Superconducting Quantum Computers
Superconductivity is a quantum phenomenon in which electrical resistance becomes zero, manifesting on a macroscale that is visible to the human eye.
By using a superconductor to create an LC resonator, which consists of an inductor L and a capacitor C, a resonator can be created with extremely low loss. The Josephson junction, which consists of a thin insulator sandwiched between superconductors, behaves as a nonlinear inductor due to the quantum tunneling effect of Cooper pairs. Application of this junction enables the creation of an artificial atom that has discrete energy levels with non-uniform gaps.
The atom–atom interactions and atom–electromagnetic wave (microwave) interactions of these huge artificial atoms are much stronger than the atom–atom interactions and atom–electromagnetic wave (light) interactions of regular atoms. These are a feature of superconducting circuits with low loss characteristics.
Toshiba is conducting research and development aimed at realizing quantum computers that utilize this characteristic of superconducting circuits. To this end, the company has previously proposed:
- Kerr parametric oscillator, which can create a Schrödinger’s cat state through the quantum bifurcation phenomenon, using the nonlinearity of Josephson junctions;
- Quantum bifurcation machine, which uses the Kerr parametric oscillators as fundamental elements and can be used as either an annealing-type or gate-type quantum computer; and
- Double-transmon coupler, which is a tunable coupler that can adjust the strength of the coupling between superconducting quantum bits and solve the problems of conventional methods of tunable coupling.
Circuit diagram of two fixed-frequency transmon qubits coupled with a double-transmon coupler
Research News
Quantum-Inspired Computing: Simulated Bifurcation Machine
Classical models of quantum bifurcation machines can be described by the Hamiltonian equations of motion in classical mechanics.
Simulated bifurcation algorithms are a method for solving combinatorial optimization problems by transforming these equations so that they can be simulated quickly by numerical computations and solving using the symplectic Euler method, which is a simple and stable method of solving numerical problems. Simulated bifurcation machines are a high-speed implementation that uses cutting-edge parallel processors such as FPGAs and GPUs.
Although the principles of simulated bifurcation machines are expected to be built on the classical bifurcation phenomenon and classical adiabatic theorem—because the principles of quantum bifurcation machines are built on the quantum bifurcation phenomenon and quantum adiabatic theorem—there remain many unknowns.
Research is now under way with the aim of understanding these principles and improving performance. Toshiba is also developing pioneering applications that utilize the speed and flexibility of these machines.
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Research News
Publications
2025
High-performance conditional-driving gate for Kerr parametric oscillator qubits
Hiroomi Chono and Hayato Goto
APL Quantum 2, 016110 (2025)
2024
Realization of high-fidelity CZ gate based on a double-transmon coupler
Rui Li, Kentaro Kubo, Yinghao Ho, Zhiguang Yan, Yasunobu Nakamura, and Hayato Goto
Physical Review X 14, 041050 (2024)
High-performance multiqubit system with double-transmon couplers: Toward scalable superconducting quantum computers
Kentaro Kubo, Yinghao Ho, Hayato Goto
Physical Review Applied 22, 024057 (2024)
Fast elementary gates for universal quantum computation with Kerr parametric oscillator qubits
Taro Kanao, Hayato Goto
Physical Review Research 6, 013192 (2024)
Ultra-High-Speed Optimization for 5G Wireless Resource Allocation by Simulated Bifurcation Machine
H. Obata, T. Nabetani, H. Goto, K. Tatsumura
Proc. of IEEE Wireless Communications and Networking Conference (WCNC) (2024)
Efficient and Scalable Architecture for Multiple-chip Implementation of Simulated Bifurcation Machines
T. Kashimata, M. Yamasaki, R. Hidaka, K. Tatsumura
IEEE Access 12, 36606-36621 (2024)
Roadmap for Unconventional Computing with Nanotechnology
G. Finocchio, K. Tatsumura, H. Goto et al.
Nano Futures 8, 012001 (2024)
Control of the ZZ coupling between Kerr cat qubits via transmon couplers
Takaaki Aoki, Taro Kanao, Hayato Goto, Shiro Kawabata, Shumpei Masuda
Physical Review Applied 21, 014030 (2024)
Lectures and Interviews
2024
Simulated Bifurcation Machines: Enabling NP-hard Optimization-based Judgement in Realtime systems by Quantum-inspiredTechnology
K. Tatsumura
IEEE International Workshop on Ising Machines(IISM), 2024.4.16
https://www.petaspin.com/isingmachines2024/
History
2024
・Published a paper on the successful realization of the Double-Transmon Coupler in collaboration with RIKEN, achieving a 99.90% two-qubit gate fidelity.
2022
・Published a paper on the theoretical proposal of the double-transmon coupler.
2021
・Theoretically discovered the double-transmon coupler.
・Published a paper on second-generation simulated bifurcation algorithms.
2019
・Published the first paper on simulated bifurcation machines.
2017
・Discovered the simulated bifurcation algorithm from the theory of quantum bifurcation machines.
2016
・Published two papers on quantum bifurcation machines (annealing type and gate type).
・Published a paper on a minimal-resource encoding method for the Steane code, a representative quantum error–correcting code.
2015
・Theoretically discovered quantum bifurcation machines by replacing two-photon loss and one- photon loss in coherent Ising machines with the Kerr effect and detuning, respectively.
2013
・Published a paper on optimal decoding of concatenated quantum codes based on probabilistic inference.
2010
・Published a paper on large improvements to the threshold value of fault-tolerant quantum computing with a cavity quantum electrodynamics system.
2006
・Published a paper on the world’s first experimental realization of stimulated Raman adiabatic passage (STIRAP) in solids.
2001
・Published a paper proposing the theory of quantum computers using electromagnetically induced transparency (EIT) in solids.
1998
・Published a paper on the observation of EIT in solids by using crystal doped with rare earth ions (Pr3+: Y2SiO5).
Related Links
Quantum-Inspired Optimization Solutions
Link to Toshiba Digital Solutions Corporation page.
Quantum Key Distribution
Link to Toshiba Digital Solutions Corporation page.