Bring In the (Nano) Noise

At the forefront of nanotechnology, researchers design miniature machines to do big jobs, from treating diseases to harnessing sunlight for energy. But as they push the limits of this technology, devices are becoming so small and sensitive that the behavior of individual atoms starts to get in the way. Now Caltech researchers have, for the first time, measured and characterized these atomic fluctuations—which cause statistical noise—in a nanoscale device.

Physicist Michael Roukes and his colleagues specialize in building devices called nanoelectromechanical systems—NEMS for short—which have a myriad of applications. For example, by detecting the presence of proteins that are markers of disease, the devices can serve as cheap and portable diagnostic tools—useful for keeping people healthy in poor and rural parts of the world. Similar gadgets can measure toxic gases in an enclosed room, providing a warning for the inhabitants.

Two years ago, Roukes's group created the world's first nanomechanical mass spectrometer, enabling the researchers to measure the mass of a single biological molecule. The device, a resonator that resembles a tiny bridge, consists of a thin strip of material 2 microns long and 100 nanometers wide that vibrates at a resonant frequency of several hundred megahertz. When an atom is placed on the bridge, the frequency shifts in proportion to the atom's mass. 

But with increasingly sensitive devices, the random motions of the atoms come into play, generating statistical noise. "It's like fog or smoke that obscures what you're trying to measure," says Roukes, who's a professor of physics, applied physics, and bioengineering. In order to distinguish signal from noise, researchers have to understand what's causing the ruckus.

So Roukes—along with former graduate student and staff scientist Philip X. L. Feng, former graduate student Ya-Tang (Jack) Yang, and former postdoc Carlo Callegari—set out to measure this noise in a NEMS resonator. They described their results in the April issue of the journal Nano Letters.

In their experiment, the researchers sprayed xenon gas onto a bridgelike resonator that's similar to the one they used to weigh biological molecules. The xenon can accumulate in a one-atom-thick layer on the surface, like marbles covering a table. In such an arrangement—a so-called monolayer—the atoms are packed so tightly together that they don't have much room to move. But to study noise, the researchers created a submonolayer, which doesn't have enough atoms to completely cover the surface of the resonator. Because of the extra space, the atoms have more freedom to move around, which generates more noise in the system.

The atoms in the submonolayer do one of three things: they stick to the surface, become unstuck and fly off, or slide off. Or in physics speak, the atoms adsorb, desorb, or diffuse. Previous theories had predicted that the noise is most likely due to atoms sticking and unsticking. But now that the researchers were able to observe what actually happens in such a device, they discovered that diffusion dominates the noise. What's noteworthy, the researchers say, is that they found that when an atom slides along the surface of the resonator, it causes the device's vibrating frequency to fluctuate. This is the first time anyone has measured this effect, since previous devices were not sensitive to this sort of diffusion. They also found new power laws in the spectra of noise frequencies—quantitative descriptions of the frequencies at which the atoms vibrate.

There's still a lot more to learn about the physics of this noise, the researchers say. Ultimately, they will need to figure out how to get rid of it or suppress it to build better NEMS devices. But understanding this noise—by measuring the random movement of individual atoms—is itself fascinating science, Roukes says. "It's a new window into how things work in the nanoscale world."

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Stone Awarded Goddard Astronautics Award

Ed Stone, the David Morrisroe Professor of Physics at Caltech and lead scientist on the Voyager 1 and Voyager deep-space probe missions since 1972, was awarded the Goddard Astronautics Award from the American Institute of Aeronautics and Astronautics (AIAA) at a gala ceremony on May 11 in Washington D.C.

The Goddard Award, which honors rocketry pioneer Robert H. Goddard, is the highest honor bestowed by the AIAA; previous winners include Theodore von Karman, James van Allen, Werner von Braun, and Charles Elachi.

Stone joined Caltech's faculty as a research fellow in 1964, the same year he received his PhD from the University of Chicago, and achieved the rank of full professor in 1976. A principal investigator on nine NASA spacecraft missions and coinvestigator on five others, he was formerly director of the Jet Propulsion Laboratory.

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Caltech Faculty Receive Early Career Grants

Four Caltech faculty members are among the 65 scientists from across the nation selected to receive five-year Early Career Research Awards from the U.S. Department of Energy (DOE). The grant winners, who were selected from a pool of about 1,150 applicants, are:

  • Guillaume Blanquart, assistant professor of mechanical engineering, who will develop a chemical model of the inner structure and of the formation of soot particles—black carbon particles formed during the incomplete combustion of hydrocarbon fuels that can cause health problems and adverse effects on the environment—that will aid the development of models that predict emissions from car and truck engines, aircraft engines, fires, and more.

  • Julia R. Greer, assistant professor of materials science and mechanics, who will use nanomechanical experimental and computational tools to isolate and understand the role of specific tailored interfaces and deformation mechanisms on the degradation of properties of materials subjected to helium irradiation. Elucidating these mechanisms will provide insight into requirements for advanced materials for current and next-generation nuclear reactors.

  • Chris Hirata, assistant professor of astrophysics, who will be conducting theoretical studies of cosmological observables—such as galaxy clustering—that are being used to probe dark energy and dark matter and to search for gravitational waves from inflation.

  • Ryan Patterson, assistant professor of physics, who will develop new techniques for readout, calibration, and particle identification for the NOvA long-baseline neutrino experiment at Fermilab, which will investigate neutrino oscillations—the conversion of neutrinos of one type (or "flavor") into another.

The Early Career Research Program, which is funded by the DOE's Office of Science, is "designed to bolster the nation's scientific workforce by providing support to exceptional researchers during the crucial early career years, when many scientists do their most formative work," according to the DOE announcement, and is intended to encourage scientists to focus on research areas that are considered high priorities for the Department of Energy.

To be eligible for an award, a researcher must have received a doctorate within the past 10 years and be an untenured, tenure-track assistant or associate professor at a U.S. academic institution or a full-time employee at a DOE national laboratory.

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Caltech's Ed Stone Profiled in the LA Times

Zipping through the cosmos for 34 years and counting, the two Voyager spacecraft have been the quintessential mission of inspiration and discovery, having revealed new alien worlds and revolutionized our view of the solar system. As the mission's project scientist since 1972, Caltech's Ed Stone has been with Voyager since the beginning, and like the robot explorers, which are now venturing into interstellar space, he's still going and going.

In a front-page story that ran on April 14, The Los Angeles Times profiled Stone, who's the David Morrisroe Professor of Physics and was the director of JPL from 1991 to 2001. The article recounts Voyager's three-plus decades of exploration, returning dazzling, unprecedented images of Saturn's rings, Jupiter's swirling clouds, breathtaking moons, and the never-before-seen worlds of Neptune and Uranus (Voyager 2 is still the only spacecraft to have visited them):
 
"What a journey, what a thrill," Stone says, sitting at his spotless, unadorned desk. "It seemed like everywhere we looked, as we encountered those planets and their moons, we were surprised.

"We were finding things we never imagined, gaining a clearer understanding of the environment Earth was part of. I can close my eyes and still remember every part of it."

But of course, as the Voyagers will soon be the first to determine the outer boundary of the solar system and measure the conditions of interstellar space, the mission isn't finished yet. And neither is Stone.

Read the whole story here.

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Physicists Discover New Way to Visualize Warped Space and Time

PASADENA, Calif.—When black holes slam into each other, the surrounding space and time surge and undulate like a heaving sea during a storm. This warping of space and time is so complicated that physicists haven't been able to understand the details of what goes on—until now.

"We've found ways to visualize warped space-time like never before," says Kip Thorne, Feynman Professor of Theoretical Physics, Emeritus, at the California Institute of Technology (Caltech).

By combining theory with computer simulations, Thorne and his colleagues at Caltech, Cornell University, and the National Institute for Theoretical Physics in South Africa have developed conceptual tools they've dubbed tendex lines and vortex lines.

Using these tools, they have discovered that black-hole collisions can produce vortex lines that form a doughnut-shaped pattern, flying away from the merged black hole like smoke rings. The researchers also found that these bundles of vortex lines—called vortexes—can spiral out of the black hole like water from a rotating sprinkler.

The researchers explain tendex and vortex lines—and their implications for black holes—in a paper that's published online on April 11 in the journal Physical Review Letters.

Tendex and vortex lines describe the gravitational forces caused by warped space-time. They are analogous to the electric and magnetic field lines that describe electric and magnetic forces.

Tendex lines describe the stretching force that warped space-time exerts on everything it encounters. "Tendex lines sticking out of the moon raise the tides on the earth's oceans," says David Nichols, the Caltech graduate student who coined the term "tendex." The stretching force of these lines would rip apart an astronaut who falls into a black hole.

Vortex lines, on the other hand, describe the twisting of space. If an astronaut’s body is aligned with a vortex line, she gets wrung like a wet towel.

When many tendex lines are bunched together, they create a region of strong stretching called a tendex. Similarly, a bundle of vortex lines creates a whirling region of space called a vortex. “Anything that falls into a vortex gets spun around and around,” says Dr. Robert Owen of Cornell University, the lead author of the paper. 

Tendex and vortex lines provide a powerful new way to understand black holes, gravity, and the nature of the universe. "Using these tools, we can now make much better sense of the tremendous amount of data that's produced in our computer simulations," says Dr. Mark Scheel, a senior researcher at Caltech and leader of the team's simulation work.

Two spiral-shaped vortexes of whirling space sticking out of a black hole, and the vortex lines (red curves) that form the vortexes.
Credit: The Caltech/Cornell SXS Collaboration

Using computer simulations, the researchers have discovered that two spinning black holes crashing into each other produce several vortexes and several tendexes. If the collision is head-on, the merged hole ejects vortexes as doughnut-shaped regions of whirling space, and it ejects tendexes as doughnut-shaped regions of stretching. But if the black holes spiral in toward each other before merging, their vortexes and tendexes spiral out of the merged hole. In either case—doughnut or spiral—the outward-moving vortexes and tendexes become gravitational waves—the kinds of waves that the Caltech-led Laser Interferometer Gravitational-Wave Observatory (LIGO) seeks to detect.

"With these tendexes and vortexes, we may be able to much more easily predict the waveforms of the gravitational waves that LIGO is searching for," says Yanbei Chen, associate professor of physics at Caltech and the leader of the team's theoretical efforts.

Additionally, tendexes and vortexes have allowed the researchers to solve the mystery behind the gravitational kick of a merged black hole at the center of a galaxy. In 2007, a team at the University of Texas in Brownsville, led by Professor Manuela Campanelli, used computer simulations to discover that colliding black holes can produce a directed burst of gravitational waves that causes the merged black hole to recoil—like a rifle firing a bullet. The recoil is so strong that it can throw the merged hole out of its galaxy. But nobody understood how this directed burst of gravitational waves is produced.

Now, equipped with their new tools, Thorne's team has found the answer. On one side of the black hole, the gravitational waves from the spiraling vortexes add together with the waves from the spiraling tendexes. On the other side, the vortex and tendex waves cancel each other out. The result is a burst of waves in one direction, causing the merged hole to recoil.

“Though we’ve developed these tools for black-hole collisions, they can be applied wherever space-time is warped,” says Dr. Geoffrey Lovelace, a member of the team from Cornell. “For instance, I expect that people will apply vortex and tendex lines to cosmology, to black holes ripping stars apart, and to the singularities that live inside black holes. They’ll become standard tools throughout general relativity.”

The team is already preparing multiple follow-up papers with new results. "I've never before coauthored a paper where essentially everything is new," says Thorne, who has authored hundreds of articles. "But that's the case here."

The other authors on the Physical Review Letters paper, "Frame-dragging vortexes and tidal tendexes attached to colliding black holes: Visualizing the curvature of spacetime," are Dr. Jeandrew Brink at the National Institute for Theoretical Physics in South Africa and Caltech graduate students Jeff Kaplan, Keith D. Matthews, Fan Zhang, and Aaron Zimmerman.

This research was supported by the National Science Foundation, the Sherman Fairchild Foundation, the Brinson Foundation, NASA, and the David and Barbara Groce Fund.

Written by Marcus Woo

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Caltech Math for the Win

March has been a good month for Caltech mathematics. For the first time since 1983, Caltech placed first in the Mathematical Association of America's William Lowell Putnam Competition, one of the premier undergraduate mathematics contests. Also this past month, Michael Aschbacher, the Shaler Arthur Hanisch Professor of Mathematics, was awarded the Rolf Schock Prize in Mathematics.

Caltech finished third and fifth in the Putnam Competition in the last two years, respectively. But this year, the team of senior Jason Bland, senior Yakov Berchenko-Kogan, and junior Brian Lawrence beat out perennial powerhouses MIT and Harvard for the win. Caltech's math department is awarded $25,000 for the top finish, and each team member receives $1,000. Lawrence finished in the top five, winning another $2,500, and was named a Putnam Fellow for a third time—one of only 19 three-time fellows. Bland was a fellow in 2007. Past Putnam Fellows include Richard Feynman (1939) and Caltech's IBM Professor of Mathematics and Theoretical Physics Barry Simon (1965).

Berchenko-Kogan and Bland, as well as junior Zarathustra Brady, placed in the top 24, receiving $250. Senior Timothy Black, junior Sam Elder, junior Jeffrey Manning, and senior Gjergji Zaimi earned honorable mentions. About 40 Caltech students participated in the contest, which took place back in December. A total of 4,296 students from 546 colleges and universities from the United States and Canada competed.

Taking place every year since 1938, the Putnam Competition is a two-part written test in which participants have a total of six hours to tackle 12 problems. Caltech has won the competition 10 times, second only to Harvard (27). Caltech has also finished in the top five 31 times, behind MIT (41) and Harvard (56).

Aschbacher won the Rolf Schock Prize "for his fundamental contributions to one of the largest mathematical projects ever, the classification of finite simple groups, notably his contribution to the quasi-thin case," according to the award citation.

Groups are one of the most fundamental objects in mathematics. You can rotate an equilateral triangle once, twice, or three times, and the triangle still looks the same. Likewise, you can rotate a dodecahedron in 60 distinct ways, and it won't look different (see the shape on the right of the top image). These rotational symmetries for the triangle and dodecahedron both form simple groups, which contain three and 60 elements, respectively.

All groups can be built from so-called finite simple groups, and over the last few decades, Aschbacher has played a leading role in constructing and determining all of the finite simple groups—a daunting task to say the least. For example, proving that the list of finite simple groups was complete took over 5,000 pages, says Barry Simon. "Many people regard it as the most complicated single result in mathematics."

Most of the finite simple groups were classified by around 1980, but mathematicians realized that there was a gap in the list, called the "quasi-thin case." Finally, in 2004, Aschbacher and Stephen Smith of the University of Illinois at Chicago completed the proof in two books that span more than 1,200 pages.

The Schock mathematics prize is awarded by the Royal Swedish Academy of Sciences and includes $75,000. Perhaps the most famous previous winner is Andrew Wiles, who proved Fermat's Last Theorem and won the prize in 1995. Founded in 1993, the Rolf Schock Prizes were awarded every two years until 2008, when they became triennial awards. The prizes are also given in the fields of logic and philosophy, visual arts, and music.

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Ellis Awarded Gold Medal

Richard Ellis, the Steele Family Professor of Astronomy, has received the Gold Medal of the Royal Astronomical Society. Awarded annually since 1824, the Gold Medal is the society's highest honor and one of the premier prizes in astronomy. Ellis joins a long list of distinguished recipients, including several from Caltech: Don Anderson, Peter Goldreich, Gerald Wasserburg, Maarten Schmidt, Fritz Zwicky, Jesse Greenstein, Ira Bowen, and George Ellery Hale.

According to the London-based society's award citation, "[Ellis] has been one of the most influential British astronomers in the past thirty years," and the Gold Medal recognizes his "outstanding personal research achievements and his leadership in astronomy." Ellis's research focuses on the large-scale distribution of matter in the universe; the cosmic expansion history; and the evolution of galaxies, through detailed studies of nearby systems and the exploration of the very earliest objects. After Ellis joined Caltech's faculty in 1999, the latter observations were accomplished in large part at the Keck Observatory.

"We are very proud that Richard continues the long tradition of outstanding achievement in astronomy at Caltech," says Tom Soifer, professor of physics and chair of the Division of Physics, Mathematics and Astronomy.

As a scientific mentor, Ellis has supervised 30 PhD students; 28 are still active in academic research. He served as director of the Palomar Observatory (now Caltech Optical Observatories) from 2000 to 2005 and has played an important role in building the science case and partnership for the upcoming Thirty Meter Telescope.

He has received several other honors, including sharing the Peter and Patricia Gruber Foundation's Cosmology Prize for his part in the discovery of the accelerating universe and the Royal Astronomical Society's Group Achievement Award for his leadership in the 2-degree-Field Galaxy Redshift Survey, one of the largest astronomical surveys ever performed. Ellis was made a Commander of the British Empire by Queen Elizabeth II for services to international science, and he is a fellow of the Royal Society, the American Association for the Advancement of Science, and the Institute of Physics.

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Marcus Woo
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Astronomers Discover Close-knit Pairs of Massive Black Holes

PASADENA, Calif.—Astronomers at the California Institute of Technology (Caltech), University of Illinois at Urbana-Champaign (UIUC), and University of Hawaii (UH) have discovered 16 close-knit pairs of supermassive black holes in merging galaxies.

The discovery, based on observations done at the W. M. Keck Observatory on Hawaii's Mauna Kea, is being presented in Seattle on January 12 at the meeting of the American Astronomical Society, and has been submitted for publication in the Astrophysical Journal.

These black-hole pairs, also called binaries, are about a hundred to a thousand times closer together than most that have been observed before, providing astronomers a glimpse into how these behemoths and their host galaxies merge—a crucial part of understanding the evolution of the universe. Although few similarly close pairs have been seen previously, this is the largest population of such objects observed as the result of a systematic search.

"This is a very nice confirmation of theoretical predictions," says S. George Djorgovski, professor of astronomy, who will present the results at the conference. "These close pairs are a missing link between the wide binary systems seen previously and the merging black-hole pairs at even smaller separations that we believe must be there."

As the universe has evolved, galaxies have collided and merged to form larger ones. Nearly every one—or perhaps all—of these large galaxies contains a giant black hole at its center, with a mass millions—or even billions—of times higher than the sun’s. Material such as interstellar gas falls into the black hole, producing enough energy to outshine galaxies composed of a hundred billion stars. The hot gas and black hole form an active galactic nucleus, the brightest and most distant of which are called quasars. The prodigious energy output of active galactic nuclei can affect the evolution of galaxies themselves.
   
While galaxies merge, so should their central black holes, producing an even more massive black hole in the nucleus of the resulting galaxy. Such collisions are expected to generate bursts of gravitational waves, which have yet to be detected. Some merging galaxies should contain pairs of active nuclei, indicating the presence of supermassive black holes on their way to coalescing. Until now, astronomers have generally observed only widely separated pairs—binary quasars—which are typically hundreds of thousands of light-years apart.

"If our understanding of structure formation in the universe is correct, closer pairs of active nuclei must exist," adds Adam Myers, a research scientist at UIUC and one of the coauthors. "However, they would be hard to discern in typical images blurred by Earth's atmosphere."

The solution was to use Laser Guide Star Adaptive Optics, a technique that enables astronomers to remove the atmospheric blur and capture images as sharp as those taken from space. One such system is deployed on the W. M. Keck Observatory's 10-meter telescopes on Mauna Kea.

The astronomers selected their targets using spectra of known galaxies from the Sloan Digital Sky Survey (SDSS). In the SDSS images, the galaxies are unresolved, appearing as single objects instead of binaries. To find potential pairs, the astronomers identified targets with double sets of emission lines—a key feature that suggests the existence of two active nuclei.

By using adaptive optics on Keck, the astronomers were able to resolve close pairs of galactic nuclei, discovering 16 such binaries out of 50 targets. "The pairs we see are separated only by a few thousands of light-years—and there are probably many more to be found," says Hai Fu, a Caltech postdoctoral scholar and the lead author of the paper.

"Our results add to the growing understanding of how galaxies and their central black holes evolve," adds Lin Yan, a staff scientist at Caltech and one of the coauthors of the study.

“These results illustrate the discovery power of adaptive optics on large telescopes,” Djorgovski says. “With the upcoming Thirty Meter Telescope, we’ll be able to push our observational capabilities to see pairs with separations that are three times closer.”

In addition to Djorgovski, Fu, Myers, and Yan, the team includes Alan Stockton from the University of Hawaii at Manoa. The work done at Caltech was supported by the National Science Foundation and the Ajax Foundation.

Images of some of the merging systems are available at http://www.astro.caltech.edu/~george/bbh.

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Make Your Own Flake

With little more than a plastic soda bottle, some fishing line, a sponge, and dry ice, anyone can make it snow, make it snow, make it snow...one flake at a time.

So says Caltech physicist-turned-snowflake-guru Ken Libbrecht, who recently walked listeners of NPR's Science Friday through a do-it-yourself snowflake-making tutorial.

The home-grown snowmaking process tends to differ from what goes on in a cloud, relying on water vapor rather than water droplets. But, in the end, creating flakes is about humidity and temperature, with the shape of the crystal depending in large part on how cold it is where it forms.

And, it turns out, snowflakes are picky about the temperatures at which they'll grow; not every spot on the snowflake-growing fishing line is conducive to creating a crystal. Why not? "It's not understood at all," Libbrecht says, "why the growth of ice depends so sensitively on temperatures." Indeed, that's one of the mysteries Libbrecht is studying in his Caltech lab, where the snowflake growing may be higher-tech, but no less entrancing, than in a soda bottle.

Want to try this at home? You can check out Libbrecht's recipe for creating a white Christmas—yes, even in Southern California—at his Snow Crystals website.

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Lori Oliwenstein
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Caltech Physicists Demonstrate a Four-Fold Quantum Memory

PASADENA, Calif. — Researchers at the California Institute of Technology (Caltech) have demonstrated quantum entanglement for a quantum state stored in four spatially distinct atomic memories.

Their work, described in the November 18 issue of the journal Nature, also demonstrated a quantum interface between the atomic memories—which represent something akin to a computer "hard drive" for entanglement—and four beams of light, thereby enabling the four-fold entanglement to be distributed by photons across quantum networks. The research represents an important achievement in quantum information science by extending the coherent control of entanglement from two to multiple (four) spatially separated physical systems of matter and light.

The proof-of-principle experiment, led by William L. Valentine Professor and professor of physics H. Jeff Kimble, helps to pave the way toward quantum networks. Similar to the Internet in our daily life, a quantum network is a quantum "web" composed of many interconnected quantum nodes, each of which is capable of rudimentary quantum logic operations (similar to the "AND" and "OR" gates in computers) utilizing "quantum transistors" and of storing the resulting quantum states in quantum memories. The quantum nodes are "wired" together by quantum channels that carry, for example, beams of photons to deliver quantum information from node to node. Such an interconnected quantum system could function as a quantum computer, or, as proposed by the late Caltech physicist Richard Feynman in the 1980s, as a "quantum simulator" for studying complex problems in physics.

Quantum entanglement is a quintessential feature of the quantum realm and involves correlations among components of the overall physical system that cannot be described by classical physics. Strangely, for an entangled quantum system, there exists no objective physical reality for the system's properties. Instead, an entangled system contains simultaneously multiple possibilities for its properties. Such an entangled system has been created and stored by the Caltech researchers.

Previously, Kimble's group entangled a pair of atomic quantum memories and coherently transferred the entangled photons into and out of the quantum memories (http://media.caltech.edu/press_releases/13115). For such two-component—or bipartite—entanglement, the subsystems are either entangled or not. But for multi-component entanglement with more than two subsystems—or multipartite entanglement—there are many possible ways to entangle the subsystems. For example, with four subsystems, all of the possible pair combinations could be bipartite entangled but not be entangled over all four components; alternatively, they could share a "global" quadripartite (four-part) entanglement.

Hence, multipartite entanglement is accompanied by increased complexity in the system. While this makes the creation and characterization of these quantum states substantially more difficult, it also makes the entangled states more valuable for tasks in quantum information science.

To achieve multipartite entanglement, the Caltech team used lasers to cool four collections (or ensembles) of about one million Cesium atoms, separated by 1 millimeter and trapped in a magnetic field, to within a few hundred millionths of a degree above absolute zero. Each ensemble can have atoms with internal spins that are "up" or "down" (analogous to spinning tops) and that are collectively described by a "spin wave" for the respective ensemble. It is these spin waves that the Caltech researchers succeeded in entangling among the four atomic ensembles.

The technique employed by the Caltech team for creating quadripartite entanglement is an extension of the theoretical work of Luming Duan, Mikhail Lukin, Ignacio Cirac, and Peter Zoller in 2001 for the generation of bipartite entanglement by the act of quantum measurement. This kind of "measurement-induced" entanglement for two atomic ensembles was first achieved by the Caltech group in 2005 (http://media.caltech.edu/press_releases/12776).

In the current experiment, entanglement was "stored" in the four atomic ensembles for a variable time, and then "read out"—essentially, transferred—to four beams of light. To do this, the researchers shot four "read" lasers into the four, now-entangled, ensembles. The coherent arrangement of excitation amplitudes for the atoms in the ensembles, described by spin waves, enhances the matter–light interaction through a phenomenon known as superradiant emission.

"The emitted light from each atom in an ensemble constructively interferes with the light from other atoms in the forward direction, allowing us to transfer the spin wave excitations of the ensembles to single photons," says Akihisa Goban, a Caltech graduate student and coauthor of the paper. The researchers were therefore able to coherently move the quantum information from the individual sets of multipartite entangled atoms to four entangled beams of light, forming the bridge between matter and light that is necessary for quantum networks.

The Caltech team investigated the dynamics by which the multipartite entanglement decayed while stored in the atomic memories. "In the zoology of entangled states, our experiment illustrates how multipartite entangled spin waves can evolve into various subsets of the entangled systems over time, and sheds light on the intricacy and fragility of quantum entanglement in open quantum systems," says Caltech graduate student Kyung Soo Choi, the lead author of the Nature paper. The researchers suggest that the theoretical tools developed for their studies of the dynamics of entanglement decay could be applied for studying the entangled spin waves in quantum magnets.

Further possibilities of their experiment include the expansion of multipartite entanglement across quantum networks and quantum metrology. "Our work introduces new sets of experimental capabilities to generate, store, and transfer multipartite entanglement from matter to light in quantum networks," Choi explains. "It signifies the ever-increasing degree of exquisite quantum control to study and manipulate entangled states of matter and light."

In addition to Kimble, Choi, and Goban, the other authors of the paper, "Entanglement of spin waves among four quantum memories," are Scott Papp, a former postdoctoral scholar in the Caltech Center for the Physics of Information now at the National Institute of Standards and Technology in Boulder, Colorado, and Steven van Enk, a theoretical collaborator and professor of physics at the University of Oregon, and an associate of the Institute for Quantum Information at Caltech.

This research was funded by the National Science Foundation, the National Security Science and Engineering Faculty Fellowship program at the U.S. Department of Defense (DOD), the Northrop Grumman Corporation, and the Intelligence Advanced Research Projects Activity.

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