New Gut Bacterium Discovered in Termite's Digestion of Wood

Caltech researchers find new species of microbe responsible for acetogenesis, an important process in termite nutrition.

When termites munch on wood, the small bits are delivered to feed a community of unique microbes living in their guts, and in a complex process involving multiple steps, these microbes turn the hard, fibrous material into a nutritious meal for the termite host. One key step uses hydrogen to convert carbon dioxide into organic carbon—a process called acetogenesis—but little is known about which gut bacteria play specific roles in the process. Utilizing a variety of experimental techniques, researchers from the California Institute of Technology (Caltech) have now discovered a previously unidentified bacterium—living on the surface of a larger microorganism in the termite gut—that may be responsible for most gut acetogenesis.

"In the termite gut, you have several hundred different species of microbes that live within a millimeter of one another. We know certain microbes are present in the gut, and we know microbes are responsible for certain functions, but until now, we didn't have a good way of knowing which microbes are doing what," says Jared Leadbetter, professor of environmental microbiology at Caltech, in whose laboratory much of the research was performed. He is also an author of a paper about the work published the week of September 16 in the online issue of the Proceedings of the National Academy of Sciences (PNAS).

Acetogenesis is the production of acetate (a source of nutrition for termites) from the carbon dioxide and hydrogen generated by gut protozoa as they break down decaying wood. In their study of "who is doing what and where," Leadbetter and his colleagues searched the entire pool of termite gut microbes to identify specific genes from organisms responsible for acetogenesis.

The researchers began by sifting through the microbes' RNA—genetic information that can provide a snapshot of the genes active at a certain point in time. Using RNA from the total pool of termite gut microbes, they searched for actively transcribed formate dehydrogenase (FDH) genes, known to encode a protein necessary for acetogenesis. Next, using a method called multiplex microfluidic digital polymerase chain reaction (digital PCR), the researchers sequestered the previously unstudied individual microbes into tiny compartments to identify the actual microbial species carrying each of the FDH genes. Some of the FDH genes were found in types of bacteria known as spirochetes—a previously predicted source of acetogenesis. Yet it appeared that these spirochetes alone could not account for all of the acetate produced in the termite gut.

Initially, the Caltech researchers were unable to identify the microorganism expressing the single most active FDH gene in the gut. However, the first authors on the study, Adam Rosenthal, a postdoctoral scholar in biology at Caltech, and Xinning Zhang (PhD '10, Environmental Science and Engineering), noticed that this gene was more abundant in the portion of the gut extract containing wood chunks and larger microbes, like protozoans. After analyzing the chunkier gut extract, they discovered that the single most active FDH gene was encoded by a previously unstudied species from a group of microbes known as the deltaproteobacteria. This was the first evidence that a substantial amount of acetate in the gut may be produced by a non-spirochete.

Because the genes from this deltaproteobacterium were found in the chunky particulate matter of the termite gut, the researchers thought that perhaps the newly identified microbe attaches to the surface of one of the chunks. To test this hypothesis, the researchers used a color-coded visualization method called hybridization chain reaction-fluorescent in situ hybridization, or HCR-FISH.

The technique—developed in the laboratory of Niles Pierce, professor of applied and computational mathematics and bioengineering at Caltech, and a coauthor on the PNAS study—allowed the researchers to simultaneously "paint" cells expressing both the active FDH gene and a gene identifying the deltoproteobacterium with different fluorescent colors simultaneously. "The microfluidics experiment suggested that the two colors should be expressed in the same location and in the same tiny cell," Leadbetter says. And, indeed, they were. "Through this approach, we were able to actually see where the new deltaproteobacterium resided. As it turns out, the cells live on the surface of a very particular hydrogen-producing protozoan."

This association between the two organisms makes sense based on what is known about the complex food web of the termite gut, Leadbetter says. "Here you have a large eukaryotic single cell—a protozoan—which is making hydrogen as it degrades wood, and you have these much smaller hydrogen-consuming deltaproteobacteria attached to its surface," he says. "So, this new acetogenic bacterium is snuggled up to its source of hydrogen just as close as it can get."

This intimate relationship, Leadbetter says, might never have been discovered relying on phylogenetic inference—the standard method for matching a function to a specific organism. "Using phylogenetic inference, we say, 'We know a lot about this hypothetical organism's relatives, so without ever seeing the organism, we're going to make guesses about who it is related to," he says. "But with the techniques in this study, we found that our initial prediction was wrong. Importantly, we have been able to determine the specific organism responsible and a location of the mystery organism, both of which appear to be extremely important in the consumption of hydrogen and turning it into a product the insect can use." These results not only identify a new source for acetogenesis in the termite gut—they also reveal the limitations of making predictions based exclusively on phylogenetic relationships.

Other Caltech coauthors on the paper titled "Localizing transcripts to single cells suggests an important role of uncultured deltaproteobacteria in the termite gut hydrogen economy," are graduate student Kaitlyn S. Lucey (environmental science and engineering), Elizabeth A. Ottesen (PhD '08, biology), graduate student Vikas Trivedi (bioengineering), and research scientist Harry M. T. Choi (PhD '10, bioengineering). This work was funded by the U.S. Department of Energy, the National Science Foundation, the National Institutes of Health, the Programmable Molecular Technology Center within the Beckman Institute at Caltech, a Donna and Benjamin M. Rosen Center Bioengineering scholarship, and the Center for Environmental Microbial Interactions at Caltech.

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Team Led by Caltech Wins Second $10 Million Award for Research in Molecular Programming

During the past century, programmable technologies evolved from spinning gears and vacuum tubes to transistors and microchips. Now, a group of Caltech researchers and their colleagues at the University of Washington, Harvard University, and UC San Francisco are exploring how biologically important molecules—like DNA, RNA, and proteins—could be the next generation of programmable devices.

Erik Winfree, professor of computer science, computation, and neural systems, and bioengineering, along with collaborators at Caltech and the University of Washington, began the Molecular Programming Project (MPP) in 2008, as part of an NSF Expeditions in Computing award to develop practices for programming biomolecules—much like a computer code—to perform designated functions. Over the past five years, the researchers have programmed DNA to carry out a number of tasks, from solving basic math and pattern-recognition problems to more mechanical tasks like programming RNA and DNA to selectively amplify fluorescent signals for biological microscopy. Through these initial experiments, the researchers have shown that it is possible to systematically encode specialized tasks within DNA molecules.

"Computer science gave us this idea that many tasks can actually be done with different types of devices," Winfree says. For example, a 19th-century cash register and a 21st-century computer can both be used to calculate sums, though they perform the same task very differently. At first glance, writing a computer program and programming a DNA molecule may seem like very different endeavors, but "each one provides a systematic way of implementing automated behaviors, and they are both based on similar principles of information technology," Winfree says.

Expanding the team to include five additional faculty who bring expertise in structural and dynamic DNA nanotechnology, synthetic biology, computer-aided design, programming languages, and compilers, Winfree and his colleagues recently received a second Expeditions in Computing award to take their work in molecular programming to the next level: from proof-of-principle demonstrations to putting the technology in the hands of users in biology, chemistry, physics, and materials science.

The researchers aim to use molecular programming to establish general-purpose, reliable, and easy-to-use methods for engineering complex nanoscale systems from biomolecules. In the hands of users, these methods could be used to create novel self-assembling electronic and optical devices, powerful nanoscale tools for the study of biology, and programmable molecular circuits for the diagnosis and treatment of disease. In one application, the researchers hope to program DNA molecules to carry out recognition and logical circuitry for exquisitely targeted drug delivery, thus reducing drug side effects and increasing efficacy.

Today, the largest synthetic molecular programs—human-designed sequences of the A, T, C, and G bases that make up DNA—contain on the order of 60,000 bases. "That's comparable to the amount of RAM memory in my first computer, a 1983 Apple II+," says Winfree. Designed systems in the future will only become more complex, a challenge that MPP researchers aim to tackle by approaching biological systems with something computer scientists call the abstraction hierarchy.

"In some sense computer science is the art of managing complexity, because you design things that have billions of components, but a single person simply cannot understand all the details and interactions," he says. "Abstraction is a way of hiding a component's details while making it easy to incorporate into higher-order components—which, themselves, can also be abstracted. For example, you don't need to know the details of a multiplication circuit in order to use it to make a circuit for factoring." In the molecular world, the task might be different—like transporting a molecular cargo to a designated location—but abstraction is still essential for combining simpler systems into larger ones to perform tasks of greater complexity.

"Over the next several decades, the MPP seeks to develop the principles and practice for a new engineering discipline that will enable the function of molecules to be programmed with the ease and rigor that computers are today, while achieving the sophistication, complexity, and robustness evident in the programmable DNA, RNA, and protein machinery of biology," says Niles Pierce, professor of applied and computational mathematics and bioengineering at Caltech and member of the MPP.

To integrate these fields, the MPP has brought together an interdisciplinary team of computer scientists, chemists, electrical engineers, physicists, roboticists, mathematicians, and bioengineers—all of whom have a strong research interest in the intersection of information, biology, and the molecular world. The team will explore the potential of molecular programming from many perspectives.

"Because of the diverse expertise that is required to work on these challenges, the participating students and faculty come from an unusual array of fields," Pierce says. "It's a lot of fun to be in a room with this group of people to see where the discussions lead."

The 2013 Expeditions award was granted for the proposal "Molecular Programming Architectures, Abstractions, Algorithms, and Applications." Winfree and Pierce are joined on the project by four other collaborators at Caltech: Jehoshua (Shuki) Bruck, Gordon and Betty Moore Professor of Computation and Neural Systems and Electrical Engineering; Richard Murray, Thomas E. and Doris Everhart Professor of Control and Dynamical Systems and Bioengineering; Lulu Qian, assistant professor of bioengineering; and Paul Rothemund, senior research associate in bioengineering, computing and mathematical sciences, and computation and neural systems. Other collaborators include Eric Klavins and Georg Seelig from the University of Washington, Peng Yin and William Shih from Harvard, and Shawn Douglas from UC San Francisco.

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Caltech to Offer Online Courses through edX

To expand its involvement in online learning, the California Institute of Technology will offer courses through the online education platform edX beginning this October.

The edX course platform is an online learning initiative launched in 2012 by founding partners Harvard University and the Massachusetts Institute of Technology (MIT). Caltech's rigorous online course offerings will join those of 28 other prestigious colleges and universities in the edX platform's "xConsortium."

This new partnership with edX comes one year after Caltech offered three courses through the online learning platform Coursera in fall 2012. The Institute will now offer courses through both platforms.

"Coursera and edX have some foundational differences which are of interest to the faculty," says Cassandra Horii, director of teaching and learning programs at Caltech. Both organizations offer their courses at no cost to participating students; edX, however, operates as a nonprofit and plans to partner with only a small number of institutions, whereas Coursera—a for-profit, self-described "social entrepreneurship company"—partners with many institutions and state university systems.

The two platforms also emphasize different learning strategies, says Horii. "Coursera has a strong organizational principle built around lectures, so a lot of the interactivity is tied right into the video," she says. Though edX still enables the use of video lectures, a student can customize when he or she would like to take quizzes and use learning resources. In addition, edX allows faculty to embed a variety of learning materials—like textbook chapters, discussions, diagrams, and tables—directly into the platform's layout.

In the future, data collected from both platforms could provide valuable information about how students best learn certain material, especially in the sciences. "Caltech occupies this advanced, really rigorous scientific education space, and in general our interest in these online courses is to maintain that rigor and quality," Horii says. "So, with these learning data, we have some potential contributions to make to the general understanding of learning in this niche that we occupy."

Even before joining edX and Coursera, Caltech had already become an example in the growing trend of Massive Open Online Courses (MOOCs). Yaser Abu-Mostafa, professor of electrical engineering and computer science, developed his own MOOC on machine learning, called "Learning from Data," and offered it on YouTube and iTunes U beginning in April 2012.

Since its debut, Abu-Mostafa's MOOC has reached more than 200,000 participants, and it received mention in the NMC Horizon Report: 2013 Higher Education Edition—the latest edition of an annual report highlighting important trends in higher education. The course will be offered again in fall 2013 on iTunes U, and is now also open for enrollment in edX.

Although Caltech is now actively exploring several outlets for online learning, the Institute's commitment to educational outreach is not a recent phenomenon. In the early 1960s, Caltech physicist Richard Feynman reorganized the Institute's introductory physics course, incorporating contemporary research topics and making the course more engaging for students. His lectures were recorded and eventually incorporated into a widely popular physics book, The Feynman Lectures on Physics, which has sold millions of copies in a dozen languages.

Continuing in the tradition set by Feynman, the MOOCs at Caltech seek to provide a high-quality learning environment that is rigorous but accessible. "No dumbing down of courses for popular consumption . . . no talking over people's heads either; at Caltech, we explain things well because we understand them well," adds Abu-Mostafa.

More information on Caltech's online learning opportunities is available on the Online Education website.

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Friday, October 4, 2013

Undergraduate Teaching Assistant Orientation

Thursday, October 17, 2013


Made-to-Order Materials

Caltech engineers focus on the nano to create strong, lightweight materials

The lightweight skeletons of organisms such as sea sponges display a strength that far exceeds that of manmade products constructed from similar materials. Scientists have long suspected that the difference has to do with the hierarchical architecture of the biological materials—the way the silica-based skeletons are built up from different structural elements, some of which are measured on the scale of billionths of meters, or nanometers. Now engineers at the California Institute of Technology (Caltech) have mimicked such a structure by creating nanostructured, hollow ceramic scaffolds, and have found that the small building blocks, or unit cells, do indeed display remarkable strength and resistance to failure despite being more than 85 percent air.

"Inspired, in part, by hard biological materials and by earlier work by Toby Schaedler and a team from HRL Laboratories, Caltech, and UC Irvine on the fabrication of extremely lightweight microtrusses, we designed architectures with building blocks that are less than five microns long, meaning that they are not resolvable by the human eye," says Julia R. Greer, professor of materials science and mechanics at Caltech. "Constructing these architectures out of materials with nanometer dimensions has enabled us to decouple the materials' strength from their density and to fabricate so-called structural metamaterials which are very stiff yet extremely lightweight."

At the nanometer scale, solids have been shown to exhibit mechanical properties that differ substantially from those displayed by the same materials at larger scales. For example, Greer's group has shown previously that at the nanoscale, some metals are about 50 times stronger than usual, and some amorphous materials become ductile rather than brittle. "We are capitalizing on these size effects and using them to make real, three-dimensional structures," Greer says.

In an advance online publication of the journal Nature Materials, Greer and her students describe how the new structures were made and responded to applied forces.

The largest structure the team has fabricated thus far using the new method is a one-millimeter cube. Compression tests on the the entire structure indicate that not only the individual unit cells but also the complete architecture can be endowed with unusually high strength, depending on the material, which suggests that the general fabrication technique the researchers developed could be used to produce lightweight, mechanically robust small-scale components such as batteries, interfaces, catalysts, and implantable biomedical devices.

Greer says the work could fundamentally shift the way people think about the creation of materials. "With this approach, we can really start thinking about designing materials backward," she says. "I can start with a property and say that I want something that has this strength or this thermal conductivity, for example. Then I can design the optimal architecture with the optimal material at the relevant size and end up with the material I wanted."

The team first digitally designed a lattice structure featuring repeating octahedral unit cells—a design that mimics the type of periodic lattice structure seen in diatoms. Next, the researchers used a technique called two-photon lithography to turn that design into a three-dimensional polymer lattice. Then they uniformly coated that polymer lattice with thin layers of the ceramic material titanium nitride (TiN) and removed the polymer core, leaving a ceramic nanolattice. The lattice is constructed of hollow struts with walls no thicker than 75 nanometers.

"We are now able to design exactly the structure that we want to replicate and then process it in such a way that it's made out of almost any material class we'd like—for example, metals, ceramics, or semiconductors—at the right dimensions," Greer says.

In a second paper, scheduled for publication in the journal Advanced Engineering Materials, Greer's group demonstrates that similar nanostructured lattices could be made from gold rather than a ceramic. "Basically, once you've created the scaffold, you can use whatever technique will allow you to deposit a uniform layer of material on top of it," Greer says.

In the Nature Materials work, the team tested the individual octahedral cells of the final ceramic lattice and found that they had an unusually high tensile strength. Despite being repeatedly subjected to stress, the lattice cells did not break, whereas a much larger, solid piece of TiN would break at much lower stresses. Typical ceramics fail because of flaws—the imperfections, such as holes and voids, that they contain. "We believe the greater strength of these nanostructured materials comes from the fact that when samples become sufficiently small, their potential flaws also become very small, and the probability of finding a weak flaw within them becomes very low," Greer says. So although structural mechanics would predict that a cellular structure made of TiN would be weak because it has very thin walls, she says, "we can effectively trick this law by reducing the thickness or the size of the material and by tuning its microstructure, or atomic configurations."

Additional coauthors on the Nature Materials paper, "Fabrication and Deformation of Three-Dimensional Hollow Ceramic Nanostructures," are Dongchan Jang, who recently completed a postdoctoral fellowship in Greer's lab, Caltech graduate student Lucas Meza, and Frank Greer, formerly of the Jet Propulsion Laboratory (JPL). The work was supported by funding from the Dow-Resnick Innovation Fund at Caltech, DARPA's Materials with Controlled Microstructural Architecture program, and the Army Research Office through the Institute for Collaborative Biotechnologies at Caltech. Some of the work was carried out at JPL under a contract with NASA, and the Kavli Nanoscience Institute at Caltech provided support and infrastructure.

The lead author on the Advanced Engineering Materials paper, "Design and Fabrication of Hollow Rigid Nanolattices Via Two-Photon Lithography," is Caltech graduate student Lauren Montemayor. Meza is a coauthor. In addition to support from the Dow-Resnick Innovation Fund, this work received funding from an NSF Graduate Research Fellowship.

Kimm Fesenmaier
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Meet DALE: Solar Decathlon 2013 Construction is Under Way

Trading in their textbooks for power tools this summer, a group of nine Caltech students and recent graduates have had a unique opportunity to apply their classroom knowledge to real-world challenges. Along with students in architectural design from the Southern California Institute of Architecture (SCI-Arc), the Caltech students have spent their summer building the Dynamic Augmented Living Environment (DALE), a joint SCI-Arc/Caltech entry in the 2013 Solar Decathlon competition. DALE marks Caltech's second collaboration with SCI-Arc, following their Compact Hyper-Insulated Prototype (CHIP), the partnership's first Solar Decathlon entry, in 2011.

Sponsored by the Department of Energy, the biennial Solar Decathlon competition challenges collegiate teams to "design, build, and operate solar-powered houses that are cost-effective, energy-efficient, and attractive." Contest rules state that each entry must be a net-zero home, meaning that its solar panels must produce at least as much energy as the home uses.

Construction on the SCI-Arc/Caltech collaboration began in March, when DALE's cement foundation was poured. In April, the home's steel frames were dropped in, allowing the students (guided by a few construction professionals) to begin nailing the lumber into place.

As of August, the home is starting to take shape; the bathroom has been framed out, the kitchen cabinets are set for installation, and soon the house will be sporting a vinyl exterior and a set of moving canopies that will hold its solar panels.

Although construction work only began a few months ago, the Caltech students began planning for DALE last fall in an engineering project course called Introduction to Multidisciplinary Systems Engineering, taught by Melany Hunt, Dotty and Dick Hayman Professor of Mechanical Engineering and a vice provost.

"I really like this project because it's very hands-on," says DALE team member Zeke Millikan (BS '13, mechanical engineering). "A lot of classes at Caltech are very theoretical, and I'm more of a hands-on type of person. It's really satisfying to actually build something and see it come together."

"Prior to this summer," says DALE team member Sheila Lo ('16), "I didn't really have a lot of experience in construction, so I spent a lot of time learning the terminology and how to use which tools in certain situations. As one of the youngest members of the team, it's been a great privilege to work with upperclassmen and recent graduates because they've taught me a lot about dedication to a project and what it means to apply the skills you learn at Caltech."

And this dedication will be important in the coming weeks, as there is still plenty of work to be done for the early-October competition. Unlike the five previous Solar Decathlons, which were held in Washington, D.C., this year's event will take place in nearby Irvine, California. "Having the competition just right down the road from us inspired the design," says DALE team member Ella Seal (BS '13, mechanical engineering).

To capitalize on Southern California's mild climate, DALE is made up of two moving modules that can glide apart on warm sunny days, creating an open indoor courtyard that can triple the home's available living space. During inclement weather—and for enhanced safety and privacy—DALE's modules can also move together, creating an enclosed home of about 600 square feet.

The home's untraditional moving design—conceived by SCI-Arc team members—is more than just eye-catching. "It also will actually save energy and money over the course of the year," says Seal. By varying the configurations of DALE's modules and shade canopies—the same ones that will hold DALE's solar panels—the Caltech students were able to optimize energy efficiency during different times of the day without sacrificing comfort. "During the summer, the air-conditioning energy consumption drops by at least half when you are able to open up the house and adjust the shading depending on the weather outside," says Millikan.

But a moving house also presents several engineering challenges, says Seal. Wires for electricity and pipes for plumbing had to be specially designed for their moving platform. Seal and Millikan were also tasked with creating a foolproof safety mechanism for DALE's movement systems. Applying their backgrounds in mechanical engineering, they created a system of laser beams, light curtains, and pressure sensors that acts "basically like a garage door sensor on steroids," says Millikan. "We think we've addressed pretty much every scenario where someone could get seriously hurt."

In addition to the movement systems, students from Caltech are responsible for designing the home's heating, ventilation, and air-conditioning system; hot water system; photovoltaic arrays; and other engineering aspects of the solar-powered home. As well as their technical contributions, the Caltech students will collaborate with their SCI-Arc teammates on publicity and fund-raising efforts and the compilation of a final written report.

"I appreciate the fact that it's not just engineering," says Seal. "I really like the fact that we have to write an engineering narrative, describing all of the really cool innovations that we've built into the house. It's not necessarily something that I would get to do if I took a different project class at Caltech."

This type of multidisciplinary and collaborative experience is important for Caltech students, notes Hunt. "Engineering students need experiences in which they design, create, build, and test," she says. "They also should have opportunities in which they work as part of a team. Most engineering projects require multiple perspectives with input coming from a range of individuals with different expertise and vision."

In addition to Millikan, Seal, and Lo, the DALE team includes current Caltech students Brynan Qui ('15), Do Hee Kim ('15), Sharon Wang ('16), as well as recent graduates Tony Wu (BS '13, mechanical engineering and business economics and management) and Christine Viveiros (BS '13, mechanical engineering), and project manager Andrew Gong (BS '12, chemical engineering [materials]). The SCI-Arc/Caltech project, along with other entries for this year's Solar Decathlon competition, will be open to the public October 3–6 and 10–13 at the Orange County Great Park in Irvine, California.

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Yeh and Schwab Named Kavli Nanoscience Institute Codirectors

Caltech professors Nai-Chang Yeh and Keith Schwab have been named codirectors of the Kavli Nanoscience Institute (KNI), a center supporting multidisciplinary nanoscience research on campus and beyond. Yeh and Schwab have both served as KNI board members and are the first to hold the title Fletcher Jones Foundation Codirector of the KNI; the position was recently endowed by a gift from The Fletcher Jones Foundation.

"I look forward to the energy and creativity that Nai-Chang and Keith will bring to the continued evolution of the KNI as a preeminent organization propelling nanoscience forward in diverse application areas ranging from medical engineering to nanophotonics," says Ares Rosakis, Otis Booth Leadership Chair of the Division of Engineering and Applied Science.

Yeh and Schwab follow in the footsteps of professors Michael Roukes and Oskar Painter (MS '95, PhD '01).

"It is an exciting time to conduct research in nanoscience and nanotechnology," Yeh says. "As the new codirectors of KNI, our vision is not only to maintain the current role of KNI but also to make KNI an intellectual hub that facilitates Caltech research in the areas of quantum frontiers, medical/bioengineering, and sustainability," she says.  "We look forward to working with the nanoresearch community on campus and the Caltech administration to advance frontiers of nanoscience and nanotechnology."

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Arnold Appointed New Director of Rosen Bioengineering Center

Now in its sixth year of exploring the intersection between biology and engineering, the Donna and Benjamin M. Rosen Bioengineering Center has chosen Caltech professor Frances Arnold as its new director. Arnold, the Dick and Barbara Dickinson Professor of Chemical Engineering, Bioengineering and Biochemistry began her tenure as director on June 1.

A recipient of the 2011 National Medal of Technology and Innovation, Arnold pioneered methods of "directed evolution" – processes now widely used to create biological catalysts that are important in the production of fuels from renewable resources. She was selected for the directorship because "of her demonstrated leadership in the field of bioengineering," says Stephen Mayo, William K. Bowes Jr. Foundation Chair of the Division of Biology and Biological Engineering.

The Rosen Center supports bioengineering research through the funding of fellows and faculty from many disciplines, including applied physics, chemical engineering, synthetic biology, and computer science.

"Bioengineering is an incredibly exciting field right now," Arnold says. "Solutions to some of the biggest problems in science, medicine, and sustainability will come from the interface between biology and engineering, and Caltech is well positioned to be at the forefront. The Rosen Center will help make that happen with innovative programs for bioengineering research and education."

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Caltech Team Produces Squeezed Light Using a Silicon Micromechanical System

One of the many counterintuitive and bizarre insights of quantum mechanics is that even in a vacuum—what many of us think of as an empty void—all is not completely still. Low levels of noise, known as quantum fluctuations, are always present. Always, that is, unless you can pull off a quantum trick. And that's just what a team led by researchers at the California Institute of Technology (Caltech) has done. The group has engineered a miniature silicon system that produces a type of light that is quieter at certain frequencies—meaning it has fewer quantum fluctuations—than what is usually present in a vacuum.

This special type of light with fewer fluctuations is known as squeezed light and is useful for making precise measurements at lower power levels than are required when using normal light. Although other research groups previously have produced squeezed light, the Caltech team's new system, which is miniaturized on a silicon microchip, generates the ultraquiet light in a way that can be more easily adapted to a variety of sensor applications.

"This system should enable a new set of precision microsensors capable of beating standard limits set by quantum mechanics," says Oskar Painter, a professor of applied physics at Caltech and the senior author on a paper that describes the system; the paper appears in the August 8 issue of the journal Nature. "Our experiment brings together, in a tiny microchip package, many aspects of work that has been done in quantum optics and precision measurement over the last 40 years."

The history of squeezed light is closely associated with Caltech. More than 30 years ago, Kip Thorne, Caltech's Richard P. Feynman Professor of Theoretical Physics, Emeritus, and physicist Carlton Caves (PhD '79) theorized that squeezed light would enable scientists to build more sensitive detectors that could make more precise measurements. A decade later, Caltech's Jeff Kimble, the William L. Valentine Professor and professor of physics, and his colleagues conducted some of the first experiments using squeezed light. Since then, the LIGO (Laser Interferometer Gravitational-Wave Observatory) Scientific Collaboration has invested heavily in research on squeezed light because of its potential to enhance the sensitivity of gravitational-wave detectors.

In the past, squeezed light has been made using so-called nonlinear materials, which have unusual optical properties. This latest Caltech work marks the first time that squeezed light has been produced using silicon, a standard material. "We work with a material that's very plain in terms of its optical properties," says Amir Safavi-Naeini (PhD '13), a graduate student in Painter's group and one of three lead authors on the new paper. "We make it special by engineering or punching holes into it, making these mechanical structures that respond to light in a very novel way. Of course, silicon is also a material that is technologically very amenable to fabrication and integration, enabling a great many applications in electronics."

In this new system, a waveguide feeds laser light into a cavity created by two tiny silicon beams. Once there, the light bounces back and forth a bit thanks to the engineered holes, which effectively turn the beams into mirrors. When photons—particles of light—strike the beams, they cause the beams to vibrate. And the particulate nature of the light introduces quantum fluctuations that affect those vibrations.

Typically, such fluctuations mean that in order to get a good reading of a signal, you would have to increase the power of the light to overcome the noise. But by increasing the power you also introduce other problems, such as introducing excess heat into the system.

Ideally, then, any measurements should be made with as low a power as possible. "One way to do that," says Safavi-Naeini, "is to use light that has less noise."

And that's exactly what the new system does; it has been engineered so that the light and beams interact strongly with each other—so strongly, in fact, that the beams impart the quantum fluctuations they experience back on the light. And, as is the case with the noise-canceling technology used, for example, in some headphones, the fluctuations that shake the beams interfere with the fluctuations of the light. They effectively cancel each other out, eliminating the noise in the light.

"This is a demonstration of what quantum mechanics really says: Light is neither a particle nor a wave; you need both explanations to understand this experiment," says Safavi-Naeini. "You need the particle nature of light to explain these quantum fluctuations, and you need the wave nature of light to understand this interference."

In the experiment, a detector measuring the noise in the light as a function of frequency showed that in a frequency range centered around 28 MHz, the system produces light with less noise than what is present in a vacuum—the standard quantum limit. "But one of the interesting things," Safavi-Naeini adds, "is that by carefully designing our structures, we can actually choose the frequency at which we go below the vacuum." Many signals are specific to a particular frequency range—a certain audio band in the case of acoustic signals, or, in the case of LIGO, a frequency intimately related to the dynamics of astrophysical objects such as circling black holes. Because the optical squeezing occurs near the mechanical resonance frequency where an individual device is most sensitive to external forces, this feature would enable the system studied by the Caltech team to be optimized for targeting specific signals.

"This new way of 'squeezing light' in a silicon micro-device may provide new, significant applications in sensor technology," said Siu Au Lee, program officer at the National Science Foundation, which provided support for the work through the Institute for Quantum Information and Matter, a Physics Frontier Center. "For decades, NSF's Physics Division has been supporting basic research in quantum optics, precision measurements and nanotechnology that laid the foundation for today's accomplishments."

The paper is titled "Squeezed light from a silicon micromechanical resonator." Along with Painter and Safavi-Naeini, additional coauthors on the paper include current and former Painter-group researchers Jeff Hill (PhD '13), Simon Gröblacher (both lead authors on the paper with Safavi-Naeini), and Jasper Chan (PhD '12), as well as Markus Aspelmeyer of the Vienna Center for Quantum Science and Technology and the University of Vienna. The work was also supported by the Gordon and Betty Moore Foundation, by DARPA/MTO ORCHID through a grant from the Air Force Office of Scientific Research, and by the Kavli Nanoscience Institute at Caltech.

Kimm Fesenmaier
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