Sunday, October 23, 2011

Laser light used to cool object to quantum ground state

For the first time, researchers at the California Institute of Technology (Caltech), in collaboration with a team from the University of Vienna, have managed to cool a miniature mechanical object to its lowest possible energy state using laser light. The achievement paves the way for the development of exquisitely sensitive detectors as well as for quantum experiments that scientists have long dreamed of conducting.

"We've taken a solid mechanical system -- one made up of billions of atoms -- and used optical light to put it into a state in which it behaves according to the laws of quantum mechanics. In the past, this has only been achieved with trapped single atoms or ions," says Oskar Painter, professor of applied physics and executive officer for applied physics and materials science at Caltech and the principal investigator on a paper describing the work that appears in the October 6 issue of the journal Nature.

As described in the paper, Painter and his colleagues have engineered a nanoscale object -- a tiny mechanical silicon beam -- such that laser light of a carefully selected frequency can enter the system and, once reflected, can carry thermal energy away, cooling the system.

By carefully designing each element of the beam as well as a patterned silicon shield that isolates it from the environment, Painter and colleagues were able to use the laser cooling technique to bring the system down to the quantum ground state, where mechanical vibrations are at an absolute minimum. Such a cold mechanical object could help detect very small forces or masses, whose presence would normally be masked by the noisy thermal vibrations of the sensor.

"In many ways, the experiment we've done provides a starting point for the really interesting quantum-mechanical experiments one wants to do," Painter says. For example, scientists would like to show that a mechanical system could be coaxed into a quantum superposition -- a bizarre quantum state in which a physical system can exist in more than one position at once. But they need a system at the quantum ground state to begin such experiments.

To reach the ground state, Painter's group had to cool its mechanical beam to a temperature below 100 millikelvin (-273.15°C). That's because the beam is designed to vibrate at gigahertz frequencies (corresponding to a billion cycles per second) -- a range where a large number of phonons are present at room temperature. Phonons are the most basic units of vibration just as the most basic units or packets of light are called photons. All of the phonons in a system have to be removed to cool it to the ground state.

Conventional means of cryogenically cooling to such temperatures exist but require expensive and, in some cases, impractical equipment. There's also the problem of figuring out how to measure such a cold mechanical system. To solve both problems, the Caltech team used a different cooling strategy.

"What we've done is used the photons -- the light field -- to extract phonons from the system," says Jasper Chan, lead author of the new paper and a graduate student in Painter's group. To do so, the researchers drilled tiny holes at precise locations in their mechanical beam so that when they directed laser light of a particular frequency down the length of the beam, the holes acted as mirrors, trapping the light in a cavity and causing it to interact strongly with the mechanical vibrations of the beam.

Because a shift in the frequency of the light is directly related to the thermal motion of the mechanical object, the light -- when it eventually escapes from the cavity -- also carries with it information about the mechanical system, such as the motion and temperature of the beam. Thus, the researchers have created an efficient optical interface to a mechanical element -- or an optomechanical transducer -- that can convert information from the mechanical system into photons of light.

Importantly, since optical light, unlike microwaves or electrons, can be transmitted over large, kilometer-length distances without attenuation, such an optomechanical transducer could be useful for linking different quantum systems -- a microwave system with an optical system, for example. While Painter's system involves an optical interface to a mechanical element, other teams have been developing systems that link a microwave interface to a mechanical element. What if those two mechanical elements were the same? "Then," says Painter, "I could imagine connecting the microwave world to the optical world via this mechanical conduit one photon at a time."

The Caltech team isn't the first to cool a nanomechanical object to the quantum ground state; a group led by former Caltech postdoctoral scholar Andrew Cleland, now at the University of California, Santa Barbara, accomplished this in 2010 using more conventional refrigeration techniques, and, earlier this year, a group from the National Institute of Standards and Technology in Boulder, Colorado, cooled an object to the ground state using microwave radiation. The new work, however, is the first in which a nanomechanical object has been put into the ground state using optical light.

"This is an exciting development because there are so many established techniques for manipulating and measuring the quantum properties of systems using optics," Painter says.

The other cooling techniques used starting temperatures of approximately 20 millikelvin -- more than a factor of 10,000 times cooler than room temperature. Ideally, to simplify designs, scientists would like to initiate these experiments at room temperature. Using laser cooling, Painter and his colleagues were able to perform their experiment at a much higher temperature -- only about 10 times lower than room temperature.

The work was supported by Caltech's Kavli Nanoscience Institute; the Defense Advanced Research Projects Agency's Microsystems Technology Office through a grant from the Air Force Office of Scientific Research; the European Commission; the European Research Council; and the Austrian Science Fund.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by California Institute of Technology, via EurekAlert!, a service of AAAS.

Journal Reference:

Jasper Chan, T. P. Mayer Alegre, Amir H. Safavi-Naeini, Jeff T. Hill, Alex Krause, Simon Gröblacher, Markus Aspelmeyer, Oskar Painter. Laser cooling of a nanomechanical oscillator into its quantum ground state. Nature, 2011; 478 (7367): 89 DOI: 10.1038/nature10461

Advance offers new opportunities in chemistry education, research

Researchers at Oregon State University have created a new, unifying method to describe a basic chemical concept called "electronegativity," first described almost 80 years ago by OSU alumnus Linus Pauling and part of the work that led to his receiving the Nobel Prize.

The new system offers simplicity of understanding that should rewrite high school and college chemistry textbooks around the world, even as it opens important new avenues in materials and chemical research, with possible applications in everything from solar energy to solid state batteries.

The findings were just published in the Journal of the American Chemical Society, in work supported by the National Science Foundation and the U.S. Department of Energy.

"This is a quantum leap forward in understanding basic tendencies in chemical bond formation," said John Wager, a professor of electrical engineering at OSU. "We can now take a concept that college students struggle with and I could explain it to a kindergarten class.

"Even advanced scientists will gain new insights and understanding into the chemical processes they study," Wager said. "Using this system, I could look at various materials being considered for use in new solar energy cells and determine quickly that this one might work, that one doesn't stand a chance."

Electronegativity, as defined by Pauling, is "the power of an atom in a molecule to attract electrons to itself." This concept is useful for explaining why some atoms tend to attract electrons, others share them and some give them away. In the 1930s, Pauling was the first to devise a method for numerically estimating the electronegativity of an atom. Other researchers later developed different approaches.

The new system developed at OSU -- the first of its type since the early 1990s -- is called an atomic "solid state energy scale." It characterizes electronegativity as the solid state energy of elements in a compound, and shows that electrons simply move from a higher energy to a lower energy.

"This is a remarkably intuitive approach to understanding electronegativity, and yet it's based on data that are absolute, not arbitrary," said Douglas Keszler, an OSU professor of chemistry, co-author on the study and an international expert in materials science research.

"This is already one of the best instruments in my tool box for predicting the properties of new materials and understanding inorganic reactions," Keszler said. "It's not only more accurate and comprehensive, it just offers a simplicity of understanding that is very important."

The electronegativity scale developed by Pauling is among the most widely known of his contributions in studies on the nature of the chemical bond, the work for which he received a Nobel Prize in chemistry.

According to Ram Ravichandran, an electrical engineering student at OSU and co-author of the study, the new approach is based on the study of how the "band gap," a fundamental property of materials, varies for a variety of compounds. This helps to derive an absolute energy reference and a new solid state energy scale, providing a surprisingly simple way to visualize the way materials will interact.

The system could aid research in new semiconductor devices, catalysts, solar cells, light emitting materials and many other uses.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Oregon State University.

Journal Reference:

Brian D. Pelatt, Ram Ravichandran, John F. Wager, Douglas A. Keszler. Atomic Solid State Energy Scale. Journal of the American Chemical Society, 2011; : 111003131629001 DOI: 10.1021/ja204670s

When water and air meet: New light shed on mysterious structure of world's most common liquid interface

Findings by researchers at the RIKEN Advanced Science Institute and their colleagues at Tohoku University and in the Netherlands have resolved a long-standing debate over the structure of water molecules at the water surface. Published in the Journal of The American Chemical Society, the research combines theoretical and experimental techniques to pinpoint, for the first time, the origin of water's unique surface properties in the interaction of water pairs at the air-water interface.

The most abundant compound on Earth's surface, water is essential to life and has shaped the course of human civilization. As perhaps the most common liquid interface, the air-water interface offers insights into the surface properties of water in everything from atmospheric and environmental chemistry, to cellular biology, to regenerative medicine. Yet despite its ubiquity, the structure of this interface has remained shrouded in mystery.

At the heart of this mystery are two broad bands in the vibrational spectrum for surface water resembling those of bulk ice and liquid water. Whether these bands are the result of hydrogen bonds themselves, of intra-molecular coupling between hydrogen bonds within a single water molecule, or of inter-molecular coupling between adjacent water molecules, is a source of heated debate. One popular but controversial hypothesis suggests one of the spectral bands corresponds to water forming an actual tetrahedral "ice-like" structure at the surface, but this interpretation raises issues of its own.

The researchers set out to resolve this debate through a comprehensive study combining theory and experiment. For their experiments, they applied a powerful spectroscopy technique developed at RIKEN to selectively pick out surface molecules and rapidly measure their spectra. To eliminate coupling effects, which are difficult to reproduce in simulations, they used water diluted with D2O (heavy water) and HOD (water with one hydrogen atom, H, replaced by deuterium, D). Doing so eliminates coupling of OH bonds within a single molecule (since there is only one OH bond) and reduces the overall concentration of OH bonds in the solution, suppressing intermolecular coupling.

With other influences removed, the researchers at last pinpointed the source of water's unique surface structure not in an "ice-like" structure, but in the strong hydrogen bonding between water pairs at the outermost surface. The extremely good match between experimental and theoretical results confirms this conclusion, at long last bringing clarity to the debate over the structure of the water surface and setting the groundwork for fundamental advances in a range of scientific fields.

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by RIKEN.

Journal Reference:

Satoshi Nihonyanagi, Tatsuya Ishiyama, Touk-kwan Lee, Shoichi Yamaguchi, Mischa Bonn, Akihiro Morita, Tahei Tahara. Unified Molecular View of the Air/Water Interface Based on Experimental and Theoretical x(2) Spectra of an Isotopically Diluted Water Surface. Journal of the American Chemical Society, 2011; 110929124813002 DOI: 10.1021/ja2053754

New technique for understanding quantum effects in water

The use of oxygen isotope substitution will lead to more accurate structural modeling of oxide materials found in everything from biological processes to electronic devices, new research suggests.

It covers over two thirds of our planet, is essential for life on Earth and its chemical formula is one of the few most people can name, but we still have much to learn about the structure of H2O. Now, scientists working in Grenoble have developed a new technique using oxygen isotopes to study in detail the structure of disordered oxide materials such as water in biological processes or glasses in lasers and telecommunication devices. This new technique allowed a team from the Institut Laue-Langevin (ILL), University of Bath, Oak Ridge National Laboratory and Stanford University to validate a new theoretical model for water's structure by measuring subtle differences between the molecular organisation of light and heavy water that result from quantum mechanics.

At ILL the structural properties of materials are probed by using neutrons, which act like "super x-rays," via a technique known as neutron scattering. As neutrons pass through materials they are often bounced (or scattered) by atomic nuclei which alters their trajectories, and these scattered neutrons can then be detected to create detailed maps of a sample's molecular structure. To find out more about the positions of particular atoms within a sample, scientists use a trick called isotopic substitution where the scattering length (or ability to bounce neutrons) of a particular element is 'tuned' by substituting one of its isotopes for another. This allows them to zero in on the structure around the atoms of the chosen element.

In modern structural analysis, researchers commonly interchange hydrogen with its heavier isotope deuterium to probe the locations of atoms in water or other hydrogen containing materials. This technique of 'H/D substitution' is also commonly used in the analysis of hydrogen-storage materials or fuel cells. However, there are problems with using H/D substitution in neutron scattering. The lighter hydrogen isotope is comparable in mass to the neutron which generates imprecise scattering data and makes determination of structure more difficult. Also, you can't use H/D substitution to study the difference between the positions of hydrogen atoms in H2O versus deuterium atoms in D2O as the technique assumes that H and D atoms have the same positions. Oxygen has three isotopes: 16O, 17O and 18O and, like hydrogen, is a ubiquitous element on Earth and plays an important role across scientific disciplines. It is often found in structurally disordered materials like silicates in planetary science, glasses for lasers and optical communications, oxide layers in silicon-based electronic devices and water in biological processes. However, it was generally believed that the difference in scattering length between these isotopes is too small to make isotopic substitution with neutron scattering feasible.

The team at ILL challenged this assumption via neutron interferometry -- a technique where neutrons, acting as coherent quantum waves, allow for a very precise measurement of the scattering lengths of atoms in a sample. With the highly sensitive equipment at ILL, the team showed that the difference between the scattering lengths of two of the oxygen isotopes was actually six times larger than the literature suggested. Professor Philip Salmon, from the University of Bath, said: "With this larger contrast, we showed the difference in the scattering lengths of the oxygen isotopes was just about large enough to make neutron scattering a plausible technique for studying the structure of oxide materials." In order to demonstrate the powerful potential of their new technique, the team turned to the structure of the best-known oxide in nature -- liquid water where the imprecise results from hydrogen isotope substitution had created some uncertainty. In particular, the team were interested in comparing structural differences between light water (H2O) and heavy water (D2O).

"The structure and dynamics of water have long been controversial subjects since they can have profound effects on biological processes, and there can be dramatic differences between heavy and light water. For example, most organisms eventually perish in a D2O environment, whereas they thrive in H2O," said Dr Henry Fischer, a physicist at ILL who worked alongside Prof Salmon on this paper.

Using oxygen isotope substitution, Prof Salmon and his team at ILL analyzed the difference between the lengths of the O-H and O-D bonds within water molecules. They found that the O-H bonds were 1 % longer than the O-D bonds -- the first time anyone had measured with such pin-point accuracy this important difference between the molecular structures of light and heavy water.

Their findings were then compared with quantum mechanics predictions using path-integral methods to see if they could clarify some uncertainty around the structural model for liquid water. Earlier mathematical models often assumed simple rigid molecules, where the bond lengths do not vary, but it turns out that such models are not sufficient to account for the quantum effects leading to the observed structural differences between H2O and D2O. Quantum mechanics gives a fuzzy uncertainty to the positions of the H and D atoms in a water molecule, and since D is twice as heavy as H, the fuzzy effect is not as strong for D as compared to H. This leads to the observed structural differences which can be predicted using a more appropriate flexible model for the water molecule. Salmon and his team thus identified the type of theoretical model that is needed for understanding the true structure of water, and confirmed that this model can explain the structural differences between H2O and D2O due to quantum mechanics.

The above story is reprinted (with editorial adaptations ) from materials provided by Institut Laue-Langevin (ILL).

Journal Reference:

Anita Zeidler, Philip Salmon, Henry Fischer, Jörg Neuefeind, J. Simonson, Hartmut Lemmel, Helmut Rauch, Thomas Markland. Oxygen as a Site Specific Probe of the Structure of Water and Oxide Materials. Physical Review Letters, 2011; 107 (14) DOI: 10.1103/PhysRevLett.107.145501