AMO

Mark Saffman awarded 2026 APS Ramsey Prize

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Mark Saffman, the Johannes Rydberg Professor of Physics and director of the Wisconsin Quantum Institute, won the American Physical Society’s 2026 Norman F. Ramsey Prize in Atomic, Molecular, and Optical Physics, and in Precision Tests of Fundamental Laws and Symmetries.

The Ramsey prize recognizes outstanding accomplishments in the two fields of Norman Ramsey: atomic, molecular, and optical (AMO) physics; and precision tests of fundamental laws and symmetries. Saffman won “for seminal developments of quantum information processing with neutral atoms that allow the investigation of many-body problems that are intractable by classical computing.” He shares the prize with Antoine Browaeys at the Institut d’Optique in France.

Mark Saffman poses in front of equipment in his lab
Mark Saffman

Saffman joined the UW–Madison physics faculty in 1999 with ideas for his research program but struggled to acquire enough funding. Then, he started reading theory papers about the relatively new field of quantum computing and how to develop qubits, or quantum bits.

“This was in an era when people were proposing all these different ideas for qubits,” Saffman says. “I read this paper about using Rydberg gates to entangle atomic qubits and thought, ‘This looks interesting, let’s do that.’ That was the smartest decision I ever made in my career.”

An atom can be induced into a Rydberg state by a strong laser, when one of its outer shell electrons is excited into a very high energy state. The atom is effectively much larger than usual, and can lead to interesting quantum properties. Relatively inexperienced in experimental atomic physics, Saffman approached Thad Walker, a professor in the department and an expert on how to laser cool atoms, about collaborating. A decade later, they had their major success: a Rydberg blockade.

“The basic interaction is that you excite one atom to a Rydberg state and then you cannot excite a second one close by,” Saffman says. “That blockade interaction lies behind the ability to do a logic gate — a CNOT gate — and entangle two qubits.”

A year later, Saffman and Walker demonstrated the first CNOT gate for atomic qubits. These qubits, also called neutral atom qubits, quickly are now one of the leading platforms for achieving fault tolerant quantum computing.

Over the next decade Saffman started to realize that building a fully functional quantum computer was not just a scientific effort, it was a major engineering effort, one that was likely outside the scope of an academic research group.

“It became clear to me that to compete at the forefront, I needed more resources. I wanted to go faster,” Saffman says. “So, I ended up joining forces with ColdQuanta (now Infleqtion), an existing small cold atom sensing and components company .”

a photograph of a room with the lights off, but the bulk of the image is taken up by a large piece of complicated equipment with many different colored laser lights visible, illuminating the shape of the equipment
The glow of red and green lasers and an array of supporting electronics fill the Saffman lab | Jacob Scott, PhD’25

Saffman brought his quantum computing ideas to the company as Chief Scientist for Quantum Information at Colorado-based Infleqtion in 2018, and the company now has a satellite office in Madison.

The partnership with Infleqtion did, in fact, accelerate Saffman’s research. In 2022, his group, including long-time scientist and group member Trent Graham, co-authored a paper with engineers at Infleqtion where they demonstrated the first quantum algorithm to be run on an atomic quantum computer. It was a huge proof of principle and significant step forward in the field.

Quantum information research has emerged as a major topic within the AMO physics community. At UW–Madison, Saffman has been a key player in that shift. In 2019, he helped develop the Wisconsin Quantum Institute, an interdisciplinary effort of all quantum information science and engineering researchers on campus. That same year, he was named the institute’s director.

“UW–Madison was one of the first places to have multiple serious efforts in qubits: Thad and I pioneered neutral atoms, (physics professor) Mark Eriksson pioneered silicon spin qubits, (physics professor) Robert McDermott has superconducting qubits,” Saffman says. “Now, a huge fraction of new faculty coming out of academia and starting their own groups are working in quantum information-related science and engineering, including many of our new faculty. The state of quantum computing at UW–Madison is very strong.”

Welcome, Prof. Josiah Sinclair!

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Josiah Sinclair

When he was younger, UW–Madison assistant professor of physics Josiah Sinclair wanted to be a scientist-inventor when he grew up. In high school, he would ask questions in biology and chemistry classes that his teachers said were really physics questions. So, when he began his undergrad at Calvin University, he majored in physics, believing that experimental physics would be at the intersection of his interests. In the end, it was quantum physics that really fascinated him, motivating him to complete a PhD in experimental quantum optics and atomic physics at the University of Toronto. He says, “The ethos of my PhD group was this idea that with modern technology, maybe we can invent an apparatus that can reproduce the essential elements of this or that classic thought experiment and learn something new.” After completing a postdoc at MIT, Sinclair joined the UW–Madison physics department as an assistant professor in August, where he will tinker in the lab as an experimental quantum physicist, and just maybe invent a new kind of neutral atom quantum computer.

Please give an overview of your research.

There’s a global race underway to build a quantum computer—a machine that operates according to the laws of quantum mechanics and uses an entirely different, more powerful kind of logic to solve certain problems exponentially faster than any classical computer can. Quantum computers won’t solve all problems, but there’s strong confidence they’ll solve some very important ones. Moreover, as we build them, we’re likely to discover new applications we can’t yet imagine.

The approach my group focuses on uses arrays of single neutral atoms as qubits. Right now, the central challenge in practical quantum computing is how to scale up quantum processors without compromising their quality. Today’s atom-array quantum computers are remarkable, hand-built systems that have reached hundreds or even thousands of qubits in recent years—a truly impressive feat and possible in part due to pioneering work done right here in Madison. However, as these systems grow larger, we’re hitting fundamental size limits that call for new strategies.

My lab is working to develop modular interconnects for neutral-atom quantum computers. Instead of trying to build a single massive machine, we aim to link multiple smaller systems together using single photons traveling through optical fibers. The challenge is that single photons are easily misplaced, so to make this work, we need to develop the most efficient atom–photon interfaces ever built—pushing the limits of our ability to control the interaction between one atom and one photon.

Once we get these quantum links working, we’ll have realized the essential building block for a truly scalable quantum computer and maybe someday the quantum internet. Beyond computing, these technologies could also enable new kinds of distributed quantum sensors, where multiple quantum systems work together to detect extremely faint signals spread across a large area, like photons arriving from distant planets.

What are the one or two main projects your new group will work on?

Our main focus will be to build two neutral atom quantum processors in adjacent rooms and link them together with an optical fiber. This project will teach us how to integrate highly efficient photonic interfaces—such as optical cavities—with atom arrays, and how to precisely control the interactions between atoms and photons. Step by step, we aim to demonstrate atom-photon entanglement and eventually send quantum information back and forth through the fiber.

We’re collaborating with a new company called CavilinQ, a Harvard spin-out supported by Argonne National Lab, to integrate a new cavity design with the geometry we want to explore for atom-photon coupling. Because we intend to iterate rapidly on the cavity design, our setup will be built on a precision translation stage, allowing us to easily slide the system in and out and swap out cavity components.

Another project in the lab will focus on developing a new kind of cold-atom quantum sensor. Most current sensors rely on magneto-optical traps, which require bulky electromagnets and impose constraints that limit performance. We plan to explore magnetic-field-free trapping techniques that could lead to simpler, more compact, and ultimately higher-performance quantum sensors.

What attracted you to Madison and the university?

Well, for me professionally, Madison’s a powerhouse in atomic physics and quantum computing. There are groups here that have been highly influential since the beginning in developing neutral atoms as a platform for quantum information science. So there’s a strong atomic physics community here that has incredible overlap with my research interests, and a thriving broader quantum information community as well. Some people work best in isolation, but that is not who I am, so the prospects of joining this vibrant collaborative environment was very appealing to me.

I also really enjoyed all my interactions with the members of the search committee and other faculty here both during my interview and subsequent visits. On the personal side, my wife’s family is all in the Chicago area, so the prospects of being so close to one side of the family were very appealing. We have a 18-month-old daughter, and when we visited, we just had such a positive impression of Madison as a place to have a family and to grow up.

What is your favorite element and/or elementary particle?

It’s rubidium. I worked with it in my PhD, I worked with it in my postdoc, and I will work with it again. It’s simple. It has one electron in the outer valence shell, which makes it easy to work with. It was one of the first atoms to be laser cooled and one of the first to be Bose condensed, but I think it still has some tricks for us up its sleeve. I believe the first quantum computers are going to be built out of rubidium atoms. Some people (and companies) think we will need a more complicated atom, like strontium or ytterbium, but I think we already have the atom we need—we just need to figure out how to make it work.

What hobbies and interests do you have?

In the last year: spending time with my eighteen-month-old daughter. It’s been a special time. I also enjoy photography. I do some photography of research labs, but mostly I do adventure photography. I don’t think of myself as a particularly talented photographer, my specialty is more being willing to lug a heavy camera up a mountain. I also really enjoy cycling, rock climbing, reading, and traveling.

 

 

Deniz Yavuz elected Fellow of the American Physical Society

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Deniz Yavuz

Congratulations to Prof. Deniz Yavuz, who was elected a 2025 Fellow of the American Physical Society!

He was elected “for outstanding experimental and theoretical contributions to nanoscale localization of atoms with electromagnetically induced transparency and collective radiation effects in atomic ensembles,” and nominated by the Division of Atomic, Molecular & Optical Physics (DAMOP).

APS Fellowship is a distinct honor signifying recognition by one’s professional peers for outstanding contributions to physics. Each year, no more than one half of one percent of the Society’s membership is recognized by this honor.

See the full list of 2025 honorees at the APS Fellows archive.

Mark Saffman part of team awarded in latest round of Research Forward funding

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This story was originally published by the OVCR

The Office of the Vice Chancellor for Research (OVCR) hosts the Research Forward initiative to stimulate and support highly innovative and groundbreaking research at the University of Wisconsin–Madison. The initiative is supported by the Wisconsin Alumni Research Foundation (WARF) and will provide funding for 1–2 years, depending on the needs and scope of the project.

Research Forward seeks to support collaborative, multidisciplinary, multi-investigator research projects that are high-risk, high-impact, and transformative. It seeks to fund research projects that have the potential to fundamentally transform a field of study as well as projects that require significant development prior to the submission of applications for external funding. Collaborative research proposals are welcome from within any of the four divisions (Arts & Humanities, Biological Sciences, Physical Sciences, Social Sciences), as are cross-divisional collaborations.

Physics professor Mark Saffman is part of a team awarded funding in Round 4 of the Research Forward competition for their project:

Quanta sensing for next generation quantum computing

Future quantum computers could open new scientific and engineering frontiers, impacting existential challenges like climate change. However, quantum information is delicate; it leaks with time and is prone to significant errors. These errors are exacerbated by imperfect reading and writing of quantum bits (qubits). These challenges fundamentally limit our ability to run quantum programs, and could hold back this powerful technology. Fast and accurate qubit readout, therefore, is essential for unlocking the quantum advantage. Current quantum computers use conventional cameras for reading qubits, which are inherently slow and noisy.

This research project will use quanta (single-photon) sensors for fast and accurate qubit readout. Quanta sensors detect individual photons scattered from qubits, thus enabling sensing qubits at 2-3 orders of magnitude higher speeds (few microseconds from ~10 milliseconds), thereby transforming the capabilities (speed, accuracy) of future quantum computers, and for the first time, paving the way for scalable and practical quantum computing.

Principal investigator: Mohit Gupta, associate professor of computer sciences

Co-PIs: Mark Saffman, professor of physics; Swamit Tannu, assistant professor of computer sciences; Andreas Velten, associate professor of biostatistics and medical informatics, electrical and computer engineering

Welcome, Professor Matthew Otten!

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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.

NASA funds Fundamental Physics proposal from Shimon Kolkowitz

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This post is adapted from a NASA news release; read the original here

NASA’s Fundamental Physics Program has selected seven proposals, including one from UW–Madison physics professor Shimon Kolkowitz, submitted in response to the Research Opportunities in Space and Earth Sciences – 2022 Fundamental Physics call for proposal.

The selected proposals are from seven institutions in seven states, with the total combined award amount of approximately $9.6 million over a five-year period. Kolkowitz’s proposal is ““Developing new techniques for ultra-high-precision space-based optical lattice clock comparisons.” 

Three of the selected projects will involve performing experiments using the Cold Atom Laboratory (CAL) aboard the International Space Station (ISS). Four of the selected proposals call for ground-based research to help NASA identify and develop the foundation for future space-based experiments.

The Fundamental Physics Program is managed by the Biological and Physical Sciences Division in NASA’s Science Mission Directorate. This program performs carefully designed research in space that advances our understanding of physical laws, nature’s organizing principles, and how these laws and principles can be manipulated by scientists and technologies to benefit humanity on Earth and in space.

Beating the diffraction limit in diamonds

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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.

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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.

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

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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.

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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.

Coherent light production found in very low optical density atomic clouds

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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.

A schematic of the experimental setup. (Top) the overall apparatus used. (A) shows the setup for the first part of the experiment, where the researchers were measuring decay rates in lower and lower density clouds. (B) shows the setup for the second part of the paper, with the addition of an interferometer

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.

profile picture of David Gold
David Gold

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.

This work was supported by National Science Foundation (NSF) Grant No. 2016136 for the QLCI center Hybrid Quantum Architectures and Networks.