Students create exotic state of matter

By Bennett McIntosh

IN THE SUMMER OF 2015, Princeton students Joseph Scherrer and Adam Bowman experienced something few undergraduates can claim: they built, from scratch, a laser system capable of coaxing lithium atoms into a rare, highly excited state of matter to reveal their quantum nature.

When they joined Assistant Professor of Physics Waseem Bakr’s lab in the spring of 2014, Scherrer and Bowman had little experience in optics or quantum physics. Their task was to convince lithium atoms to enter a state of matter known as the Rydberg state. In this state, each atom has a very high-energy electron located far from the atom’s nucleus. The separation of the electron’s negative charge from the nucleus’ positive charge creates a dipole, like a magnet’s north and south poles.

To give the electrons the right amount of energy to create the Rydberg state, Scherrer and Bowman hit the atoms with two carefully tuned lasers, first blue and then red. To prove that the lithium atoms had indeed entered the Rydberg state, the two researchers needed a way to detect them. They trawled the scientific literature for a sensitive enough detection method, and eventually implemented a technique called electromagnetically induced transparency. With this technique, the Rydberg atoms interfere with the absorbance of certain wavelengths of light, so if the gas is transparent in those wavelengths, the Rydberg atoms are present.

The undergraduates designed and built the device independently, Bakr said. “I wasn’t planning on starting this, and suddenly it grew into a whole project, largely due to their efforts,” he said.

“It was a turning point in our scientific development,” said Scherrer, who graduated in 2016 with a degree in physics. “For me, it was a realization of what you can do with quantum optics.” Scherrer was awarded a Fulbright grant to join a team in Munich, Germany, where he is building electron microscopes to image the brain. He will next head to the Massachusetts Institute of Technology to pursue a Ph.D. in physics. Bowman, a physics major in the Class of 2017, continues to study the physics of electronically interesting materials, and spent his junior year and the summer of 2016 working on a new project with Ali Yazdani, Princeton’s Class of 1909 Professor of Physics. There, Bowman built a device that works like an inkjet printer for atoms to print superconductors layer-by-layer.

Atom catcher: With lasers and magnets, Waseem Bakr traps atoms for study under the microscope

By Bennett McIntosh

THE COLDEST SPOT on the Princeton campus is a cluster of a few thousand atoms suspended above a table in Waseem Bakr’s laboratory. When trapped in a lattice of intersecting lasers at just millionths of a degree above absolute zero — and roughly one-millionth the density of air — atoms become very still, enabling Bakr, an assistant professor of physics, to study them through a microscope.

At these frigid temperatures and ultralow densities, atoms begin to act very strangely. They function less like individual particles and instead behave like waves that blur and overlap, losing their individual identity and trading the physics of the everyday world for the laws of quantum mechanics. The resulting state, known as a degenerate Fermi gas, can yield insights into new states of matter that someday may lead to applications such as superconductors and quantum computers.

Bakr uses a system of lasers and magnetic fields to cool and trap the ultracold atoms in a crystal-like lattice made from light. He then manipulates and observes the atoms using a quantum-gas microscope, a device that he helped invent during his graduate studies with Markus Greiner at Harvard University, and further improved when he was a postdoctoral researcher with Martin Zwierlein at the Massachusetts Institute of Technology.

“We use lasers to create artificial crystals in which we place these quantum-mechanical atoms where the spacing between atoms is 10,000 times larger than what you find in real crystals,” Bakr said. “We are essentially engineering the behaviors of atoms using light.”

Bakr and his team first heat a block of lithium to 800 degrees Fahrenheit to liberate individual atoms that then fly into a long tube. There, the particles collide head-on with a laser beam pointed in the opposite direction, which rapidly slows and cools them. The atoms then flow into a chamber where the intersection of several laser beams creates an electromagnetic field that confines the atoms in an “optical trap.” The trap allows the fastest-moving (and warmest) atoms to escape, further cooling the ultracold gas. The resulting cluster of atoms, Bakr said, is “the coldest stuff you can find in the universe.”

Using the microscope, Bakr can agitate a single atom to watch the disturbance propagate, or he can rearrange the entire system to simulate a different material. “If I decide I want to study graphene today,” he said, “I can arrange my lasers to make a graphene-like lattice, and suddenly the physics that I’m looking at are very different.” This precise control could hold the key to another advance, he said. “If you have 1,000 atoms, and you have control over every single atom and their interactions, these are the basic building blocks of a quantum computer,” Bakr said.

Cold atoms

Trapped by lasers and magnets, lithium atoms form a fluorescent red ball at the center of this image. In this initial stage of laser cooling, about 1 billion atoms are brought from a temperature of 350 degrees Celsius to a thousandth of a degree above absolute zero.

Bakr and his team are using ultracold atoms to study the behavior of superfluids with imbalanced spin populations. In a paper published in the August 24, 2016, issue of Physical Review Letters, Bakr and his team showed that the two-dimensional gas separates into two phases, a superfluid in the center of the trap and a  normal gas at its periphery, like the phase  separation that happens when mixing oil and  water. “Observing this phase separation is the first step in a search for exotic types of superfluidity that were predicted over 50 years ago,”

The Bakr lab’s work is supported by grants from the Air Force Office of Scientific Research, the National Science Foundation and the Alfred P.  Sloan Foundation.

F. Duncan Haldane receives Nobel Prize in Physics

F. Duncan Haldane, Princeton’s Eugene Higgins Professor of Physics, was awarded the 2016 Nobel Prize in Physics “for theoretical discoveries of topological phase transitions and topological phases of matter.” Considered key to finally realizing highly efficient and powerful quantum computers, topological materials exhibit unique properties, particularly great stability and efficient particle movement. Haldane shares the prize with David Thouless of the University of Washington and J. Michael Kosterlitz of Brown University.

 

 

JEREMIAH OSTRIKER and LYMAN PAGE receive Gruber Cosmology Prize

Lyman Page

Lyman Page. PHOTO BY DENISE APPLEWHITE

The 2015 Gruber Foundation Cosmology Prize has been awarded to Jeremiah Ostriker and Lyman Page for “individual and collective contributions to the study of the universe on the largest scales.”

The two share the prize with John Carlstrom of the University of Chicago. Half of the $500,000 prize went to Ostriker, while Carlstrom and Page divided the other half. Each also received a gold medal at the XXIX General Assembly of the International Astronomical Union in Honolulu, Hawaii, on Aug. 3, 2015. Ostriker is the Charles A. Young Professor of Astronomy on the Class of 1897 Foundation, Emeritus, and Page is the James S. McDonnell Distinguished University Professor in Physics.

Jeremiah Ostriker

Jeremiah Ostriker. PHOTO BY DENISE APPLEWHITE

According to the award citation, Ostriker was honored for his “groundbreaking body of work over a five-decade career,” while Carlstrom and Page “have each overseen ground-based experiments providing a wealth of information about the origins and evolution of the universe. Together the theoretical and experimental work of these three scientists has contributed to, clarified and advanced today’s standard cosmological model.”

–By Catherine Zandonella

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Cosmic background: 51 years ago, an accidental discovery sparked a big bang in astrophysics

SPIDER

The balloon-borne spacecraft, SPIDER, prior to launch. PHOTO BY ZIGMUND KERMISH

ON NEW YEAR’S DAY 2015, A BALLOON-BORNE SPACECRAFT ascended above Antarctica and snapped crisp photos of space, unobscured by the humidity of Earth’s atmosphere. Meanwhile, a telescope located 4,000 miles to the north, in the desolate Chilean desert, scanned roughly half of the visible sky.

By air and by land, physicists have staked out the best vantage points on the globe, not for stargazing, but for peering between the stars at the thermal traces of the Big Bang.

Spread nearly evenly across the universe is a sea of invisible radiation called the Cosmic Microwave Background (CMB) that keeps space a chilly 2.7 degrees above absolute zero. The now-cold CMB, however, is a remnant of a much hotter, more violent cosmic epoch. About 13.8 billion years ago, immediately after the Big Bang, the universe was filled with a hot gas of ionized particles and radiation. As space expanded, the waves of radiation were stretched and diluted into their current low-energy state. The boiling plasma has since cooled and clumped into galaxies, stars, planets and human beings, all drifting through the faint afterimage of the first flash.

The prediction, discovery and study of the CMB 50 years ago comprise a story that is deeply intertwined with several generations of faculty at the Princeton physics department. The story continues today as University researchers probe the microwave background with the goal of understanding the past and future of our cosmos.

The discovery of the background radiation was a serendipitous one. In 1964, Bell Laboratories technicians Robert Wilson and Arno Penzias racked their brains for an explanation of the noisy signal recorded by their radio antenna. When it turned out that the “noise” was actually radiation from the CMB, the two engineers found themselves unexpectedly pulled into the growing field of modern cosmology. The detection of the CMB earned them the Nobel Prize.

Yet the discovery wouldn’t have been possible without the work of physicists at Princeton, 40 miles down the road from Bell Labs. Back then, the branch of science known as cosmology was ignored by most serious researchers. The physics community viewed the origin and development of the universe as dead-end topics, yet a few at Princeton had dared to tackle it.

At the time, P. James Peebles was a physics postdoctoral researcher at Princeton. “When I started working in this field, everyone was saying, ‘There’s no evidence. Why are you studying this?’” said Peebles, who today is the Albert Einstein Professor of Science, Emeritus. Instead, the mainstream focus was particle physics, which studies the subatomic particles that make up the universe.

Guyot Hall

David Wilkinson and Peter Roll used this experimental setup on the roof of Guyot Hall, which housed the Department of Geology (now Geosciences), to search for the CMB, at Bob Dicke’s suggestion. Wilkinson is holding a screwdriver, and Roll is almost obscured by the instrument. Photo by Robert Matthews circa 1964-65

Two Princeton professors, John Wheeler and Peebles’ mentor, Robert Dicke, decided that research on the cosmic scale should not be neglected. Since 1915, when Einstein developed the theory of general relativity to explain the behavior of large objects in space, hardly any further research had been done on gravity or the structure of the universe. This was due in part to respect for Einstein’s picture of the cosmos, and in part to the difficulty of devising fruitful experiments. “In the mid-1950s, Bob started a serious program of laboratory and extraterrestrial tests for general relativity,” said Peebles. “John started a school for the theoretical study of the subject. These changes marked a renaissance.”

While everyone else was thinking small, Dicke and Wheeler were thinking big. More specifically, Dicke was thinking about the Big Bang, a concept that dated back to the 1920s, when it was first observed that the universe is expanding. Yet extrapolating the current expansion back in time to a tiny, hot, dense state from which it all began was not a widely accepted leap. Peebles said: “Until the ’60s, the evidence that this is what happened was minimal. It was still just an idea, popular in some circles, detested in others.”

Dicke took the Big Bang theory from guesswork firmly into the realm of empirical physics when he proposed the CMB as evidence for a hot, dense beginning. Peebles recalls how Dicke almost casually set the course for his career and that of his peers: “He persuaded Dave Wilkinson and Peter Roll [Princeton physics faculty members] to build a device called a Dicke radiometer to look for this radiation, and he told me with a wave of his hand, ‘Why don’t you go think about the theory.’ And I’ve been doing it ever since.”

By 1970, the scientific community had accepted that the CMB had the properties that made it undeniable evidence for the Big Bang. Physicists then shifted their attention to more detailed scrutiny of the remnant radiation as a way of deepening our understanding of the birth of the universe, its expansion and its fate.

One area of scrutiny is whether the universe went through a period of rapid expansion, or inflation, after the Big Bang. To look for signs of inflation and to map the CMB in our region of space, NASA in partnership with Princeton and other universities launched the Wilkinson Microwave Anisotropy Probe (WMAP) satellite, named postmortem in honor of Wilkinson’s contribution to experimental cosmology.

The inflationary model predicts a particular pattern to the fluctuations of the CMB. When WMAP released its first set of results in 2003, they neatly matched the predictions of inflation. Among the many Princeton researchers who played significant roles in WMAP were Lyman Page, the James S. McDonnell Distinguished University Professor in Physics; Norman Jarosik, senior research physicist; and David Spergel, the Charles A. Young Professor of Astronomy on the Class of 1897 Foundation. Thanks to WMAP, Spergel said, “We have a coherent cosmological model that fits all the data.”

Since then, several other projects, including most recently the European Space Agency’s Planck space telescope, have mapped the CMB and provided evidence for inflation. But scientists are looking for additional evidence in the form of long undulations — called gravitational waves — stretching across the fabric of space. The remnants of these waves could be detected as a faint pattern in the CMB known as B-mode polarization. A reported detection of gravitational waves earlier this year from another project, BICEP2, created a stir in the astrophysics community but turned out to be an artifact of interstellar dust.

Detecting the remnants of gravitational waves is one of the goals of the Atacama Cosmology Telescope (ACT), an international project funded by the National Science Foundation and led by Princeton’s Suzanne Staggs, the Henry DeWolf Smyth Professor of Physics. The team includes Lyman Page, the James S. McDonnell Distinguished University Professor of Physics; Spergel, the Charles A. Young Professor of Astronomy on the Class of 1897 Foundation; and many colleagues at collaborating institutions.

The data collected during the flight of the balloon-borne SPIDER mission in Antarctica — funded by NASA, the National Science Foundation, the David and Lucile Packard Foundation, and the Natural Sciences and Engineering Research Council of Canada; and led by Assistant Professor of Physics William Jones — could also reveal evidence of these waves in the CMB.

The search for gravitational waves is just one of the ways in which the CMB provides opportunities for studying the early universe. The ACT collaboration is also looking for evidence of dark energy, a mysterious force that is speeding up the expansion of the universe, and answers to even bigger questions about the cosmological model. It is clear that the CMB is an important tool for the foreseeable future of cosmology. However faint, it illuminates the distant past, which in turn illuminates the future.

–By Takim Williams

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Imaging system tracks brain activity of a freely moving worm

TO EXPLORE HOW THE BRAIN controls behavior, researchers have for the first time captured the whole-brain activity of a freely moving animal, in this case a nematode worm called Caenorhabditis elegans.

Using an imaging system they designed, Andrew Leifer, a Lewis-Sigler Fellow, and Joshua Shaevitz, an associate professor of physics and the Lewis-Sigler Institute for Integrative Genomics, measured the activity of 78 of the worm brain’s 125-plus neurons, which they engineered to turn green when active.

The setup consists of cameras that monitor the worm’s position and a motorized stage that adjusts to track the worm as it roams freely. The researchers were able to show significant correlations between neuron activity patterns and behaviors such as moving backward or forward, and turning. The team included Jeffrey Nguyen, a postdoctoral research associate and first author on the study, and colleagues at the Lewis-Sigler Institute and the Princeton Neuroscience Institute.

The study was posted on the preprint server arXiv.org and was funded by Princeton’s Dean for Research Innovation Fund for New Ideas in the Natural Sciences, the Simons Foundation and the National Institutes of Health.

–By Catherine Zandonella

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Elusive particles found

IN THE PAST YEAR, PRINCETON PHYSICISTS have detected two particles that were predicted decades ago to exist but had not been found until now. Both particles were detected using a scanning-tunneling microscope to image the particles inside a crystal. The particles may someday enable powerful computers based on quantum mechanics.

A team led by Ali Yazdani, the Class of 1909 Professor of Physics, detected the “Majorana fermion,” which behaves simultaneously like matter and antimatter and was first proposed in 1937 by Italian physicist Ettore Majorana. The team, which received funding from the National Science Foundation and the Office of Naval Research, included B. Andrei Bernevig, an associate professor of physics, and other colleagues at Princeton and at the University of Texas-Austin. They published their results in the Oct. 2, 2014, issue of the journal Science.

A few months later, an international team led by M. Zahid Hasan, professor of physics, detected another elusive particle, the “Weyl fermion,” first theorized by the mathematician and physicist Hermann Weyl in 1929. The particle is massless and can also behave like matter and antimatter. The research team, which received support from the Gordon and Betty Moore Foundation and the U.S. Department of Energy, published their work in Science on July 16, 2015.

–By Steven Schultz and Morgan Kelly

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Striking resemblance: A physical law may govern very different biological activities

Starlings

FLOCKS OF BIRDS FLY ACROSS THE SKY in shifting configurations. In the retina of an eye, millions of neurons ignite in ever-changing combinations, translating light into meaningful images. Yet both of these seemingly random behaviors have an underlying order that can be described by mathematics.

Like these cells and birds, when atoms and molecules come together they can display coordinated behaviors that are more than the sum of their parts. At a critical point, such as the boundary between liquid and gas, local interactions between molecules propagate through an entire material, changing its essential properties.

Princeton biophysicist William Bialek thinks criticality may also underlie collective behaviors in living organisms, and he’s using real-world data to test this hypothesis. Recently, Bialek and his colleagues have analyzed the flocking behaviors of birds, the genetic networks of fruit fly embryos and the activation patterns of salamander neurons.

“In physics, we use the same mathematical language to describe many seemingly different behaviors,” said Bialek, the John Archibald Wheeler/Battelle Professor in Physics and the Lewis-Sigler Institute for Integrative Genomics. “So we understand that the emergence of collective behavior from all the individual interactions has a kind of universality.”

To explore the possibility that this universality might extend to living systems, Bialek made use of a large dataset on the changing positions and velocities of thousands of individual birds in a flock of starlings. A group of Italian physicists used multiple cameras to record the birds and calculate their exact locations over time in three dimensions — “a technical triumph,” according to Bialek.

The researchers, including former Princeton postdoctoral fellows Thierry Mora and Aleksandra Walczak, analyzed the deviations of each bird from the flock’s average speed and direction of movement. They found not only that these variations were correlated between nearby birds, but also that the fluctuations from the average propagated through the group over long distances. This pattern of rapid, remote signal transmission echoes the changes that occur among molecules during a phase change from solid to liquid or liquid to gas. At a critical point, this could allow information to spread swiftly through the group, enabling the whole flock to nimbly change direction.

“The model you build just by keeping track of what each bird does relative to its neighbors predicts what happens throughout the entire flock,” Bialek said. “And it does so with an accuracy that is beyond what we had any reason to expect. It’s really a very precise prediction.”

Other biological examples of criticality play out on a microscopic scale. Bialek has an ongoing collaboration with Princeton’s Squibb Professor in Molecular Biology Eric Wieschaus, a Howard Hughes Medical Institute researcher and Nobel Prize winner, who has uncovered many of the genes involved in the embryonic development of the fruit fly — a model biological system.

Bialek has found signatures of criticality in gene activation patterns during the first few hours of fly embryo development. The synchronized actions of “gap genes” establish the fly’s 14-segment body plan. Mutations in these genes lead to gaps between segments, whose effects are reflected in the names of the genes: two examples are “hunchback” and “giant.”

Recently, Thomas Gregor, an assistant professor of physics and also a member of the Lewis-Sigler Institute, has developed experimental tools to precisely measure the activity of many gap genes at once, all along the halfmillimeter length of the fly embryo. These measurements allowed Bialek and physics graduate student Dmitry Krotov to test whether the patterns of gene activity across the embryo fit a model of criticality. Indeed, using data from 24 embryos, they found that fluctuations from the average level of gene activity at each point along the embryo were correlated between certain pairs of gap genes, which regulate one another’s activity like on/off switches. They mapped the locations of these switch points, which appear to act like signals that spread over long distances, just as changes in velocity are correlated in a flock of birds.

Bialek has also looked for signatures of criticality among the activation patterns in a patch of 160 nerve cells from a salamander retina, a model system for studying this light-sensing layer of the eye. In collaboration with Michael Berry, an associate professor of molecular biology and the Princeton Neuroscience Institute, Bialek and his colleagues showed how the coordinated activity of the neurons could be tuned to a critical state.

Bialek thinks critical systems may be common features of life that have repeatedly evolved in different organisms and at different levels — both molecular and behavioral. This could explain why, though systems of cells or groups of organisms could be organized in any number of possible ways, networks with similar properties continue to emerge.

“Is there anything special about the way nature has organized things in living systems?” Bialek wondered. He said much more work is necessary to claim criticality as a general biological principle. “But I do think we’re seeing in the data, somehow, signs of that specialness — things that it seems you can only get if the system has been set up in particular ways and not in others,” he said. “That I find very appealing.”

This work was supported in part by the National Science Foundation, the National Institutes of Health, the Howard Hughes Medical Institute, the W.M. Keck Foundation and the Swartz Foundation.

–By Molly Sharlach

Telescopes take the universe’s temperature

Two telescope projects are measuring cosmic microwave background radiation with the goal of understanding more about the universe’s early history. The telescopes (pictured) are located on a peak in the Atacama Desert in Chile. (Image courtesy of ACT Collaboration)

Two telescope projects are measuring cosmic microwave background radiation with the goal of understanding more about the universe’s early history. The telescopes (pictured) are located on a peak in the Atacama Desert in Chile. (Image courtesy of ACT Collaboration)

Two telescopes on a Chilean mountaintop are poised to tell us much about the universe in its infancy. They are surveying the faint temperature fluctuations left over from the explosive birth of the universe, with the goal of piecing together its early history and understanding how clusters of galaxies evolved.

The telescopes are measuring these temperature fluctuations, known as cosmic microwave background radiation or CMB for short, from their perch 17,000 feet above sea level in Chile’s desolate Atacama Desert, where a dry atmosphere permits radiation to reach Earth with relatively little attenuation. In contrast to backyard telescopes that help us see visible light from stars and planets, these telescopes collect invisible microwave radiation.

Lyman Page

Lyman Page

These invisible waves are mostly uniform but contain slight differences in intensity and polarization that hold a wealth of information for cosmologists, said Lyman Page, the Henry De Wolf Smyth Professor of Physics. Page and Professor of Physics Suzanne Staggs co-lead two telescope projects, the Atacama Cosmology Telescope (ACT) and the Atacama B-mode Search telescope (ABS), which are funded by the National Science Foundation.

“If you imagine the temperature perturbations as a distant mountain range, the peaks and valleys correspond to the temperature variations,” Page said. “By looking at the patterns — the spacing between peaks, and whether they are narrow or fat — we are able to answer questions about the composition and evolution of the universe,” Page said.

ACT, which is about 18 feet across and looks like a giant metal bowl, has already made new discoveries, and confirmed and extended the findings of other CMB surveys, including two space-based telescopes, NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and the European Space Agency’s Planck mission. A new, more sensitive receiver is currently being mounted on the ACT telescope, which is a collaborative effort with David Spergel, Princeton’s Charles A. Young Professor of Astronomy on the Class of 1897 Foundation, along with researchers at the University of Pennsylvania, National Institutes of Standards and Technology, the University of British Columbia, and 10 other institutions contributing significantly to the instruments and analysis.

Suzanne Staggs

Suzanne Staggs

The CMB originated in the hot plasma soon after the Big Bang, which cosmologists consider to be the birth of the universe. As the universe expanded, the radiation propagated, carrying the secrets of the early universe with it. One of the questions is why the CMB on opposite sides of the universe is so similar in temperature. The leading explanation of this observation is the inflation model, which posits that the universe underwent a rapid acceleration of its expansion just after the Big Bang.

This is where the lower-resolution, second telescope comes in. Co-led by Staggs, the ABS is looking for signs of inflation. “Inflation should produce gravitational waves which create patterns in the CMB called ‘B modes,’” said Staggs. B modes are extremely faint — to measure them requires an instrument that can detect temperature changes of just billionths of a degree. To obtain these sensitivities, ABS mirrors, which are relatively small at about two feet across, sit inside a cryogenically cooled barrel.

The two telescopes can be operated remotely, but require frequent trips to the Chilean peak, which often include Princeton students and postdocs. The team at Princeton includes Senior Research Physicist Norm Jarosik, Associate Research Scholar Jonathan Sievers, postdoctoral researchers Matthew Hasselfield, Rénee Hložek, Akito Kusaka and Laura Newburgh, and graduate students Farzan Beroz, Kang Hoon (Steve) Choi, Emily Grace, Colin Hill, Shuay-Pwu (Patty) Ho, Christine Pappas, Lucas Parker, Blake Sherwin, Sara Simon, Katerina Visnjic and Sophie Zhang.

–By Catherine Zandonella

Alexander Polyakov wins Fundamental Physics Prize

Alexander Polyakov

Alexander Polyakov (Photo by Martin Rocek)

Alexander Polyakov, the Joseph Henry Professor of Physics, received the $3 million Fundamental Physics Prize in 2013 for his work in field theory and string theory. His ideas have dominated work in these fields during the past decades, according to the Fundamental Physics Prize Foundation. The award recognizes Polyakov’s influential work in string theory, which looks to find common ground between quantum mechanics and general relativity. In addition, he was honored for his work in quantum field theory, a framework for modeling the dynamics of particles.

The prize is awarded to researchers who have made transformative advances in physics. Polyakov, who received the prize during a ceremony in Geneva, was selected for the honor by the recipients of the 2012 Fundamental Physics Prize.