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The Photon Science Center The University of Tokyo
MAKOTO GONOKAMI DIRECTOR THE P
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The Photon Science Center The University of Tokyo

MAKOTO GONOKAMI
DIRECTOR
THE PHOTON SCIENCE CENTER, THE UNIVERSITY OF TOKYO

1. THE UNIVERSITY OF TOKYO

Founded in 1877, the University of Tokyo (UTokyo) is Japan's oldest national university. During its long and distinguished history, UTokyo has produced numerous outstanding scholars, scientists, and social and political leaders of Japan, including eight Nobel laureates*, 16 prime ministers of Japan, and one Fields Medal winner**. UTokyo not only ranks as the top university in Japan but also is highly regarded as a world-class university in terms of education and research.

UTokyo is a vast educational and research university that encompasses about 5,800 faculty members and researchers, and nearly 28,000 students at its 10 faculties, 15 graduate schools, 11 institutes, 13 university-wide centers, and two Todai Institutes for Advanced Study (as of May 2013). Students are roughly evenly divided between undergraduate and graduate students. Graduate students themselves are roughly evenly divided between master's and doctoral programs. Approximately 9 % of the students are international students, and most of these international students are enrolled in graduate programs. UTokyo offers courses in essentially all academic disciplines and conducts research across the full spectrum of academic activity. The university aims to provide its students with a rich and varied academic environment that ensures opportunities for both intellectual development and the acquisition of professional knowledge and skills.

Fig. 1: Yasuda Auditorium and Akamon (the Red Gate).

Today, UTokyo continues to strengthen its international outlook. The university is actively expanding its network with leading universities worldwide and increasing degree programs that can be completed entirely in English.

* Ei-ichi Negishi (2010 Chemistry), Yoichiro Nambu (2008 Physics), Masatoshi Koshiba (2002 Physics), Kenzaburo Oe (1994 Literature), Eisaku Sato (1974 Peace), Leo Esaki (1973 Physics), Yasunari Kawabata (1968 Literature), and Sin-Itiro Tomonaga (1965 Physics).
** Kunihiko Kodaira (1954)

2. THE PHOTON SCIENCE CENTER

The Photon Science Center (PSC) was established in 2008 as a university-wide core and international center for research into and education of optical science. Photon science is a foundation of modern science and technology and a frontier field accelerating technological revolution. PSC aims to systematically reconstruct the knowledge of photon science through advanced research outcomes. Our emphasis is also on training of PhD students and young researchers.

PSC's particular focus is to open up novel technologies to generate, manipulate, and utilize optical waves and photons. Additionally, we are conducting education and research to develop advanced photon science in an interdisciplinary manner through collaborations with both domestic and overseas research centers. Through these activities, PSC is rapidly emerging as a global center-of-excellence in photon science.

PSC is leading the Advanced Photon Science Alliance (APSA), a network of five core universities and institutes in the Tokyo area to develop new laser technologies and applications under the slogan of "Photon Science by the Ultimate Control of Light." APSA in turn belongs to the Photon Frontier Network, a nation-wide ten-year program (2008-2017) to explore the frontiers of photon science and technology, foster young researchers, and promote collaboration with industry.

3. RESEARCH

Many-body Quantum Physics by Light-matter Interaction

We are trying to explore new aspects of many-body quantum systems and their exotic quantum optical effects through designed light-matter interactions. Our current target consists of a wide variety of matter, including excitons and electron-hole ensembles in semiconductors, antiferromagnetic magnons and ultracold atomic gases. In particular, we have been investigating the Bose-Einstein condensation phase of excitons, which is considered the ground state of electron-hole ensembles but has, as yet, not been demonstrated experimentally. Based on quantitative spectroscopic measurements, the temperature and density are determined for an exciton gas in a quasi-equilibrium condition trapped inside a high purity crystal kept below 1 K. We are now investigating a stable and quantum degenerate state of dark exciton gas at such very low temperatures.

We have also been investigating "universal" properties of interacting quantum systems using ultracold atomic gases. Our 6Li-7Li mixed system is ideal for simulating both few- and many-body systems of bosons, fermions and their mixture.

Optical Phenomena in Artificial Micro- and Nano- Structures

We investigate novel optical and terahertz-wave responses for some artificial nanostructures obtained by advanced micro-fabrication technologies. We have been investigating giant optical activity induced in quasi twodimensional chiral structures, and active control of it by photo-excitation technique or micro-electro-mechanical- systems (MEMS) technology.

THz Technology

Electromagnetic radiation in the terahertz (THz) spectral range is a powerful tool to sensitively probe the response of metal and semiconductor conduction electrons and to identify molecules. We are developing new THz technology, especially for THz polarization control. Recently, we succeeded in realizing THz polarization pulse shaping with arbitrary field control, and also in generating broadband THz cylindrical vector beams. These unique methods can be applied to many scientific fields.

Quantum Information

Photons are microscopic objects which clearly manifest bizarre properties of quantum mechanics, and they can be transmitted over a long distance through optical fibers with small decoherence. As a result, photons play an important role in the field of quantum information. We are conducting experimental and theoretical research toward better manipulation of quantum information on photons and a deeper understanding of the underlying quantum physics.

Fig. 2: Experimental setup for exciton Bose-Einstein Condensation.

Fig. 3: Schematic picture of experimental setup for THz polarization pulse shaping with arbitrary field control.

Quantum cryptography or quantum key distribution (QKD) is currently the most promising application of quantum information. We are conducting theoretical research aiming at establishing security proofs for various QKD protocols. Recently, we succeeded in proving the security of a protocol based on binary phase modulation of a laser pulse train, which has been known to be particularly suited for implementation.

We are tackling various issues arising in quantum communication through optical fibers in collaboration with Osaka University. This includes research on very efficient schemes for protecting quantum information from the noises in the optical fibers. We have also developed a quantum interface to convert the wavelength of a photon from the visible range to telecommunication wavelengths without destroying quantum information encoded on it.

We are also interested in the strong interaction between photons and matter, and are in the process of building a cavity quantum electrodynamics (cavity QED) experiment. An optical cavity formed with a pair of highly reflective mirrors provides a means to enhance the electromagnetic field of even a single photon to the extent that it can efficiently interact with a single atom. We are aiming to exploit this strong interaction to observe and make use of nonlinear optical processes at a few-photon level, which are crucial for quantum information technology.

Precision Metrology

Laser-based measurement techniques are widely used in basic research, and also in applications in industry. In particular, interferometry, which makes use of the high coherence of laser light, is very useful as a tool for high precision measurements of mechanical displacements, shapes of objects, very small variations of the optical properties of a medium, and so on. There are also huge interferometers all over the world built for the detection of the waves of the distortion of space-time; the "gravitational waves" predicted by the general theory of relativity. With the aim of improving the sensitivity of interferometers, we are working on the reduction of noise, such as thermal noise and shot noise, which limit their ultimate sensitivity. For example, the sensitivity determined by the shot noise grows in proportion to the square root of the power of the light source; so, we are developing a high-power laser system that employs the coherent addition technology to bind the output from multiple high-power fiber-lasers.

In order to handle this very intense light, the performance of mirrors and other optical elements also needs to be improved. In particular, heat that is generated when the light is incident at such elements becomes serious because even little heat can distort their optical properties at this level of precision. We are developing an optical absorption measurement apparatus that employs a Michelson interferometer to quantitatively observe this small optical absorption that occurs in a transparent optical material such as sapphire or synthetic silica, so as to help improve the characteristics of these basic optical materials.

Fig. 4: Fiber lasers.

Fig. 5: Coherent addition system.

Cold Molecules

Since the advent of laser cooling, the field of cold atoms and molecules expanded quite rapidly, enabling research on condensed matter systems using dilute gases isolated in an ultrahigh vacuum chamber. Our primary interest is to produce the first Bose-Einstein condensate of polar molecules. Unlike atoms, polar molecules have a large electrical dipole moment and interact with each other through long-range forces. Being confined in an optical lattice, quantum degenerate gas of polar molecules should show novel quantum phases, not seen in other systems. People also envision constructing a quantum computer based on ultracold polar molecules confined in an electromagnetic potential.

The strategy that we took for producing a gas of ultracold polar molecules was somewhat ambitious. We cooled rubidium and potassium atoms to ultracold temperatures, and then tried to combine those atoms to molecules using light pulses. The first pulse combines atoms into loosely bound molecules and the next pulse turns them into tightly bound, ground state molecules. The second process was especially difficult to realize experimentally, since the process connects two quantum states directly: any source of decoherence should be eliminated from the experiment. After years of experimental work, in 2010 we succeeded in producing ultracold KRb molecules whose temperature was about 100 micro Kelvin.

Ultracold molecules have made an impact on various fields of physics. One of the areas of focus of the group right now is to perform a high precision measurement on the frequency of vibrational transition of ultracold molecules. The goal is to set an upper limit for change in electron-to-proton mass ratio over time, which is one of the good tests for checking the validity of Einstein's equivalence principle.

Strong-Field and Ultrafast Phenomena

When exposed to visible-to-mid-infrared pulses with intensity typically higher than 1014 W/cm2, atoms and molecules exhibit extreme nonlinear response such as tunneling ionization and high-harmonic generation (HHG). HHG as well as free-electron lasers represent ultrashort coherent light sources in the extreme-ultraviolet and soft x-ray ranges. We are one of the frontrunners in the theory of femtosecond and attosecond motion of electrons on the atomic scale under these pulses.

Our starting point is the time-dependent Schr철dinger equation (TDSE). By numerically solving it exactly, we are looking into the slow motion of an "instant." We have successfully revealed how the electrons interact with each other during photoionization, previously considered just instantaneous. Direct TDSE simulations become rapidly prohibitive as the number of electrons increases. Our recent achievements include the development of new theories to allow compact and, at the same time, accurate representation of multielectron dynamics. We are also tackling the problem of how to trace electronic motion hidden in thus computationally wave functions.

Fig. 6: Ultracold potassium atoms in a magneto-optical trap (the orange-looking object at the center of the image).

Fig. 7: Computer-simulated birth of ionizing double electrons after attosecond two-photon absorption.

Fig. 8: Photon Ring.

Photon Ring Facilities

In collaboration with RIKEN, an APSA member institute, PSC is now constructing a totally new type of compact coherent light source, called "Photon Ring." Its circulating coherent high-power laser pulse with multiport wave length conversion capability will be able to deliver photons spanning from terahertz radiation to soft x-ray pulses with MHz repetition rates. Photon Ring will enable imaging and measurements of ultrafast and faint phenomena in the wide energy scale, which would have previously required a huge device such as synchrotron radiation source.

Generated photons will be used, for example, for soft x-ray microscopy to image living cells in real time by 4 and 13 nm high-harmonic photons with high resolution that visible light can never achieve. Other applications include high-resolution time- and angular resolved laser photoelectron spectrometry and real-time terahertz imaging. A wide spectrum of researchers from nanotechnology to medicine will be able to use Photon Ring Facilities, contributing more efficient solar cells, lowcarbon technology, bio-imaging, and a myriad of other advances.

4. COLLABORATION AND COOPERATION

PSC advances education and research through collaboration, acting as a hub of different projects, departments, and institutes in UTokyo, such as the Department of Physics, the Department of Applied Physics, the Department of Nuclear Engineering and Management, the Advanced Leading Graduate Course for Photon Science (ALPS), the Consortium on Education and Research on Advanced Laser Science (CORAL), Laser Alliance, the Laser and Synchrotron Research Center (LASOR), the Center for Ultrafast Intense Laser Science (CUILS), the Research Center for Spectrochemistry, the Institute for Photon Science and Technology (IPST), and the Innovative Center for Coherent Photon Technology (ICCPT).

CORAL is a collaborative initiative to strengthen master's course training in photon science. Two other universities and more than twenty industrial partners participate in CORAL, giving various lectures and practices. ALPS is a graduate course whose objective is to foster PhDs with wide-ranging interdisciplinary viewpoints and the ability to apply their knowledge. Its curriculum includes course work on innovation management, training by industrial engineers, and international internship.

LASOR has satellites of Photon Ring Facilities, which have recently developed a high-power extreme ultraviolet light source by an incredible 30-m enhancement cavity, and also successfully generated water-window highharmonics using intense infrared lasers.

IPST, founded in October 2013, is committed to further expansion of the network in photon science inside and outside UTokyo, and even worldwide. Together with CUILS and the Research Center for Spectrochemistry, PSC will support the activity of IPST.

We have also just launched a nine-year ICCPT project. Together with RIKEN and our industry partners, we will use coherent photon technologies to establish the science and technology of manufacturing that is reliable, innovative, personalized, and eco-friendly. The goal of this project is to build a "personalized and sustainable society," driven by rich diversity and ideas arising from individuals.

 

Mokoto Gonokami received a doctorate in physics from the University of Tokyo in 1985. He became a professor of the Department of Applied Physics at the University of Tokyo in 1998 and joined the Department of Physics, Graduate School of Science in 2012. His research fields are optical science, solid state physics, quantum electronics, and laser spectroscopy. He has been a vice president of the University of Tokyo since 2012.

 
AAPPS Bulletin        ISSN: 2309-4710
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