arXiv subjects

Takafumi Miyanaga

Publications and source records attributed to Takafumi Miyanaga.

4 recordsLinked to original sources

arXiv2026

QuBE/Qubex: an integrated hardware-software system for superconducting qubit experiments with broadband control

Achieving high-fidelity operation in large-scale superconducting qubit systems requires not only control hardware with broad frequency coverage, low crosstalk, and tight synchronization but also software that coordinates system configuration, experiment execution, and data analysis. Here we present an integrated qubit-control system that combines broadband microwave hardware with a pulse-level software stack for scalable superconducting qubit experiments. The hardware provides broadband microwave coverage, including an instantaneous span of up to 1.6 GHz from a control output, while the software reduces setup and calibration overhead through automated configuration and built-in experiment workflows. We validate the system on a 64-qubit fixed-frequency transmon chip through full-chip frequency identification and representative demonstrations, including multi-unit far-detuned cross-resonance calibration and benchmarking that yields a measured two-qubit gate fidelity of 98.34%, and multilevel readout beyond the computational subspace. By disclosing the hardware architecture and releasing the software stack as open source, this work provides an inspectable hardware-software foundation for scalable superconducting qubit control experiments.

quant-ph↗

arXiv2025

A Practical Open-Source Software Stack for a Cloud-Based Quantum Computing System

Since the late 2010s, quantum computers have become commercially available, and the number of services that users can run remotely via cloud servers is increasing. In Japan, several domestic superconducting quantum computing systems, including our own, began operation in 2023. However, the design of quantum computing systems, especially in the most critical areas near quantum computers, remains largely undisclosed, creating a significant barrier to entry into the quantum computing field. If this situation continues, progress toward standardization, which is essential for guiding quantum computer development, will stall, and it will be difficult to develop a practical quantum computing system that can perform calculations on a supercomputer scale. To address this issue, we propose Open Quantum Toolchain for OPerators and USers (OQTOPUS), a full-stack quantum computing system developed from research with real quantum computers. OQTOPUS is one of the world's largest open-source software projects, covering operational software from cloud-based execution environment construction to system operation. Furthermore, to perform quantum computing effectively and efficiently, it implements key features, such as transpilers, multiprogramming, and error mitigation, in an area as close as possible to a quantum computer, an area that system vendors rarely disclose. Finally, this study presents experimental results of applying OQTOPUS to a real quantum computer. OQTOPUS is publicly available on GitHub and will notably lower the barrier to entry into the quantum computing field, contributing to the formation of a quantum computing developer community through open discussion.

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arXiv2025

Auxiliary-field quantum Monte Carlo method with quantum selected configuration interaction

We propose using the wave function generated by the quantum selected configuration interaction (QSCI) method as the trial wave function in phaseless auxiliary-field quantum Monte Carlo (ph-AFQMC). In the QSCI framework, electronic configurations are sampled from the quantum state realized on a quantum computer. These configurations serve as basis states for constructing an effective Hamiltonian, which is then diagonalized to obtain the corresponding eigenstate. Using this wave function, ph-AFQMC is performed to recover the dynamical electron correlation across the whole orbital space. The use of the QSCI trial wave function is expected to improve the feasibility of the quantum-classical (QC) hybrid quantum Monte Carlo approach [Nature, 603, 416 (2022)]. We call this integrated approach QC-QSCI-AFQMC, or QSCI-AFQMC for short. This method is validated across several molecular systems. For H2O and a linear H4 chain, we achieved chemical accuracy in most investigations relative to full configuration interaction while utilizing superconducting quantum computers at Osaka University and RIKEN. Additionally, the application of QSCI-AFQMC to the O-H bond dissociation in an organic molecule highlights the complementary synergy between capturing static correlation on quantum hardware and incorporating dynamical correlation via classical post-processing. For the N2, when QSCI-AFQMC is executed with a noiseless simulator, it ranks among the most accurate methods compared to various multireference electronic structure theories. Although the proposed method is demonstrated using small active spaces on current quantum devices, the concept is not limited to few-qubit problems. The QSCI-AFQMC can compete with state-of-the-art classical computational techniques, particularly in larger active spaces, displaying considerable potential for resolving classically intractable problems in quantum chemistry.

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arXiv2021

Ultrastrong tunable coupler between superconducting LC resonators

We investigate the ultrastrong tunable coupler for coupling of superconducting resonators. Obtained coupling constant exceeds 1 GHz, and the wide range tunability is achieved both antiferromagnetics and ferromagnetics from $-1086$ MHz to 604 MHz. The ultrastrong coupler is composed of rf-SQUID and dc-SQUID as tunable junctions, which connected to resonators via shared aluminum thin film meander lines enabling such a huge coupling constant. The spectrum of the coupler obviously shows the breaking of the rotating wave approximation, and our circuit model treating the Josephson junction as a tunable inductance reproduces the experimental results well. The ultrastrong coupler is expected to be utilized in quantum annealing circuits and/or NISQ devices with dense connections between qubits.

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