Faculty Profiles - KOBAYASHI KOHEI

写真a

KOBAYASHI KOHEI

Organization

School of Science Researcher

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Degree

博士 (工学)

Research Interests

量子制御
量子力学

Research Areas

Natural Science / Mathematical physics and fundamental theory of condensed matter physics

Education

Keio University Graduate School of Science and Technology

2018.4 - 2021.3

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Country： Japan

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慶応義塾大学大学院理工学研究科基礎理工学専攻修士課程

2016.4 - 2018.3

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Country： Japan

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慶応義塾大学理工学部物理情報工学科

2012.4 - 2016.3

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Country： Japan

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Research History

東京科学大学理学院物理学系研究員

2026.4

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国立情報学研究所量子情報国際研究センター特任研究員

2023.4 - 2026.3

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Country：Japan

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Keio University

2021.4 - 2023.3

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Country：Japan

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Professional Memberships

電子情報通信学会

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Papers

Saturable quantum speed limits for imaginary-time evolution Reviewed

Kohei Kobayashi

Physical Review A 113 ( 3 ) 032618 2026.3

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Publisher：American Physical Society (APS)

DOI： 10.1103/bln8-8xzy

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Quantum speed limit under decoherence: unitary, dissipative, and fluctuation contributions Reviewed

Kohei Kobayashi

Journal of Physics Communications 9 ( 11 ) 115002 2025.11

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Publisher：IOP Publishing

Abstract

We present a new quantum speed limit (QSL) for open quantum systems governed by Markovian dynamics. Our approach is based on analyzing the time derivative of the Bures angle between the initial pure state and its time-evolved state, leading to an analytically computable upper bound on the evolution speed. This bound naturally decomposes into three distinct physical contributions: a unitary term, a dissipative deformation term, and a fluctuation term. Building on this structure, we establish a general inequality that connects the QSL to the quantum Fisher information (QFI) in the short-time regime. This reveals a fundamental trade-off between evolution speed and estimation precision, and clarifies how decoherence can both accelerate and constrain information acquisition in open quantum systems.

DOI： 10.1088/2399-6528/ae1bf5

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Other Link： https://iopscience.iop.org/article/10.1088/2399-6528/ae1bf5/pdf
Inequality for von Neumann entropy change under measurement and dissipation

Kohei Kobayashi

arXiv:2506.12603 [quant-ph] 2025.6

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Norm Inequality for Perturbed Quantum Evolutions and Its Application to Grover's Algorithm

Kohei Kobayashi

arXiv:2506.14230 [quant-ph] 2025.6

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Analytical evaluation of the effect of deterministic control error on isolated quantum system

Kohei Kobayashi

arXiv:2408.17442 [quant-ph] 2024.8

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Time evolution of the von Neumann entropy in open quantum system

Kohei Kobayashi

arXiv:2405.11824 [quant-ph] 2024.5

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Bound on the distance between controlled quantum state and target state under decoherence Reviewed

Kohei Kobayashi

Journal of Physics Communications 8 ( 1 ) 015005 2024.1

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Publishing type：Research paper (scientific journal) Publisher：IOP Publishing

Abstract

To implement quantum information technologies, carefully designed control for preparing a desired state plays a key role. However, in realistic situation, the actual performance of those methodologies is severely limited by decoherence. Therefore, it is important to evaluate how close we can steer the controlled state to a desired target state under decoherence. In this paper, we provide an upper bound of the distance between the two controlled quantum systems in the presence and absence of decoherence. The bound quantifies the degree of achievement of the control for a given target state under decoherence, and can be straightforwardly calculated without solving any equation. Moreover, the upper bound is applied to derive a theoretical limit of the probability for obtaining the target state under decoherence.

DOI： 10.1088/2399-6528/ad1f74

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Other Link： https://iopscience.iop.org/article/10.1088/2399-6528/ad1f74/pdf
Control limit for the quantum state preparation under stochastic control errors Reviewed

Kohei Kobayashi

Proceedings of the Royal Society A 479 ( 2277 ) 2023.9

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Language：English

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Reachable-set characterization of an open quantum system by the quantum speed limit Reviewed

Kohei Kobayashi

Physical Review A 105 ( 4 ) 2022.4

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Publishing type：Research paper (scientific journal) Publisher：American Physical Society (APS)

DOI： 10.1103/physreva.105.042608

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevA.105.042608/fulltext
Quantum speed limit for robust state characterization and engineering Reviewed

Kohei Kobayashi, Naoki Yamamoto

Physical Review A 102 ( 4 ) 2020.10

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Publishing type：Research paper (scientific journal) Publisher：American Physical Society (APS)

DOI： 10.1103/physreva.102.042606

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevA.102.042606/fulltext
Control limit on quantum state preparation under decoherence Reviewed

Kohei Kobayashi, Naoki Yamamoto

Physical Review A 99 ( 5 ) 2019.5

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Authorship：Lead author Language：English Publishing type：Research paper (scientific journal) Publisher：American Physical Society (APS)

DOI： 10.1103/physreva.99.052347

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Other Link： https://link.aps.org/article/10.1103/PhysRevA.99.052347

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Presentations

制御された被デコヒーレンス量子系と目標量子状態間の距離に対する理論的限界

小林幸平

第52回量子情報技術研究会 2025.5

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オンライン教材の多角的利用～リョーシカ先生と学ぶ超ミニマム線形代数～

小林幸平

NIIオープンハウス2025 2025.6

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オンライン教材の多角的利用

小林幸平, 根本香絵

Q-LEAP第7回シンポジウム 2025.2

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QOLSオンライン講義の開発と共同利用

吾妻広夫, 小林幸平, 根本香絵

Q-LEAP第7回シンポジウム 2025.2

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Analytical evaluation of stochastic control error on quantum operation

小林幸平

第50回量子情報技術研究会 2024.5

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多角的利活用の実例紹介（リョーシカ先生の教材）

小林幸平

第1回量子技術高等教育拠点ユーザーフォーラム 2024.11

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オンライン教材の多角的利用～リョーシカ先生と学ぶ超ミニマム線形代数～

小林幸平, 根本香絵

Q-LEAP第8回シンポジウム 2026.2

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散逸下における量子状態制御の限界

小林幸平

第35回量子情報技術研究会 2016.11

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散逸下における量子状態制御の限界

小林幸平

第36回量子情報技術研究会 2017.5

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教材データベースのご紹介

小林幸平

第6回Q-Leapシンポジウム 2024.2

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Presentation type：Poster presentation

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TractableなQuantum Speed Limitとロバスト化に向けたハミルトニアン設計

小林幸平

第41回量子情報技術研究会 2019.11

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量子情報技術の発展を支える人材育成教材データベースのご紹介

小林幸平

NIIオープンハウス2023 2023.6

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Presentation type：Poster presentation

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散逸下における量子状態制御の限界

小林幸平

第37回量子情報技術研究会 2017.11

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Quantum Speed Limitによる量子系のロバスト性の特徴付け

小林幸平

第39回量子情報技術研究会 2018.11

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Awards

義塾賞

2016.3 慶應義塾大学

小林幸平

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Research Projects

量子技術高等教育拠点標準プログラムの開発

Grant number：JPMXS0120351339 2020.4 - 2026.3

文部科学省光・量子飛躍フラッグシッププログラム（Q-LEAP）人材育成プログラム[共通的コアプログラム]

根本香絵

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Authorship：Coinvestigator(s)

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