慶應義塾研究者情報データベース

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• Quantum key distribution and its applications

  Takeoka M., Quantum Photonics, 2024年01月

  　概要を見る

  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)

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• Rate-fidelity tradeoff in cavity-based remote entanglement generation

  Tanji K., Takahashi H., Roga W., Takeoka M.

  Physical Review A 110 （ 4 ）  2024年10月

  ISSN  24699926

  　概要を見る

  The qubit scalability imposes a paramount challenge in the field of quantum computing. Photonic interconnects between distinct quantum computing modules provide a solution to deal with this issue. The fundamental part of this approach is entanglement distribution via traveling photons emitted by matter qubits. However, randomness of the spontaneous emission in the matter qubits limits both the entanglement fidelity and the generation rate. In this paper, by numerical and analytical methods, we investigate the relationship between the entanglement affected by the spontaneous emission and the waveform of the pump pulse used in the photon generation. We confirm and analyze a rate-fidelity trade-off in the entanglement swapping with Gaussian pump pulses and show that a simple extension to non-Gaussian pump pulses improves the trade-off in a certain parameter region. Furthermore we extend our analysis to entanglement distribution in the general multipartite setting and show that the analysis of the bipartite entanglement can be straightforwardly applied in this case as well.

  • Access to Document (DOI)

• Spectrally resolved Franson interference

  Jin R.B., Zeng Z.Q., Xu D., Yuan C.Z., Li B.H., Wang Y., Shimizu R., Takeoka M., Fujiwara M., Sasaki M., Lu P.X.

  Science China: Physics, Mechanics and Astronomy 67 （ 5 ）  2024年05月

  ISSN  16747348

  　概要を見る

  Franson interference can be used to test the nonlocal features of energy-time entanglement and has become a standard in quantum physics. However, most of the previous Franson interference experiments were demonstrated in the time domain, and the spectral properties of Franson interference have not been fully explored. Here, we theoretically and experimentally demonstrate spectrally resolved Franson interference using biphotons with different correlations, including positive correlation, negative correlation, and non-correlation. It is found that the joint spectral intensities of the biphotons can be modulated along both the signal and idler directions, which has potential applications in generating high-dimensional frequency entanglement and time-frequency grid states. This work may provide a new perspective for understanding the spectral-temporal properties of the Franson interferometer.

  • Access to Document (DOI)

• Multimode Quantum Correlations in Supercontinuum Pulses

  Aruto Hosaka, Shintaro Niimura, Masaya Tomita, Akihito Omi, Masahiro Takeoka, Fumihiko Kannari

  2024年03月

  　概要を見る

  Suprecontinuum (SC) light contains complex spectral noise structure and its
  accurate characterization is important for fundamental understanding of its
  physics as well as for its applications. Several experimental and theoretical
  noise characterizations have been performed so far. However, none of them takes
  into account the quantum mechanical properties. Here, we demonstrate
  experimental characterisation of quantum noise and its spectral correlations
  formed in the SC light generated from a photonic crystal fiber. Moreover, by
  applying an appropriate basis transformation to these correlations, we
  demonstrate that the SC noise amplitude can be squeezed below the shot-noise
  limit in some bases, even in the presence of excessively large nonlinearities.
  

• Modelocked Thermal Frequency Combs for Ultrashort Chaotic Quantum Optics

  Wakui K., Tsujimoto Y., Kishimoto T., Fujiwara M., Sasaki M., Hosaka A., Kannari F., Takeoka M.

  Advanced Quantum Technologies 2024年

  　概要を見る

  Modelocked thermal frequency combs (MTCs) are generated by employing spectrally narrowed amplified spontaneous emission (ASE) seeded into an electro-optic frequency comb generator. The MTC emits 2-ps duration ultrashort pulses at a repetition rate of 10 GHz. Autocorrelation of the MTC pulses confirms a reduced coherence time, (Formula presented.) ps, aligning with the narrowed bandwidth of the ASE seed. Intensity correlations of optically gated MTC pulses at a repetition rate of 250 MHz reveal nearly ideal thermal photon statistics with an experimental (Formula presented.), yielding an intrinsic (Formula presented.) after background noise removal. As a practical application, second harmonic generation (SHG) is performed utilizing the optically gated MTC pulses as a pump and experimental intensity correlations, (Formula presented.), are examined for the SH photons. An entire transition in (Formula presented.), continuously changing from six to two by increasing the pump strength, agrees with the single-mode analytical model. Furthermore, time-resolved pulse height correlations allow to simultaneously acquire power variations in SHG and third harmonic generation against the pump. With the maximum peak intensity, (Formula presented.), realized in a periodically poled (Formula presented.) waveguide for SHG, the demonstration highlights the potential for various applications in chaotic quantum optics experiments that necessitate ultrashort, high-intensity, single-spatiotemporal-mode thermal pulses.

  • Access to Document (DOI)

• Quantum resource theory of Bell nonlocality in Hilbert space

  Gennaro Zanfardino, Wojciech Roga, Masahiro Takeoka, Fabrizio Illuminati

  2023年11月

  　概要を見る

  We introduce a Hilbert space based resource theory of Bell nonlocality with
  the aim of providing bona fide measures of quantum nonlocality depending only
  on the intrinsic properties of the quantum states being considered. We
  construct our theory by defining the set of local (or free) states, i.e., the
  states that do not violate the Clauser-Horne-Shimony-Holt inequality; the set
  of free operations, i.e., the transformations that do not create the
  nonlocality resource, which includes local operations and shared randomness;
  and suitable measures of nonlocality based either on geometric distances or
  relative entropies with respect to the set of local states. We discuss the
  basic axiomatic structure that is needed for a meaningful characterization and
  quantification of Bell nonlocality and we illustrate the general resource
  theory so developed by applying it to specific classes of quantum states,
  including two-qubit Werner states, Bell-diagonal states, and Bell-diagonal
  states at fixed convexity.
  

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担当授業科目 【 表示 ／ 非表示 】

担当授業科目 【 表示 ／ 非表示 】

• 電気情報工学セミナーⅡ

  2024年度

• 電気情報工学輪講

  2024年度

• 量子インターネットと量子情報理論

  2024年度

• 量子工学

  2024年度

• 理工学基礎実験

  2024年度

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