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Two-qubit gate of combined single spin rotation and inter-dot spin exchange in a double quantum dot, a useful technique for entanglement control

Researchers have studied various types of quantum element in the search for the components with which to implement quantum computation. Most promising among the candidates are semiconductor quantum dots (QDs) based on electron spin and highly-rated for their potential scalability. Single qubits, the smallest logical unit of a quantum circuit, have been realized by several groups around the world. In contrast, two-qubit gates, which operate on four computational basis states to control quantum entanglement and are essential for the construction of a set of universal quantum gates, have not yet been demonstrated for QD spin qubits.

Here, we demonstrate an electron spin-based all-electrical two-qubit gate consisting of single-spin rotations and interdot spin exchange in a double QD. A partially entangled output state is obtained by setting the two-qubit gate to an initial, uncorrelated state. We find that the degree of entanglement is controllable by the exchange operation time. The approach represents a key step towards the realization of a set of universal multiple-qubit gates necessary for quantum computation.

This research was carried out in collaboration with NTT Basic Research laboratories.
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R. Brunner, Y.-S. Shin1, T. Obata, M. Pioro-Ladriere, T. Kubo, K. Yoshida, T. Taniyama, Y. Tokura, and S. Tarucha, "Two-qubit gate of combined single spin rotation and inter-dots spin exchange in a double quantum dot", Physical Review Letters 107 (2011), DOI: 10.1103.
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School of Engineering

Keep an Eye on PPi: The Vacuolar-Type Proton-Pyrophosphatase Regulates Postgerminative Development in Arabidopsis

Pyrophosphate (PPi), being toxic to the cell itself at high levels, is efficiently hydrolyzed by enzymes called pyrophosphatases (PPases). In plants, unlike animals or yeast, there is no cytosolic PPases, instead a unique vacuolar membrane bound H+-PPase is responsible for PPi degradation and vacuole acidification as well. A previous report claimed that the latter function, namely the vacuolar acidification, is the major role of this PPase.
Here, we discovered that upon seed germination, seedlings of the H+-PPase defective fugu5 mutant failed to produce sucrose from lipid stores due to high cytosolic PPi, which is released by active metabolism in germinating seeds. The addition of sucrose or specific removal of cytosolic PPi in the fugu5 mutant, by the expression of the yeast cytosolic PPase IPP1, rescued both cellular and morphological phenotypes of fugu5. Therefore, in contrast to the previous belief that favored a major role of plant H+-PPase as a proton pump, here we suggest based on solid evidence, that the removal of PPi is the central role of the plant H+-PPase.
Importantly, the growth of the transgenic lines was significantly improved (AVP1pro:IPP1; Figure bottom panels), suggesting that the IPP1 gene may represent a powerful tool for genetic engineering of plants with higher yield and increased biomass production.
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Ali Ferjani, Shoji Segami, Gorou Horiguchi, Yukari Muto, Masayoshi Maeshima, and Hirokazu Tsukaya, "Keep an Eye on PPi: The Vacuolar-Type H+-Pyrophosphatase Regulates Postgerminative Development in Arabidopsis", The Plant Cell, published online on 29 August 2011.(Japan time)
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School of Science

Traffic Jams Reduce Hydrolytic Efficiency of Cellulase on Cellulose Surface

Cellulose is a major component of the cell wall of plants and the most abundant form of biomass on earth. A linear polymer of glucose, it can be converted into biofuels and precursor materials of bioplastics. The efficient degradation of cellulose would thus make it possible to produce fuels and chemicals from plant resources. However, the slow rate of the biochemical conversion of cellulose by the enzyme cellulase is a bottleneck in the process.

Associate Professor Kiyohiko Igarashi in the Graduate School of Agricultural and Life Sciences in the University of Tokyo and co-workers have succeeded in observing "traffic jams" of cellulase molecules on the surface of cellulose using high-speed atomic force microscopy.

This "molecular congestion" is not only a new finding as basic science, but also is the first proof that this traffic jam of cellulase molecules is responsible for reducing the rate of cellulase hydrolysis. The findings of the present study provide a useful guide to developing an efficient system for the utilization of cellulose biomass near future.

This research was carried out in collaboration with Kanazawa University and VTT Technical Research Center, Finland.
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Kiyohiko Igarashi, Takayuki Uchihashi, Anu Koivula, Masahisa Wada, Satoshi Kimura, Tetsuaki Okamoto, Merja Penttilä, Toshio Ando, Masahiro Samejima, "Traffic Jams Reduce Hydrolytic Efficiency of Cellulase on Cellulose Surface," Science 333 (2011):1279-1282, doi:10.1126/science.1208386.

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Graduate School of Agricultural and Life Sciences

Three classical Cepheids and their impact on the history of star formation in the Galactic Nuclear Bulge.

The region within a few hundred light years of the central black hole of our Galaxy, often called the Nuclear Bulge, presents us with various interesting objects and phenomena. We discovered in this region classical Cepheid variable stars, pulsating supergiants, whose ages can be derived accurately from their periods. All three of our Cepheids have pulsation periods near 20 days and ages close to 25 million years (Myr), suggesting that active star formation occurred at the period around their births. In contrast, the absence of shorter-period Cepheids shows that the star formation rate was lower between 30 and 70 Myr ago. We suggest that the rate of star formation in this region has changed on a time scale of a few tens of millions of years possibly due to a stochastic gas inflow into the Nuclear Bulge.

Figure: One of the classical Cepheid variable stars discovered

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Matsunaga, N., Kawadu, T., Nishiyama, S., Nagayama, T., Kobayashi, N., Tamura, M., Bono, G., Feast, M. W. & Nagata, T.
"Three classical Cepheid variable stars in the Nuclear Bulge of the Milky Way"
Nature, published online on 24 August 2011 (DOI: 11.1038/nature10359)

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School of Science

Long-life artificial hip joint reaches the bedside -- Interdisciplinary research making strides

In a critical step toward helping sufferers of joint pain and walking disability, University of Tokyo researchers have developed what could be the first in a new generation of artificial joints. Artificial hip joints have helped many people with walking trouble, but long-term use sometimes causes loss of the surrounding bone. This bone loss is known to result from immune reaction to tiny particles that form as a result of wear of the joint's polyethylene liner. In these cases, an operation to replace the entire joint is sometimes necessary.

The new artificial joint, a result of on-campus collaboration between materials science and medical researchers, is to reach the patient soon. It produces remarkably few wear particles, even after simulating 70 years of walking. Researchers say that the use of a polymer very close in structure to the cell membrane has proved a breakthrough in the creation of the new artificial joint.

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Japanese eel eggs tracked down!

Did you know that eels were born at sea? Even though the eel is a very familiar animal to us (particularly in Japan), its life-cycle is still shrouded in mystery. In fact, until recently, it wasn't even known where they went to spawn. That was until 2009, when a research group centered on the University of Tokyo's Atmosphere and Ocean Research Institute succeeded in collecting eel eggs from the wild for the first time. On this summer's research voyage, the team collected 147 eel eggs, even more than last time, by accurately predicting the spawning site and timing. The day when the mystery of the eel's spawning is unraveled is not far off.
A sample of the eggs collected on this voyage is now on display at the University Museum in a special exhibition titled "Eel Expo Tokyo 2011 - Mysterious Creature" from Saturday, 16 July to Sunday, 16 October (admission free).
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Atmosphere and Ocean Research Institute
University Museum "Eel Expo - the strangest of creatures"

Aging proteins Formation of allergic substances by air pollution

The reaction between pollen proteins and mixtures of ozone and nitrogen oxides, particularly in air masses exposed to traffic-related pollution and high levels of sunlight in summer (photochemical smog), leads to a strengthening of the allergy-inducing effect of pollen. This research explains for the first time the affect of humidity and temperature on the chemical aging of proteins by reactive gases and air pollutants. With increased humidity pollen proteins change phase from glassy into a gel-like state, in which state they are much more easily chemically aged. This study indicates that protein particles might have enhanced allergenicity in hot and humid polluted urban air in summer.
This research was conducted by Manabu Shiraiwa, a PhD student at the Max Planck Institute for Chemistry on the University of Tokyo-Japanese Ministry of Education, Culture, Sports, Science and Technology Support for Long-term Study Abroad Program, and Dr. Ulrich Poschl, his supervising professor and research group leader, in collaboration with Dr. Markus Ammann (Paul Scherrer Institute, Switzerland) and Prof. Thomas Koop (Bielefeld University, Germany).

Figure: Fluorescence microscope picture of birch pollen.
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Manabu Shiraiwa, Markus Ammann, Thomas Koop and Ulrich Poschl,
"Gas uptake and chemical aging of semi-solid organic aerosol particles",
Proceedings of the National Academy of Sciences of the United States of America (PNAS).Published online on 20 June 2011
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School of Science

How did plants explore unique intracellular trafficking routes?

Endosomal trafficking plays an integral role in various eukaryotic cell activities. In animal cells, a member of the RAB GTPase family, RAB5, is known as a key regulator of various endosomal functions, including endosomal fusion, endosomal signaling, and endosomal motility. In addition to orthologs of animal RAB5, land plants harbor the plant-unique RAB5, conserved in all land plant lineages examined thus far. Previously, it was unknown and of much interest why plants evolved two distinct types of RAB5 and how they differed functionally. Plants also acquired the unique machinery for membrane fusion (SNARE protein) during evolution. The presence of plant-specific RAB and SNARE suggested diversification of membrane trafficking pathways was achieved by molecular evolution of RAB and SNARE in a lineage specific manner; however, this has not been demonstrated experimentally. Using Arabidopsis thaliana, we showed the plant-unique RAB5 (ARA6) promotes membrane fusion between endosomes and the plasma membrane, with seed plant-unique R-SNARE (VAMP727). This pathway has not been described in plant cells thus far. We further demonstrated that ARA6 has a functional role in the salinity stress response. Our results indicate that plants explored a new intracellular trafficking route from endosomes to the plasma membrane during evolution, which plays an important role in responding to environmental cues.

Figure: Colocalization of ARA6 (red, plant-unique RAB GTPase) and SYP121 (green, SNARE protein on the plasma membrane)
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Kazuo Ebine, Masaru Fujimoto, Yusuke Okatani, Tomoaki Nishiyama, Tatsuaki Goh, Emi Ito, Tomoko Dainobu, Aiko Nishitani, Tomohiro Uemura, Masa H. Sato, Hans Thordal-Christensen, Nobuhiro Tsutsumi, Akihiko Nakano, and Takashi Ueda "A membrane trafficking pathway regulated by the plant-specific RAB GTPase ARA6" Nature Cell Biology Online Edition: 2011/6/13 (Japan time), doi:10.1038/ncb2270
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School of Science

Discovery of a huge new deposit of rare-earth elements in the Pacific Ocean

Rare-earth elements are crucial for novel electronic equipment and green-energy technologies and world demand is rapidly increasing. Associate Professor Yasuhiro Kato and his team at the Department of Systems Innovation of the University of Tokyo's Graduate School of Engineering have discovered a new type of mineral resource, named REY (rare-earth elements and yttrium)-rich mud, distributed in vast quantities throughout a large part of the Pacific Ocean. REY-rich mud containing up to approximately 0.2 percent by weight total REY occurs across the central north and southeastern Pacific Ocean in average thicknesses of approximately 24 m and 8 m, respectively. Our data show that REY stored in these Pacific mud deposits amounts to a possible resource 100 to 1,000 times greater than the world's current land reserves of 110,000,000 tonnes of REY oxides, depending on local stratigraphic continuity and thickness of the REY-rich mud. Uptake by materials such as hydrothermal Fe-oxyhydroxides and phillipsite seems to be responsible for the high REY content, and consequently REY are readily recovered by simple acid leaching and are a suitable resource for development. The newly discovered REY-rich mud may constitute a highly promising source of rare earth elements.
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Yasuhiro Kato, Koichiro Fujinaga, Kentaro Nakamura, Yutaro Takaya, Kenichi Kitamura, Junichiro Ohta, Ryuichi Toda, Takuya Nakashima and Hikaru Iwamori, "Deep-sea mud in the Pacific Ocean as a potential resource for rare-earth elements" Nature Geoscience Online Edition: 2011 / 7 / 4 at 2:00 (Japan time) (doi:10.1038/ngeo1185)
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School of Engineering

Optical switching magnet composed of iron ion and organic molecules: Discovery of a light-induced spin-crossover magnet

The research group of Prof. Shin-ichi Ohkoshi (Department of Chemistry, School of Science, The University of Tokyo) has reported the discovery of a new type of photomagnet which transitions from paramagnetic state (nonmagnetically ordered phase) to ferromagnetic state (magnetically ordered phase) on irradiation by blue light. The photomagnet has a ferromagnetic phase transition temperature of 20K (-253 centigrade). Transition is caused by a light-induced spin-crossover phenomenon, and the magnet is returned to the original paramagnetic state by thermal annealing. The material is composed of iron and niobium ions, organic molecules, and cyano groups. This light-induced spin-crossover magnet can contain a large proportion of organic molecules and may be the first step toward a flexible optical-magnetic material.
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Shin-ichi Ohkoshi, Kenta Imoto, Yoshihide Tsunobuchi, Shinjiro Takano and Hiroko Tokoro, "Light-induced Spin-crossover magnet" Nature Chemistry Online Edition: 2011/6/6 at 2:00 AM (Japan time)
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School of Science

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