arXiv subjects

Ryo Ito

Publications and source records attributed to Ryo Ito.

6 recordsLinked to original sources

arXiv2025

High-temperature helical edge states in BiSbTeSe$_2$/graphene van der Waals heterostructure

Van der Waals heterostructures have been used to tailor atomic layers into various artificial materials through interactions at heterointerfaces. The interplay between the band gap created by the band folding of the interfacial potential and the band inversion driven by enhanced spin-orbit interaction (SOI) through band hybridization enables us to realize a two-dimensional topological insulator (2D-TI). Here we report the realization of graphene 2D-TIs by epitaxial growth of three-dimensional topological insulator (3D-TI) BiSbTeSe$_2$ ultrathin films on graphene. By increasing the BiSbTeSe$_2$ thickness from 2 nm to 9 nm to enhance SOI on graphene, the electronic state is altered from the trivial Kekul${\'e}$ insulator to the 2D-TI. The nonlocal transport reveals the helical edge conduction which survives up to 200 K at maximum. Our graphene 2D-TI is stable, easy to make electrical contacts, and of high quality. It offers various applications including spin-current conversion and platforms for Majorana fermions in junctions to superconductors.

cond-mat.mes-hall↗

arXiv2024

Semiconductor Circuits for Quantum Computing with Electronic Wave Packets

Standard approaches to quantum computing require significant overhead to correct for errors. The hardware size for conventional quantum processors in solids often increases linearly with the number of physical qubits, such as for transmon qubits in superconducting circuits or electron spin qubits in quantum dot arrays. While photonic circuits based on flying qubits do not suffer from decoherence or lack of potential scalability, they have encountered significant challenges to overcome photon loss in long delay circuits. Here, we propose an alternative approach that utilizes flying electronic wave packets propagating in solid-state quantum semiconductor circuits. Using a novel time-bin architecture for the electronic wave packets, hardware requirements are drastically reduced because qubits can be created on-demand and manipulated with a common hardware element, unlike the localized approach of wiring each qubit individually. The electronic Coulomb interaction enables reliable coupling and readout of qubits. Improving upon previous devices, we realize electronic interference at the level of a single quantized mode that can be used for manipulation of electronic wavepackets. This important landmark lays the foundation for fault-tolerant quantum computing with a compact and scalable architecture based on electron interferometry in semiconductors.

cond-mat.mes-hall↗

arXiv2024

Electrical control of a Kondo spin screening cloud

Quantitative analysis of quantum many-body systems, consisting of numerous itinerant electrons that interact with localized spins or electrons, is a long-standing issue. The Kondo cloud, a quantum many-body object of conduction electrons that screens a single localized spin, is the building block of such strongly correlated electronic systems. While quantitative analysis of the Kondo cloud associated with a single magnetic impurity is well established for uniform conduction electrons, the fundamental properties of a deformed Kondo cloud influenced by conduction electrons with a modulated density of states remain unsolved. Here we report engineering of the Kondo cloud deformation by confining a part of the cloud into a quantum box called the Kondo box that mimics realistic material systems. We demonstrate quantitative control of the Kondo cloud by developing a way of tuning quantum interference in the box and monitoring the Kondo entanglement. The temperature dependence of the entanglement reveals counterintuitively that the cloud shape is altered mainly outside the box although the quantum interference in the box is tuned. Our work provides a way to simulate various strongly correlated systems by integrating the Kondo cloud, which is not possible in the current theoretical framework.

cond-mat.mes-hall↗

arXiv2021

Lower bounds on the error probability of multiple quantum channel discrimination by the Bures angle and the trace distance

Quantum channel discrimination is a fundamental problem in quantum information science. In this study, we consider general quantum channel discrimination problems, and derive the lower bounds of the error probability. Our lower bounds are based on the triangle inequalities of the Bures angle and the trace distance. As a consequence of the lower bound based on the Bures angle, we prove the optimality of Grover's search if the number of marked elements is fixed to some integer $\ell$. This result generalizes Zalka's result for $\ell=1$. We also present several numerical results in which our lower bounds based on the trace distance outperform recently obtained lower bounds.

quant-ph↗

arXiv2019

Determining the optimal coefficient of the spatially periodic Fisher-KPP equation that minimizes the spreading speed

This paper is concerned with the spatially periodic Fisher-KPP equation $u_t=(d(x)u_x)_x+(r(x)-u)u$, $x\in \mathbb{R}$, where $d(x)$ and $r(x)$ are periodic functions with period $L>0$. We assume that $r(x)$ has positive mean and $d(x)>0$. It is known that there exists a positive number $c^*_d(r)$, called the minimal wave speed, such that a periodic traveling wave solution with average speed $c$ exists if and only if $c \geq c^*_d(r)$. In the one-dimensional case, the minimal speed $c^*_d(r)$ coincides with the ``spreading speed'', that is, the asymptotic speed of the propagating front of a solution with compactly supported initial data. In this paper, we study the minimizing problem for the minimal speed $c^*_d(r)$ by varying $r(x)$ under a certain constraint, while $d(x)$ arbitrarily. We have been able to obtain an explicit form of the minimizing function $r(x)$. Our result provides the first calculable example of the minimal speed for spatially periodic Fisher-KPP equations as far as the author knows.

math.AP↗

arXiv2014

Stream Processor Generator for HPC to Embedded Applications on FPGA-based System Platform

This paper presents a stream processor generator, called SPGen, for FPGA-based system-on-chip platforms. In our research project, we use an FPGA as a common platform for applications ranging from HPC to embedded/robotics computing. Pipelining in application-specific stream processors brings FPGAs power-efficient and high-performance computing. However, poor productivity in developing custom pipelines prevents the reconfigurable platform from being widely and easily used. SPGen aims at assisting developers to design and implement high-throughput stream processors by generating their HDL codes with our domain-specific high-level stream processing description, called SPD.With an example of fluid dynamics computation, we validate SPD for describing a real application and verify SPGen for synthesis with a pipelined data-flow graph. We also demonstrate that SPGen allows us to easily explore a design space for finding better implementation than a hand-designed one.

cs.OH↗