KEIO RESEARCHERS INFORMATION SYSTEM

Books 【 Display / hide 】

Books 【 Display / hide 】

• Quantum key distribution and its applications

  M Takeoka, Quantum Photonics, 2024.01

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  Quantum cryptography, with its key technology, quantum key distribution (QKD), can unconditionally achieve secure data communication in the sense that encrypted data can never be cracked by any computation attacks, including with large-scale quantum computers. Quantum cryptography is one of the quantum information technologies and has technically matured for deployment in real uses. This chapter describes the basic principles of QKD, a QKD network, and their applications. We discuss the current QKD network architecture and how to integrate it with other modern security technologies to develop a total security solution that is useful in our society.

  • Access to Document (DOI)

• Quantum communication for the ultimate capacity and security

  M Sasaki, M Fujiwara, M Takeoka, Lecture Notes in Physics, 2016

  • Access to Document (DOI)

Papers 【 Display / hide 】

Papers 【 Display / hide 】

• Geometric measures of quantum nonlocality: characterization, quantification, and comparison by distances and operations

  G Zanfardino, W Roga, G Tartaglione, M Takeoka, F Illuminati

  Journal of Physics A: Mathematical and Theoretical 59 ( 5 )  2026.02

  Accepted,  ISSN  1751-8113

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  We introduce a geometric framework for studying Bell nonlocality in Hilbert space, where, for a given quantum state, nonlocality is quantified by the distance between the state and the set of local states. This approach applies to any Bell inequality and any measurement scenario. Whenever the local set is characterized, the proposed nonlocality measure can be computed explicitly.
  As a general result, we prove that for any scenario in arbitrary dimension the closest local state to a Werner state is itself a Werner state, and analogously, the closest local state to an isotropic state is again isotropic. In the two-qubit case, we further show that the closest local state to a Bell-diagonal state is Bell-diagonal as well. These structural results are independent of the specific Bell inequality considered, thus revealing intrinsic geometric features of these families of states and providing significant simplifications for computing the proposed measures.
  For the Clauser-Horne-Shimony-Holt (CHSH) inequality in two-qubit systems and the Collins-Gisin-Linden-Massar-Popescu (CGLMP) inequality for two qudits of arbitrary finite dimension, we derive explicit geometric measures of nonlocality for Bell-diagonal, Werner, and isotropic states using various distance metrics, including the trace, Hellinger, Hilbert-Schmidt distances, and relative entropy. Furthermore, we prove in all generality that for all scenarios in which the local set is not fully characterized, the geometric measures provide rigorous lower bounds on nonlocality

  • Access to Document (DOI)

• Fault-tolerant modular quantum computing with surface codes using single-shot emission-based hardware

  S Singh, R Kashiwagi, K Tanji, W Roga, D Bhatti, M Takeoka, D Elkouss

  arXiv preprint arXiv:2601.07241  2026.01

  Accepted

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  Fault-tolerant modular quantum computing requires stabilizer measurements across the modules in a quantum network. For this, entangled states of high quality and rate must be distributed. Currently, two main types of entanglement distribution protocols exist, namely emission-based and scattering-based, each with its own advantages and drawbacks. On the one hand, scattering-based protocols with cavities or waveguides are fast but demand stringent hardware such as high-efficiency integrated circulators or strong waveguide coupling. On the other hand, emission-based platforms are experimentally feasible but so far rely on Bell-pair fusion with extensive use of slow two-qubit memory gates, limiting thresholds to $\approx 0.16\%$. Here, we consider a fully distributed surface code using emission-based entanglement schemes that generate GHZ states in a single shot, i.e., without the need for Bell-pair fusions. We show that our optical setup produces Bell pairs, W states, and GHZ states, enabling both memory-based and optical protocols for distilling high-fidelity GHZ states with significantly improved success rates. Furthermore, we introduce protocols that completely eliminate the need for memory-based two-qubit gates, achieving thresholds of $\approx 0.19\%$ with modest hardware enhancements, increasing to above $\approx 0.24\%$ with photon-number-resolving detectors. These results show the feasibility of emission-based architectures for scalable fault-tolerant operation.

• Reception device, quantum key distribution system, and method for detecting quantum signal

  K Yoshino, M Fujiwara, A Tomita, M KOASHI, M Takeoka, M Sasaki

  US Patent 12,519,623  2026

  Accepted

• Long-distance device-independent quantum key distribution with standard optics tools

  M Ishihara, A Brendan, W Roga, UL Andersen, M Takeoka

  Optica Quantum 3 (6), 535-551 3 ( 6 ) 535 - 551 2025.12

  Accepted

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  Device-independent quantum key distribution (DI-QKD) enables information-theoretically secure key exchange between remote parties without any assumptions on the internal workings of the devices used for its implementation. However, its practical deployment remains severely constrained by the need for loophole-free Bell inequality violations, which are highly susceptible to losses and detection efficiencies. In this paper, we propose two long-distance DI-QKD protocols based on a heralding scheme using single-photon interference. Our protocols consist of only standard quantum optics tools such as two-mode squeezed states, displacement operations and on-off detectors, making them experimentally accessible. To further enhance robustness against realistic imperfections, we integrate a classical noisy preprocessing technique during post-processing. We calculate key rates of the protocols by numerical optimization and show the supremacy of this implementation over existing protocols in terms of communication distances.

  • Access to Document (DOI)

• A Compressive Sensing Inspired Monte-Carlo Method for Combinatorial Optimization

  B Chevalier, S Yamaguchi, W Roga, M Takeoka

  arXiv preprint arXiv:2510.24755  2025.10

  Accepted

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  In this paper, we present the Monte-Carlo Compressive Optimization algorithm, a new method to solve a combinatorial optimization problem that is assumed compressible. The method relies on random queries to the objective function in order to estimate generalized moments. Next, a greedy algorithm from compressive sensing is repurposed to find the global optimum when not overfitting to samples. We provide numerical results giving evidences that our methods overcome state-of-the-art dual annealing. Moreover, we also give theoretical justification of the algorithm success and analyze its properties. The practicality of our algorithm is enhanced by the ability to tune heuristic parameters to available computational resources.

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Reviews, Commentaries, etc. 【 Display / hide 】

Reviews, Commentaries, etc. 【 Display / hide 】

• Bridging discrete and continuous variables in quantum information

  JS Neergaard-Nielsen, M Takeuchi, K Wakui, H Takahashi, K Hayasaka, ...

  Optics and Photonics News 21 (12), 46 21 ( 12 )  2010.12

  ISSN  10476938

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  A study was conducted to bridge discrete and continuous variables (CV) in quantum information. The generated states were best understood in terms of continuous variables when looked at individually. They consisted of contributions from a range of different photon number states and occupied a larger space than the two-dimensional qubit space. It was demonstrated they were reduced to the same squeezed vacuum or squeezed photon superposition space. It was observed the qubit state was conveniently visualized on a Poincaré sphere with the two basis states represented on the north and south poles, while squeezed states were represented by their continuous distributions in a phase space of the two conjugate variables.

  • Access to Document (DOI)

Courses Taught 【 Display / hide 】

Courses Taught 【 Display / hide 】

• SEMINAR IN ELECTRONICS AND INFORMATION ENGINEERING(2)

  2025

• SEMINAR IN ELECTRONICS AND INFORMATION ENGINEERING(1)

  2025

• RECITATION IN ELECTRONICS AND INFORMATION ENGINEERING

  2025

• QUANTUM INTERNET AND QUANTUM INFORMATION THEORY

  2025

• QUANTUM ENGINEERING

  2025

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