Research Team Describes "Somersaulting" Photons

Spinning or rotating objects are commonplace, from toy tops and fidget spinners to spinning figure skaters. And from water circling a drain to far less welcome tornadoes and hurricanes.

In physics, there are two kinds of rotational motion, spin rotation or orbital rotation. Earth’s motion in our solar system nicely illustrates these: The daily 360 degree rotation of earth around its own axis is ‘spin’ rotation, while earth’s yearly trip around the sun is ‘orbital’ rotation. STOV pulse (on the left) moving through a nonlinear crystal undergoes second harmonic generation, generating the pulse on the right.A STOV pulse (on the left) moving through a nonlinear crystal undergoes second harmonic generation, generating the pulse on the right.

The quantity in physics defined to describe such rotational motion is “angular momentum” (AM). The important thing about AM is that it is a conserved quantity. Given an initial amount of it, it can be broken up and redistributed among particles (such as atoms, photons, pebbles, M&Ms) but the total AM must remain the same. Angular momentum is a vector. It is a quantity that has a direction, and this direction is perpendicular to the plane in which the rotational circulation occurs.

For the particles of light in laser beams—photons—these two kinds of AM are present. Photons have spin, but we can’t think of a photon as rotating on its own axis. Instead, the spin angular momentum (SAM) comes from the rotation of the photon’s electric field, and the SAM can only point forward or backward with respect to the beam direction. Photons in laser beams can also have orbital angular momentum (OAM). The simplest laser beam where the photons have OAM is the ‘donut beam’---if you shine such a beam on the wall, it will look like a bright donut or ring with a dark centre. In this case, the OAM vector also points forward or backward. The amazing fact, courtesy of quantum mechanics, is that the OAM is the same for every photon in the beam.

In a paper published today in the Journal Optica, Professor Howard Milchberg’s group (IREAP/ECE/Physics) demonstrates the surprising result that photons in vacuum can have orbital angular momentum vectors pointing sideways—at 90 degrees to the direction of propagation—a result literally orthogonal to the many decades-long expectation that OAM vectors could only point forward or backward. The research team, including graduate student and lead author Scott Hancock, postdoc Sina Zahedpour (EE Ph.D. '17), and Milchberg, did this by generating a donut pulse they dub an “edge-first flying donut”, depicted in the diagram (its more technical name is “spatio-temporal optical vortex”—STOV). Here, the donut hole is oriented sideways, and because the rotational circulation now occurs around the ring, the angular momentum vector points at right angles to the plane containing the ring. To prove that this sideways-pointing OAM is associated with individual photons and not just the overall shape of the flying donut, the team sent the pulse through a nonlinear crystal (shown in diagram) to undergo a well-known process called “second harmonic generation”, where 2 red photons are converted into a single blue photon with double the frequency. This reduces the number of photons by a factor of 2, which means each blue photon should have twice the sideways-pointing OAM—and this is exactly what the measurements showed. As seen in the diagram, the angular momentum of the flying donut (or STOV) –represented by the red and twice-longer blue arrows—is the composite effect of a swarm of photons somersaulting in lockstep.

There are numerous potential applications of STOVs. For example, the angular momentum conservation embodied by somersaulting photons may make STOV beams resistant to breakup by atmospheric turbulence, with potential application to free-space optical communications. In addition, because STOV photons must occur in pulses of light, such pulses could be used to dynamically excite a wide range of materials or to probe them in ways that exploit the OAM and the donut hole. “STOV pulses could play a big role in nonlinear optics,” says Milchberg, "where beams can control the material they propagate in, enabling novel applications in beam focusing, steering, and switching.”

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A Handy New Platform for Majorana Fermions

A new, experimentally-feasible approach to generating Majorana fermions has been identified in iron-based superconducting thin films, potentially paving theQuasiparticle platform A visualization of the Fermi surface depicting the momenta of electrons in the newly identified quantum computing platform. (Courtesy: Ruixing Zhang)Quasiparticle platform A visualization of the Fermi surface depicting the momenta of electrons in the newly identified quantum computing platform. (Courtesy: Ruixing Zhang) way for Majorana-based quantum computation. The research, conducted by CMTC and JQI postdoc Ruixing Zhang and Distinguished University Professor Sankar Das Sarma, was published in Physical Review Letters and highlighted in a recent story in Physics World.



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Gorshkov to Receive Flemming Award

Adjunct Associate Professor Alexey Gorshkov is among 12 exceptional public servants chosen to receive the Arthur S. Flemming Award for 2020. The awardees will be honored during a virtual celebration this summer. Gorshkov, a physicist at the National Institute of Standards and Technology(link is external) (NIST), is also a Fellow of the Joint Quantum Institute (JQI) and the Joint Center for Quantum Information and Computer Science (QuICS),

The award is presented annually(link is external) by the Arthur S. Flemming Commission, in partnership with the George Washington University Trachtenberg School of Public Policy and Public Administration and the National Academy of Public Administration. It recognizes federal employees who have provided outstanding service in the fields of applied science and engineering, basic science, leadership and management, legal achievement, and social science.

Established in 1948, the award is named after Arthur Sherwood Flemming, a distinguished government official who served seven presidential administrations of both parties, most notably as Secretary of Health, Education, and Welfare under President Dwight Eisenhower.

Gorshkov, who joined NIST’s Quantum Measurement Division in 2013 and has been embedded at the University of Maryland since that time, was specifically noted for his pioneering research at the crossroads of quantum optics and atomic and condensed matter physics. His research team is engineering strong interactions between photons, providing a practical basis for a new generation of technologies, where instead of electrons, circuits of light are used to perform logical operations and computations.

Expanding upon this successful demonstration, which was hailed as one of Physics World’s Top 10 breakthroughs, Gorshkov has shown novel ways to control strongly coupled atom-light systems and is laying the theoretical foundation for a new suite of enabling quantum technologies.

“I am honored to receive this award in recognition of science and service that will ultimately benefit the public good,” says Gorshkov. “As we continue to develop and implement the ideas and technologies needed to build and deploy quantum systems, we should see a rapid increase in their uses for many practical applications, from secure communication, to accurate time-keeping, to optimizing supply chain logistics and traffic flow.”

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Mohapatra Authors Book on Neutrino's Importance

Rabi MohapatraRabi Mohapatra

Distinguished University Professor Rabi Mohapatra recently published The Neutrino Story: One Tiny Particle’s Grand Role in the Cosmos, a book describing the importance of the mysterious particle that Mohapatra has studied for decades.

“The idea for writing this book came to me when I realized how little common people knew about the neutrino and its role in building the universe” said Mohapatra. The book should be understandable to non-scientists with an interest in physics.

In 2020, his paper Neutrino masses and mixings in gauge models with spontaneous parity violation was named one of the three most influential titles in the first fifty years of Physical Review D, which was established to cover the fields of particles, fields, gravitation, and cosmology. This “neutrino mass seesaw" paper, written with Mohapatra’s student Goran Senjanović (then a UMD post-doc) has helped theorists better assess neutrinos and has inspired various experimental quests, as noted in Physics magazine.

Mohapatra has also written two textbooks. The first, Unification and Supersymmetry, appeared in 1986; a second, Massive Neutrinos in Physics and Astrophysics (with Palash B. Pal)in 1989. Each has gone through three editions, and each remains a standard reference in its subfield.

Mohapatra is a fellow of the American Physical Society, a fellow of the National Academy of Sciences, India, and a recipient of the Alexander von Humboldt Prize. He has written more than 450 papers, with over 45,000 citations. At the University of Maryland, he was named a Distinguished Scholar-Teacher  in 2001 and a Distinguished University Professor in 2016.