Eric Sembrat's Test Bonanza

A simple pendulum has two equilibrium points: hanging in the “down” position and perfectly inverted in the “up” position. While the “down” position is a stable equilibrium, the inverted position is definitely not stable. Any infinitesimal deviation from perfectly inverted is enough to cause the pendulum to eventually swing down.

It has been known for more than 100 years, though, that an inverted pendulum can be stabilized by vibrating the pivot point. This non-intuitive phenomenon is known as dynamic stabilization, and it has led to a broad range of applications including charged particle traps, mass spectrometers and high-energy particle accelerators.

Many-body quantum systems can also be placed into unstable non-equilibrium states, and like the inverted pendulum of classical physics, they typically evolve away from these states. Now, researchers at the Georgia Institute of Technology have demonstrated a way to maintain an unstable quantum system by applying bursts of microwave radiation – a quantum analog to vibrating the inverted pendulum.

In an experiment that could have implications for quantum computers and quantum simulators, the researchers used microwave pulses of varying amplitudes and frequencies to control a quantum system composed of a cloud of approximately 40,000 rubidium atoms cooled nearly to absolute zero.

The research, sponsored by the National Science Foundation and reported online August 27 by the journal Physical Review Letters, experimentally demonstrated dynamical stabilization of a non-equilibrium many-body quantum system. The paper is scheduled to appear in the journal's August 30 print issue.

“In this work, we have demonstrated that we can control the quantum dynamics of a cloud of atoms to maintain them in a non-equilibrium configuration analogous to the inverted pendulum,” said Michael Chapman, a professor in the Georgia Tech School of Physics. “What we actually control is the internal spins of the atoms that give each atom a small magnetic moment. The spins are oriented in an external magnetic field against their will such that they would prefer to flip their orientation to the equilibrium position.”

Mathematically, the state of the rubidium atoms is virtually identical to that of the simple mechanical pendulum, meaning that Chapman and his students have controlled what could be called a “quantum inverted pendulum.”

In their experiment, the researchers began with a spin-1 atomic Bose-Einstein condensate (BEC) that is initialized in an unstable, fixed point of the spin-nematic phase space – comparable to an inverted pendulum. If allowed to freely evolve, interactions between the atoms would give rise to squeezing, quantum spin mixing and eventually relaxation to a stable state – comparable to a pendulum hanging straight down from a pivot point.

By periodically applying bursts of microwave radiation, the researchers rotated the spin-nematic many-body fluctuations, halting the squeezing and the relaxation toward stability. The researchers investigated a range of pulse periods and phase shifts to map a stability diagram that compares well with what they expected theoretically.

“The net effect is that the many-body system basically returns to the original state,” said Chapman. “We reverse the squeezing of the condensate, and after it again evolves toward squeezing, we cause it to return. If we do this periodically, we can maintain the Bose-Einstein condensate in this unstable point indefinitely.”

The control technique differs from active feedback, which measures the direction in which a system is moving and applies a force counter to that direction. The open-loop technique used by Chapman’s group applies a constant input that doesn’t vary with the activity of the system being controlled.

“We are periodically kicking the system to keep it in a state where it doesn’t want to be,” he said. “This is the first time we have been able to make a many-body spin system that we can stabilize against its natural evolution.”

Controlling and manipulating single-particle quantum systems or simple collections of atoms, electrons and photons has been a focus of the physics community over recent decades. These capabilities have formed the foundation for technologies such as lasers, magnetic resonance imaging, atomic clocks and new atomic sensors for magnetic fields and inertial guidance.

Now, researchers are studying more complex systems that involve many additional interacting particles, perhaps thousands of them. Chapman and his group hope to help extend their knowledge of these more complex many-body systems, which could lead to developments in quantum computing, quantum simulations and improved measurements.

“The long-range goal of our work is to further the understanding of quantum mechanics and to develop new technologies that exploit the often counterintuitive realities of the quantum world,” Chapman said. “Quantum many-body systems are being actively explored, and one of the things you’d like to do is be able to control them. I think this is one of the cleanest examples of being able to control a quantum many-body system in a manifestly unstable configuration.”

In addition to Chapman, other co-authors of the paper include T.M. Hoang, C.S. Gerving, B.J. Land, M. Anquez and C.D. Hamley.

This research is supported by the National Science Foundation (NSF) under Award PHY-1208828. Any conclusions or opinions are those of the authors and do not necessarily represent the official views of the NSF.

CITATION: T.M. Hoang, et at., “Dynamic stabilization of a quantum many-body spin system, (Physical Review Letters, 2013). http://link.aps.org/doi/10.1103/PhysRevLett.111.090403

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Having videos available so that she could learn at her own pace — that’s what Theresa Sorrentino enjoyed most about her recent online class experience.

“The video lectures helped me to better understand the course material because I  could watch and pause each one whenever I needed to,” added Sorrentino, a third-year Biomedical Engineering student.

Sorrentino was one of 11 Georgia Tech students who made up an on-campus contingent of this summer’s Introductory Physics I massive open online course (MOOC). (There were a total of 17,000 students around the world enrolled in the course.) The on-campus students actually took the course through Tech and earned credit.

“This flipped classroom model allowed the Tech students to watch lectures and complete homework assignments online, which freed up class time to work on problems and do other activities together,” said Mike Schatz, the physics professor who led the MOOC.

For example, as part of the course, students were asked to complete five video labs where they recorded a moving object, analyzed it using software, and created a five-minute lab report to share with the class.

When on campus, students were able to do a dry run of their lab reports during the face-to-face time with Schatz, allowing for them to get feedback before uploading their final videos to YouTube.

“This exercise was valuable, because we were able to catch some wrong turns and help students improve along the way,” Schatz said.    

Sorrentino is quick to share that being part of the small cohort of on-campus students was a plus.

“There was good camaraderie among us,” she said. “Also, there was greater accountability. If you didn’t get your work done, it was easily noticed, which was a good incentive to keep up with the class.”   

Aside from the flipped model and the video labs, this course experimented with video white board illustrations as another way to teach the material. The illustrations cover everything from the differences between length and time measurements to friction.

“I thought they were great,” Sorrentino said. “I don’t know if it was because they were a novelty or if I am just a visual learner, but the video illustrations made it easier to understand the information being taught.”

The five- to 15-minute videos were primarily created by several undergraduate students, which allowed the students to become engaged in the teaching process, Schatz said.

From writing the script and creating the storyboard to editing the footage, each video took about eight to 10 hours to complete. The team is still producing videos, with the goal being to have a library of about 100.   

The MOOC will be offered again beginning Aug. 19, and will run for 16 weeks. Schatz’s approach to teaching the course will be similar but with a few changes.

One change will include more frequent testing. This summer, peer-evaluated lab reports, homework, and a final exam contributed to the students’ final grades. In the fall, there will be more frequent testing (weekly quizzes and a midterm) and less weight placed on the lab reports.

“Testing will be spread out, so students will know where they stand in the course, and we will be able to see if they’re grasping the material,” Schatz said.

Also, the number of on-campus students taking the course will increase to six sections of 30.
“We want to find out what it takes to successfully scale up the course to handle all the Tech students who may want to take the course,” Schatz said.

For more information, email Schatz.

Figuring out how to offer the lab component of a course has been a challenge for faculty members as they develop massive open online courses (MOOCs) — until now.

Georgia Tech’s Introductory Physics MOOC, which launched on May 20, is using video labs to simulate the experience students would typically have in the classroom. This topic has become the focus of one of seven mini innovation hubs that are researching questions related to MOOCs and online learning.  

“In some ways, the video labs provide students and instructors with a better experience than being in a traditional lab,” said Ed Greco, an instructor in the School of Physics and “champion” of the innovation hub that is examining the question of labs in MOOCs. “The videos single students out in a way that forces them to demonstrate their knowledge in a brief period of time, and it’s easier for instructors to hone in on who is getting the material and who isn’t.”  

There are 17,000 students enrolled in the MOOC, 11 of which are a part of a for-credit Georgia Tech version of the course where students have both online and on-campus experiences. (More details on the structure of this MOOC will be featured in a future Whistle article.)

All students are asked to complete five labs as part of the course, which will wrap up the last week of July. Each lab requires students to do the following:

• Record a moving object (using any device that will take video).
• Analyze the video using the free video analysis package, Tracker.
• Create models of motion using computer programs written in Python/VPython.
• Compare the observations to the models.
• Create a five-minute video lab report.
• Upload the video to YouTube.

Videos are then graded by fellow classmates based on a six-item rubric that includes questions such as “Does the author state the problem and show a result?” and “Is the video easy to follow?”

But, there have been a few challenges when it comes to the labs. For example, many of the students enrolled in the course live in countries that ban YouTube.

“Students living in places like Pakistan and China where they don’t have access to YouTube have been pretty frustrated with the lack of an alternative,” said Mike Schatz, the professor leading the MOOC. “So we’re going to have to think of a way to work around this with future versions of the course.”

Then there’s the issue of engagement. Schatz estimates that of the thousands of students enrolled, about 1,000 are actually regularly participating in some aspect of the course, whether it’s watching lectures, completing homework or quizzes, or participating in the online forum. But he estimates that only 300 to 400 are doing the labs.

Both Schatz and Greco agree that this is an issue that this hub will be considering as they tweak the course, which they hope to offer again in the fall.

“We’re still trying to figure out what we should expect of MOOC students when it comes to things like time spent on assignments and the money we should expect them to pay for a textbook,” Greco said. “Once we get a better handle on this, it will help us address the poor retention numbers that MOOCs typically have.”

To join the hub, email Greco. For more about the MOOC, email Schatz.

Using clouds of ultra-cold atoms and a pair of lasers operating at optical wavelengths, researchers have reached a quantum network milestone: entangling light with an optical atomic coherence composed of interacting atoms in two different states. The development could help pave the way for functional, multi-node quantum networks.

The research, done at the Georgia Institute of Technology, used a new type of optical trap that simultaneously confined both ground-state and highly-excited (Rydberg) atoms of the element rubidium. The large size of the Rydberg atoms – which have a radius of about one micron instead of a usual sub-nanometer size – gives them exaggerated electromagnetic properties and allows them to interact strongly with one another.

A single Rydberg atom can block the formation of additional Rydberg atoms within an ensemble of atoms, allowing scientists to create single photons on demand. Georgia Tech professor Alex Kuzmich and collaborators published a report on the Rydberg single-photon source in the journal Science in April 2012, and in a subsequent Nature Physics article, demonstrated for the first time many-body Rabi oscillations of an atomic ensemble.

In the new research, the state-insensitive trap allowed the researchers to increase the rate at which they could generate photons by a factor of 100 compared to their previous work.

“We want to allow photons to propagate to distant locations so we can develop scalable protocols to entangle more and more nodes,” said Kuzmich, a professor in Georgia Tech’s School of Physics. “If you can have coherence between the ground and Rydberg atoms, they can interact strongly while emitting light in a cooperative fashion. The combination of strong atomic interactions and collective light emissions results in entanglement between atoms and light. We think that this approach is quite promising for quantum networking.”

The research was reported June 19 in the early edition of the journal Nature. The research has been supported by the Atomic Physics Program and the Quantum Memories Multidisciplinary University Research Initiative (MURI) of the Air Force Office of Scientific Research, and by the National Science Foundation.

Generating, distributing and controlling entanglement across quantum networks are the primary goals of quantum information science being pursued at research laboratories around the world. In earlier work, ground states of single atoms or atomic ensembles have been entangled with spontaneously-emitted light, but the production of those photons has been through a probabilistic approach – which generated photons infrequently.

This spontaneous emission process requires a relatively long time to create entanglement and limits the potential quantum network to just two nodes. To expand the potential for multi-mode networks, researchers have explored other approaches, including entanglement between light fields and atoms in quantum superpositions of the ground and highly-excited Rydberg electronic states. This latter approach allows the deterministic generation of photons that produces entanglement at a much higher rate.

However, until now, Rydberg atoms could not be excited to that state while confined to optical traps, so the traps had to be turned off for that step. That allowed the confined atoms to escape, preventing realization of atom-light entanglement.

Based on a suggestion from MURI colleagues at the University of Wisconsin, the Georgia Tech team developed a solution to that problem: a state-insensitive optical trap able to confine both ground-state and Rydberg atoms coherently. In this trap, atoms persist for as much as 80 milliseconds while being excited into the Rydberg state – and the researchers believe that can be extended with additional improvements. However, even the current atomic confinement time would be enough to operate complex protocols that might be part of a quantum network.

“The system we have realized is closer to being a node in a quantum network than what we have been able to do before,” said Kuzmich. “It is certainly a promising improvement.”

Key to the improved system is operation of an optical trap at wavelengths of 1,004 and 1,012 nanometers, so-called “magic” wavelengths tuned to both the Rydberg atoms and the ground state atoms, noted Lin Li, a graduate student in the Kuzmich Laboratory.

“We have experimentally demonstrated that in such a trap, the quantum coherence can be well preserved for a few microseconds and that we can confine atoms for as long as 80 milliseconds,” Li said. “There are ways that we can improve this, but with the help of this state-insensitive trap, we have achieved entanglement between light and the Rydberg excitation.”

The rate of generating entangled photons increased from a few photons per second with the earlier approaches to as many as 5,000 photons per second with the new technique, Kuzmich said. That will allow the researchers to pursue future research goals – such as demonstration of quantum gates – as they optimize their technique.

Experimentally, the research worked as follows: (1) an ultra-cold gas of rubidium atoms was confined in a one-dimensional optical lattice using lasers operating at 1,004-nanometer and 1,012-nanometer wavelengths. The atomic ensemble was driven from the collective ground state into a single excited state; (2) By applying a laser field, an entangled state was generated. The retrieved field was mixed with the coherent field using polarizing beam-splitters, followed by measurement at single-photon detectors; (3) The remaining spin wave was mapped into a field by a laser field.

According to Kuzmich, the success demonstrates the value of collaboration through the MURI supported by the Air Force Office of Scientific Research, which in 2012 awarded $8.5 million to a consortium of seven U.S. universities that are working together to determine the best approach for creating quantum memories based on the interaction between light and matter.

Through the MURI, a team of universities is considering three different approaches for creating entangled quantum memories that could facilitate long-distance transmission of secure information. Among the collaborators in the five-year program are Mark Saffman and Thad Walker at the University of Wisconsin, Mikhail Lukin of Harvard, and Luming Duan of the University of Michigan, who at the beginning of this century made pioneering proposals which formed the basis of the approach that Kuzmich, Li and colleague Yaroslav Dudin used to create the entanglement between light and the Rydberg excitation.

This research was supported by the Air Force Office of Scientific Research (AFOSR) under contract FA9550-12-1-0025 and the by National Science Foundation under award PHY-1105994. The conclusions and opinions expressed in this article are those of the principal investigator and do not necessarily reflect the official views of the AFOSR or the NSF.

CITATION: Lin Li, Yaroslav Dudin and Alexander Kuzmich, “Entanglement between light and an optical atomic excitation,” (Nature 2013). http://dx.doi.org/10.1038/nature12227

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What do swimmers like trout, eels and sandfish lizards have in common? According to a new study, the similar timing patterns that these animals use to contract their muscles and produce undulatory swimming motions can be explained using a simple model. Scientists have now applied the new model to understand the connection between electrical signals and body movement in the sandfish.

Most swimming creatures rely on an undulating pattern of body movement to propel themselves through fluids. Though differences in body flexibility may lead to different swimming styles, scientists have found “neuromechanical phase lags” in nearly all swimmers. These lags are characterized by a wave of muscle activation that travels faster down the body than the wave of body curvature.

A study of the sandfish lizard – which “swims” through sand – led to development of the new model, which researchers believe could also be used to study other swimming animals. Beyond assisting the study of locomotion in a wide range of animals, the findings could also help researchers design efficient swimming robots.

“A graduate student in our group, Yang Ding, who is now at the University of Southern California, was able to develop a theory that could explain the kinematics of how this animal swims as well as the timing of the nervous system control signals,” said Daniel Goldman, an associate professor in the School of Physics at the Georgia Institute of Technology. “For animals swimming in fluids using an undulating movement, there are basic physical constraints on how they must activate their muscles. We think we have uncovered an important mechanism that governs this kind of swimming.”

The research was reported June 3 in the early edition of the journal Proceedings of the National Academy of Sciences. It was sponsored by the National Science Foundation’s Physics of Living Systems program, the Micro Autonomous Systems and Technology (MAST) program of the Army Research Office, and the Burroughs Wellcome Fund.

Undulatory locomotion is a gait in which thrust is produced in the opposite direction from a traveling wave of body bending. Because it is so commonly used by animals, this mode of locomotion has been widely used for studying the neuromechanical principles of movement.

Sarah Sharpe, the paper’s second author and a graduate student in Georgia Tech’s Interdisciplinary Bioengineering Program, led laboratory experiments studying undulatory swimming in sandfish lizards. She used X-ray imaging to visualize how the animals swam through sand that was composed of tiny glass spheres.

At the same time their swimming movements were being tracked, a set of four hair-thin electrodes implanted in the lizards’ bodies were providing information on when their muscles were activated. The two information sources allowed the researchers to compare the electrical muscle activity to the lizards’ body motion.

“The lizards propagate a wave of muscle activations, contracting the muscles close to their heads first, then the muscles at the midpoint of their body, then their tail,” said Sharpe. “They send a wave of muscle of contraction down their bodies, which creates a wave of curvature that allows them to swim. This wave of activation travels faster than the wave of curvature down the body, resulting in different timing relationships, known as phase differences, between muscle contracts and bending along the body.”

Sand acts like a frictional fluid as the sandfish swims through it. However, a sandfish swimming through sand is simpler to model than a fish swimming through water because the sand lacks the vortices and other complex behavior of water – and the friction of the sand eliminates inertia.

“Theoretically, it is difficult to calculate all of the forces acting on a fish or an eel swimming in a real fluid,” said Goldman. “But for a sandfish, you can calculate pretty much everything.”
The relative simplicity of the system allowed the research team – which also included Georgia Tech professor Kurt Wiesenfeld – to develop a simple model showing how the muscle activation relates to motion. The model showed that combining synchronized torques from distant points in the lizards’ bodies with local traveling torques is what creates the neuromechanical phase lag.

“This is one of the simplest, if not the simplest, models of swimming that reproduces the neuromechanical phase lag phenomenon,” Sharpe said. “All we really had to pay attention to was the external forces acting on an animal’s body. We realized that this timing relationship would emerge for any undulatory animal with distributed forces along its body. Understanding this concept can be used as the foundation to begin understanding timing patterns in all other swimmers.”

The sandfish swims using a simple single-period sinusoidal wave with constant amplitude. A key finding that facilitated the model’s development was that the sandfish’s body is extremely flexible, allowing internal forces – body stiffness – to be ignored.

“This animal turns out to be like a little limp noodle,” said Goldman. “Having that result in the theory makes everything else pop out.”

The model shows that the waveform used by the sandfish should allow it to swim the farthest with the least expenditure of energy. Swimming robots adopting the same waveform should therefore be able to maximize their range.

Goldman and his colleagues have been studying the sandfish, a native of the northern African desert, for more than six years.

“Sandfish are among the champions of all sand diggers, swimmers and burrowers,” said Goldman. “This lizard has provided us with an interesting entry point into swimming because its environment is surprisingly simple and behavior is simple. It turns out that this little sand-dweller may be able to tell us things about swimming more generally.”

This research has been supported by the National Science Foundation Physics of Living Systems (PoLS) under grants PHY-0749991 and PHY-1150760, by the U.S. Army Research Laboratory’s (ARL) Micro Autonomous Systems and Technology (MAST) Program under cooperative agreement W911NF-11-1-0514, and by the Burroughs Wellcome Fund Career Award. Any conclusions are those of the authors and do not necessarily represent the official views of the NSF or ARL.

CITATION: Yang Ding, Sarah Sharpe, Kurt Wiesenfeld and Daniel Goldman, “Emergence of the advancing neuromechanical phase in resistive force dominated medium,” (Proceedings of the National Academy of Sciences, 2013).

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A fried breakfast food popular in Spain provided the inspiration for the development of doughnut-shaped droplets that may provide scientists with a new approach for studying fundamental issues in physics, mathematics and materials.

The doughnut-shaped droplets, a shape known as toroidal, are formed from two dissimilar liquids using a simple rotating stage and an injection needle. About a millimeter in overall size, the droplets are produced individually, their shapes maintained by a surrounding springy material made of polymers. Droplets in this toroidal shape made of a liquid crystal – the same type of material used in laptop displays – may have properties very different from those of spherical droplets made from the same material.

While researchers at the Georgia Institute of Technology don’t have a specific application for the doughnut-shaped droplets yet, they believe the novel structures offer opportunities to study many interesting problems, from looking at the properties of ordered materials within these confined spaces to studying how geometry affects how cells behave.

“Our experiments provide a fresh approach to the way that people have been looking at these kinds of problems, which is mainly theoretical. We are doing experiments with toroids whose geometry can be precisely controlled in the lab,” said Alberto Fernandez-Nieves, an assistant professor in the Georgia Tech School of Physics. “This work opens up a new way to experimentally look at problems that nobody has been able to study before. The properties of toroidal surfaces are very different, from a general point of view, from those of spherical surfaces.”

Development of these “stable nematic droplets with handles” was described May 20 in the early edition of the journal Proceedings of the National Academy of Sciences (PNAS). The research has been sponsored by the National Science Foundation (NSF), and also involves researchers at the Lorentz Institute for Theoretical Physics at Leiden University in The Netherlands and at York University in the United Kingdom.

Droplets normally form spherical shapes to minimize the surface area required to contain a given volume of liquid. Though they appear to be simple, when an ordered material like a crystal or a liquid crystal lives on the surface of a sphere, it provides interesting challenges to mathematicians and theoretical physicists.

A physicist who focuses on soft condensed matter, Fernandez-Nieves had long been interested in the theoretical aspects of curved surfaces. Working with graduate research assistant Ekapop Pairam and postdoctoral fellow Jayalakshmi Vallamkondu, he wanted to extend the theoretical studies into the experimental world for a system of toroidal shapes.

But could doughnut-shaped droplets be made in the lab?

The partial answer came from churros Fernandez-Nieves ate as a child growing up in Spain. These “Spanish doughnuts” – actually spirals – are made by injecting dough into hot oil while the dough is spun and fried.

In the lab at a much smaller size scale, the researchers found they could use a similar process with two immiscible liquids such as glycerine or water and oil, a needle and a magnetically-controlled rotating stage. A droplet of glycerine is injected into the rotating stage containing the oil. In certain conditions, a jet forms at the needle, which closes up into a torus because of the imposed rotation.

“You can control the two relevant curvatures of the torus,” explained Fernandez-Nieves. “You can control how large it is because you can move the needle with respect to the rotation axis. You can also infuse more volume to make the torus thicker.”

If the stage is then turned off, however, the drop of glycerine quickly loses its doughnut shape as surface tension forces it to become a traditional spherical droplet. To maintain the toroidal shape, Fernandez-Nieves and his collaborators replace the surrounding oil with a springy polymeric material; the springy character of this material provides a force that can overcome surface tension forces.

“When you are making the toroid, the forces on the needle are large enough that the surrounding material behaves as a fluid,” he explained.  “Once you stop, the elasticity of the outside fluid overcomes surface tension and that freezes the structure in place.”

The researchers have been using the doughnut shapes to study how liquid crystal materials, which are well known for their applications in laptop displays, organize inside the torus. These materials have degrees of order beyond those of simple liquids such as water. For these materials, the toroidal shape provides a new set of study opportunities from both theoretical and experimental perspectives.

“This changes how you think about a liquid inside a container,” said Fernandez-Nieves. “The materials will still adopt the shape of the container, but its energy will be different depending on the shape. The materials feel distortions and will try to minimize them. In a given shape, the molecules in these materials will rearrange themselves to minimize these distortions.”

Among the surprises is that the nematic droplets created with toroidal shapes become chiral, that is, they adopt a certain twisting direction and break their mirror symmetry.

“In our case, the materials we are using are not chiral under normal circumstances,” he noted. “This was a surprise to us, and it has to do with how we are confining the molecules.”

Beyond looking at the dynamics of creating the droplets and how ordered materials behave when the torus transforms into a sphere, Fernandez-Nieves and colleagues are also exploring potential biological applications, applying electrical fields to the droplets, and sharing the unique structures with scientists at other institutions.

“This is the first time that stable nematic droplets have been generated with handles, and we have exploited that to look at the nematic organization inside those spaces,” said Fernandez-Nieves. “Our experiments open up a versatile new approach for generating handled droplets made of an ordered material that can self-assemble into interesting and unexpected structures when confined to these non-spherical spaces. Now that theoreticians realize we can generate and study these systems, there may be much more development in this area.”

In addition to those already mentioned, the paper’s authors included V. Koning, B.C. van Zuiden and V. Vitelli from Leiden University, M.A. Bates from the University of York in the United Kingdom, and P.W. Ellis from Georgia Tech.

The research described here has been sponsored by the National Science Foundation under CAREER award DMR-0847304. The findings and conclusions are those of the authors and do not necessarily represent the official views of the National Science Foundation.

CITATION: E. Pairam, et al., “Stable nematic droplets with handles,” (Proceedings of the National Academy of Sciences, 2013)

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Future teams of subterranean search and rescue robots may owe their success to the lowly fire ant, a much despised insect whose painful bites and extensive networks of underground tunnels are all-too-familiar to people living in the southern United States.

• Watch a YouTube video of this project.

By studying fire ants in the laboratory using video tracking equipment and X-ray computed tomography, researchers have uncovered fundamental principles of locomotion that robot teams could one day use to travel quickly and easily through underground tunnels. Among the principles is building tunnel environments that assist in moving around by limiting slips and falls, and by reducing the need for complex neural processing.

Among the study’s surprises was the first observation that ants in confined spaces use their antennae for locomotion as well as for sensing the environment.

“Our hypothesis is that the ants are creating their environment in just the right way to allow them to move up and down rapidly with a minimal amount of neural control,” said Daniel Goldman, an associate professor in the School of Physics at the Georgia Institute of Technology, and one of the paper’s co-authors. “The environment allows the ants to make missteps and not suffer for them. These ants can teach us some remarkably effective tricks for maneuvering in subterranean environments.”

The research was reported May 20 in the early edition of the journal Proceedings of the National Academy of Sciences. The work was sponsored by the National Science Foundation’s Physics of Living Systems program.

In a series of studies carried out by graduate research assistant Nick Gravish, groups of fire ants (Solenopsis invicta) were placed into tubes of soil and allowed to dig tunnels for 20 hours. To simulate a range of environmental conditions, Gravish and postdoctoral fellow Daria Monaenkova varied the size of the soil particles from 50 microns on up to 600 microns, and also altered the moisture content from 1 to 20 percent.

While the variations in particle size and moisture content did produce changes in the volume of tunnels produced and the depth that the ants dug, the diameters of the tunnels remained constant – and comparable to the length of the creatures’ own bodies: about 3.5 millimeters.

“Independent of whether the soil particles were as large as the animals’ heads or whether they were fine powder, or whether the soil was damp or contained very little moisture, the tunnel size was always the same within a tight range,” said Goldman. “The size of the tunnels appears to be a design principle used by the ants, something that they were controlling for.”

Gravish believes such a scaling effect allows the ants to make best use of their antennae, limbs and body to rapidly ascend and descend in the tunnels by interacting with the walls and limiting the range of possible missteps.

“In these subterranean environments where their leg motions are certainly hindered, we see that the speeds at which these ants can run are the same,” he said. “The tunnel size seems to have little, if any, effect on locomotion as defined by speed.”

The researchers used X-ray computed tomography to study tunnels the ants built in the test chambers, gathering 168 observations. They also used video tracking equipment to collect data on ants moving through tunnels made between two clear plates – much like “ant farms” sold for children – and through a maze of glass tubes of differing diameters.

The maze was mounted on an air piston that was periodically fired, dropping the maze with a force of as much as 27 times that of gravity. The sudden movement caused about half of the ants in the tubes to lose their footing and begin to fall. That led to one of the study’s most surprising findings: the creatures used their antennae to help grab onto the tube walls as they fell.

“A lot of us who have studied social insects for a long time have never seen antennae used in that way,” said Michael Goodisman, a professor in the Georgia Tech School of Biology and one of the paper’s other co-authors. “It’s incredible that they catch themselves with their antennae. This is an adaptive behavior that we never would have expected.”

By analyzing ants falling in the glass tubes, the researchers determined that the tube diameter played a key role in whether the animals could arrest their fall.

In future studies, the researchers plan to explore how the ants excavate their tunnel networks, which involves moving massive amounts of soil. That soil is the source of the large mounds for which fire ants are known.

While the research focused on understanding the principles behind how ants move in confined spaces, the results could have implications for future teams of small robots.

“The problems that the ants face are the same kinds of problems that a digging robot working in a confined space would potentially face – the need for rapid movement, stability and safety – all with limited sensing and brain power,” said Goodisman. “If we want to build machines that dig, we can build in controls like these ants have.”  

Why use fire ants for studying underground locomotion?

“These animals dig virtually non-stop, and they are good, repeatable study subjects,” Goodisman explained. “And they are very convenient for us to study. We can go outside the laboratory door and collect them virtually anywhere.”

The research described here has been sponsored by the National Science Foundation (NSF) under grant POLS 095765, and by the Burroughs Wellcome Fund. The findings and conclusions are those of the authors and do not necessarily represent the official views of the NSF.

CITATION: Nick Gravish, et al., “Climbing, falling and jamming during ant locomotion in confined environments,” (Proceedings of the National Academy of Sciences, 2013).

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For sea turtle hatchlings struggling to reach the ocean, success may depend on having flexible wrists that allow them to move without disturbing too much sand. A similar wrist also helps a robot known as “FlipperBot” move through a test bed, demonstrating how animals and bio-inspired robots can together provide new information on the principles governing locomotion on granular surfaces.

Both the baby turtles and FlipperBot run into trouble under the same conditions: traversing granular media disturbed by previous steps. Information from the robot research helped scientists understand why some of the hatchlings they studied experienced trouble, creating a unique feedback loop from animal to robot – and back to animal.

The research could help robot designers better understand locomotion on complex surfaces and lead biologists to a clearer picture of how sea turtles and other animals like mudskippers use their flippers. The research could also help explain how animals evolved limbs – including flippers – for walking on land.

The research was published April 24 in the journal Bioinspiration & Biomimetics. The work was supported by the National Science Foundation, the U.S. Army Research Laboratory’s Micro Autonomous Systems and Technology (MAST) Program, the U.S. Army Research Office, and the Burroughs Wellcome Fund.

“We are looking at different ways that robots can move about on sand,” said Daniel Goldman, an associate professor in the School of Physics at the Georgia Institute of Technology. “We wanted to make a systematic study of what makes flippers useful or effective. We’ve learned that the flow of the materials plays a large role in the strategy that can be used by either animals or robots.”

The research began in 2010 with a six-week study of hatchling loggerhead sea turtles emerging at night from nests on Jekyll Island, one of Georgia’s coastal islands. The research was done in collaboration with the Georgia Sea Turtle Center.

Nicole Mazouchova, then a graduate student in the Georgia Tech School of Biology, studied the baby turtles using a trackway filled with beach sand and housed in a truck parked near the beach. She recorded kinematic and biomechanical data as the turtles moved in darkness toward an LED light that simulated the moon.

Mazouchova and Goldman studied data from the 25 hatchlings, and were surprised to learn that they managed to maintain their speed regardless of the surface on which they were running.

“On soft sand, the animals move their limbs in such a way that they don’t create a yielding of the material on which they’re walking,” said Goldman. “That means the material doesn’t flow around the limbs and they don’t slip. The surprising thing to us was that the turtles had comparable performance when they were running on hard ground or soft sand.”

The key to maintaining performance seemed to be the ability of the hatchlings to control their wrists, allowing them to change how they used their flippers under different sand conditions.

“On hard ground, their wrists locked in place, and they pivoted about a fixed arm,” Goldman explained. “On soft sand, they put their flippers into the sand and the wrist would bend as they moved forward. We decided to investigate this using a robot model.”

That led to development of FlipperBot, with assistance from Paul Umbanhowar, a research associate professor at Northwestern University. The robot measures about 19 centimeters in length, weighs about 970 grams, and has two flippers driven by servo-motors. Like the turtles, the robot has flexible wrists that allow variations in its movement. To move through a track bed filled with poppy seeds that simulate sand, the robot lifts its flippers up, drops them into the seeds, then moves the flippers backward to propel itself.

Mazouchova, now a Ph.D. student at Temple University, studied many variations of gait and wrist position and found that the free-moving mechanical wrist also provided an advantage to the robot.

“In the robot, the free wrist does provide some advantage,” said Goldman. “For the most part, the wrist confers advantage for moving forward without slipping. The wrist flexibility minimizes material yielding, which disturbs less ground. The flexible wrist also allows both the robot and turtles to maintain a high angle of attack for their bodies, which reduces performance-impeding drag from belly friction.”

The researchers also noted that the robot often failed when limbs encountered material that the same limbs had already disturbed. That led them to re-examine the data collected on the hatchling turtles, some of which had also experienced difficulty walking across the soft sand.

“When we saw the turtles moving poorly, they appeared to be suffering from the same failure mode that we saw in the robot,” Goldman explained. “When they interacted with materials that had been previously disturbed, they tended to lose performance.”

Mazouchova and Goldman then worked with Umbanhowar to model the robot’s performance in an effort to predict how the turtle hatchlings should respond to different conditions. The predictions closely matched what was actually observed, closing the loop between robot and animal.

“The robot study allowed us to test how principles applied to the animals,” Goldman said.

While the results may not directly improve robot designs, what the researchers learned should contribute to a better understanding of the principles governing movement using flippers. That would be useful to the designers of robots that must swim through water and walk on land.

“A multi-modal robot might need to use paddles for swimming in water, but it might also need to walk in an effective way on the beach,” Goldman said. “This work can provide fundamental information on what makes flippers good or bad. This information could give robot designers clues to appendage designs and control techniques for robots moving in these environments.”

The research could ultimately provide clues to how turtles evolved to walk on land with appendages designed for swimming.

“To understand the mechanics of how the first terrestrial animals moved, you have to understand how their flipper-like limbs interacted with complex, yielding substrates like mud flats,” said Goldman. “We don’t have solid results on the evolutionary questions yet, but this certainly points to a way that we could address these issues.”

This research has been supported by the National Science Foundation under grant CMMI-0825480 and the Physics of Living Systems PoLS program, the U.S. Army Research Laboratory’s (ARL) Micro Autonomous Systems and Technology (MAST) Program under cooperative agreement W911NF-08-2-0004, the U.S. Army Research Office (ARO) and the Burroughs Wellcome Fund Career Award. Any conclusions are those of the authors and do not necessarily represent the official views of the NSF, ARL or ARO.

CITATION: Nicole Mazouchova, Paul B. Umbanhowar and Daniel I. Goldman, “Flipper-driven terrestrial locomotion of a sea turtle-inspired robot, (Bioinspiration & Biomimetics, 2013).

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Video available - http://www.youtube.com/watch?v=CkHA3tL4z5U

H. Fukuyo Outstanding Physics Undergraduate Award to Benjamin Land and Rita Garrido:
Award of $2,500 given to the most outstanding undergraduate academic student(s) in
the School of Physics.

H. Fukuyo Memorial Scholarship Award in Physics to Alan Pryor and Greg Douthit:
This award consisting of $2,500.

The Joyce M. and Glenn A. Burdick Award to Edward Dannemiller:
Recognizes rising seniors in the School of Physics who demonstrate scholastic
achievement and leadership, and possess characteristics that embody the mission
of Georgia Tech. The award consists of $500.

Current SoP undergraduate student Benjamin Land is a recipient of the Robert A. Pierotti Memorial Scholarship and the H. Fukuyo Outstanding Physics Undergraduate Award. Benjamin Land is a graduating Physics major and works under the supervision of Prof. Michael Chapman. He will be pursuing a Ph.D in Physics at the University of California, Berkeley in the Fall. Join us in wishing him congratulations for this great accomplishment.