Oxford Instruments announced Feb 15 that Tiancheng Song, who will join the UW–Madison physics department as an assistant professor in May, has been awarded the 2024 Lee Osheroff Richardson Science Prize. He is currently an experimental physicist and Dicke Fellow at Princeton University.
Dr. Song is recognized for his efforts in developing and employing various measurement techniques at low temperatures and in magnetic fields to study 2D superconductivity and magnetism in van der Waals heterostructures. His works have uncovered a series of emergent quantum phenomena in 2D superconducting and magnetic systems.
The Lee Osheroff Richardson Science Prize promotes and recognises the novel work of young scientists working in the fields of low temperatures and/or high magnetic fields or surface science in North and South America.
“I am thrilled to be the recipient of the prestigious Lee Osheroff Richardson Science Prize this year! I feel this is a special honour because I am joining the ranks of remarkable scientists who have been awarded this prize for their famous experiments and achievements,” commented Dr. Song.
Tiancheng Song is currently a Dicke Fellow in the Department of Physics at Princeton University. Working with Prof. Sanfeng Wu, Dr. Song recently developed a new technique to investigate 2D superconductivity, strongly correlated phases and the associated unconventional quantum phase transition.
In his work at Princeton, Dr. Song successfully measured superconducting quantum fluctuations of monolayer WTe2 based on the vortex Nernst effect. The result led to the discovery of a new type of quantum critical point beyond the conventional Ginzburg-Landau theory and demonstrated a new sensitive probe to 2D superconductivity and superconducting phase transitions.
Dr. Song’s results have been well recognized by the community with his work being cited over 4,000 times. Dr. Song’s original contributions are demonstrated by the faculty offers he has subsequently received; he will join the University of Wisconsin–Madison as an assistant professor in May 2024.
As part of the prize, Dr. Song will receive $8000 as well as support to attend the APS March Meeting in Minneapolis where he will be presented his award.
The 2024 LOR Science Prize selection committee is chaired by Professor Laura Greene, NHMFL and FSU and includes: Professor Hae-Young Kee, Toronto University; Professor Collin Broholm, Johns Hopkins University; Professor Paula Giraldo-Gallo, University of the Andes; and Dr Xiaomeng Liu, Princeton (2023 winner).
About the LOR Science Prize
Oxford Instruments is aware that there is a critical and often difficult stage for many scientists between completing a PhD and gaining a permanent research position. The company is pleased to help individuals producing innovative work by offering financial assistance and suitably promoting their research work, through sponsoring the LOR Science Prize for North and South America for the past 19 years. The Prize is named in honour of Professors David M. Lee, Douglas D. Osheroff and Robert C. Richardson, joint recipients of The Nobel Prize in Physics 1996 for their discovery of ‘superfluidity in helium-3’.
The previous winners of the LOR Science Prize are Dr Xiaomeng Liu, Dr James Nakamura, Dr Matthew Yankowitz, Dr Sheng Ran, Dr Paula Giraldo-Gallo, Dr Kate Ross, Dr Brad Ramshaw, Dr Mohamad Hamidian, Dr Cory Dean, Dr Chiara Tarantini, Dr Lu Li, Dr Kenneth Burch, Dr Jing Xia, Dr Vivien Zapf, Dr Eunseong Kim, Dr Suchitra Sebastian, Dr Jason Petta, and Dr Christian Lupien.
Welcome, Professor Matthew Otten!
Atomic, molecular and optical and quantum theorist Matthew Otten will join the UW–Madison physics department as an assistant professor on January 3, 2024. He joins us most recently from HRL Laboratories. Prior to HRL, Otten earned his PhD from Cornell University, and then was the Maria Goeppert Mayer fellow at Argonne National Laboratory.
Please give an overview of your research.
Very generally, my goal is to make utility scale quantum computing a reality, and to get there faster than we would otherwise without my help. We have a lot of theoretical reasons to believe that quantum algorithms will be faster in certain areas; in practice, we need to know how expensive it’s going to be. It could be that a back of the envelope calculation says a quantum computer might be better, but because quantum computers are very expensive to build and have a lot of overhead, you could find that once you crunch the numbers really carefully, it turns out to cost more money or more energy or more time than just doing it on a supercomputer. In that case, it’s not worth the investment to build it, or at least not at this point. Part of my research is to understand and develop quantum algorithms and count how expensive they are. Once you do that, you can figure out the reason it’s so expensive is A and B. Then we go and we try to fix A and B, and then whack-a-mole all these bottlenecks down and eventually you go from, “It’ll never work,” to “Okay, it’ll work in twenty years.”
Another part of my research is looking at the physical qubits. These devices all have a lot of deep physics inside of them. If you just look at it from the quantum algorithm level, you might get so far. But if you dig down and try to understand the underlying physics, I think you can get further. You might be able to make devices cheaper, faster, or more performant in general. I do a lot of simulations of the underlying physics of these various types of qubits to understand what their properties are, what causes the noise that ruins computation, and what we can do to fix that noise. Through simulations on classical computers, sometimes very large ones, we come up with ways to tweak the system so that you get better performance, by coming up with better quantum algorithms and better qubits. Put those together and hopefully you get to a better quantum computer.
Once you arrive in Madison, what are one or two research projects you think your group will focus on first?
I’ll be bringing a few projects with me. The first is part of a DARPA program called Quantum Benchmarking, which I was part of while at HRL. We found really high-value computational tasks, not specifically quantum, that Boeing, which owns HRL, would like calculated: for instance, reducing corrosion. Corrosion causes planes to be grounded for maintenance, which is costly. Reducing corrosion will reduce maintenance costs and increase uptime. We’ve been developing ways to ask and answer the question, how close are today’s quantum computers to solving that problem? How big do quantum computers need to be to solve that problem? The specific task is understanding what it takes to solve such a large-scale problem, counting the quantum resources that are necessary and coming up with tests so that you could go to a quantum computer, run the tests, and hopefully be able to predict how much bigger or how much faster they would need to be to solve the problem.
Another one comes from the Wellcome Leap Foundation. We are trying to do the largest, most accurate calculation of biological objects — a molecule, string of carbon, something like this — possible on a real-life quantum computer. We’re trying to take techniques that have already been developed or develop new techniques to make circuits smaller, which means a less expensive quantum computer, and faster. That one is a competition, they gave us funding to do it, but if we complete the task better than other competitors, we get more funding to do more.
What attracted you to UW–Madison?
The strength of the science that’s happening in the physics and broader Wisconsin community is very attractive. When I visited, everyone was very nice, it’s a very collegial department. And being from St. Louis, I like the Midwest. I’ve lived in Southern California for a couple of years now and I haven’t seen snow, and that’s sad. Madison is a lovely area. Great people.
What is your favorite element and/or elementary particle?
I think it has to be silicon. Silicon is used in classical computing and potentially has use in quantum computing. And you’re carrying around silicon right now, just like everyone else.
What hobbies and interests do you have?
I have a Siberian Husky puppy and we’ll be very happy to go to Madison and do a lot of skijoring, which is cross country skiing, but the dog pulls you. I started running recently and I was jazzed up for my first half marathon and then I got COVID and I didn’t do it, so I’m still jazzed up for my first half marathon. I play a lot of board games and have a very large board game collection. And my daughter just turned one. She’s become a new hobby.
Ben Woods and team named finalists in 2023 WARF Innovation Awards
Each fall the WARF Innovation Awards recognize some of the best inventions at UW–Madison. WARF receives hundreds of new invention disclosures each year. Of these disclosures, the WARF Innovation Award finalists are considered exceptional in the following criteria:
Has potential for high long-term impact
Presents an exciting solution to a known important problem
Could produce broad benefits for humankind
One of the six finalists comes from Physics. Research Associate Benjamin Woods and a team including Distinguished Scientist Mark Friesen, John Bardeen Prof. of Physics Mark Eriksson, Honorary Associate Robert Joynt, and Graduate Student Emily Joseph developed a quantum device that shows a significant increase in valley splitting, a key property needed for error-free quantum computing. The device features a novel structural composition that turns conventional wisdom on its head.
Two winners, selected from the six finalists, will be announced in WARF’s annual holiday greeting; sign up to receive the greeting here. Each of the two Innovation Award winners receive $10,000, split among UW inventors.
Choy leads team awarded National Science Foundation Quantum Sensing Challenge Grant
The National Science Foundation has selected a proposal “Compact and robust quantum atomic sensors for timekeeping and inertial sensing” by an interdisciplinary team led by University of Wisconsin-Madison researchers for...
New quantum sensing technique reveals magnetic connections
By Leah Hesla, Q-NEXT
A research team supported by the Q-NEXT quantum research center demonstrates a new way to use quantum sensors to tease out relationships between microscopic magnetic fields.
Say you notice a sudden drop in temperature on both your patio and kitchen thermometers. At first, you think it’s because of a cold snap, so you crank up the heat in your home. Then you realize that while the outside has indeed become colder, inside, someone left the refrigerator door open.
Initially, you thought the temperature drops were correlated. Later, you saw that they weren’t.
Recognizing when readings are correlated is important not only for your home heating bill but for all of science. It’s especially challenging when measuring properties of atoms.
Now scientists — including those from UW–Madison physics professor Shimon Kolkowitz‘s group — have developed a method, reported in Science, that enables them to see whether magnetic fields detected by a pair of atom-scale quantum sensors are correlated or not.
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.
“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).
Welcome, Roman Kuzmin, the Dunson Cheng Assistant Professor of Physics
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 thingsyou 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
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.
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.
“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.
Opening doors to quantum research experiences with the Open Quantum Initiative
This past winter, Katie Harrison, then a junior physics major at UW–Madison, started thinking about which areas of physics she was interested in studying more in-depth.
“Physics is in general so broad, saying you want to research physics doesn’t really cut it,” Harrison says.
She thought about which classes she enjoyed the most and talked to other students and professors to help figure out what she might focus on. Quantum mechanics was high on her list. During her search for additional learning opportunities, she saw the email about the Open Quantum Initiative (OQI), a new fellowship program run by the Chicago Quantum Exchange (CQE).
“This could be something I’m interested in, right?” Harrison thought. “I’ll apply and see what happens.”
What happened was that Harrison was one of 12 undergraduate students accepted into the inaugural class of OQI Fellows. These students were paired with mentors at CQE member institutions, where they conducted research in quantum science information and engineering. OQI has a goal of connecting students with leaders in academia and industry and increasing their awareness of quantum career opportunities. The ten-week Fellowship ran through August 19.
OQI also places an emphasis on establishing diversity, equity, and inclusion as priorities central to the development of the quantum ecosystem. Almost 70% of this year’s fellowship students are Hispanic, Latino, or Black, and half are the first in their family to go to college. In addition, while the field of quantum science and engineering is generally majority-male, the 2022 cohort is half female.
This summer, UW–Madison and the Wisconsin Quantum Institute hosted two students: Harrison with physics professor Baha Balantekin and postdoc Pooja Siwach; and MIT physics and electrical engineering major Kate Arutyunova with engineering physics professor Jennifer Choy, postdoc Maryam Zahedian and graduate student Ricardo Vidrio.
Harrison and Arutyunova met at OQI orientation at IBM’s quantum research lab in New York, and they hit it off immediately. (“We have the most matching energies (of the fellows),” Arutyunova says, with Harrison adding, “The synergy is real.”)
Despite their very different research projects — Harrison’s was theoretical and strongly focused on physics, whereas Arutyunova’s was experimental and with an engineering focus — they leaned on each other throughout the summer in Madison. They met at Union South nearly every morning at 7am to read and bounce ideas off each other. Then, after a full day with their respective research groups, they’d head back to Union South until it closed.
Modeling neutrino oscillations
Harrison’s research with Balantekin and Siwach investigated the neutrinos that escape collapsing supernovae cores. Neutrinos have a neutral charge and are relatively small particles, they make it out of cores without interacting with much — and therefore without changing much — so studying them helps physicists understand what is happening inside those stars. However, this is a difficult task because neutrinos oscillate between flavors, or different energy levels, and therefore require a lot of time and resources to calculate on a classical computer.
Harrison’s project, then, was to investigate two types of quantum computing methods, pulse vs circuit based, and determine if one might better fit their problem than the other. Previous studies suggest that pulsed based is likely to be better, but circuit based involves less complicated input calculations.
“I’ve been doing calibrations and calculating the frequencies of the pulses we’ll need to send to our qubits in order to get data that’s as accurate as a classical computer,” Harrison says. “I’m working with the circuit space, the mathematical versions of them, and then I’ll send my work to IBM’s quantum computers and they’ll calculate it and give results back.”
While she didn’t fully complete the project, she did make significant progress.
“(Katie) is very enthusiastic and she has gone a lot further than one would have expected an average undergraduate could have,” Balantekin says. “She started an interesting project, she started getting interesting results. But we are nowhere near the completion of the project, so she will continue working with us next academic year, and hopefully we’ll get interesting results.”
Developing better quantum sensors
Over on the engineering side of campus, Arutyunova was studying different ways to introduce nitrogen vacancy (NV) centers in diamonds. These atomic-scale defects are useful in quantum sensing and have applications in magnetometry. Previous work in Choy’s group made the NV centers by a method known as nitrogen ion beam implantation. Arutyunova’s project was to compare how a different method, electron beam irradiation, formed the NV centers under different starting nitrogen concentrations in diamond.
Briefly, she would mark an edge of a very tiny (2 x 2 x 0.5 millimeter), nitrogen-containing diamond, and irradiate the sample with a scanning electron microscope. She used confocal microscopy to record the initial distribution of NV centers, then moved the sample to the annealing step, where the diamond is heated up to 1200 celsius in a vacuum annealing furnace. The diamonds are then acid washed and reexamined with the confocal microscope to see if additional NV centers are formed.
“It’s a challenging process as it requires precise coordinate-by-coordinate calculation for exposed areas and extensive knowledge of how to use the scanning electron microscope,” says Arutyunova, who will go back to MIT after the fellowship wraps. “I think I laid down a good foundation for future steps so that the work can be continued in my group.”
Kate made significant strides in her project and her work has put us on a great path for our continued investigation into effective ways of generating color centers in diamond. In addition to her research contributions, our group has really enjoyed and benefited from her enthusiasm and collaborative spirit. It’s wonderful to see the relationships that Kate has forged with the rest of the group and in particular her mentors, Maryam and Ricardo. We look forward to keeping in touch with Kate on matters related to the project as well as her academic journey.
Beyond the summer fellowship
Both Harrison and Arutyunova think that this experience has drawn them to the graduate school track, likely with a focus on quantum science. More importantly, it has helped them both to learn what they like about research.
“I would prefer to work on a problem and see the final output rather than a question where I do not have an idea of the application,” Arutyunova says. “And I realized how much I like to collaborate with people, exchange ideas, propose something, and listen to people and what they think about research.”
They also offer similar advice to other undergraduate students who are interested in research: do it, and start early.
“No matter when you start, you’re going to start knowing nothing,” Harrison says. “And if you start sooner, even though it’s scary and you feel like you know even less, you have more time to learn, which is amazing. And get in a research group where they really want you to learn.”
Coherent light production found in very low optical density atomic clouds
No atom is an island, and scientists have known for decades that groups of atoms form communities that “talk” to each other. But there is still much to learn about how atoms — particularly energetically excited ones — interact in groups.
In a study published in PRX Quantum, physicists from the University of Wisconsin–Madison observed communication between atoms at lower and lower densities. They found that the atoms influence each other at 100 times lower densities than probed before, exhibiting slow decay rates and emitting coherent light.
“It seems that (low-density) groups of excited atoms spontaneously organize to then produce light that is coherent,” says David Gold, a postdoctoral fellow in Deniz Yavuz’s group and lead author of the study. “These findings are pretty interesting from a basic science standpoint, and in terms of quantum computing, the takeaway is that even with very low numbers of atoms, you can see significant amounts of (these effects).”
A well-established property of atoms is found in electron excitation: when a specific wavelength of light hits an atom of a specific element, an electron is excited to a higher orbital level. As that electron decays back to its initial state, a photon of a specific wavelength is emitted. A single atom has a characteristic decay rate for that process. When groups of atoms are studied, their interactions are observed: the initial decay rate is very fast, or superradiant, then transitions to a slower, or subradiant, rate.
Though well-established in dense clouds, this group-talk has never been studied in less dense clouds of atoms, which could have impacts on applications such as quantum computing.
In their first set of experiments, Gold and colleagues asked what the decay rate of lower-density clouds looked like. They supercooled the atoms in a cloud, hit them with an excitation laser, and recorded the decay rates as an intensity of emitted light over time. They observed the characteristic subradiance. In this case, they did not always see superradiance, likely due to the reduced number of atoms available to measure.
Next, they asked what happened if they let the cloud expand — or decrease in density — for varying periods of time before repeating their experiment. They found that as the cloud become less and less dense, the amount of subradiance decreased, until eventually a density was reached where the atoms stopped behaving like a group and instead displayed single-atom decay rates.
“The most subradiance that we observed was at around a hundred times lower optical density than it had previously been observed before,” Gold says.
Now that the researchers knew that a less dense cloud still decays subradiantly to a point, they asked if the decay was happening in an isolated manner, or if the atoms were really acting as a group. If acting as a group, the emitted light would be coherent, or more laser-like, with some structure between the atoms.
They used the same experimental setup but added an interferometer, where light is split and recombined before the photons are detected. They first set the baseline interference pattern by moving the mirror closer or further away from the splitter — changing the path length of one of the beams — and mapping the interference pattern of the split light waves that were emitted from the same atom.
If there were no relationship between the two atoms and the light they emit, then they would have expected to see no interference pattern. Instead, they saw that for some distance of mirror displacement, the lightwaves did interfere, indicating that different atoms being measured were nonetheless producing coherent light.
“I think this is the more exciting thing we found: that the light that’s being emitted is coherent and it has more of the properties of a laser than you would expect,” Gold says. “The atoms are influenced by each other and not in a way we would have expected.”
Aside from the interesting physics seen in the study, Gold says the work is also applicable to quantum computing, particularly as those computers grow bigger in the future.
“Even if everything in a quantum computer is running perfectly and the system was completely isolated, there’s still this inherent thing of, well, the atoms just might decay down from [the computational] state,” Gold says.