KEIO RESEARCHERS INFORMATION SYSTEM

Career 【 Display / hide 】

Career 【 Display / hide 】

• 2007.04

  -

  2009.08

  RIKEN, Condensed Matter Theory Laboratory, Special Postdoctoral Researcher

• 2009.09

  -

  2011.06

  University of Toronto, Department of Physics, Postdoctoral Fellow

• 2011.07

  -

  2012.08

  University of Tokyo, Department of Physics, Postdoctoral Researcher

• 2012.08

  -

  2019.03

  University of Tokyo, Department of Physics, Assistant Professor

• 2019.04

  -

  2025.03

  Keio University, Department of Physics, Assistant Professor

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Academic Background 【 Display / hide 】

Academic Background 【 Display / hide 】

• 1999.04

  -

  2002.03

  Tokyo Institute of Technology, School of Science, Department of Physics

  University, Skipped grade(s)

• 2002.04

  -

  2004.03

  Tokyo Institute of Technology, Graduate School of Science, Department of Physics

  Graduate School, Completed, Master's course

• 2004.04

  -

  2007.03

  Tokyo Institute of Technology, Graduate School of Science, Department of Physics

  Graduate School, Completed, Doctoral course

Academic Degrees 【 Display / hide 】

Academic Degrees 【 Display / hide 】

• 博士（理学）, Tokyo Institute of Technology, Coursework, 2007.03

  Characterization of Exotic Orders in Quantum Spin Systems

Research Areas 【 Display / hide 】

Research Areas 【 Display / hide 】

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

• Natural Science / Semiconductors, optical properties of condensed matter and atomic physics

Research Keywords 【 Display / hide 】

Research Keywords 【 Display / hide 】

• topological states of matter

• ultracold atomic systems

• magnetism

Research Themes 【 Display / hide 】

Research Themes 【 Display / hide 】

• Competition of topological and ordered phases in spin ladder systems, 

  2022.07

  -

  Present

• Vortex lattices in Bose-Einstein condensates in synthetic gauge fields, 

  2017.01

  -

  2022.05

• Quantum critical phenomena in non-Hermitian quantum many-body systems, 

  2015.12

  -

  Present

• Theory of frustrated magnetism of volborthite based on a coupled-trimer model, 

  2015.03

  -

  Present

• Quantum Hall physics of ultracold atomic systems in synthetic gauge fields, 

  2011.07

  -

  Present

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Papers 【 Display / hide 】

Papers 【 Display / hide 】

• High-Field Magnetism of the Spin-1/2 Two-Leg-Ladder Antiferromagnet Cu(DEP)Cl2 under High Pressure

  Morimoto T., Morikawa E., Nihongi K., Kida T., Narumi Y., Honda Z., Furukawa S., Uwatoko Y., Kindo K., Hagiwara M.

  Journal of the Physical Society of Japan 94 ( 2 )  2025.02

  ISSN  00319015

  　View Summary

  We report the magnetic properties of powder samples of the two-leg-ladder antiferromagnet Cu(DEP)Cl2 (DEP: 2,3-diethylpyradine) measured in magnetic fields of up to 40 T under pressures (P) of up to 1.7 GPa. Measured magnetic properties (magnetic susceptibility and high-field magnetization with two transition fields) at ambient P are compared with those calculated by a quantum Monte Carlo method and are well explained by the two-leg-ladder antiferromagnetic model with large rung exchange interaction (Jrung). A peak of magnetic susceptibility shifts largely to a higher temperature with a significant decrease in height, and both transition fields shift largely to higher fields at a critical pressure Pc of approximately 0.2 GPa. The results of an evaluation of these exchange constants below and above Pc indicate that antiferromagnetic two-leg ladders (Jrung > Jleg) with a very weak Jint changed into strongly coupled (Jrung) antiferromagnetic dimers with weak Jleg and Jint

  • Access to Document (DOI)

• Phase Diagram of a Spin-1/2 Heisenberg Ladder with a Chirality–Chirality Interaction

  Fontaine M., Furukawa S.

  Journal of the Physical Society of Japan 93 ( 12 )  2024.12

  ISSN  00319015

  　View Summary

  A spin-12 Heisenberg model with a chirality–chirality interaction (CCI) on a two-leg ladder provides a minimal setup to explore an interplay between spin and chirality degrees of freedom. This model is potentially relevant for understanding the effects of a four-spin ring exchange and a phonon-mediated interaction on a ladder. By applying the Abelian bosonization and two types of spin-chirality duality, we derive a comprehensive ground-state phase diagram that includes previously unexplored cases of a ferromagnetic rung exchange Jτ < 0 or a negative CCI Kch < 0. The phase diagram consists of the rung singlet (RS) phase, the Haldane phase, and three non-magnetic ordered phases. Moreover, the RS and Haldane phases consist of several regions with different dominant correlations in terms of spin, dimer, or chirality. We perform numerical simulations to confirm the predicted phase structure and critical properties.

  • Access to Document (DOI)

• System-environment entanglement phase transitions

  Ashida Y., Furukawa S., Oshikawa M.

  Physical Review B 110 ( 9 )  2024.09

  ISSN  24699950

  　View Summary

  Entanglement in quantum many-body systems can exhibit universal phenomena governed by long-distance properties. We study universality and phase transitions of the entanglement inherent to open many-body systems, namely, the entanglement between a system of interest and its environment. Specifically, we consider the Tomonaga-Luttinger liquid (TLL) under a local measurement and analyze its unconditioned nonunitary evolution, where the measurement outcomes are averaged over. We quantify the system-environment entanglement by the Rényi entropy of the post-measurement density matrix, whose size-independent term encodes the universal low-energy physics. We develop a field-theoretical description to relate the universal term to the effective ground-state degeneracy known as the g function in a boundary conformal field theory, and use the renormalization group method to determine its value. We show that the universal contribution is determined by the TLL parameter K and can exhibit singularity signifying an entanglement phase transition. Surprisingly, in certain cases the size-independent contribution can increase as a function of the measurement strength in contrast to what is naïvely expected from the g-theorem. We argue that this unconventional behavior could be attributed to the dangerously irrelevant term which has been found in studies of the resistively shunted Josephson junction. We also check these results by numerical calculations in the spin-12 XXZ chain subject to a site-resolved measurement. Possible experimental realization in ultracold gases, which requires no postselections, is discussed.

  • Access to Document (DOI)

• Symmetry, topology, duality, chirality, and criticality in a spin-1/2 XXZ ladder with a four-spin interaction

  Fontaine M., Sugimoto K., Furukawa S.

  Physical Review B (American Physical Society)  109 ( 13 ) 134413 2024.04

  Research paper (scientific journal), Joint Work, Accepted,  ISSN  24699950

  　View Summary

  We study the ground-state phase diagram of a spin-12 XXZ model with a chirality-chirality interaction (CCI) on a two-leg ladder. This model offers a minimal setup to study an interplay between spin and chirality degrees of freedom. The spin-chirality duality transformation allows us to relate the regimes of weak and strong CCIs. By applying the Abelian bosonization and the duality, we obtain a rich phase diagram that contains distinct gapped featureless and ordered phases. In particular, Néel and vector chiral orders appear for easy-axis anisotropy, while two distinct symmetry-protected topological (SPT) phases appear for easy-plane anisotropy. The two SPT phases can be viewed as twisted variants of the Haldane phase. We also present an effective description in terms of (spinor) hard-core bosons, which reveals critical behavior on the self-dual line in the easy-axis and easy-plane regimes. We perform numerical simulations to confirm the predicted phase structure and critical properties. We further demonstrate that the two SPT phases and a trivial phase are distinguished by topological indices in the presence of certain symmetries. A similar phase structure is expected in a spin-12 XXZ ladder with four-spin ring exchange.

  • Access to Document (DOI)

• Ground-state phase diagram of a spin-1/2 frustrated XXZ ladder

  Ogino T., Kaneko R., Morita S., Furukawa S.

  Physical Review B (American Physical Society)  106 ( 15 ) 155106 2022.10

  Research paper (scientific journal), Joint Work, Accepted,  ISSN  24699950

  　View Summary

  We study the ground-state phase diagram of a spin-12 frustrated XXZ ladder, in which two antiferromagnetic chains are coupled by competing rung and diagonal interactions, J⊥ and J×. Previous studies on the isotropic model have revealed that a fluctuation-induced effective dimer attraction between the legs stabilizes the columnar dimer (CD) phase in the highly frustrated regime J⊥≈2J×, especially for ferromagnetic J⊥,×<0. By means of effective field theory and numerical analyses, we extend this analysis to the XXZ model, and we obtain a rich phase diagram. The diagram includes four gapped featureless phases with no symmetry breaking: the rung singlet (RS) and Haldane phases as well as their twisted variants, the RS∗ and Haldane∗ phases, which are all distinct in the presence of certain symmetries. Significantly, the Haldane-CD transition point in the isotropic model turns out to be a crossing point of two transition lines in the XXZ model, and the stripe Néel and RS∗ phases appear between these lines. This indicates a nontrivial interplay between the effective dimer attraction and the exchange anisotropy. In the easy-plane regime, the four featureless phases and two critical phases are found to compete in a complex manner depending on the signs of J⊥,×.

  • Access to Document (DOI)

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Papers, etc., Registered in KOARA 【 Display / hide 】

Papers, etc., Registered in KOARA 【 Display / hide 】

• Theoretical study of many-body effects in Bose-Einstein condensates in synthetic gauge fields

  Furukawa, Shunsuke

  学事振興資金研究成果実績報告書 (慶應義塾大学)  2019

Reviews, Commentaries, etc. 【 Display / hide 】

Reviews, Commentaries, etc. 【 Display / hide 】

• Entanglement entropy and conformal field theory

  S. Furukawa, J. Shiraishi

  BUTSURI 69 ( 8 ) 541 - 546 2014

  Article, review, commentary, editorial, etc. (scientific journal), Joint Work

  • Access to Document (DOI)

Presentations 【 Display / hide 】

Presentations 【 Display / hide 】

• Symmetry-protected topological phases and entanglement swapping in spin-1/2 ladder systems

  Shunsuke Furukawa

  高次にもつれた量子相探索の展開 (基礎物理学研究所、京都大学、京都) , 

  2025.09

  , 

  Oral presentation (general)

  　View Summary

  The concept of symmetry-protected topological (SPT) phases has been proposed and extensively developed over the past 15 years. While bosonic SPT phases in one spatial dimension is fully classified by projective representations of the symmetry group, it is interesting to examine when nontrivial SPT phases emerge and how they compete with conventional ordered phases in concrete physical systems. Here we explore this issue in spin-1/2 XXZ ladders with four-spin interactions. We find that weak XXZ anisotropy crucially affects the phase structure, inducing SPT phases in the easy-plane regime and magnetic and chiral ordered phases in the easy-axis regime. The SPT phases identified here can be regarded as twisted variants of the well-known Haldane phase, and are distinguished by topological indices linked to the system's symmetries. Motivated by the deepening interplay between quantum magnetism and quantum information, we further explore a many-body extension of the entanglement swapping protocol, in which entangled spin pairs are replaced by quantum phases of spin-1/2 ladders. We show that Bell measurements performed between legs of topological and trivial ladder states, followed by postselection of uniform outcomes, lead to the generation of a topological state on the unmeasured legs.

• Topological entanglement swapping in spin-ladder systems

  Mizuki Hamada, Shunsuke Furukawa

  SQAI-NCTS Workshop on Quantum Technologies and Machine Learning (National Taiwan University, Taipei) , 

  2025.08

  , 

  Poster presentation

  　View Summary

  We study how quantum measurements can transform quantum phases in spin ladder systems through an entanglement swapping protocol. Consecutive Bell measurements are performed between the legs of two independent ladders, followed by uniform post-selection of the measurement outcomes. We analyze the resulting phase realized in the unmeasured outer legs.
  Our analysis is based on topological indices protected by the D_2=Z_2×Z_2 spin rotational symmetry as well as the lattice translation symmetry. We find that these indices remain unchanged through the measurement process, leading to a nontrivial inheritance rule. Namely, the output state is topological if only one of the ladders is initially topological, while it becomes trivial if both are.
  To illustrate this, we explicitly construct post-measurement states using matrix product states (MPS), in which the rung singlet (trivial) and Haldane (SPT) phases are represented by a product of singlets and a valence bond solid state, respectively. The MPS results are consistent with the index characterization and further reveal that the correlation length increases after measurement, indicating enhanced fluctuations.
  Finally, we perform a field-theoretical analysis using bosonization. By identifying the locking positions of bosonic fields associated with string order parameters, we determine the resulting phase and confirm consistency with both the index argument and MPS analysis., Interestingly, the post-measurement correlation length is approximately equal to the sum of those before measurement.

• スピン梯子系におけるトポロジカルエンタングルメントスワッピング

  濱田瑞貴, 古川俊輔

  第70回物性若手夏の学校 (白浜荘、滋賀) , 

  2025.07

  -

  2025.08

  , 

  Poster presentation

  　View Summary

  近年、量子測定が量子多体系に与える影響に広く関心が集まっている。本研究ではその一例として、二つのスピン梯子系に対してエンタングルメントスワッピングを施し、測定後に実現される量子相の特定を試みる。具体的には、独立な二つのスピン梯子系の各脚にベル測定を行い、測定結果が一様となるよう事後選択を行った上で、直接測定を施していない梯子の脚に現れる状態が属する相を解析する。スピン梯子系は、トポロジカル相（Haldane 相）や自明相（rung singlet 相; RS 相）を実現するだけでなく、異方性や多体相互作用の導入によって多彩な相構造を示す。これらの相は局所的な秩序変数では検出できないが、特定の対称性を課している限り、トポロジカル指数を用いて区別することが可能である。本研究では、離散的なスピン回転対称性D2=Z2 X Z2 と並進対称性に保護された三つのトポロジカル指数に注目し、二系の合成系における指数が測定前後で不変であるという事実を基に、測定後の相の同定を行った。この議論は一般的であるが、測定後の状態の詳細が不明であるため、行列積状態（matrix product state; MPS）およびボソン化による具体的な解析も実施した。MPS を用いた解析では、Haldane 相の代表例であるvalence bond solid 状態がMPS で表現できることを利用し、測定後の状態を明示的に構成してトポロジカル指数を評価した。その結果は、トポロジカル指数のみに基づく一般的議論と整合的であり、さらに転送行列の解析を通じて、測定後の状態では相関長が増加する傾向も確認された。一方、ボソン化の枠組みでは、RS 相やHaldane 相などのギャップのある状態は、有効ハミルトニアン中のcos 項によってボソン場がロックされることに対応し、ロックされる場の種類や位置がストリング秩序変数と密接に関係することを用いた。ボソン化した有効ハミルトニアンの基底状態を求め、測定後の状態を計算し、ボソン場がロックされる位置を解析することで測定後に実現される相を特定した。ボソン化による結果も、トポロジカル指数やMPS を用いた解析と整合する。更に、測定後の状態の相関長が、測定前の各梯子の相関長の和で与えられることを解析的に導出し、MPS により観測された相関長の増加を説明した。

• Topological entanglement swapping in spin-ladder systems

  Mizuki Hamada, Shunsuke Furukawa

  The 4th Young Researchers' Workshop of the Extreme Universe Collaboration (Kyukamura Irago, Aichi) , 

  2025.06

  -

  2025.07

  , 

  Oral presentation (general)

  　View Summary

  We study how quantum measurements can transform quantum phases in spin ladder systems through an entanglement swapping protocol. Consecutive Bell measurements are performed between the legs of two independent ladders, followed by uniform post-selection of the measurement outcomes. We analyze the resulting phase realized in the unmeasured outer legs.
  Our analysis is based on topological indices protected by the D_2=Z_2×Z_2 spin rotational symmetry as well as the lattice translation symmetry. We find that these indices remain unchanged through the measurement process, leading to a nontrivial inheritance rule. Namely, the output state is topological if only one of the ladders is initially topological, while it becomes trivial if both are.
  To illustrate this, we explicitly construct post-measurement states using matrix product states (MPS), in which the rung singlet (trivial) and Haldane (SPT) phases are represented by a product of singlets and a valence bond solid state, respectively. The MPS results are consistent with the index characterization and further reveal that the correlation length increases after measurement, indicating enhanced fluctuations.
  Finally, we perform a field-theoretical analysis using bosonization. By identifying the locking positions of bosonic fields associated with string order parameters, we determine the resulting phase and confirm consistency with both the index argument and MPS analysis., Interestingly, the post-measurement correlation length is approximately equal to the sum of those before measurement.

• System-environment entanglement phase transitions for open boundary conditions

  Yuheng Sui, Shunsuke Furukawa

  The 4th Young Researchers' Workshop of the Extreme Universe Collaboration (Kyukamura Irago, Aichi) , 

  2025.06

  -

  2025.07

  , 

  Oral presentation (general)

  　View Summary

  A quantum channel described by a CPTP map generally turns a pure quantum state into a mixed state, which can be viewed as development of system-environment entanglement. We study a phase transition in the system-environment Renyi entanglement entropy (EE) when the XXZ chain with open boundary conditions is subject to continuous density measurements. We show that the EE exhibits a volume law followed by a subleading logarithmic term; the coefficient of the latter changes from 0 to 1/4 as a function of the measurement strength. We also study the expectation value of a certain twist operator in the doubled Hilbert space formalism, that is expected to detect the phase transition. We compare our results with previous results for the periodic boundary condition as well as the boundary renormalization group analysis.

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Research Projects of Competitive Funds, etc. 【 Display / hide 】

Research Projects of Competitive Funds, etc. 【 Display / hide 】

• Investigation of topological phases and transitions via quantum control, quantum entanglement, and duality

  2024.04

  -

  2026.03

  Grants-in-Aid for Scientific Research, Grant-in-Aid for Transformative Research Areas (A), No Setting

  　View Summary

  液相から固相への転移が分子の自発的な秩序化として理解される一方、そのような秩序化では捉えられない奇妙な物質相としてトポロジカル相が知られ、量子磁性体や半導体などにおいて盛んに研究されている。本研究では、トポロジカル相研究の新しい舞台として、急速に技術革新の進む量子コンピュータに着目する。クラウド利用できる量子コンピュータを利用してトポロジカル状態をつくり出し、量子誤り軽減法によりノイズの影響を抑えたもとで非局所相関関数などの量子計算を行う。また、トポロジカル相に内在する量子もつれを量子情報の視点から特徴づける方法を発展させ、物性物理学と量子情報の融合領域の開拓を推進する。

• Theoretical study of many-body effects in Bose-Einstein condensates in synthetic gauge fields

  2019.04

  -

  2020.03

  Keio Gijuku, Academic Development Funds, Shunsuke Furukawa, Research grant, Principal investigator

• Symmetry-enriched topological phases: theoretical foundation and proposal for realization

  2018.04

  -

  2024.03

  MEXT,JSPS, Grant-in-Aid for Scientific Research, Grant-in-Aid for Scientific Research (C), Principal investigator

• Phase frustration in Bose-Einstein condensates in optical lattices

  2016.11

  -

  2019.03

  Matsuo Foundation, Shunsuke Furukawa, Research grant, Principal investigator

• Exploration of topological orders in strongly correlated ultracold atomic systems and development of their fundamental theory

  2013.04

  -

  2017.03

  The University of Tokyo, FURUKAWA SHUNSUKE, Grant-in-Aid for Young Scientists (B), No Setting

  　View Summary

  Topologically ordered states exhibit a number of intriguing properties such as quantized responses and quasiparticles obeying fractional statistics. In this study, we have theoretically explored novel topologically ordered states that can be realized in highly controlled ultracold atomic systems, and investigated their physical properties. In particular, we have determined detailed ground-state phase diagrams of two-component Bose gases in synthetic gauge fields, and revealed (i) the appearance of a bosonic integer quantum Hall state with unusual edge states and (ii) a nontrivial role of time-reversal symmetry in the interaction effect. We have also shown that Bose-Einstein condensate in optical lattices can host an excitation band with nontrivial topology, and that edge states originating from it can be observed by exploiting a macroscopic quantum interference.

Awards 【 Display / hide 】

Awards 【 Display / hide 】

• The 2nd High Magnetic Field Forum Frontier Incentive Award

  Shunsuke Furukawa, 2020.12, The High Magnetic Field Forum of Japan, Theoretical studies on quantum magnets with competing ferromagnetic and antiferromagnetic interactions

  Type of Award： Award from Japanese society, conference, symposium, etc.

  　View Description

  http://www.issp.u-tokyo.ac.jp/labs/mgsl/himag-forum/frontier.html

• The 7th Young Scientist Award of the Physical Society of Japan

  Shunsuke Furukawa, 2013.03, Physical Society of Japan, Entanglement entropy in quantum many-body systems

  Type of Award： Award from Japanese society, conference, symposium, etc.

  　View Description

  https://www.jps.or.jp/activities/awards/jusyosya/wakate2013.html#r11
  http://www.r11.div.jps.or.jp/wakate07.html
  https://ss.scphys.kyoto-u.ac.jp/TQP/achievements/newsletters.html#no3

Courses Taught 【 Display / hide 】

Courses Taught 【 Display / hide 】

• THERMODYNAMICS

  2025

• STATISTICAL PHYSICS B

  2025

• PHYSICS SEMINAR 4

  2025

• PHYSICS B

  2025

• OFF-CAMPUS WORK-STUDY PROGRAM

  2025

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Memberships in Academic Societies 【 Display / hide 】

Memberships in Academic Societies 【 Display / hide 】

• Physical Society of Japan, 

  2004.01

  -

  Present

Committee Experiences 【 Display / hide 】

Committee Experiences 【 Display / hide 】

• 2022.04

  -

  2023.03

  新著紹介小委員会 委員長（兼 会誌編集委員）, 日本物理学会

• 2021.04

  -

  2022.03

  新著紹介小委員会 委員, 日本物理学会

• 2020.04

  -

  2021.03

  領域３（磁性）運営委員, 日本物理学会