座長：王 越、青柳 弓槻
| 氏名： 住谷
指導教員名： 小芦 雅斗 教授
発表題目（英語）： Security analysis of quantum key distribution under pattern effects of pulse amplitudes
要旨（英語）： Quantum key distribution (QKD) is a procedure which allows two parties (Alice and Bob) to share a secret key by utilizing a quantum channel even under the presence of an eavesdropper (Eve) who can perform anything physically possible on the quantum channel. One practical method for QKD is the decoy-state BB84 protocol, in which Alice randomly switches the amplitude of each laser pulse in order to reveal Eve’s attack. The security proof of the decoy-state BB84 protocol is based on the premise that each switching is independently performed.
In one of the recent demonstrations of the decoy-state BB84 protocol, though, it was revealed that it is difficult in principle to switch each pulse independently, and correlations between amplitudes of adjacent two pulses (we call them “pattern effects”) exist. This contradicts the premise of the security proof. To address this problem, we propose a classical post processing method to eliminate influence of the pattern effects and prove its validity.
| 氏名： 髙梨
指導教員名： 古澤 明 教授
発表題目（英語）： Entanglement Swapping for Continuous Variables on a Waveguide Chip
要旨（英語）： In this presentation, first I will explain quantum teleportation and then talk about my chip design. At the end, I will explain the significance of the entanglement swapping experiment which is scheduled to be carried out on my chip for my master thesis.
In Furusawa laboratory, we are working on quantum information processing of continuous variable using light. However, free space optics is too large and not extensible. So I am trying to integrate same optics on a chip.
Such attempts had been done in our laboratory, but we had not been able to get satisfactory results. End of 2016, I redesigned the chip design and succeeded in designing a promising mask pattern that overcomes the conventional drawbacks.
Coupling mismatch at the input section has been thought to be one of the most severe optical losses of the waveguide. In this chip, a tapered waveguide is installed at the input port. By dicing the chip at the appropriate position, the problem can be solved fundamentally.
Regarding the size of the waveguide, I succeeded in downsizing the size to less than 25% of the conventional size by using custom photodiode and optimization using a computer.
The chip is currently being manufactured at NTT Advanced Device Research Laboratories in Atsugi City, Kanagawa Prefecture. After the manufacturing, the chip will be transported to the University of Tokyo and attached to the waveguide control device, which was also developed by me.
| 氏名： 諏訪
指導教員名： 為ヶ井 強 准教授
発表題目（英語）： Enhancement of critical current density in (Sr,Na)Fe2As2 tapes by densification of the core
要旨（英語）： '122 types' iron-based superconductors are extensively studied for both basic science and applications. In particular, (AE,K)Fe2As2 (AE = Ba, Sr), superconducting wires and tapes have been widely studied for its very large critical densities (Jc) even at high fields. Recently, a new material joined this kind of research, namely (Sr,Na)Fe2As2. In the former research, we fabricated (Sr,Na)Fe2As2/Ag tapes by optimizing the sintering temperature and time, and succeeded in fabricating very field-insensitive tapes.
In this research, we will report on enhancement of critical current density in (Sr,Na)Fe2As2 tapes by densification of the core. First, we further optimized sintering condition, and magnetic Jc for (Sr,Na)Fe2As2/Ag?tape reached 150 kA/cm2 (H⊥tape surface), and 350 kA/cm2 (H//tape surface) under self-field, which was higher than that of the former group’s research. Second, we fabricated superconducting (Sr,Na)Fe2As2?tapes by using AgSn alloy as a sheath material instead of Ag sheath. AgSn alloy sheath is harder than Ag sheath, so it leads to densification of the core and improvement of connection between grains. The magnetic Jc for (Sr,Na)Fe2As2/AgSn?tape sintered at 750℃ reached 180 A/cm2 (H⊥tape surface), and 460 A/cm2 (H//tape surface) under self-field. Third, we fabricated first superconducting wires in this material, and transport Jc reached 50 kA/cm2 under self-field. We will report these three points by using compositional distributions and magneto-optical images of current distribution in tapes.
| 氏名： 高橋
指導教員名： 石坂 香子 准教授
発表題目（英語）： Observation of band structure in the misfit compound (PbSe)1.16(TiSe2)2
要旨（英語）： The series (MX)1+δ(TX2)n, where MX is a double rock-salt layer; TX2 is a transition metal dichalcogenide layer; δ is a small number between 0.08 and 0.28, are called misfit compounds because of their structural mismatch along one in-plane direction originating from symmetric and periodic differences between MX and TX2 layers.
Recently, misfit compound (PbSe)1.16(TiSe2)2 was reported as a superconductor with a Tc of 2.3 K, while its parent compound TiSe2 shows a charge-density wave transition and no superconductivity.
In this talk, some ARPES(angle-resolved photoemission spectroscopy) data will be shown and new Fermi surface unique to misfit structure will be reported.
| 氏名： 関
指導教員名： 鹿野田 一司 教授
発表題目（英語）： In-plane and out-of-plane charge transport of the quasi-two-dimensional organic conductors, k-(ET)2X (X=Cu[N(CN)2]Cl and Cu2(CN)3)
要旨（英語）： Mott physics is a central issue of strongly correlated electron systems and a family of organic conductors, ｋ-(ET)2X, play an important role in the research of Mott physics. ｋ-(ET)2X has a layered structure of ET layers and anion (X) layers. In the ET layers, the 2D electronic bands are half filled and the lattices are modeled to anisotropic triangular lattices, in which the geometrical frustration works against spin ordering.
ｋ-(ET)2Cu2(CN)3 (ｋ-Cu2(CN)3), a member in this family, is a Mott insulator with a nearly right triangular lattice, which means that spins are highly frustrated. In actuality, the localized spins show no magnetic ordering even at low temperatures. Therefore, the ground state of this system is thought to be a Quantum Spin Liquid (QSL). The electronic state of ｋ-Cu2(CN)3 as a QSL material has been intensively studied, but is far from being fully understood. Theoretically, one of the pictures of electronic state in QSL is Fermi-liquid of charge neutral spin-1/2 fermions with localized charges. This picture leads to difference in temperature dependence between in-plane and out-of-plane resistivities. To explore the nature of charge transport in the spin liquid experimentally, we have investigated the anisotropy of the in-plane and out-of-plane resistivities. In this presentation, I introduce how to estimate the resistivity of highly anisotropic materials, and compare the results obtained for the spin liquid system, ｋ-Cu2(CN)3, and the antiferromagnet, ｋ-(ET)2Cu[N(CN)2]Cl (ｋ-Cl).
指導教員名： 香取 秀俊 教授
発表題目（英語）： spectroscopy of the Sr clock transition inside Hollow-core fiber
要旨（英語）： An optical lattice clock had been proposed to improve the clock stability as 1 / √N by applying a large number of atoms. However, atoms easily lose their coherence via atom- atom interaction which deteriorates the performance of atomic clocks. To increase the number of the atoms without atom- atom interaction, a new platform of precise spectroscopy using a lattice guide inside hollow-core photonics crystal fiber (HC-PCF) had been demonstrated. In the previous experiment, a 8-kHz-wide spectrum was observed for the 1S0-3P1 transition solely limited by the natural lifetime (21s) of the 3P1 state.
We present the undergoing experiment to further investigate the coherence time of the system by interrogating the 1S0-3P0 clock transition with the natural lifetime of 150 s. In our experiment, we observe the 1S0- 3P0 clock transition by measuring the absorption on the 1S0- 1P1 transition. This electron shelving technique enables us to observe population of atoms on the clock states while keeping atoms inside the fiber.