.

The University of Wisconsin–Madison’s Department of Physics contributed to this article.

Experimental condensed matter physics professor Marshall Onellion has passed away

Posted on December 20, 2022

UW–Madison physics professor Marshall Onellion passed away November 20, 2022. He was 72.

After completing his BS in mathematics and physics at West Virginia University in 1972, Onellion served in the U.S. Air Force until he was honorably discharged with the rank of Captain in 1979. He then began graduate studies in physics at Rice University, earning his PhD in 1984 before completing postdoctoral research at the University of Texas, Austin and Harvard University. Onellion joined the UW–Madison physics faculty as an assistant professor in 1987.

A condensed matter experimentalist, Onellion established a vigorous research program that primarily utilized the Aladdin ring at the UW-Madison Synchrotron Radiation Center (SRC) located in Stoughton, WI, for innovative studies of correlated electron materials of various types, including high-temperature superconductors, thin films, and magnetic multi-layers. His workhorse experimental tool was angle-resolved photoemission that was ideally suited to the stable and bright UV SRC synchrotron source.

Over the course of the next 15 years, his work was prolific. He published over 180 peer-reviewed articles, was a thesis advisor to many graduate and undergraduate students, and trained several postdoctoral researchers.

Onellion garnered numerous awards over his career, including being named a Hertz Fellow in graduate school and earning a National Science Foundation Presidential Young Investigator award in 1987. In 1996, he was named a UW–Madison Vilas Research Associate.

For many years Marshall actively volunteered to work with science students in area high schools, primarily Stoughton High School. In recognition of this outstanding service, in 2000 Marshall received a State of Wisconsin Certificate of Commendation for Public Service from Governor Tommy Thompson.

Special thanks to Prof. Thad Walker and Robert Sundling for contributing to this piece

Please visit the department’s tribute page to Marshall Onellion to submit and/or read stories from Marshall’s colleagues.