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.
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.
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
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.
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.
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.
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.
Thad Walker honored with Vilas Distinguished Achievement Professorship
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Thad Walker
Extraordinary members of the University of Wisconsin–Madison faculty, including physics professor Thad Walker, have been honored during the last year with awards supported by the estate of professor, U.S. senator and UW Regent William F. Vilas (1840-1908).
Walker was one of seventeen professors were named to Vilas Distinguished Achievement Professorships, an award recognizing distinguished scholarship as well as standout efforts in teaching and service. The professorship provides five years of flexible funding — two-thirds of which is provided by the Office of the Provost through the generosity of the Vilas trustees and one-third provided by the school or college whose dean nominated the winner.
In addition, nine professors received Vilas Faculty Mid-Career Investigator Awards and six professors received Vilas Faculty Early Career Investigator Awards.
Detailed analysis of old star provides template for heavy element formation
The fusion furnaces that are the universe’s stars create the elements from helium up to iron. But iron is only number 26 on the periodic table out of well over 100 known elements. So the heavier ones, like gold, lead and uranium, must come from somewhere other than fusion.
Scientists have long known that those heavy elements come from neutron capture, where neutrons are added to an element that make it unstable, then it radioactively decays and its atomic number increases by one. Nearly 70 years ago, they confirmed one site, or event, of a neutron capture method known as the slow, or s-process. The rapid, or r-process, was not confirmed with a site until 2017, when the LIGO/VIRGO collaboration detected a neutron star merger.
“With a neutron star merger, the neutron stars are ripped apart and they throw out neutrons, and you can build lots of heavy elements out of these neutron stars,” says Jim Lawler, a professor of physics at the University of Wisconsin–Madison. “The mystery arises when we look at the total r-process inventory of our home galaxy: Can we explain all that with neutron star mergers or are there additional sites?”
In a new study led by astronomers from the University of Michigan, Lawler and colleagues identified the elemental composition of HD 222925, a Milky Way star located over 1400 lightyears from earth. Their analysis confirmed that the star was rich in r-process elements, and they were able to identify and calculate the relative abundance of each element. They also found that the star is iron- and metal-poor, a proxy for age that indicates HD 222925 is relatively old and provides information about early star formation.
“We were able to determine a complete r-process abundance pattern for what we think is probably one event that happened early in the beginning of the universe,” Lawler says. “So that r-process template now can be used to screen various models of the nuclear physics that produce the r-process and see if the models for all sites are physically correct.”
At UW–Madison, Lawler and scientist Elizabeth Denhartog contributed the spectroscopic analysis that identified the elements in the star. Every element has a unique electromagnetic spectrum that can be separated into spectral lines using a diffraction grating — just like a prism separates white light into a rainbow. HD 222925 is a relatively bright star, meaning it provided stronger spectra to analyze. It was also identified by the Hubble Space Telescope, providing access to data in the ultraviolet range that is normally blocked by the ozone layer and undetectable by telescopes on Earth.
THIS STUDY WAS SUPPORTED IN PART BY NASA (GRANTS GO-15657, GO-15951, AND 80NSSC21K0627); U.S. NATIONAL SCIENCE FOUNDATION (NSF, GRANTS PHY 14-30152, OISE 1927130, AST 1716251 AND AST 1815403); AND THE U.S. DEPARTMENT OF ENERGY (GRANT DE-FG02-95-ER40934); AND NOIRLAB, WHICH IS MANAGED UNDER A COOPERATIVE AGREEMENT WITH THE NSF.
Thirty-two members of the University of Wisconsin–Madison faculty — including physics professor Mark Saffman — have been awarded fellowships from the Office of the Vice Chancellor for Research and Graduate Education for 2022-23. The awardees span the four divisions on campus: arts and humanities, physical sciences, social sciences and biological sciences.
“These awards provide an opportunity for campus to recognize our outstanding faculty,” says Steve Ackerman, vice chancellor for research and graduate education. “They highlight faculty efforts to support the research, teaching, outreach and public service missions of the university.”
The awards are possible due to the research efforts of UW–Madison faculty and staff. Technology that arises from these efforts is licensed by the Wisconsin Alumni Research Foundation and the income from successful licenses is returned to the OVCRGE, where it’s used to fund research activities and awards throughout the divisions on campus.
Mark Saffman was awarded a WARF professorship. These professorships come with $100,000 and honor faculty who have made major contributions to the advancement of knowledge, primarily through their research endeavors, but also as a result of their teaching and service activities. Award recipients choose the names associated with their professorships. Saffman, the Johannes Rydberg Professor of Physics and director of The Wisconsin Quantum Institute, first began work on atomic physics and initiated a long-term effort to develop quantum computers. He is known for his research as a leader in the ongoing development of atomic quantum computers based on the Rydberg blockade mechanism.
In addition, physics affiliate professor Mikhail Kats received a Romnes Faculty Fellowship.
Congratulations to Professor Lawler on his retirement!
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After 42 years on the UW–Madison faculty, Jim Lawler, the Arthur and Aurelia Schawlow Professor of Physics, has announced his retirement. Lawler is an atomic, molecular & optical physicist with a focus developing and applying laser spectroscopic techniques for determining accurate absolute atomic transition probabilities. His retirement is official as of May 22.
“What we’ve really done gradually over four-plus decades is make atomic spectroscopy more quantitative so that people can use it to really learn the detailed physics and chemistry of the remote universe,” Lawler says.
Lawler received his MS (’74) and PhD (’78) from this department, studying with now-professor emeritus 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.
“There was a little bit of a disadvantage to come back to a place where I had recently been as a student,” Lawler says. “But I knew I would get extremely good graduate students and I would have access to a lot of infrastructure, and that combination really drew me back.”
He had extremely good graduate students and postdocs. 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.
Lawler served as department chair from 1994-1997. He also accumulated numerous awards and honors over his distinguished career. He is 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.
Longtime collaborator Blair Savage, UW–Madison professor emeritus of astronomy, says:
“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.”
And Wilmer Anderson, Lawler’s doctoral advisor, says:
“He was a very good graduate student, and he of course has turned out to be a really great scientist and colleague. His lifetime measurements on atomic physics played a key role in understanding the neutron star collisions. I’m sorry to see him retiring but I’m sure that he will leave a legacy behind that’s really fantastic. It’s going to be a big loss to the department not to have him around.”
Lawler has collaborated with his AMO colleagues over the years, but in more of an intellectual capacity than in research. As he notes, much of AMO is headed in the quantum information and quantum computing direction, with public and private funding helping to drive it. Still, he does not see AMO headed solely in the quantum direction.
“Decades from now the currently Hot areas of physics will be less glamorous, but those stars are still going to be light years away,” Lawler says. “I think the connection of astronomy and spectroscopy — the way we learn about the physics and chemistry of the remote universe — is strong enough that it will survive. And helping make spectroscopy in astronomy more quantitative is what we’ve done that will have some lasting significance.”
Ultraprecise atomic clock poised for new physics discoveries
Their instrument, known as an optical lattice atomic clock, can measure differences in time to a precision equivalent to losing just one second every 300 billion years and is the first example of a “multiplexed” optical clock, where six separate clocks can exist in the same environment. Its design allows the team to test ways to search for gravitational waves, attempt to detect dark matter, and discover new physics with clocks.
From one sphere of supercooled strontium atoms, Kolkowitz’s group multiplexes them into six separate spheres, each of which can be used as an atomic clock.
“Optical lattice clocks are already the best clocks in the world, and here we get this level of performance that no one has seen before,” says Shimon Kolkowitz, a UW–Madison physics professor and senior author of the study. “We’re working to both improve their performance and to develop emerging applications that are enabled by this improved performance.”
Atomic clocks are so precise because they take advantage of a fundamental property of atoms: when an electron changes energy levels, it absorbs or emits light with a frequency that is identical for all atoms of a particular element. Optical atomic clocks keep time by using a laser that is tuned to precisely match this frequency, and they require some of the world’s most sophisticated lasers to keep accurate time.
By comparison, Kolkowitz’s group has “a relatively lousy laser,” he says, so they knew that any clock they built would not be the most accurate or precise on its own. But they also knew that many downstream applications of optical clocks will require portable, commercially available lasers like theirs. Designing a clock that could use average lasers would be a boon.
Shimon Kolkowitz one of four UW professors awarded Sloan Fellowship
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Four University of Wisconsin–Madison professors, including assistant professor of physics Shimon Kolkowitz, have been named to Sloan Research Fellowships — competitive, prestigious awards given to promising researchers in the early stages of their careers.
“Today’s Sloan Research Fellows represent the scientific leaders of tomorrow,” says Adam F. Falk, president of the Alfred P. Sloan Foundation, which has awarded the fellowships since 1955. “As formidable young scholars, they are already shaping the research agenda within their respective fields—and their trailblazing won’t end here.”
Kolkowitz, an assistant professor of physics, builds some of the most precise clocks in the world by trapping ultracold atoms of strontium — clocks so accurate they could be used to test fundamental theories of physics and search for dark matter.
UW–Madison’s other 2022 Sloan Fellows are Tatyana Shcherbina (math), Zachary K. Wickens (chemistry) and Andrew Zimmer (math).
The UW–Madison professors are among 118 researchers from the United States and Canada honored by the New York-based philanthropic foundation. The four new fellows join 110 UW–Madison researchers honored in the past.
Each fellow receives $75,000 in research funding from the foundation, which awards Sloan Research Fellowships in eight scientific and technical fields: chemistry, computer science, economics, mathematics, computational and evolutionary molecular biology, neuroscience, ocean sciences and physics.
