座長：岩村 顕也、大塚 悠介
|氏名： 三浦 彰太
指導教員名： 吉岡 孝高 准教授
発表題目（英語）： Development of femtosecond optical frequency comb in the ultraviolet via highly efficient wavelength conversion using passive optical cavity
要旨（英語）： Optical frequency comb (OFC) is a light source whose spectrum consists of equally spaced frequency line and is used for direct measurement of optical frequency and for ultrafast spectroscopy. To expand the spectrum region, optical frequency comb in the ultraviolet (UV) region has been developed. UV-OFC can be used as a light source not only to measure optical spectra but also to perform laser cooling with atoms such as carbon and oxygen which are important in a wide range of research fields.
Nonlinear crystals are often used to do wavelength conversion. For efficient wavelength conversion in nonlinear crystals, high intensity light is necessary. By using a passive optical cavity, pulses can be coherently stored inside the cavity and the intracavity power can be enhanced so that strong short wavelength femtosecond pulses can be generated.
In my presentation, I will explain how to store femtosecond pulses in the passive optical cavity and to do efficient wavelength conversion and talk about an optimized design of the cavity for our OFC.
|氏名： 松山 幹尚
指導教員名： 古澤 明 教授
発表題目（英語）： Photon Number Resolving Detection by Time Domain Multiplexed Array
要旨（英語）： Photon number resolving detection (PNRD) plays an essential role as a resource of non-linearity in quantum optics experiments. There are various applications, but we are focusing on its application to quantum computing. In order to realize a practicable fault-tolerant universal quantum computation, the generation of an extremely non-classical quantum state called a non-Gaussian state using PNRD is the most important task.
In the past, PNRD was approximately performed by multiplexed on/off detectors array, but we have developed superconducting transition edge sensor that acts as a true photon number resolving detector. To show its superiority, it is necessary to compare these two methods. For that purpose, I set up the experimental system of former method.
In this presentation, I will talk about the mathematical model of the PNRD from the quantum mechanical point of view and explain the experimental setup I constructed and the measurement result.
|氏名： 三橋 洋亮
指導教員名： 沙川 貴大 准教授
発表題目（英語）： The second law of thermodynamics under symmetry constraints
要旨（英語）： In the context of the second law of thermodynamics phrased in terms of work extraction, it is known that no work can be extracted from any number of copies of a quantum state by any unitary transformation, if and only if the state is the Gibbs states. This property is called complete passivity in the literature of quantum thermodynamics.? In the resource theory of thermodynamics, which provides a quantum information-theoretic foundation of thermodynamics, completely passive states play crucial roles as they are regarded as freely available states without any additional resources.
Given that possible operations are often restricted due to symmetries in real experimental setups, a fundamental problem is what states are completely passive if we are only allowed to perform unitary operations that respect given symmetry.? For symmetry described by a connected compact Lie group, we proved that completely passive states are only given by the generalized Gibbs states constructed from the associated Lie algebra. This statement is valid even if the symmetry operators are noncommutative, which gives an unconventional extension of the generalized Gibbs states. Furthermore, we proved the same result even if we explicitly introduce work storage as a quantum system, which is sometimes referred to as a "fully quantum" setup.? We also discuss the fundamental difference between our result and a previous study in N. Y. Halpern et al., Nature Communications 7, 12051 (2016). Our result lays the foundation of the second law of thermodynamics under symmetry constraints in the quantum regime.
|氏名： 山嶋 大地
指導教員名： 古澤 明 教授
発表題目（英語）： Toward Practical Realization of a Quantum Computer
要旨（英語）： The ultimate goal of our research is to realize a quantum computer with continuous variables. Recently, many demonstration experiments in this field have been making progress, but the optical system in such advanced experiments becomes very large and complicated. In this presentation, I will talk about how to solve the problem, my experiment based on that idea and a future plan for practical realization of a quantum computer.
|氏名： 宮崎 優
指導教員名： 塩見 雄毅 准教授
発表題目（英語）： Electric-Field Control of Surface Transport in Topological Semimetal Nanowires
要旨（英語）： Topological spintronics is spintronics using topological materials. Topological materials have special surface states called spin-momentum locking. This is a phenomenon that the electron spin is locked at the orthogonal direction of the momentum of electron. Mutual highly efficient conversion between electric current and spin current becomes possible using such surface states. However, conventional topological spintronics devices using topological insulators has following problems: 1) Expensive equipment or facilities (such as molecular beam epitaxy (MBE), pulse laser deposition (PLD) etc.) are necessary to synthesize high quality thin films of topological insulators. 2) To impede bulk transport, it can only be used at low temperature. 3) There are few topological spintronics researches oriented toward practical devices.
In this study, we aim to fabricate the spin transistor using nanowires of topological Dirac semimetals (TDSM), Cd3As2 (Fig.1). Cd3As2 nanowires can be synthesized by chemical vapor deposition (CVD), which is very low-cost equipment. This spin transistor will be operated at higher temperature because this device uses both the special surface transport and the high mobility bulk transport. Our final goal is to fabricate our topological spin transistor and to show a load map of practical realization of topological spintronics.
In this presentation, I will explain the principle of operation and the expected feature of our spin transistor and discuss the research plan and the results so far.
|氏名： 山辺 雄暉
指導教員名： 求 幸年 教授
発表題目（英語）： Skyrmion in a chiral Kondo necklace model
要旨（英語）： Topological spin textures, such as skyrmions and hedgehogs, have attracted attention since they bring about unconventional electronic, transport, and optical phenomena. There are several mechanisms for stabilizing such textures, e.g., the Dzyaloshinskii-Moriya interaction originating from the spin-orbit coupling in noncentrosymmetric systems, long-range dipolar interactions, frustrated exchange interactions, and the coupling between itinerant electrons and localized spins. The last one, the spin-charge coupling, has recently been featured as an origin of topological spin textures with short periods, even in centrosymmetric systems. Despite the discovery of several candidates, the effect of the spin-charge coupling has not been fully elucidated thus far.
In this study, in order to investigate the role of itinerant electrons on the topological spin textures, we study an extended Kondo necklace model including the Dzyaloshinskii-Moriya interaction. Treating both itinerant electron spins and localized spins as classical unit vectors, we study the ground state of this model on a triangular lattice by the conjugate gradient method. We find that the model exhibits a skyrmion crystal in an external magnetic field, regardless of the sign of the Kondo coupling. When the Kondo coupling is ferromagnetic, the localized spins are almost parallel to itinerant electron spins. On the other hand, when the Kondo coupling is antiferromagnetic, we show that the localized spin texture is affected by the competition between the Kondo coupling and the external magnetic field; in particular, the period of the skyrmion crystal changes depending on the value of the Kondo coupling even for a fixed magnetic field.
|氏名： 橋本 幸典
指導教員名： 山地 洋平 特任准教授
発表題目（英語）： Analysis of hidden structures in amorphous solids
要旨（英語）： In this seminar, we will introduce an application of machine-learning techniques to amorphous solids. Amorphous solids such as silica glass and amorphous silicon are typical examples of complex many-body systems. Because of their irregularity, characterization of the structures has attracted much attention for several decades. By combining molecular dynamics simulation and a newly developed machine-learning technique called neural canonical transformation, we extract low-energy collective motions of atoms in amorphous silicon. The low-energy motions will provide a clue to find hidden structures that characterize amorphous silicon and to reveal origin of a typical low-energy structure in excitation spectra of amorphous solids, which is called boson peak.
We will also analyze the irregular structures by using a data-driven science technique called topological data analysis.