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2Physics

2Physics Quote:
"Many of the molecules found by ROSINA DFMS in the coma of comet 67P are compatible with the idea that comets delivered key molecules for prebiotic chemistry throughout the solar system and in particular to the early Earth increasing drastically the concentration of life-related chemicals by impact on a closed water body. The fact that glycine was most probably formed on dust grains in the presolar stage also makes these molecules somehow universal, which means that what happened in the solar system could probably happen elsewhere in the Universe."
-- Kathrin Altwegg and the ROSINA Team

(Read Full Article: "Glycine, an Amino Acid and Other Prebiotic Molecules in Comet 67P/Churyumov-Gerasimenko"
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Saturday, August 29, 2009

IBM Researchers First to Image the 'Anatomy' of a Molecule at Atomic Resolution

IBM Research - Zurich scientists Nikolaj Moll, Reto Schlittler, Gerhard Meyer, Fabian Mohn and Leo Gross (L to R) standing behind a scanning tunneling/atomic force microscope similar to the one they used to image the "anatomy" of the Pentacene molecule at an atomic resolution. [Photo by Michael Lowry. Image courtesy of IBM Research - Zurich]


In a paper published in the August 28 issue of Science magazine [1], IBM Research – Zurich scientists Leo Gross, Fabian Mohn, Nikolaj Moll and Gerhard Meyer, in collaboration with Peter Liljeroth of Utrecht University -- Netherlands, have reported capturing the first image of the “anatomy” -- or chemical structure -- inside a molecule with unprecedented resolution, using a complex technique known as noncontact atomic force microscopy (AFM). The results push the exploration of using molecules and atoms at the smallest scale and could greatly impact the field of nanotechnology.

"Though not an exact comparison, if you think about how a doctor uses an x-ray to image bones and organs inside the human body, we are using the atomic force microscope to image the atomic structures that are the backbones of individual molecules," said Gerhard Meyer. "Scanning probe techniques offer amazing potential for prototyping complex functional structures and for tailoring and studying their electronic and chemical properties on the atomic scale.”

The team’s current publication follows on the heels of another experiment published just two months ago [2] where IBM scientists measured the charge states of atoms using an AFM. These breakthroughs will open new possibilities for investigating how charge transmits through molecules or molecular networks. Understanding the charge distribution at the atomic scale is essential for building smaller, faster and more energy-efficient computing components than today’s processors and memory devices.

The team used an AFM operated in an ultrahigh vacuum and at very low temperatures ( –2680C or –4510F) to image the chemical structure of individual pentacene molecules. With their AFM, the IBM scientists, for the first time ever, were able to look through the electron cloud and see the atomic backbone of an individual molecule. While not a direct technological comparison, this is reminiscent of x-rays that pass through soft tissue to enable clear images of bones.

Imaging the "anatomy" of a pentacene molecule - 3D rendered view: By using an atomically sharp metal tip terminated with a carbon monoxide molecule, IBM scientists were able to measure in the short-range regime of forces which allowed them to obtain an image of the inner structure of the molecule. The colored surface represents experimental data. [Image courtesy of IBM Research – Zurich]

The AFM uses a sharp metal tip to measure the tiny forces between the tip and the sample, such as a molecule, to create an image. In the present experiments, the molecule investigated was pentacene. Pentacene is an oblong organic molecule consisting of 22 carbon atoms and 14 hydrogen atoms measuring 1.4 nanometers in length. The spacing between neighboring carbon atoms is only 0.14 nanometers—roughly 1 million times smaller then the diameter of a grain of sand. In the experimental image, the hexagonal shapes of the five carbon rings as well as the carbon atoms in the molecule are clearly resolved. Even the positions of the hydrogen atoms of the molecule can be deduced from the image.

“The key to achieving atomic resolution was an atomically sharp and defined tip apex as well as the very high stability of the system,” said Leo Gross. To image the chemical structure of a molecule with an AFM, it is necessary to operate in very close proximity to the molecule. The range, where chemical interactions give significant contributions to the forces, is less than a nanometer. To achieve this, the IBM scientists were required to increase the sensitivity of the tip and overcome a major limitation: Similar to the way two magnets would attract or repel each other when getting close, the molecules would easily be displaced by or attach to the tip when the tip was approached too closely—rendering further measurements impossible.

Gross added, “We prepared our tip by deliberately picking up single atoms and molecules and showed that it is the foremost tip atom or molecule that governs the contrast and resolution of our AFM measurements.” A tip terminated with a carbon monoxide (CO) molecule yielded the optimum contrast at a tip height of approximately 0.5 nanometers above the molecule being imaged and—acting like a powerful magnifying glass—resolved the individual atoms within the pentacene molecule, revealing its exact atomic-scale chemical structure.

Furthermore, the scientists were able to derive a complete three-dimensional force map of the molecule investigated. “To obtain a complete force map the microscope needed to be highly stable, both mechanically and thermally, to ensure that both the tip of the AFM and the molecule remained unaltered during the more than 20 hours of data acquisition,” says Fabian Mohn, who is working on his Ph.D. thesis at IBM Research – Zurich.

A topography of forces: The forces exerted on the tip above the pentacene molecule create a landscape that resembles a mountain ridge in this 3-D force map. The overall elevation represents the attractive forces between the tip and the molecule. The finer features on the ridge stem from repulsive forces between the tip and the pentacene molecule at closer tip–molecule distance. [Courtesy: IBM Research –Zurich]

To corroborate the experimental findings and gain further insight into the exact nature of the imaging mechanism, IBM scientist Nikolaj Moll performed first-principles density functional theory calculations of the system investigated. He explains, “The calculations helped us understand what caused the atomic contrast. In fact, we found that its source was Pauli repulsion between the CO and the pentacene molecule.” This repulsive force stems from the quantum mechanical effect of Pauli exclusion principle which states that two identical electrons can not approach each other too closely.

Reference
[1] “The Chemical Structure of a Molecule Resolved by Atomic Force Microscopy”,

Leo Gross, Fabian Mohn, Nikolaj Moll, Peter Liljeroth, and Gerhard Meyer,
Science, Volume 325, Issue 5944, pp. 1110 – 1114 (28 August 2009). Abstract.
[2] "Measuring the Charge State of an Adatom with Noncontact Atomic Force Microscopy",

Leo Gross, Fabian Mohn, Peter Liljeroth, Jascha Repp, Franz J. Giessibl, Gerhard Meyer,
Science, Volume 324, Issue 5933, pp. 1428 – 1431 (June 12, 2009). Abstract.

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Saturday, August 22, 2009

Tiniest Laser Developed -- Confirming 'Spaser' Concept

In a paper published in 'Nature' [1], a team of scientists reported the creation of the tiniest laser since its invention nearly 50 years ago, paving the way for a host of innovations, including superfast computers that use light instead of electrons to process information, advanced sensors and imaging. The research was conducted by Norfolk State University researchers Mikhail A. Noginov, Guohua Zhu and Akeisha M. Belgrave; Purdue University researchers Reuben M. Bakker, Vlad Shalaev and Evgenii E. Narimanov; and Cornell University researchers Samantha Stout, Erik Herz, Teeraporn Suteewong and Ulrich B. Wiesner.

Because the new device, called a "spaser" (which stands for 'Surface Plasmon Amplification by Stimulated Emission of Radiation') is the first of its kind to emit visible light, it represents a critical component for possible future technologies based on "nanophotonic" circuitry, said Vladimir Shalaev of Purdue University.

[Image courtesy: Birck Nanotechnology Center, Purdue University] The color diagram (a) shows the nanolaser's design: a gold core surrounded by a glasslike shell filled with green dye. When a light was shined on the spheres, plasmons generated by the gold core were amplified by the dye. The plasmons were then converted to photons of visible light, which was emitted as a laser. Scanning electron microscope images (b and c) show that the gold core and the thickness of the silica shell were about 14 nanometers and 15 nanometers, respectively. A simulation of the SPASER (d) shows the device emitting visible light with a wavelength of 525 nanometers.

Nanophotonics may usher in a host of radical advances, including powerful "hyperlenses" resulting in sensors and microscopes 10 times more powerful than today's and able to see objects as small as DNA; computers and consumer electronics that use light instead of electronic signals to process information; and more efficient solar collectors.

Nanophotonic circuits will require a laser-light source, but current lasers can't be made small enough to integrate them into electronic chips. Now researchers have overcome this obstacle, harnessing clouds of electrons called "surface plasmons," instead of the photons that make up light, to create the tiny spasers.

The "spaser-based nanolasers" created in the research were spheres 44 nanometers in diameter - more than 1 million could fit inside a red blood cell. The spheres were fabricated at Cornell, with Norfolk State and Purdue performing the optical characterization needed to determine whether the devices behave as lasers.

Past 2Physics article by Vlad Shalaev and his collaborators:
"Large Broadband Invisibility Cloak for Visible Light"


To act like lasers, Spasers require a "feedback system" that causes the surface plasmons to oscillate back and forth so that they gain power and can be emitted as light. Conventional lasers are limited in how small they can be made because this feedback component for photons, called an optical resonator, must be at least half the size of the wavelength of laser light.

The researchers, however, have overcome this hurdle by using not photons but surface plasmons, which enabled them to create a resonator 44 nanometers in diameter, or less than one-tenth the size of the 530-nanometer wavelength emitted by the spaser.

The findings confirm work by physicists David Bergman (Tel Aviv University) and Mark Stockman (Georgia State University), who first proposed the spaser concept in 2003 [2].

"It's fitting that we have realized a breakthrough in laser technology as we are getting ready to celebrate the 50th anniversary of the invention of the laser," Shalaev said.

The first working laser was demonstrated in 1960.

Reference
[1]
"Demonstration of SPASER-based Nanolaser" , M. A. Noginov, G. Zhu, A. M. Belgrave, R. Bakker, V. M. Shalaev, E. E. Narimanov, S. Stout, E. Herz, T. Suteewong, U. Wiesner,
Nature, doi:10.1038/nature08318, (Published online 16th August, 2009),
Abstract
[2] "Surface Plasmon Amplification by Stimulated Emission of Radiation: Quantum Generation of Coherent Surface Plasmons in Nanosystems", David J. Bergman and Mark I. Stockman, Phys. Rev. Lett. 90, 027402(2003). Abstract.

[Our presentation of this work is based upon a write-up by Emil Venere of Purdue University]

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Saturday, August 15, 2009

NIST Physicists Demonstrate Sustained Quantum Processing in Step Toward Building Quantum Computers

Jonathan Home

In a paper published online in Science Express, physicists at the Time and Frequency Division of the National Institute of Standards and Technology (NIST, Boulder, CO) have demonstrated sustained, reliable information processing operations on electrically charged atoms (ions). Their new device is able to perform a complete set of quantum logic operations without significant loss of information in transit. This path-breaking work overcomes significant hurdles in scaling up ion-trapping technology from small demonstrations to larger quantum processors.

“The significant advance is that we can keep on computing, despite the fact we’re doing a lot of qubit transport,” says first author Jonathan Home.

In the new demonstration, the team of researchers repeatedly performed a combined sequence of five quantum logic operations and 10 transport operations while reliably maintaining the 0s and 1s of the binary data stored in the ions, which serve as quantum bits (qubits) for a hypothetical quantum computer, and retaining the ability to subsequently manipulate this information. Previously, scientists at NIST and elsewhere have been unable to coax any qubit technology into performing a complete set of quantum logic operations while transporting information without disturbances degrading the later processes.

[Image Credit: J. Jost, NIST] NIST team demonstrated sustained, reliable quantum information processing in the ion trap at the left center of this photograph, improving prospects for building a practical quantum computer. The ions are trapped inside the dark slit (3.5 mm long and 200 micron wide) between the gold-covered alumina wafers. By changing the voltages applied to each of the gold electrodes, scientists can move the ions between the six zones of the trap.

The NIST group performed some of the earliest experiments on quantum information processing and has previously demonstrated many basic components needed for computing with trapped ions. The new research combines previous advances with two crucial solutions to previously chronic vulnerabilities: cooling of ions after transport so their fragile quantum properties can be used for subsequent logic operations and storing data values in special states of ions that are resistant to unwanted alterations by stray magnetic fields.

As a result, the NIST researchers have now demonstrated on a small scale all the generally recognized requirements for a large-scale ion-based quantum processor. Previously they could perform all of the following processes a few at a time, but now they can perform all of them together and repeatedly: (1) “initialize” qubits to the desired starting state (0 or 1), (2) store qubit data in ions, (3) perform logic operations on one or two qubits, (4) transfer information between different locations in the processor, and (5) read out qubit results individually (0 or 1).

Through its use of ions, the experiment showcases one promising architecture for a quantum computer, a potentially powerful machine that theoretically could solve some problems that are currently intractable, such as breaking today’s most widely used encryption codes. Relying on the unusual rules of the submicroscopic quantum world, qubits can act as 0s and 1s simultaneously, unlike ordinary digital bits, which hold only one value at any given time. Quantum computers also derive their power from the fact that qubits can be “entangled,” so their properties are linked, even at a distance. Ions are one of a number of different types of quantum systems under investigation around the world for use as qubits in a quantum computer. There is no general agreement on which system will turn out to be the best.

These experiments stored the qubits in two beryllium ions held in a trap with six distinct zones. Electric fields are used to move the ions from one zone to another in the trap, and ultraviolet laser pulses of specific frequencies and duration are used to manipulate the ions’ energy states. The scientists demonstrated repeated rounds of a sequence of logic operations (four single-qubit operations and a two-qubit operation) on the ions and found that operational error rates did not increase as they progressed through the series, despite transporting qubits across macroscopic distances (960 micrometers, or almost a millimeter) while carrying out the operations.

The researchers applied two key innovations to quantum-information processing. First, they used two partner magnesium ions as “refrigerants” for cooling the beryllium ions after transporting them, thereby allowing logic operations to continue without any additional errors due to heating incurred during transport. The strong electric forces between the ions enabled the laser-cooled magnesium to cool down the beryllium ions, and thereby remove heat associated with their motion, without disturbing the stored quantum information. The new experiment is the first to apply this “sympathetic cooling” in preparation for successful two-qubit logic operations.

The other significant innovation was the use of three different pairs of energy states within the beryllium ions to hold information during different processing steps. This allowed information to be held in ion states that were not altered by magnetic field fluctuations during ion storage and transport, eliminating another source of processing errors. Information was transferred to different energy levels in the beryllium ions for performing logic operations or reading out their data values.

The experiment began with two qubits held in separate zones of the ion trap, so they could be manipulated individually to initialize their states, perform single-qubit logic operations, and read out results. The ions were then combined in a single trap zone for a two-qubit logic operation and again separated and transported to different trap regions for subsequent single-qubit logic operations. To evaluate the effectiveness of the processes, the scientists performed the experiment 3,150 times for each of 16 different starting states. The experimental results for one and two applications of the sequence of operations were then compared to each other, as well as to a theoretical model of perfect results.

The NIST quantum processor worked with an accuracy of 94%, averaged over all iterations of the experiment. In addition, the error rate was the same for each of two consecutive repeats of the logical sequence, demonstrating that the operations are insulated from errors that might have been introduced by ion transport. The error rate of 6 percent is not yet close to the 0.01% threshold identified by experts for fault-tolerant quantum computing, Home notes. Reducing the error rate is a focus of current NIST research. Another issue in scaling up the technology to build a practical computer will be controlling ions in large, complex arrays of traps—work also being pursued by the group.

There are also more mundane challenges: The researchers successfully performed five rounds of the logic and transport sequence (a total of 25 logic operations plus 4 preparation and analysis steps), but an attempt to continue to a sixth round crashed the conventional computer used to control the lasers and ions of the quantum processor. Nonetheless, the new demonstration moves ion-trap technology significantly forward on the path to a large quantum processor.

Reference
J.P. Home, D. Hanneke, J.D. Jost, J.M. Amini, D. Leibfried and D.J. Wineland. "Complete methods set for scalable ion trap quantum information processing". Science Express (Published Online August 6, 2009). Abstract.

[We thank NIST for materials used in this posting]

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Saturday, August 08, 2009

Walking in the Quantum World

Michal Karski, Artur Widera, and Dieter Meschede (Left to Right)

[This is an invited article based on recent works of the authors -- 2Physics.com]

Authors: Artur Widera, Michal Karski and Dieter Meschede

Affiliation: Institut für Angewandte Physik der Universität Bonn, Germany

While the random motion of classical particles is well understood and such random walks have found their way into most fields of modern science, quantum particles are expected to behave differently. The intriguing new properties of these quantum walks may lead to novel applications in quantum information science as quantum search algorithms, for example, or yield insight into the transition from the classical to the quantum regime. A quantum walk in position space has recently been observed with single Caesium (Cs) atoms by fluorescence microscopy [1].

Imagine a walker, e.g. a particle, which can move stepwise on a line. In each time step, let the walker now move randomly to the right or to the left, just as the diffusive Brownian motion of a particle. The probability of finding the particle at a certain position is given by a binomial distribution, with high probability at the initial position and a width that scales with the square-root of the number of steps taken. This well known scaling serves as the basis for numerous models in modern science, for example to estimate the speed of searching algorithms.

In the quantum world, two effects change the particle’s motion drastically: First, quantum particles can be in so-called coherent superpositions, for example, of moving to the left and to the right. This sounds weird, but atoms in coherent superpositions are routinely used, for instance, in atomic clocks where the atoms are in a superposition of two spin states. As a consequence of these coherent superpositions, the quantum particle is delocalized over two lattice sites as it moves simultaneously to the left and to the right. If this delocalization is successively repeated for more and more steps, the particle delocalizes over more and more sites of the line. At certain positions, two parts of the delocalized atom can be re-combined at a common site. Here, the second quantum effect becomes important:
Quantum mechanical objects are described by wave functions and as such they can interfere. Depending on their respective path, they can amplify or extinguish each other. This leads to a drastically changed probability distribution of finding a particle at a certain position. In particular, for a quantum walk it is unlikely to find the particle at the initial position. Its distribution rather shows pronounced peaks with large probability at the outermost edges. The width of the resulting distribution scales linearly with the number of steps. This ballistic scaling is envisioned to speed up search algorithms in quantum search devices or quantum computers.

Figure 1: (a) A single atom is trapped at an initial site of an optical lattice and prepared in a coherent superposition of two states, red and blue. (b) The two states are selectively shifted into opposite directions along the lattice, delocalizing the atom (c) over two sites. (d) After another step of coherent superposition and state-dependent shifting, two parts of the atomic wave function are re-combined, giving rise to matter wave interference.

Experimentally, we realized a quantum walk using single Cs atoms. In an ultra-high vacuum, the atoms were cooled by laser light to approximately 10 µK and then trapped in a so-called optical lattice. This is generated by two counter propagating laser beams forming a standing wave which provides a periodic intensity pattern in space. The Cs atoms are trapped in the intensity maxima of this standing wave. To create coherent superpositions, we used microwave radiation which allows us manipulating the internal states of the atom, similar to those used in atomic clocks. The superposition created is then transferred to position space by using the fact that the optical lattice can be state-selectively moved [2].

This means one of the two internal states is moved to the left, the other to the right. Experimentally this is realized by controlling the polarization of the counter propagating laser beams. After a shifting step, each part of the wave function is again brought into a coherent superposition before a next shifting step and so forth. Finally, after a certain number of steps the system is illuminated and imaged onto a CCD camera [3]. Due to the measurement, the delocalized wave function collapses to one position where the Cs atom is detected. To reconstruct the distribution, hundreds of identical measurements were performed.

Figure 2: Reconstructed wave function of a single atom in the optical lattice. (a) The atom is localized at a lattice site. (b) The atom has performed a 24 step random walk. (c) The atom has performed a 24 step quantum walk.

From the measurements we find that a particle performing a quantum walk shows the expected linear spreading. If the coherence of the process is intentionally destroyed, the classical random walk behaviour is recovered. Our system shows the quantum regime for approximately ten steps of the walk, where the particle is delocalized over more than twenty lattice sites. Then, imperfections, noise and uncontrolled interaction with the environment turns the quantum walk gradually into a random walk.

The quantum walk not only illustrates the mind-boggling laws of quantum mechanics; it might serve as a first step towards the development of novel search algorithms exploiting the properties of quantum mechanics and as a precursor for quantum information processing devices, such as quantum cellular automata [4-6]. Moreover, it can yield deeper insight into the transition from the microscopic quantum world to our every-day classical world.

References
[1]
M. Karski, L. Förster, J. Choi, A. Steffen, W. Alt, D. Meschede, and A. Widera, "Quantum Walk in Position Space with Single Optically Trapped Atoms", Science 325, 174 (2009). Abstract.
[2] O. Mandel, M. Greiner, A. Widera, T. Rom, T.W. Hänsch, and I. Bloch, "Coherent transport of neutral atoms in spin-dependent optical lattice potentials", Phys. Rev. Lett. 91, 010407 (2003). Abstract.
[3] M. Karski, L. Förster, J. Choi, W. Alt, A. Widera, and D. Meschede, "Nearest-Neighbor Detection of Atoms in a 1D Optical Lattice by Fluorescence Imaging", Phys. Rev. Lett. 102, 053001 (2009). Abstract.
[4] R. Raussendorf, "Quantum cellular automaton for universal quantum computation", Phys. Rev. A 72, 022301 (2005). Abstract.
[5] D. J. Shepherd, T. Franz, and R. F. Werner, "Universally Programmable Quantum Cellular Automaton", Phys. Rev. Lett. 97, 020502 (2006). Abstract.
[6] K. G. H. Vollbrecht and J. I. Cirac, "Reversible universal quantum computation within translation-invariant systems", Phys. Rev. A 73, 012324 (2006). Abstract.

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Saturday, August 01, 2009

Upcoming Physics Conferences

[To add an upcoming physics conference to this list, please send an email to 2Physics@gmail.com ]

Aug 17-23: Astrophysics and Cosmology after Gamow: Recent progress and new horizons: 4-th Gamow International Conference and 9-th Gamow Summer School "Astronomy and beyond: Astrophysics, Cosmology, Radioastronomy, High Energy Physics and Astrobiology" (Odessa, Ukraine)
Aug 18-28: International Summer School on Astroparticle Physics (Nijmegen, Netherlands)
Aug 21-27: Bogolyubov Conference "Problems of Theoretical and Mathematical Physics" (Moscow-Dubna, Russia)
Aug 25-28: Primordial Gravitational Waves (Cambridge, UK)
Aug 30-Sep 03: Joint International Hadron Structure Conference (Tatranska Strba, Slovak Republic)
Sep 02-05: Challenges in Cosmology (Talloires, France)
Sep 07-11: Cosmo-09 (CERN, Geneva)
Sep 11-14: Particle Cosmology (CERN, Geneva)
Sep 14-19: Intl Workshop on Weak Interactions and Neutrinos (L'Aquila, Italy)
Sep 15-18: Bogolyubov Kyiv Conference "Modern Problems of Theoretical and Mathematical Physics" (Kyiv, Ukraine)
Sep 24-28: International Conference on Electron Dynamics in Semiconductors, Optoelectronics and Nanostructures (Montpellier, France)
Sep 28-Oct 02: Physical Properties of Nanosystems (Yalta, Ukraine)
Oct 02-03: Colloquium in memory of Jan Stern: From Current Algebra to the Standard Model and Beyond (Paris, France)
Oct 08-09: Space, Time and Beyond (Golm, Germany)
Oct 11-14: 50 years of the Aharonov-Bohm Effect: Concepts and Applications (Tel Aviv, Israel)
Oct 26-30: Galileo - Xu Guangqi Meeting on the Sun, the Stars, the Universe and General Relativity (Shanghai, China)
Nov 29-Dec 04: Intl Conference on Hadron Spectroscopy (Tallahassee, FL, USA)
Nov 30-Dec 03: International Symposium on Computational Mechanics (Hong Kong and Macau, China)
Dec 15-20: Topical Conference on Elementary Particles, Astrophysics, and Cosmology (Fort Lauderdale, Florida, USA)
Jan 04-08: 3rd Intl Workshop On High Energy Physics in the LHC Era (Valparaiso, Chile)
Jan 05-08: 39th Winter Meeting on Statistical Physics (Taxco, Guerrero, Mexico)
Feb 08-13: 46th Winter School of Theoretical Physics: Quantum Dynamics and Information: Theory and Experiment (Ladek Zdroj, Poland)
Feb 23-26: Gravitational Wave Symposium (John Hopkins U., USA)
Jun 28-Jul 02: LISA 8 (Stanford U., USA)
Jul 05-09: GR19 (Mexico City, Mexico)

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