慶應義塾研究者情報データベース

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• Equilibration of noninteracting photons and quantum signatures of chaos

  Bastidas V.M., Nourse H.L., Sakurai A., Hayashi A., Nishio S., Nemoto K., Munro W.J.

  Physical Review B 112 （ 13 ） 1343041 - 13430417 2025年10月

  ISSN  24699950

  　概要を見る

  Equilibration plays a fundamental role in our understanding of statistical mechanics and the long-time dynamics of many-body systems. In quantum systems, the route to equilibration is intimately related to level repulsion and quantum signatures of chaos that are encoded in their unitary evolution. Chaotic quantum systems exhibit level statistics characteristic of ensembles of random matrices. In this work, we demonstrate that single-particle chaos leads to equilibration of many noninteracting photons. We show that the underlying mechanisms for equilibration are operator spreading and quantum interference. More specifically, we demonstrate that the unitary dynamics of a general Floquet system implemented using single-mode phase shifters and multiport beamsplitters leads to equilibration of photons. We propose a realistic photonic implementation of the multiparticle kicked rotor, which is a Floquet system that we use as a concrete example of our general approach.

  • Access to Document (DOI)

• Multiplexed Quantum Communication with Surface and Hypergraph Product Codes

  Nishio S., Connolly N., Lo Piparo N., Munro W.J., Scruby T.R., Nemoto K.

  Quantum 9 2025年

  　概要を見る

  Connecting multiple processors via quantum interconnect technologies could help overcome scalability issues in single-processor quantum computers. Transmission via these interconnects can be performed more efficiently using quantum multiplexing, where information is encoded in high-dimensional photonic degrees of freedom. We explore the effects of multiplexing on logical error rates in surface codes and hypergraph product codes. We show that, although multiplexing makes loss errors more damaging, assigning qubits to photons in an intelligent manner can minimize these effects, and the ability to encode higher-distance codes in a smaller number of photons can result in overall lower logical error rates. This multiplexing technique can also be adapted to quantum communication and multimode quantum memory with high-dimensional qudit systems.

  • Access to Document (DOI)

• Online Job Scheduler for Fault-Tolerant Quantum Multiprogramming

  Wakizaka R., Nishio S., Sakuma D., Ueno Y., Suzuki Y.

  Proceedings IEEE Quantum Week 2025 Qce 2025 1   779 - 790 2025年

  　概要を見る

  Fault-tolerant quantum computers are expected to be offered as cloud services due to their significant resource and infrastructure requirements. Quantum multiprogramming, which runs multiple quantum jobs in parallel, is a promising approach to maximize the utilization of such systems. A key challenge in this setting is the need for an online scheduler capable of handling jobs submitted dynamically while other programs are already running. In this study, we formulate the online job scheduling problem for fault-tolerant quantum computing systems based on lattice surgery and propose an efficient scheduler to address it. To meet the responsiveness required in an online environment, our scheduler approximates lattice surgery programs, originally represented as polycubes, by using simpler cuboid representations. This approximation enables efficient scheduling while improving overall throughput. In addition, we incorporate a defragmentation mechanism into the scheduling process, demonstrating that it can further enhance the utilization of quantum processing unit.

  • Access to Document (DOI)

• Impact of the form of weighted networks on the quantum extreme reservoir computation

  Hayashi A., Sakurai A., Nishio S., Munro W.J., Nemoto K.

  Physical Review A 108 （ 4 ）  2023年10月

  ISSN  24699926

  　概要を見る

  The quantum extreme reservoir computation (QERC) is a versatile quantum neural network model that combines the concepts of extreme machine learning with quantum reservoir computation. Key to QERC is the generation of a complex quantum reservoir (feature space) that does not need to be optimized for different problem instances. Originally, a periodically driven system Hamiltonian dynamics was employed as the quantum feature map. In this work we capture how the quantum feature map is generated as the number of time-steps of the dynamics increases by a method to characterize unitary matrices in the form of weighted networks. Furthermore, to identify the key properties of the feature map that has sufficiently grown, we evaluate it with various weighted network models that could be used for the quantum reservoir in image classification situations. At last, we show how a simple Hamiltonian model based on a disordered discrete time crystal with its simple implementation route provides nearly optimal performance while removing the necessity of programming of the quantum processor gate by gate.

  • Access to Document (DOI)

• Resource reduction in multiplexed high-dimensional quantum Reed-Solomon codes

  Nishio S., Lo Piparo N., Hanks M., Munro W.J., Nemoto K.

  Physical Review A 107 （ 3 ）  2023年03月

  ISSN  24699926

  　概要を見る

  Quantum communication technologies will play an important role in quantum information processing in the near future as we network devices together. However, their implementation is still a challenging task due to both loss and gate errors. Quantum error-correcting codes are one important technique to address this issue. In particular, the quantum Reed-Solomon codes well suited to communication tasks as photons can naturally carry more than one qubit of information. The high degree of physical resources required, however, makes such a code difficult to use in practice. A recent technique called quantum multiplexing has been shown to reduce resources by using multiple degrees of freedom of a photon. In this work, we propose a method to decompose multicontrolled gates using fewer controlled-x (CX) gates via this quantum multiplexing technique. We show that our method can significantly reduce the required number of CX gates needed in the encoding circuits for the quantum Reed-Solomon code. Our approach is also applicable to many other quantum error-correcting codes and quantum algorithms, including Grovers and quantum walks.

  • Access to Document (DOI)

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