Physicists observe the quantum of heat flow

Physicists at the California Institute of Technology have announced the first observation of the quantum of thermal conductance. This discovery reveals a fundamental limit to the heat that can be conducted by objects of atomic dimensions.

The findings, reported in the April 27 issue of the journal Nature, could have profound implications for the future design of microscopic electronic devices and for the transmission of information, according to the research team leader, Caltech physics professor Michael Roukes.

The quantum of thermal conductance is best understood by beginning with a simple explanation of heat flow. In the everyday world, the amount of heat carried by an object can vary in a smooth and continuous way. Heat actually flows by means of collective, wavelike vibrations of the atoms that make up a solid material. Usually immense numbers of such waves, each inducing a unique type of synchronous motion of the atoms, act simultaneously to carry heat along a material.

Physicists know that waves sometimes act like particles and vice versa, so they've given these vibrations the particle-like name phonon (reminiscent of "electron" but named after the Greek root phon for sound.) For heat flow in the macroworld, since each phonon is just one among a sea of many others, an individual phonon's contribution alters the total only imperceptibly.

But in the nanoworld, this "phonon sea" is actually rather finite, quantum effects rule, and the heat conduction can become radically different. When an object becomes extremely small, only a limited number of phonons remain active and play a significant role in heat flow within it. In fact, in small devices at temperatures close to absolute zero, most types of motion become almost completely "frozen out," and heat must then be carried by only the several remaining types of wavelike motions that persist.

It has recently become apparent that, in this regime, a strict limit exists to the amount of heat that can be conducted in a small structure or device. Although never before observed, this maximum value is actually a fundamental law of nature, independent of composition or material. It stipulates that the only way thermal conductance can be increased in a very small device is simply to make the conductor larger.

The Nature paper reports that this fundamental limiting value, called the quantum of thermal conductance, can be observed by using tiny devices with specially patterned features only 100 billionths of a meter across (about 300 atoms wide). To carry out this work, Keith Schwab, a postdoctoral fellow in Roukes's group, developed special devices from silicon nitride with assistance from research staff member Erik Henriksen. The work was carried out in the group's nanofabrication and ultralow-temperature laboratories in Pasadena, in collaboration with University of Utah research professor, John Worlock, a visiting associate at Caltech.

The Roukes team has demonstrated that the maximum possible value of energy transported per wavelike motion (phonon mode) is a number composed of only fundamental physical constants and absolute temperature itself. (The relation is given by the product of pi squared, Boltzmann's constant squared, and absolute temperature, over three times Planck's constant.)

Numerically, at an ambient temperature of one kelvin, this quantized conductance roughly translates into a temperature rise of one kelvin upon the application of only a thousandth of a billionth of a watt of power (its precise value is 9.4 x 10^-13 W/K).]

Their new result has important implications for nanotechnology as well as for the transmission of information. Moore's Law, a popularized rule-of-thumb, can be used to loosely describe the continuous decrease in size of the individual building blocks (the transistors) that populate, now in the tens of millions, the integrated circuits forming today's powerful computer chips.

In the unrelenting technological drive toward increased function and decreased size, these individual transistor components have been scaled downward in size to a realm where the underlying physics of their operation can change. In the most extreme cases at the smallest scales, conventional operation may completely break down.

One example is the so-called "power dissipation problem" stemming from the fact that when each individual transistor on a microchip is turned on, each gives off a little heat. This accumulates to become a very significant problem when millions of such transistors, each in effect a microscopic heat generator, are placed in close proximity.

"This will become especially serious for future molecular-scale devices," says Roukes. "No matter how small it is, you always have to put a finite amount of power into a device to turn it on. In this quantum regime, when only a limited number of modes are capable of transferring heat, it will be crucial to take this fundamental limitation into account."

Separate theoretical studies carried out elsewhere indicate that this quantum of thermal conductance is universal, and independent of whether the heat is carried by electrons, phonons, or any other mechanism. "It would seem there is no way of escaping this fundamental law of nature," says Roukes.

These other studies indicate that the maximum thermal conductance, observed in this work, is linked to the maximum rate that information can flow into a device having a single quantum "channel." This surprising connection between information theory and thermodynamics is a manifestation of a deep connection between information and entropy.

"As we engineer smaller and higher speed computational elements, we will also encounter this fundamental quantum limitation in the rate of information flow," Schwab says.

The group's three-year effort followed upon work of Thomas Tighe, a previous postdoctoral fellow in the group, and culminated in new techniques for creating the miniature devices studied. At the heart of each device is an isolated heat reservoir, which the researchers term a "phonon cavity." It resembles a miniature plate freely suspended by four narrow beams. Each beam acts as a quasi one-dimensional "phonon waveguide" for heat flow, and it is precisely this reduced-dimensional flow that is the focus of the researchers' measurements.

On top of the cavity, Schwab and Henriksen patterned two small, separate patches of thin-film gold, described by Roukes as "puddles of electrons." In the course of a measurement, one of these is heated by passing a very small electrical current through it. Electrical connections allowing this current to flow were made using superconducting leads (patterned on top of the phonon waveguides).

This insures that heat is deposited only within the resistive gold film and, therefore, transferred only to the phonon cavity. To escape from the suspended device, the heat must eventually flow through the phonon waveguides. Since the waveguides' thermal conductance is weak, the phonon cavity temperature ultimately rises to a new, and hotter, steady-state level that directly reflects the thermal conductance of the phonon waveguides.

Measurement of the current-induced temperature rise within the small devices is a significant challenge in its own right, and required both ingenuity and the investment of a significant portion of the researchers' efforts. Most available thermometry techniques applicable at the nanoscale are electrical, and thus involve power levels that greatly exceed that used by the researchers in their measurements.

"The power level we used to carry out these experiments, about a femtowatt, is equivalent to the power your eye would receive from a 100-watt light bulb at a distance of about 60 miles," says Schwab. Instead of the standard electrical methods, the researchers coupled the second "electron puddle" to extremely sensitive dc SQUID (superconducting quantum interference device) circuitry.

This allowed them to observe the feeble current fluctuations that have a magnitude directly proportional to the absolute temperature of the nanoscale device. This so-called Johnson/Nyquist noise, which is also the origin of the electrical noise causing background hiss in audio systems, here plays a pivotal role by allowing the local temperature of the phonon cavity to be measured without perturbing the ultraminiature device.

In the end, because the researchers know the precise amount of heat deposited, and can directly measure the absolute temperature reached by the phonon cavity in response to it, they can directly measure the thermal conductance of the narrow beams acting as phonon waveguides. Simply stated, the ratio of the heat flowing through the waveguides to the rise in cavity temperature is the phonon thermal conductance of the quasi one-dimensional waveguides.

This work was carried out over the past three years within the research laboratories of Caltech Professor of Physics, Michael Roukes. Schwab, formerly a Sherman Fairchild Distinguished Postdoctoral Scholar within Roukes' group, is the principal author of the paper.

Schwab's life as a young postdoctoral scientist, and his role in the efforts to observe the quantum of thermal conductance, are the subjects of an upcoming documentary film by independent filmmaker Toni Sherwood. The title of the film is The Uncertainty Principle: Making of an American Scientist.

Coauthors of the paper are John Worlock, visiting associate at Caltech and research professor of physics at the University of Utah, a long time collaborator with Professor Roukes; and former research staff member Erik Henriksen.

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RT
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Cosmologists reveal first detailed images of early universe

PASADENA—Caltech cosmologists and other scientists involved in an international collaboration have released the first detailed images of the universe in its infancy. The images reveal the structure that existed in the universe when it was 50,000 times younger and 1,000 times smaller and hotter than it is today.

Detailed analysis of the images is already shedding light on some of cosmology's outstanding mysteries, including the nature of the dark matter and energy that dominate intergalactic space, and whether space is "curved" or "flat." The team's results are being published in the April 27 issue of the scientific journal Nature.

Cosmologists believe that the universe was created approximately 12–15 billion years ago in an enormous explosion called the Big Bang. The intense heat that filled the embryonic universe is still detectable today as a faint glow of microwave radiation that is visible in all directions. This radiation is known as the cosmic microwave background (CMB).

Since the CMB was first discovered by a ground-based radio telescope in 1965, scientists have eagerly sought to obtain high-resolution images of this radiation. NASA's COBE (Cosmic Background Explorer) satellite discovered the first evidence for structures, or spatial variations, in the CMB in 1991.

The new experiment, dubbed BOOMERANG (Balloon Observations of Millimetric Extragalactic Radiation and Geophysics), obtained the images using a telescope suspended from a balloon that circumnavigated the Antarctic in late 1998.

The balloon carried the telescope at an altitude of almost 37 kilometers (120,000 feet) for 10 1/2 days. As it flew, an extremely sensitive detector system developed at Caltech recorded the faint signals from the early universe.

The BOOMERANG images are the first to bring the CMB into sharp focus. The images reveal hundreds of complex regions that are visible as tiny variations—typically only one ten-thousandth of a degree (0.0001 C)— in the temperature of the CMB. The complex patterns visible in the images confirm predictions of the patterns that would result from sound waves racing through the early universe, creating the structures that by now have evolved into giant clusters and super-clusters of galaxies.

"These images represent the ultimate limit of our vision," said U.S. team leader Andrew Lange, physics professor at Caltech.

"The enormous structures that they reveal predate the first star or galaxy in the universe."

Lange and Italian team leader Paolo deBernardis of the University of Rome, La Sapienza, together led the international team that developed the sophisticated experiment. The entire payload was integrated at Caltech for months of extensive testing before it was taken to Antarctica.

Already, analysis of the size of the structures has produced the most precise measurements to date of the geometry of space-time, which strongly indicate that the geometry of the universe is flat, not curved.

"It is really exciting to obtain such strong evidence for a flat universe. This result is in agreement with a fundamental prediction of the 'inflationary' theory of the universe," said Caltech Postdoctoral Scholar Eric Hivon.

The theory hypothesizes that the entire universe grew from a tiny sub-atomic region during a period of violent expansion that occurred a split second after the Big Bang. The enormous expansion stretched the geometry of space till it was precisely flat.

"These measurements represent a watershed event in cosmology" commented Mark Kamionkowski, professor of theoretical astrophysics at Caltech. "The results suggest that we are on the right track with inflation—a hitherto speculative theory for the origin of the universe—and thus open up a path toward scientifically addressing what happened in the very first micro-micro-second after the Big Bang."

"The key to BOOMERANG's ability to obtain these powerful new images," explained Lange, "is the marriage of a powerful new detector technology developed at Caltech and the Jet Propulsion Lab with the superb microwave telescope and cryogenic systems developed in Italy."

The telescope optics focus the radiation from the early universe onto button-size "bolometric" detectors cooled to a fraction of a degree above absolute zero. Extremely sensitive thermometers embedded in each detector record tiny changes in temperature as the telescope scans across the sky.

"These detectors can 'see' tiny differences in the temperature of the early universe in much the same way as the back of your hand responds to the heat from the sun," explained Caltech graduate student Brendan Crill.

"What really sets this detector system apart," continued Viktor Hristov, a senior electronics engineer at Caltech, "is the stability of the detectors and the electronics used to record the faint signals."

Caltech and JPL are responsible for fabricating a similar detector system for the Planck Surveyor, a satellite that will someday image the CMB over the entire sky from a vantage point 1 million miles from Earth.

In a complementary effort, another Caltech team led by Professor Anthony Readhead is now obtaining images of the CMB at even sharper resolution, using a specially built radio telescope, the Cosmic Background Imager (CBI), from a remote site in the Chilean Andes. BOOMERANG and CBI herald a new era of precision cosmological measurement that promises to provide new insights into fundamental physics.

The 36 BOOMERANG team members come from 16 universities and organizations in Canada, Italy, the United Kingdom, and the United States. Primary support for the BOOMERANG project comes from the Italian Space Agency, Italian Antarctic Research Programme; and the University of Rome, La Sapienza; from the Particle Physics and Astronomy Research Council in the United Kingdom; and from the National Science Foundation and NASA in the United States.

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Robert Tindol
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Caltech scientists develop first microscopic system of pumps and valves made from soft materials

PASADENA—Researchers at the California Institute of Technology have developed a pump that is less than one-half the width of a human hair. The device is a breakthrough in the 3-D microfabrication of soft materials and could be applied to revolutionize and simplify many technologies, including drug discovery and delivery, according to Caltech applied physics professor Stephen R. Quake and his colleagues, who report their findings in the April 7 issue of Science.

Unlike the silicon-based micromachining techniques used for computer chips, this team has developed a technique called multilayer soft lithography, which is essentially an intricate casting of soft rubber. The work is an extension of soft lithography casting, originally developed by George Whitesides at Harvard University.

"Basically, it's plumbing on a very small scale," says Quake. "We are trying to show that it is useful to make microdevices out of soft rubber for certain applications, rather than the hard materials like glass or silicon used in traditional micromachining. In order to make a valve, one needs to figure out how to make it seal, which is usually done with a rubber washer. We made the entire valve out of the sealing material."

The pump is made possible because of the material's softness and pliability. Embedded in a small clear rubber chip the size of a postage stamp, the pump is actually a series of tiny, multilayer channels that each measure 50 by 30 by 10 microns. By contrast, a human hair is about 100 microns wide.

Operation of the pump is similar to the peristaltic motions that make human digestion possible. By applying pressure in one of the channels, another channel above it or below it in the 3-D matrix can be closed off, thereby allowing the channel to act either as a pump or as a valve.

While the research is basic and mainly aimed at demonstrating the feasibility of the technique, Quake says the pump could have a number of practical applications, including drug delivery, one day possibly enabling doctors to implant a biocompatible device about the size of a postage stamp into a patient's body to deliver drugs for chronic disorders such as allergies, pain, diabetes and cancer.

The device may allow the drug to be delivered in a time-released manner customized for each patient. In addition to delivering the drug, the device could also contain a microsized component that would enable regular monitoring of the patient's condition.

Quake's own lab intends to use the microfabricated valves and pumps in two devices: a DNA sizer, which is a replacement for the current technique known as gel electrophoresis; and a cell sorter, a machine that physically separates microscopic materials such as bacteria or viruses. Both devices originated from research in Quake's lab. Caltech has licensed this technology to Mycometrix Corporation of South San Francisco, which will apply it to develop a variety of commercial products.

In addition to Quake, the others involved in the research are Axel Scherer, a professor of electrical engineering, applied physics, and physics at Caltech; Marc Unger, a postdoctoral scholar in applied physics; Hou-Pu Chou, a graduate student in electrical engineering; and Todd Thorsen, a graduate student in biochemistry.

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Robert Tindol
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Using quantum atomistic computer simulations to solve industrial problems

PASADENA—In the world of engineering and applied science, ideas that look good on the drawing board often turn out to have annoying real-world problems, even though the finished products still look pretty good. An example is the aluminum car engine, which has the advantage of being lightweight, but tends to wear out more quickly than its heavier steel counterpart.

To solve such bedeviling problems, experts often find it necessary to go back to "first principles," which in the case of the aluminum engine may include a computer simulation of how the individual atoms slide around under wear and tear.

California Institute of Technology chemistry professor Bill Goddard had this type of problem in mind when he established a special center a decade ago within the campus's Beckman Institute. Christened the Materials and Process Simulation Center (MSC), Goddard's group set as their goals the development of computer simulation tools necessary to deal with materials and process issues, and the transfer of solutions to government and industry for the creation and improvement of products.

"We started the center to follow the dream of being able to predict chemical, biological, and materials processes with a computer," says Goddard. "The idea was to get a simulation that was close enough so that you wouldn't have to do the experiment."

Now that the MSC is celebrating its 10th anniversary, Goddard says the group has made some genuine progress on a number of real industrial problems—much to the satisfaction of corporate collaborators and sponsors, which at present are underwriting about 10 new projects each year.

In addition, the conference celebrates the 100th birthday of Arnold Beckman, the founder of Beckman Instruments and the benefactor of the Beckman Institute.

Since technology transfer and real-world results are a high priority, Goddard and his colleagues sponsor an annual meeting in which the collaborators showcase all their activities. This year's meeting, to be held March 23–24 at the Beckman Institute on campus, is also the 10th anniversary celebration of the center itself.

"There are several new accomplishments we'll discuss at this year's meeting," Goddard says. "We've had the first prediction of the structure of a membrane-bound protein, we've shown how to grow a new class of semiconductors to make real-world devices, and with our local collaborator Avery Dennison we've had success in predicting gas diffusion polymers.

"The bottom line is that it has worked out," he says. "In this center we have probably the most complete group of theorists in the world—about 40 people—and we've continued to have a flow of excellent grad students and postdocs who have gone on to be leaders in their fields."

A unique feature of the MSC is its emphasis in starting out with first principles, using quantum mechanics (the Schrödinger equation) to describe what is happening between atoms. For example, if the real-world problem is how best to lubricate a certain type of moving part (which is an actual industrially funded project the center has worked on), then the researchers would use the Schrodinger wave equation to build a simulation to show precisely how the electrons of a certain lubricant would interact with other electrons, how variable factors such as temperature and pressure would enter into the picture, and how a host of other interactions at the atomic level would play out.

But the quantum level is only the first in a hierarchy of regimes the center researchers might use in investigating complex problems. The quantum level with its Schrödinger equation is good for a system of about 100 atoms, but currently no computer can use quantum mechanics to predict the structure of hemoglobin, the protein that carries oxygen to our muscles.

Rather, for systems with up to about a million atoms, the center uses molecular dynamics techniques, essentially solving Newtonian equations.

For the billion or so atoms or particles that compose a "segment" of material, the MSC investigators employ the techniques of coarse-grain meso-scale modeling and tools such as phase diagrams. Beyond this point, for process simulation, materials applications, and engineering design involving the entire object, the center has developed yet another set of techniques.

This hierarchy of materials modeling is not describable merely by the number or size scale of particles. Time scales are also involved, with quantum mechanics operating at the femtosecond scale (a millionth of a billionth of a second), molecular dynamics at the nanosecond scale (a billionth of a second), coarse-grain meso-scale modeling at the millisecond scale, process simulation at the scale of minutes, and engineering design over periods ranging up to years.

Finally, the hierarchy has many crossover points, which particularly allow the center's research to be innovative and interdisciplinary.

"So you start with fundamentals of quantum mechanics, and imbed this in the next steps at all length scales and time scales," Goddard says. "The idea is to figure out why these things happen, and how looking at first principles can solve industrial problems."

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Robert Tindol
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Caltech grad student's team first to detect radio emission from a brown dwarf

A graduate student in astronomy from the California Institute of Technology recently led a team of researchers in finding the first radio emission ever detected from a brown dwarf, an enigmatic object that is neither star nor planet, but something in between.

The discovery, reported in the March 15 issue of the journal Nature by lead author Edo Berger and his colleagues, demonstrates that brown dwarfs can flare 10,000 times more intensely than theory predicted. The results will likely force experts to rethink their theories about magnetism in brown dwarfs and gas giants, says Berger's supervisor, Shri Kulkarni, who is John E. and Catherine T. MacArthur Professor of Astronomy and Planetary Science at Caltech.

Berger was leader of a student team that made the discovery during a special National Science Foundation student summer program at the NSF's Very Large Array (VLA) near Socorro, New Mexico. The brown dwarf they observed is named LP944-20.

Berger and his colleagues decided to make a long-shot gamble in attempting to observe a brown dwarf from which X-ray flares had been recently discovered with NASA's Chandra X-ray satellite.

"We did some background reading and realized that, based on predictions, the brown dwarf would be undetectable with the VLA," said Berger. "But we decided to try it anyway."

After consulting with Dale Frail, an astronomer at the National Radio Astronomy Observatory (NRAO), Berger and his colleagues decided to utilize a block of observing time traditionally dedicated to the summer students.

The day after they collected their data, the students gathered at the NRAO array operations center in Socorro to process the data and make the images. Berger, who had prior experience processing VLA data, worked alone in the same room as the other students, who were working together on another computer. Berger finished first and was shocked at his image.

I saw a bright object at the exact position of the brown dwarf, and was pretty sure I had made a mistake," Berger said.

He waited for the others, who were working under the guidance of another NRAO astronomer. Ten minutes later, the others also produced an image on the screen in which the same bright object showed up at the brown dwarf's location.

Berger then began breaking up the approximately 90 minutes' worth of data into smaller segments. His results showed that the brown dwarf's radio emission had risen to a strong peak, then weakened. This demonstrated that the brown dwarf had flared.

"Then we got real excited," Berger said, adding that the students immediately sought and received additional observing time. Soon they had captured two more flares.

"The radio emission these students discovered coming from this brown dwarf is 10,000 times stronger than anyone expected," Frail said. "This is going to open up a whole new area of research for the VLA."

The existence of brown dwarfs—objects with masses intermediate between stars and planets—had long been suspected but never confirmed until 1995, when Kulkarni made the first observation at Caltech's Palomar Observatory. Since then, a large number of brown dwarfs have been identified in systematic surveys of the sky. Astronomers now believe that there are as many brown dwarfs as stars in our galaxy.

Flaring and quiescent radio emissions have been seen previously from stars and from the giant planets of our solar system, but never before from a brown dwarf. Moreover, the strength of the magnetic field near the brown dwarf—as inferred from the radio observations—is well below that of Jupiter and orders of magnitude below that of low-mass stars, said Kulkarni.

Conventional wisdom would require large magnetic fields to accelerate the energetic particles responsible for the radio emissions. The same conventional wisdom says that brown dwarfs are expected to generate only short-lived magnetic fields.

However, the persistence of the radio emission of LP944-20 shows that the picture is not complete, Kulkarni said.

"I am very pleased that a first-year Caltech graduate student was able to spearhead such an undertaking, which led to this big discovery," said Kulkarni. "This discovery will spur theorists into obtaining a better understanding of magnetism in stars and planets."

In addition to Berger and Frail, the other authors of the paper are Steven Ball of New Mexico Institute of Mining and Technology, Kate Becker of Oberlin University, Melanie Clark of Carleton College, Therese Fukuda of the University of Denver, Ian Hoffman of the University of New Mexico, Richard Mellon of Penn State, Emmanuel Momjian of the University of Kentucky, Michael Murphy of Amherst College, Stacy Teng of the University of Maryland, Timothy Woodruff of Southwestern University, Ashley Zauderer of Agnes Scott College, and Bob Zavala of New Mexico State University.

[Editors: Additional information on this discovery is available at the NRAO Web site at http://www.nrao.edu/pr/browndwarf.html]

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East and West Antarctica once began separating but then stopped, new research shows

PASADENA—Earth was well on its way to having two Antarcticas long ago, but a tectonic separation between the eastern and western portions of the continent suddenly stopped after 17 million years of spreading, researchers say.

In the March 9 issue of Nature, lead author Steve Cande of the Scripps Institution of Oceanography, Joann Stock of Caltech, and their colleagues in Australia and Japan report that the rift between East and West Antarctica began about 43 million years ago, then ended 17 million years later, after the seafloor had spread about 180 kilometers. The researchers discovered the motion after making several cruises over a period of years in the waters off the Antarctic coast and after gathering data on the seafloor itself.

"The two pieces of Antarctica pulled apart and then stopped," says Stock, a professor of geology and geophysics at Caltech. "If it had kept on going, there would eventually have been two Antarcticas."

The primary scientific value of the study is that it answers some nagging questions about the "missing" motion in the Antarctic region. For a variety of reasons, geophysicists have had a hard time getting a handle on the precise directions and amounts of motion there, and how the motion fits into the grand scheme of global plate tectonics.

"It's like a jigsaw puzzle," Stock says. "You have to know how one piece moved relative to the other pieces to understand how it all fits together.

"A lot of the tectonic plate history for western North America, for example, depends on what happened in Antarctica. You wouldn't think so, but that's the way plate tectonic movements work."

The key to the new results was the researchers' discovery of an underwater feature off Cape Adare that they have named the Adare Trough. This trough is about 230 kilometers long and runs roughly northwest-southeast near the 170th meridian. The sharp break in the direction of the magnetic lines on either side of the trough allows the researchers to infer the ancient relative motions of the plates, and the age and shape of the trough and seafloor around it indicates the period when the spreading occurred.

Seafloor spreading in the area accounts for the "missing" motion in the plate circuit linking the Australia, Antarctic, and Pacific plates, the researchers also found. Too, the 180-kilometer-wide zone of extension is most likely related to the uplift that has occurred in the Transantarctic Mountains to the west, and explains other geological features that have hitherto been puzzling.

And finally, the new results could shed new light on global issues such as the motion between hotspots in the Pacific and Indo-Atlantic oceans.

In addition to Cande and Stock, the other authors are Dietmar Müller of the University of Sidney and Takemi Ishihara of the Geological Survey of Japan.

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Robert Tindol
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Physicists create atom-cavity microscope, track single atoms bound in orbit with single photons

PASADENA—In a promising development with applications to science at the single-atom level, physicists have constructed an "atom-cavity microscope" that tracks the motion of individual atoms.

California Institute of Technology physics professor H. Jeff Kimble, his Caltech colleagues, and collaborators from New Zealand report in the February 25 issue of Science that they have succeeded in monitoring the motion of individual cesium atoms bound in orbit by single photons inside a high-quality optical resonator. The atom is trapped in orbit by a weak light field, and the same light field can be used to observe the atom's motion within the cavity.

This advance is an important development toward the eventual realization of quantum technologies, which would enable quantum computation and communication.

The stage for this microscopic dance is the optical cavity, a pair of highly reflective mirrors that face each other only 10 microns (0.0004 inches) apart. The mirrors are so reflective that a photon, the fundamental quantum of light, enters the cavity and bounces back and forth between the mirrors hundreds of thousands of times before it escapes again through one of the mirrors.

In this way a single photon confined in the cavity builds up an electric field strong enough to influence the motion of an atom and even to bind the atom in orbit within the cavity.

Collaborating theorists A. Scott Parkins and Andrew Doherty in New Zealand first recognized the potential of this trapping technique, in which the atom and the cavity share a quantum of excitation.

"I like to think of it as an atom-cavity molecule," says Christina Hood, a Caltech graduate student and primary author of the paper. "In a molecule, two atoms give up their own electron orbits, their separate identities, to share electrons and form something qualitatively different. In the same sense, in our experiment the atom and the cavity field are bound together strongly by sharing a series of single photons."

How do the scientists actually "see" what is going on inside the tiny optical system? The Caltech group and others had already used similar cavities to sense single atoms whizzing through the cavity. To do this, they illuminate one mirror of the cavity and measure the light escaping from the opposite mirror. "The cavity is a resonator for light, like a half-filled soda bottle is for sound," says Theresa Lynn, a Caltech graduate student and coauthor of the paper. "What we do is similar to holding a tuning fork up to the bottle and listening to hear it resonate. You'll only hear a ring if the right amount of water is in the bottle."

In this case, amazingly, it's a single atom that plays the role of the water in the bottle, dramatically altering the resonance properties of the cavity by its presence or absence. By measuring the amount of light emerging from the cavity, the researchers can tell whether an atom is in the cavity or not.

The major step of the current work is that now they can determine precisely where the atom is located within the light field, creating "movies" of atomic motion in the space between the cavity mirrors. Examples of these movies can be viewed at the group's web site:

http://www.its.caltech.edu/~qoptics/atomorbits/

The movies show atoms trapped in the cavity as they orbit in a plane parallel to the cavity mirrors. The atoms have orbital periods of about 150 microseconds and are typically confined to within about 20 microns of the cavity's center axis.

The Kimble team was able to measure the atomic position to within about 2 microns in measurement times of about 10 microseconds. Continuous position measurements at this level of accuracy and speed allowed them to capture the orbital motions of the atoms.

"The interaction of the atom with the cavity field gives us advantages in two distinct ways," says Kimble. "On the one hand, it provides forces sufficient to trap the atom within the cavity at the level of single photons. On the other hand and more importantly, the strong interaction enables us to sense atomic motion in a fashion that has not been possible before," he says.

Both aspects are important to physicists who probe the limits of our ability to observe and to control the microscopic world, in which the rules and regulations are set by quantum mechanics. According to one basic rule of quantum mechanics, the Heisenberg uncertainty principle, any measurement performed on a system inherently disturbs the future evolution of that system. The principle presents a challenge to physicists who strive to control or "servo" individual quantum systems for use in quantum computation and other quantum technologies.

In collaboration with Caltech assistant professor Hideo Mabuchi, the Caltech team is pursuing extensions of the current research to implement real-time quantum feedback to control atomic motion within the cavity. The operating principles for such "quantum servos" are a topic of contemporary theoretical investigation at Caltech being pursued by Mabuchi, Doherty, and their colleagues.

The cavity as a powerful sensing tool by itself also presents possibilities outside the quantum realm. The same techniques that produce movies of orbiting atoms could be adapted to more general settings, such as to "watch" the dynamics of molecules engaged in chemical and biochemical reactions. Mabuchi is pursuing an independent effort along these lines to monitor single molecules engaged in important biological processes such as conformationally gated electron transfer.

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Robert Tindol
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Astrobiologists should look for both water and energy sources when searching for life on other worlds, researcher says

PASADENA—When planetary scientists first saw evidence of a water ocean beneath the frozen surface of Europa, everyone immediately began pondering the likelihood that the Jovian moon could harbor advanced life forms—perhaps even fishlike creatures.

But last summer a group of planetary scientists from the California Institute of Technology and Jet Propulsion Laboratory threw water on the theory—so to speak—when they took a novel approach and concluded that advanced life forms were not likely.

"Water is a good place to look for life, but is only one ingredient for life," says Kenneth Nealson, an astrobiologist who holds joint appointments at Caltech and JPL, and who was a coauthor of the 1999 paper on Europa.

"You also need energy and, probably, organic carbon."

Nealson and his colleagues Eric Gaidos and Joseph Kirschvink (both of Caltech) wrote in the controversial 1999 Science paper that life on Earth is not necessarily the best analogy for life on another world. In other words, astrobiologists should be prepared to use chemistry and physics to analyze the possibilities for extraterrestrial life, rather than merely assuming life will exist wherever there is water.

Specifically, the authors showed that nearly all forms of energy used by life on Earth would be unavailable to the organisms that might live beneath Europa's surface ice layer. This did not preclude primitive unicellular organisms, but boded poorly for anyone hoping to someday see Europan creatures with gills and backbones.

"There is a trap in the thinking, because on Earth, virtually everywhere you find water you also find life," Nealson says. "And conversely, on Earth, about the only thing you can associate with lifelessness is the lack of water.

"But on another planet, just because you find water doesn't mean you're necessarily going to find life there."

Nealson says that a very likely place to look for life forms is any place where there is an energy gradient of some sort. Some potential energy gradients that might be available on Europa might arise from the gravitational and magnetic fields of Jupiter, which would almost certainly grind things around inside the moon and result in a heat source.

But when Nealson and his colleagues last year analyzed the closed system beneath Europa, they concluded that this source of energy alone was probably insufficient for multicellular life to survive. Also, they concluded that the redox energy (or available chemical energy) of the moon would also be inadequate for complex life of the kind we are familiar with on Earth.

"Still, I think Europa is a great place to look for very simple organisms," Nealson says today.

Another salubrious way to look for life is to look carefully at any place there is a water cycle, however small. If any of the other Jovian moons, such as Ganymede or Callisto, have a hydrological cycle in which moisture precipitates and runs underground, is heated by an internal source, and ultimately is returned to the surface, then the planet or moon would have the potential for energy gradients, energy flow, and geochemical cycling. All of these may be key to the existence of global life.

And the water cycle could be entirely subterranean and could even be a very limited, closed loop, Nealson says. For example, Mars may still have frozen subterranean waters that are occasionally melted by the planet's internal heat, but never result in water vapor actually surfacing. In such a case, there could be bacterial life that has lived in a closed loop beneath the Martian surface for billions of years.

"There's certainly no present-day atmospheric water cycle on Mars—no rain, no aquifers to collect the rainfall, no recycling," he says. "So if there's life on Mars, it has a hard time existing, and we'd have a hard time finding it without drilling."

While a drilling excavation to Mars is still a few decades in the future, Nealson hopes that one of the orbiters to Mars will soon include a deep-sounding radar instrument. Such an instrument can detect either liquid or frozen water beneath the surface.

The Mars orbiter scheduled for launch in 2003 by the European Space Agency (in conjunction with scientists from JPL) is scheduled to have deep-sounding radar for the detection of subsurface liquid water. A similar device will eventually be sent to Europa.

Perhaps later, the search could be extended to other Jovian moons, as well as the moons of Saturn and even Uranus.

"The moons of Jupiter have changed the way I feel about life in the solar system," Nealson says. "Each of the four large moons has different properties, different energy flows, different likelihoods of water.

"It's important to keep an open mind," he says.

Writer: 
Robert Tindol
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Snowball Earth episode 2.4 billion years ago was hard on life, but good for modern industrial economy, research shows

PASADENA-For the primitive organisms unlucky enough to be around 2.4 billion years ago, the first global freeze was a real wipeout, likely the worst in the history of life on Earth. Few of the organisms escaped extinction, and those that did were forced into an evolutionary bottleneck that altered the diversity of life for eons.

But 2.4 billion years later, an unlikely winner has emerged from that first planetary deep-freeze, and it's none other than us modern industrial humans. New research from the California Institute of Technology reveals that the world's largest deposit of manganese (a component of steel) was formed by the cascade of chemical reactions caused when the planet got so cold that even the equators were icy-a condition now known as "Snowball Earth."

In a special issue of the Proceedings of the National Academy of Sciences on global climatic change published February 14, Caltech geobiology professor Joe Kirschvink and his team show that the huge Kalahari Manganese Field in southern Africa was a consequence of a long Snowball Earth episode. Kirschvink, who originated the Snowball Earth concept more than a decade ago, says the new study explains how the drastic climatic changes in a Snowball Earth episode can alter the course of biological evolution, and can also account for a huge economic resource.

According to Kirschvink and his team, the planet froze over for tens of millions of years, but eventually thawed when a greenhouse-induced effect kicked in. This warming episode led to the deposit of iron formations and carbonates, providing nutrients to the blue-green algae that were waiting in the wings for a good feeding.

The algae bloom during the melting period resulted in an oxygen spike, which in turn led to a "rusting" of the iron and manganese. This caused the manganese to be laid down in a huge 45-meter-thick deposit in the Kalahari to await future human mining and metallurgy. Today, about 80 percent of the entire world's known manganese reserves are found in that one field, and it is a major economic resource for the Republic of South Africa.

The Snowball Earth's cascade of climatic chemical reactions also probably forced the living organisms of the time to mutate in such a way that they were protected from the excess oxygen. Because free radicals can cause DNA damage, the organisms adapted an enzyme known as the superoxide dismutase to compensate.

Kirschvink points out that the enzyme and its evolutionary history are well known to biologists, but that a global climate change apparently has never been suggested as a cause of the enzyme's diversification.

"To our knowledge, this is the first biochemical evidence for this adaptation," says Kirschvink, adding that the data shows that the adaptation can be traced back to the Snowball Earth episode 2.4 billion years ago.

Kirschvink, his former doctoral student Dave Evans (now at the University of Western Australia in Perth), and Nicolas J. Beukes of Rand Afrikaans University proposed the Snowball Earth episode in a 1997 paper in Nature. Their evidence for the freeze of 2.4 billion years ago was based on their finding evidence of glacial deposits in a place in southern Africa that in ancient times was within 11 degrees of the equator, according to magnetic samples also gathered there.

The other authors of the PNAS paper are Eric Gaidos of the Jet Propulsion Laboratory, who also holds an appointment in geobiology at Caltech; L. Elizabeth Bertani and Rachel E. Steinberger, both of the Division of Biology at Caltech; and Nicholas J. Beukes and Jans Gutzmer, both of Rand Afrikaans University in Johannesburg.

The work was supported by the NASA National Astrobiology Institute.

A detailed article on the Snowball Earth phenomenon was published in the January 2000 issue of Scientific American.

Writer: 
Robert Tindol
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Thunderstorms found to be an energy source for Jupiter's Great Red Spot

PASADENA-Using data from the Galileo spacecraft currently in orbit around Jupiter, scientists have discovered that thunderstorms beneath the upper cloud cover are supplying energy to the planet's colorful large-scale weather patterns-including the 300-year-old Great Red Spot.

In two articles in the February 10 issue of the British journal Nature and an article in the current issue of the journal Icarus, Caltech planetary science professor Andrew Ingersoll and his colleagues from Cornell, NASA, and UCLA write that lightning storms on the giant planet are clearly associated with the eddies that supply energy to the large-scale weather patterns.

Their conclusion is possible because Galileo can provide daytime photos of the cloud structure when lightning is not visible, and nighttime photos of the same area a couple of hours later clearly showing the lightning.

"You don't usually see the thunderstorms or the lightning strikes because the ammonia clouds in the upper atmosphere obscure them," says Ingersoll.

"But when Galileo passes over the night side, you can see bright flashes that let you infer the depth and the intensity of the lightning bolts."

Especially fortuitous are the Jovian nights when there is a bit of moonshine from one of the large moons such as Io, says Ingersoll. When there is no moonshine, the Galileo images show small blobs of glow from the lightning flashes, but nothing else. But when the upper cloud covers are illuminated at night by moonshine, the pictures show both the glow from the lightning some 100 kilometers below as well as eddies being roiled by the turbulence of the thunderclouds.

The association of the eddies with lightning is especially noteworthy in the new papers, Ingersoll says. Planetary scientists have known for some years that Jupiter had lightning; and in fact they have known since the Voyager flyby that the zonal jets and long-lived storms are kept alive by soaking up the energy of smaller eddies. But they did not know until now that the eddies themselves were fed by thunderstorms below.

"The lightning indicates that there's water down there, because nothing else can condense at a depth of 80 or 100 kilometers," he says. "So we can use lightning as a beacon that points to the place where there are rapidly falling raindrops and rapidly rising air columns-a source of energy for the eddies.

"The eddies, in turn, get pulled apart by shear flow and give up their energy to these large-scale features. So ultimately, the Great Red Spot gets its energy and stays alive by eating these eddies."

Adding credence to the interpretation is the fact that the anticyclonic rotation (clockwise in the northern hemisphere and counterclockwise in the southern) of the eddies is consistent with the outflow from a convective thunderstorm. Their poleward drift is consistent with anticyclones being sucked into Jupiter's powerful westward jets.

Ingersoll is lead author of the Nature paper that interprets the new Galileo data. The other authors are Peter Gierasch and Don Banfield of Cornell University; and Ashwin Vasavada of UCLA. (Banfield and Vasavada are Ingersoll's former doctoral students at Caltech).

Gierasch is lead author of the other Nature paper, which announces the discovery of moist convection on Jupiter. The other authors are Ingersoll; Banfield; Vasavada; Shawn Ewald of Caltech; Paul Helfenstein and Amy Simon-Miller, both of Cornell; and Herb Breneman and David Senske, both of NASA's Jet Propulsion Laboratory (JPL).

The authors of the Icarus paper are Ingersoll; Vasavada; Senske; Breneman; William Borucki of NASA Ames Research Center; Blane Little and Clifford Anger, both of ITRES Research in Calgary, Alberta; and the Galileo SSI Team.

The Galileo spacecraft has been orbiting Jupiter and its moons for the past four years, and the mission has begun an additional one-year extension.

JPL, a division of Caltech, manages the Galileo mission for NASA's Office of Space Science, Washington, D.C.

Writer: 
Robert Tindol
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