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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Intriguing New Result Announced by the LHCb Experiment at CERN

The Standard Model of particle physics explains the most fundamental forces and particles in the universe with unprecedented precision. However, a recent announcement from theThe decay of a B0 meson into a K*0 and an electron–positron pair in the LHCb detector, which is used for a sensitive test of lepton universality in the Standard Model. Credit: CERNThe decay of a B0 meson into a K*0 and an electron–positron pair in the LHCb detector, which is used for a sensitive test of lepton universality in the Standard Model. Credit: CERN Large Hadron Collider beauty (LHCb) experiment at CERN raises the tantalizing prospect of new physics beyond the Standard Model picture.

Scientists analyzed all the data collected by the LHCb detector over the last decade —trillions of collisions recorded during the experiment’s first two runs. This cumulative study showed that beauty quarks are not decaying into equal numbers of muons and electrons when accompanied by a kaon, as the Standard Model would predict; electrons occur at a 15% higher frequency. If confirmed, this raises the captivating possibility that a particle or force not previously known could be involved and affecting the decays and leading to lepton universality violation. The significance of the effect is currently around three standard deviations, that is, less than 0.15% chance that it is simply a fluctuation.

Assistant Professor Manuel Franco Sevilla will present the new LHCb result in a plenary talk at the 2021 April APS meeting.

This is not the first time that LHCb data has shown such a discrepancy; in addition to earlier anomalies in similar beauty decays, a 2015 finding also hinted at a violation of lepton universality in decays involving muons and tauons, a study in which the UMD LHCb team played a leading role.  

Scientists will proceed with caution before deciding that the newly announced finding contradicts the Standard Model, which has proven resilient for five decades. Even more precise data is expected from LHCb’s Run 3, which will begin after a major detector upgrade is completed in 2022. 

To learn more, see the CERN press release:

In the Guardian:

Switchbacks Science: Explaining Parker Solar Probe’s Magnetic Puzzle

When NASA’s Parker Solar Probe sent back the first observations from its voyage to the Sun, scientists found signs of a wild ocean of currents and waves quite unlike the near-Earth space much closer to our planet. This ocean was spiked with what became known as switchbacks: rapid flips in the Sun’s magnetic field that reversed direction like a zig-zagging mountain road. 

Scientists think piecing together the story of switchbacks is an important part of understanding the solar wind, the constant stream of charged particles that flows from the Sun. The solar wind races through the solar system, shaping a vast space weather system, which we regularly study from various vantage points around the solar system – but we still have basic questions about how the Sun initially manages to shoot out this two-million-miles-per-hour gust. 

animated illustration of solar switchbacks
Parker Solar Probe observed switchbacks — traveling disturbances in the solar wind that caused the magnetic field to bend back on itself — an as-yet unexplained phenomenon that might help scientists uncover more information about how the solar wind is accelerated from the Sun.
Credits: NASA's Goddard Space Flight Center/Conceptual Image Lab/Adriana Manrique Gutierrez

Solar physicists have long known the solar wind comes in two flavors: the fast wind, which travels around 430 miles per second, and the slow wind, which travels closer to 220 miles per second. The fast wind tends to come from coronal holes, dark spots on the Sun full of open magnetic field. Slower wind emerges from parts of the Sun where open and closed magnetic fields mingle. But there is much we’ve still to learn about what drives the solar wind, and scientists suspect switchbacks – fast jets of solar material peppered throughout it – hold clues to its origins. 

Since their discovery, switchbacks have sparked a flurry of studies and scientific debate as researchers try to explain how the magnetic pulses form. 

“This is the scientific process in action,” said Kelly Korreck, Heliophysics program scientist at NASA Headquarters. “There are a variety of theories, and as we get more and more data to test those theories, we get closer to figuring out switchbacks and their role in the solar wind.”

Magnetic fireworks

On one side of the debate: a group of researchers who think switchbacks originate from a dramatic magnetic explosion that happens in the Sun’s atmosphere. 

Signs of what we now call switchbacks were first observed by the joint NASA-European Space Agency mission Ulysses, the first spacecraft to fly over the Sun’s poles. But decades later when the data streamed down from Parker Solar Probe to the Johns Hopkins Applied Physics Lab in Laurel, Maryland, which manages the mission, scientists were surprised to find so many. 

As the Sun rotates and its superheated gases churn, magnetic fields migrate around our star. Some magnetic field lines are open, like ribbons waving in the wind. Others are closed, with both ends or “footpoints” anchored in the Sun, forming loops that course with scorching hot solar material. One theory – initially proposed in 1996 based on Ulysses data – suggests switchbacks are the result of a clash between open and closed magnetic fields. An analysis published last year by scientists Justin Kasper and Len Fisk of the University of Michigan further explores the 20-year-old theory. 

When an open magnetic field line brushes against a closed magnetic loop, they can reconfigure in a process called interchange reconnection – an explosive rearrangement of the magnetic fields that leads to a switchback shape. “Magnetic reconnection is a little like scissors and a soldering gun combined into one,” said Gary Zank, a solar physicist at the University of Alabama Huntsville. The open line snaps onto the closed loop, cutting free a hot burst of plasma from the loop, while “gluing” the two fields into a new configuration. That sudden snap throws an S-shaped kink into the open magnetic field line before the loop reseals – a little like, for example, the way a quick jerk of the hand will send an S-shaped wave traveling down a rope. 

Other research papers have looked at how switchbacks take shape after the fireworks of reconnection. Often, this means building mathematical simulations, then comparing the computer-generated switchbacks to Parker Solar Probe data. If they’re a close match, the physics used to create the models may successfully help describe the real physics of switchbacks. 

Zank led the development of the first switchbacks model. His model suggests not one, but two magnetic whips are born during reconnection: One travels down to the solar surface and one zips out into the solar wind. Like an electric wire made from a bundle of smaller wires, each magnetic loop is made of many magnetic field lines. “What happens is, each of these individual wires reconnects, so you produce a whole slew of switchbacks in a short period of time,” Zank said. 

Zank and his team modelled the very first switchback Parker Solar Probe observed, on Nov. 6, 2018. This first model fit the observations well, encouraging the team to develop it further. The team’s results were published in The Astrophysical Journal on Oct. 26, 2020. 

Another group of scientists, led by University of Maryland physicist James Drake, agrees on the import of interchange reconnection. But they differ when it comes to the nature of switchbacks themselves. Where others say switchbacks are a kink in a magnetic field line, Drake and his team suggest what Parker is observing is the signature of a kind of magnetic structure, called a flux rope. 

In Drake’s simulations, the kink in the field didn’t travel very far before fizzling out. “Magnetic field lines are like rubber bands, they like to snap back to their original shape,” he explained. But the scientists knew the switchbacks had to be stable enough to travel out to where Parker Solar Probe could see them. On the other hand, flux ropes – which are thought to be core components of many solar eruptions – are sturdier. Picture a magnetic striped candy cane. That’s a flux rope: strips of magnetic field wrapped around a bundle of more magnetic field. 

Drake and his team think flux ropes could be an important part of explaining switchbacks, since they should be stable enough to travel out to where Parker Solar Probe observed them. Their study – published in Astronomy and Astrophysics on Oct. 8, 2020 – lays the groundwork for building a flux rope-based model to describe the origins of switchbacks. 

What these scientists have in common is they think magnetic reconnection can explain not only how switchbacks form, but also how the solar wind is heated and slings out from the Sun. In particular, switchbacks are linked to the slow solar wind. Each switchback shoots a gob of hot plasma into space. “So we’re asking, ‘If you add up all those bursts, can they contribute to the generation of the solar wind?’” Drake said. 

infographic explaining five theories explaining switchbacks
Illustration of five current theories explaining how switchbacks form. Image is not to scale.
Credits: NASA’s Goddard Space Flight Center/Miles Hatfield/Lina Tran/Mary-Pat Hrybyk Keith


Going with the flow

On the other side of the debate are scientists who believe that switchbacks form in the solar wind, as a byproduct of turbulent forces stirring it up.

Jonathan Squire, space physicist at New Zealand’s University of Otago, is one of them. Using computer simulations, he studied how small fluctuations in the solar wind evolved over time. “What we do is try and follow a small parcel of plasma as it moves outwards,” Squire said. 

Each parcel of solar wind expands as it escapes the Sun, blowing up like a balloon. Waves that undulate across the Sun create tiny ripples in that plasma, ripples that gradually grow as the solar wind spreads out.

“They start out first as wiggles, but then what we see is as they grow even further, they turn into switchbacks,” Squire said. “That's why we feel it's quite a compelling idea – it just happened by itself in the model.”  The team didn’t have to incorporate any guesses about new physics into their models – the switchbacks appeared based on fairly agreed-upon solar science.

Squire’s model, published on Feb. 26, 2020, suggests switchbacks form naturally as the solar wind expands into space. Parts of the solar wind that expand more rapidly, he predicts, should also have more switchbacks – a prediction already testable with the latest Parker dataset.

Other researchers agree that switchbacks begin in the solar wind, but suspect they form when fast and slow streams of solar wind rub against one another. One October 2020 study, led by Dave Ruffolo at Mahidol University in Bangkok, Thailand, outlined this idea.

Bill Matthaeus, a co-author on the paper and space physicist at the University of Delaware in Newark, points to the shearing at the boundary between fast and slow streams. This shearing between fast and slow creates characteristic swirls seen all over in nature, like the eddies that form as river water flows around a rock. Their models suggest that these swirls ultimately become switchbacks, curling the magnetic field lines back on themselves.

Animation of Parker passing through switchback
Illustration of Parker Solar Probe flying through a switchback in the solar wind.
Credits: NASA's Goddard Space Flight Center/Conceptual Image Lab/Adriana Manrique Gutierrez

But the swirls don’t form immediately – the solar wind has to be moving pretty fast before it can bend its otherwise rigid magnetic field lines. The solar wind reaches this speed about 8.5 million miles from the Sun. Mattheaus’ key prediction is that when Parker gets significantly closer to the Sun than that – which should happen during its next close pass 6.5 million miles from the Sun, on April 29, 2021 – the switchbacks should disappear.

“If this is the origin, then as Parker moves into the lower corona this shearing can't happen,” Mattheaus said. “So, the switchbacks caused by the phenomenon we're describing should go away.”

One aspect of switchbacks that these solar wind models haven’t yet successfully simulated is the fact that they tend to be stronger when they twist in a particular direction – the same direction of the Sun’s rotation. However, both simulations were created with a Sun that was still, not rotating, which may make the difference. For these modelers, incorporating the actual rotation of the Sun is the next step. 

Twisting in the wind

Finally, some scientists think switchbacks stem from both processes, starting with reconnection or footpoint motion at the Sun but only growing into their final shape once they get out into the solar wind. A paper published today by Nathan Schwadron and David McComas, space physicists at the University of New Hampshire and Princeton University, respectively, adopts this approach, arguing that switchbacks form when streams of fast and slow solar wind realign at their roots.

After this realignment fast wind ends up “behind” slow wind, on the same magnetic field line. (Imagine a group of joggers on a race track, Olympic sprinters at their heels.) This could happen in any case where slow and fast wind meet, but most notably at the boundaries of coronal holes, where fast solar wind is born. As coronal holes migrate across the Sun, scooting underneath streams of slower solar wind, the footpoint from the slow solar wind plugs into a source of fast wind. Fast solar wind races after the slower stream ahead of it. Eventually the fast wind overtakes the slower wind, inverting the magnetic field line and forming a switchback. 

Schwadron thinks the motion of coronal holes and of solar wind sources across the Sun is also a key puzzle piece. Reconnection at the leading edge of coronal holes, he suggests, could explain why switchbacks tend to “zig” in a way that’s aligned with the Sun’s rotation.

“The fact that these are oriented in this particular way is telling us something very fundamental,” Schwadron said.

Though it starts with the Sun, Schwadron and McComas think those reconnecting streams only become switchbacks within the solar wind, where the Sun’s magnetic field lines are flexible enough to double-back on themselves. 

As Parker Solar Probe swoops closer and closer to the Sun, scientists will eagerly look for clues that will support – or debunk – their theories. “There are different ideas floating around,” Zank said. “Eventually something will pan out.” 

Parker Solar Probe is part of NASA’s Living With a Star program to explore aspects of the Sun–Earth system that directly affect life and society. The Living With a Star flight program is managed by the agency’s Goddard Space Flight Center in Greenbelt, Maryland, for NASA’s Science Mission Directorate in Washington, D.C. The Johns Hopkins University Applied Physics Laboratory implements the mission for NASA. Scientific instrumentation is provided by teams led by the Naval Research Laboratory, Princeton University, the University of California, Berkeley, and the University of Michigan.

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NASA’s Goddard Space Flight Center, Greenbelt, Md.

Researchers Comb Atoms into a Novel Swirl

When you brush your hair in the morning, there’s a pretty good chance you’re not thinking about quantum physics. But the way your hair swirls as you brush is actually related to some features of the quantum world.

Important properties of quantum particles are described by topology—a field of mathematics that classifies objects according to how many holes they have. For instance, a coconut is topologically the same as a pizza (no holes), but different from a donut (one hole). Importantly, topology also dictates the kinds of hairdos you can style on these shapes. On a fuzzy coconut, no matter how much you brush, you’ll end up with at least one whorl—a spot that all the nearby hairs swirl around. On the other hand, a hairy donut can be brushed down flat without incurring any unsightly tufts or whorls. It can also have pairs of whorls, swirling in opposite directions, but never a single whorl.

This is not merely a question of fashion. In physical materials, topology can determine many interesting properties, like whether a material conducts electricity or not—particularly at its edges. Additionally, these edge properties are extremely precise and stable—so stable that they now serve as the standard for measuring electrical resistance.

Physicists don’t usually re-shape the materials they study into donuts or attach hairs to them. Instead, they often look at the topology of the quantum states of a particle moving around in the material. The coconut or donut shapes are found in abstract landscapes, inhabited by the possible speeds and directions the particle is moving—in technical language, each point in this space corresponds to a particular momentum. At each momentum, a “hair” sticks out, representing the internal quantum state of the particle. For solid materials, whose atoms line up into repeating crystal structures, the landscape is usually donut-shaped, and quantum whorls either don’t form at all or appear in pairs.Scientist created a swirl around a central point (white dot) in the quantum state of atoms (left), akin to a whorl of hair on a baby’s head (right). (Credit: Ana Valdes/JQI)Scientist created a swirl around a central point (white dot) in the quantum state of atoms (left), akin to a whorl of hair on a baby’s head (right). (Credit: Ana Valdes/JQI)

Now, a team of JQI researchers has engineered a new kind of topological matter—one with a single whorl—by breaking free from the constraints of crystalline solids. They managed to do this by grooming their atomic states into a whorl situated in an abstract, infinite plane, rather than a coconut or donut shape. The team was led by former UMD graduate student Ana Valdes-Curiel (currently a postdoc at the University of California, Los Angeles) in the group of Adjunct Professor and JQI Fellow Ian Spielman, a fellow at the National Institute of Standards and Technology. They reported their findings in a recent paper published in Nature Communications.(link is external)

In most solids, be they metals, insulators or superconductors, atoms arrange themselves in a repeating grid. Some electrons in the atoms can travel around in this grid, moving up and down or left and right. But no matter where an electron travels, it will end up experiencing deja vu: The grid of atoms repeats, so any given spot in the crystal looks exactly like many others.

This repeating structure has an effect on an electron’s momentum, too. In fact, topologically speaking, it’s as if the electron’s momentum is constrained to the surface of a donut. And since the donut can be brushed smooth, most materials have quantum state “hairs” that are swirl free. (They can also form two whorls, with hairs spiraling around them in opposite directions.)

To create their quantum whorl without the benefit of a neatly arranged crystal, the JQI team took a cloud of rubidium atoms and cooled it down to extremely low temperatures—so cold that quantum effects take over. At these temperatures, the atoms can inhabit only a few distinct quantum states. But, crucially, without the regular order of a crystal, there was no restriction on their momenta—they could move at any speed, up and down as well as left and right, forming a topological landscape of possible momenta akin to an infinite sheet.

Once the atoms were cooled, the team used three laser beams with precisely chosen colors and orientations to shuffle the atoms between three of the possible quantum states in sequence: first to second to third and back to first again. With each hop between states, the lasers imparted a small kick to the atoms, causing atoms in different states to move with slightly different speeds (and thus different momenta). This ensured that the quantum “hairs” pointed in different directions at different momenta and created a swirl around a point in the momentum landscape—the quantum whorl.

“And it turns out with this kind of atom, implementing that idea in a robust way is just super hard,” Spielman says.

Not all states can be manipulated to hop in a circle, and the ones that can are pretty unstable, quickly decaying and heating the cold atomic cloud. They are also easily disturbed by the tiniest changes in the lab’s magnetic field—something as small as a person walking by with a metal keychain can throw the whole thing off.

To mitigate these issues, the researchers employed a technique that they recently pioneered(link is external). They bathed the atoms in a strong, carefully chosen radio-frequency signal before the lasers were turned on. In this field, three stable states are transformed into quantum superpositions of each other, with parts of the other states mixed in. It’s normally impossible to make an atom hop in a circle through these stable states, but once they are jumbled together this restriction is lifted. On top of that, the strong magnetic field of the radio waves inoculated the atoms against small magnetic field disturbances.

They confirmed the novel topology of their cloud of atoms by directly measuring the quantum states they created, at a host of different atom momenta. Traversing their momentum landscape in a loop, they measured what mixture of the constituent states (the first, second, and third states in the circle) the atoms lived in, analogous to looking which direction the hairs were pointing. They confirmed that there was a single swirl at the loop’s center.

One of the most important features of topological materials is the edge effects that occur at an interface with a material that has a different topology. For example, if an insulator happens to have two swirls on its momentum donut, there will be a sharp change whenever it’s bordered by swirl-free air or a vacuum. At the place where materials with different swirl counts meet, something discontinuous has to happen. In this case, the boundary becomes a conductor sandwiched between two insulators. The number of these conducting edge channels exactly corresponds to the topology in the bulk, and they cannot be easily disrupted or destroyed. The discovery and topological explanation of these edge effects was the subject of the 2016 Nobel Prize in Physics(link is external), and has inspired much recent research and development of new types of devices.

“The whole big thing about topology is you get these edge states that are very robust,” says Valdes-Curiel “And now you have this system that is topological, but it doesn’t have the usual topological features. So what happens with the usual bulk-edge correspondence? What kind of edge states do you have? And what kind of topological devices can you build?”

Story by Dina Genkina

In addition to Valdes-Curiel and Spielman, co-authors of this paper included Dimitios Trypogeorgos, a former JQI postdoc now at the at the Institute of Nanotechnology in Lecce, Italy; Qiyu Liang, a postdoc at the JQI; and Russel Anderson, a former visiting researcher at the JQI now at La Trobe University and Q-CTRL.

Research Contact:  Ian Spielman, This email address is being protected from spambots. You need JavaScript enabled to view it.
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