Quantum Computer Architecture Work in Japan

 A couple of weeks ago, I attended The First Fault Tolerant Quantum Architecture Kenkyuukai, held in Takamatsu, Shikoku, Japan. I came away very optimistic about the future of the field in Japan. There is a good cohort of talented, ambitious, (mostly) young researchers:

• Yutaka Tabuchi, RIKEN
• Teruo Tanimoto, Kyushu (Kyuudai)
• Makoto Negoro, Osaka (Handai)
• Yosuke Ueno, RIKEN
• Ilkwon Byun, Kyushu
• Yasunari Suzuki, NTT
• Shota Nagayama, Keio/Mercari
• Takahiko Satoh, Keio

All but Shota and Takahiko were in Takamatsu and gave good talks. A selection of some of the recent architecture papers from this gang (most recent first, except ours last):

• Yosuke Ueno, Taku Saito, Teruo Tanimoto, Yasunari Suzuki, Yutaka Tabuchi, Shuhei Tamate, Hiroshi Nakamura, High-Performance and Scalable Fault-Tolerant Quantum Computation with Lattice Surgery on a 2.5D Architecture, arXiv:2411.17519 (not yet peer reviewed?). Conducts a detailed analysis of the hazards in execution of lattice surgery (an important architecture analysis technique imported from classical computer architecture), and proposes an architecture with a few extra qubits (hence the 2.5D) that substantially short-circuits some of the key bottlenecks.
• F Battistel, C Chamberland, K Johar, R W J Overwater, F Sebastiano, L Skoric, Y Ueno and M Usman, Real-time decoding for fault-tolerant quantum computing: progress, challenges and outlook (Nano Futures 2023). As advertised, useful info.
• Ilkwon Byun, Junpyo Kim, Dongmoon Min, Ikki Nagaoka, Kosuke Fukumitsu, Iori Ishikawa, Teruo Tanimoto, Masamitsu Tanaka, Koji Inoue, Jangwoo Kim, XQsim: modeling cross-technology control processors for 10+K qubit quantum computers (ISCA 2022). This team is right concerned about the scalability of classical control systems, so they have developed a simulator that assesses this. They have also proposed a control processor that is reportedly capable of handling 2D surface code for up to 59,000 physical qubits, depicted in diagrams that will make circuit lovers swoon.
• Yosuke Ueno, Masaaki Kondo, Masamitsu Tanaka, Yasunari Suzuki, Yutaka Tabuchi, QECOOL: On-Line Quantum Error Correction with a Superconducting Decoder for Surface Code (DAC 2021). Architecture, simulation and analysis (especially power) for a classical superconducting processor to sit near a surface code solid-state system such as a transmon processor using lattice surgery; I assume it would probably also work for quantum dot. This is one of those papers that I wish I had gotten the chance to be involved in.
• Daisuke Sakuma, Amin Taherkhani, Tomoki Tsuno, Toshihiko Sasaki, Hikaru Shimizu, Kentaro Teramoto, Andrew Todd, Yosuke Ueno, Michal Hajdušek, Rikizo Ikuta, Rodney Van Meter, Shota Nagayama, An Optical Interconnect for Modular Quantum Computers, arXiv:2412.09299 -- our brand-new paper! We're pretty proud of this one. See my recent blog post.

For more papers, see their respective Google Scholar profiles, linked above. Also, don't forget to look up the other collaborators (mostly physicists) I didn't name in the list at the top.

And the senior leadership, all with proper backgrounds in classical computer architecture:

• Hideharu Amano, Keio / U. Tokyo (one of Japan's most important classical computer architecture researchers)
• Koji Inoue, Kyushu (not old, but senior)
  
• Shigeru Yamashita, Ritsumeikan
• Masaaki Kondo, Keio (bridging the generations)
• rdv (not yet officially old, but not very far from it)

There are also quite a few theoretical physicists besides just those linked to above who are keeping Japan on the quantum map. I'll do a separate posting about them sometime.

There is a serious shortcoming here: all of the above (including yours truly) are (or at least present as) men. Out of about fifty-five people in the room, exactly one is (or at least presents as) a woman, a Keio undergrad (from Satoh's lab, not mine). Without solving this problem, on top of the human rights issue, Japan is throwing away half its brainpower and an even larger fraction of its creativity (due to the homogeneity and groupthink of a cluster of men, all with similar backgrounds and skills), at a time when demographics show it can ill afford to.