応用物理学輪講 I
10月3日
[注意事項]
発表の10日前までに　office[at]ap.t.u-tokyo.ac.jp　宛てに「氏名」「指導教員」「発表題目（英語）」「要旨（英語）」「発表言語（英語または日本語）」を送付して下さい。
発表日
2025年10月3日(金) 16：50～18：50

Aグループ

座長
石岡　陸
指導
教員名
関　真一郎　准教授
座長
岩垣　貴祐
指導
教員名
金澤　直也　准教授
発表者名 山根　悠都
指導教員名 小芦　雅斗　教授
発表題目（英語） From Click Strategies to Key Rates: A Composable Security Proof for Passive-Biased BB84
要旨（英語） We analyze BB84 with passive, biased basis choice, where multiphoton signals inevitably produce double and cross clicks. We treat any click handling rule as a “strategy” and derive an effective measurement model for each photon-number sector. A virtual decomposition of the Z-basis outcome in the X basis enforces support matching between Z and X measurements. This yields bitwise operator dominance relations that convert directly into an inequality between the number of Z-basis errors and X-basis errors. After normalizing by the X/Z sampling imbalance, we obtain a practical upper bound on the phase-error rate solely from observable X-basis data.

We provide a general scaling law under a recommended “X-heavy” strategy that assigns all double/cross clicks to the X arm and, if needed, applies a mild virtual bit symmetrization. For finite data, we replace the observed X-error rate, the X/Z sampling ratio, and the Z-error rate with one-sided confidence limits, producing a finite-size phase-error bound. Plugging this into a Koashi-style complementarity analysis yields a composable, implementation-ready key-length bound.

Operationally, key rates improve by (i) increasing the X-line share while maintaining a minimum Z sample, (ii) boosting X-line detection efficiency, and (iii) avoiding discarding double/cross clicks or fixing their bit labels. Decoy states further tighten the bound by weighting photon-number contributions. The framework is device-realistic (no squashing assumption required), transparent, and reproducible.
発表言語 日本語
発表者名 渡邉　楓花
指導教員名 福谷　克之　教授
発表題目（英語） Evaluation of Ni thin film and thermal desorption spectroscopy of H2
要旨（英語） Electron spin of a molecule plays an important role in molecular interaction with surfaces. We recently demonstrated that the adsorption probability of a hydrogen atom is spin-dependent on Ni(111) with a spin-polarized hydrogen atom beam using a hexapole magnet [1] [2]. At the Ni(111) surface, the magnetization was in-plane. On the other hand, whereas the magnetization of a Ni thin film is in-plane at Ni thicknesses below 7ML, it is oriented in the perpendicular direction above 7ML [3][4]. We aim to investigate the spin dependence on the adsorption probability on a Ni thin film, where we expect the spin dependence of adsorption changes depending on the Ni thickness. In the present study, we report the growth of Ni thin films on a Cu layer formed on a SrTiO3(100) substrate and adsorption characteristic of H on Ni/Cu(100).

A SrTiO3(100) substrate was annealed at 923 K under oxygen atmosphere at 6.5×10-4 Pa prior to film fabrication. Subsequently, a Cu film was grown in a three-step process. Initial deposition at a substrate temperature of 673 K to grow seeds with an epitaxial orientation, followed by deposition at 373 K to cover full substrate, both at a constant rate of 7Å/min. After that, the sample was annealed at 573 K for 1 hour. The resulting Cu film exhibited a sharp (1×1) LEED pattern. The film grew epitaxially on the SrTiO3 (100) substrate, with its crystallographic orientation aligned accordingly. Following the Cu deposition, a Ni film was deposited at room temperature at a rate of 1 Å/min. LEED observation of this surface showed a  R45°(√2 ×√2) pattern. For this Ni film, we performed thermal desorption spectroscopy for spin polarized H beam and H2 gas. In the presentation, the difference between the thin film and bulk adsorption energy would be shown.

 
[1] Y. Nagaya, et al., J. Chem. Phys. 155, 194201 (2021)

[2] H.Ueta,et al., submitted

[3] Bochi G, et al., Mat. Res. Soc. Symp. Proc. 313 309 (1993)

[4] R.Vollmer, et al., PRB, 60, 9 (1999)
発表言語 日本語
発表者名 陳　正力
指導教員名 中村　泰信　教授
発表題目（英語） Developing cos μφ elements in superconducting circuits
要旨（英語） In superconducting circuits, Josephson junctions (JJ) are widely used to construct superconducting qubits and various devices. Compared to conventional inductors, Josephson junctions exhibit nonlinear properties, with their potential energy being proportional to the cosine of gauge-invariant phase difference φ. Generalized Josephson junctions(cosμφ elements), where μ is an integer, exhibit a 2π/μ-periodic potential in the phase space. A novel way to create cos μφ elements is by combining several two-Josephson-junction (2JJ) arms in parallel and applying phase shifts properly. The cosμφ elements are expected to extend circuit design flexibility, enabling a wide range of applications. One use of the cos μφ elements is to magnify the quantum phase fluctuation of qubits, which could reduce the dephasing rate as the energy dispersion of the qubits is suppressed.
In this presentation, we show the application of cos μφ elements to a 0-π qubit, called a 0-π-3φ qubit, where the normal JJs in the 0-π qubit are replaced by cos 3φ elements. First, the theoretical parameters of cos 3φ elements formed by 2JJ arms and phase shifts elements are calculated analytically. Second, the effective parameter regimes regarding the energies associated with the JJs, inductors and capacitors of 0-π-3φ qubits are derived and compared with
those of 0-π qubits. Lastly, considering the currently realizable parameter regimes in fabrication, we numerically show that 0- π - 3φ qubits will have better T coherence time than the (soft) 0- π qubit, by taking advantage of the cos 3φ elements.
発表言語 英語

Bグループ

座長
植田　大雅
指導
教員名
金澤　直也
座長
岡村　圭太
指導
教員名
香取　秀俊　教授・山口　敦史　委嘱准教授
発表者名 尹　建
指導教員名 齊藤　英治　教授
発表題目（英語） Learning dynamics of generative models based on spin dynamics
要旨（英語） This study reinterprets machine learning as an open thermodynamic system of magnetic matter, proposing a framework that links learning dynamics with physical principles. Focusing on the equivalence between regularization and magnetization order formation, we aim to unify perspectives from information theory and condensed matter physics. We construct a generative model—termed the spin autoencoder—comprising an encoder, a spin relaxation layer based on the stochastic Landau-Lifshitz-Gilbert equation, and a decoder. Learning is achieved by minimizing energy defined via reconstruction error and effective magnetic fields, with temperature controlling regularization strength.
発表言語 日本語
発表者名 LAURIENZO Joseph Thomas
指導教員名 求　幸年　教授
発表題目（英語） Noncoplanar Tetrahedral Magnetic Orders on a Triangular Lattice: Stability and Quantum Fluctuation Effects
要旨（英語） Magnetic properties are governed by underlying magnetic structures. While collinear magnetic orders such as ferromagnetism and antiferromagnetism, and coplanar textures like magnetic vortices, are well established, recent attention has shifted to noncoplanar spin structures, including magnetic skyrmions, hedgehogs, and hopfions. These structures impart a nonzero Berry phase on itinerant electrons and in turn give rise to unconventional quantum transport phenomena such as the topological Hall effect. Particularly interesting are tetrahedral orders on triangular lattices. They can be viewed as the small-size limit of a skyrmion crystal, with four-sublattice spins pointing toward the vertices of a tetrahedron, potentially leading to a gigantic Berry-phase-effect. Recent experimental realizations of such ordering in CoTa3S6, MnTe2, and GdGaI motivate a deeper understanding of their stability and the role of quantum fluctuations.  Here, we review theoretical background and recent experimental advances on the tetrahedral orders, and discuss perspectives for further research.
発表言語 英語
発表者名 HA　Gyeongmin
指導教員名 古澤　明　教授
発表題目（英語） Generating High-Level Squeezed Vacuum States of Light via an Optical Parametric Amplifier
要旨（英語） The continuous-variable model of optical quantum computing offers a promising path to building scalable and high-speed quantum processors. This architecture relies on large-scale entangled resources called cluster states, which are generated from squeezed vacuum states of light. The performance of such a computer is critically dependent on two figures of merit of the squeezed states: the squeezing level and the bandwidth.
This work details the generation of a record-high squeezed vacuum state from an Optical Parametric Amplifier (OPA), an architecture chosen for its potential for terahertz-level bandwidth and thus faster quantum clock speeds. We report a measured squeezing level of 10.1dB, surpassing the previous world record for an OPA. We will present the experimental setup, including a novel low-loss phase-locking method, and provide a detailed analysis of the loss budget and phase noise that limit performance.
発表言語 英語
応用物理学輪講 I一覧ページへ戻る