RIKEN Center for
Quantum Computing

Annual Report
FY2022

RIKEN
1

Contents
Director’s Greetings................................................................................................................................4
Overview of RIKEN Center for Quantum Computing............................................................................5
Collaborations........................................................................................................................................8
Press Releases.......................................................................................................................................9Awards..................................................................................................................................................10
Overview of Quantum Technology Innovation Hubs..........................................................................12
RQC FY2022 Pick Up Topics...............................................................................................................15
Team Outlines and Achievements• Superconducting Quantum Electronics Research Team.................................................................16• Superconducting Quantum Simulation Research Team..................................................................18• Superconducting Quantum Electronics Joint Research Unit..........................................................20• Superconducting Quantum Computing System Research Unit......................................................22• Hybrid Quantum Circuits Research Team........................................................................................24• Optical Quantum Computing Research Team.................................................................................26• Quantum Many-Body Dynamics Research Team............................................................................28• Floating-Electron-Based Quantum Information RIKEN Hakubi Research Team.............................30• Semiconductor Quantum Information Device Research Team........................................................32• Semiconductor Quantum Information Device Theory Research Team............................................34• Quantum Computing Theory Research Team.................................................................................36• Quantum Information Physics Theory Research Team....................................................................38• Quantum Computational Science Research Team..........................................................................40• Analytical Quantum Complexity RIKEN Hakubi Research Team.....................................................42• RIKEN RQC-FUJITSU Collaboration Center...................................................................................44• Office of the Center Director............................................................................................................46
RESEARCH HIGHLIGHTS / PEOPLE...................................................................................................47

Director’s Greetings

Pioneering the frontier ofquantum information science
The RIKEN Center for Quantum
Computing (RQC) was establishedin April 2021 to realize quantum
computers.

Quantum mechanics, which was born in the early 20th
century, has evolved through the efforts of numerous
researchers and contributed to the development of scienceand technology over a broad range of areas. However, fromthe perspective of quantum information science, which has
rapidly gained attention since the end of the last century, it
has become clear that humans have not yet fully masteredquantum mechanics. That is, quantum mechanics andquantum technology are expanding their scope to information
science and technology, fields that they have had limited
contact with thus far, as they seek to uncover potentiallytransformative computing resources.
Even in the research and development of quantum
computers, the major physical platform has not yet been
narrowed down worldwide. At RQC, we have invited leadersin the fields of superconducting, optical, and semiconductor
quantum computers, which are relatively advanced. By
conducting research and development on different approaches
simultaneously, we hope to enhance opportunities for mutual
discovery and learning and create new ideas through thesynergy effect. In addition, RQC has other experimental
research teams developing basic technology for quantum
information processing using atoms and electrons, as well as
several theoretical teams working on research fields such as
4quantum computing theory, quantum algorithms, quantum
architecture, and quantum software. Diverse talents collaborate
from basics to applications, from experiments to theories,
aiming for breakthrough research every day.
Furthermore, RQC serves as the headquarters andquantum computer development hub at the core of all tenof the Quantum Technology Innovation Hubs that were set
up based on the National Quantum Technology Innovation
Strategy promoted by the Japanese government. As such, it isengaging in activities for promoting advances and collaborationsin Japanese quantum technology research and development.
Through the cross-industry-government-academia baseactivities, by sharing knowledge across specialized fields
and deepening lateral collaboration, we aim to accelerate theinnovation cycle, contribute to the development of science andtechnology, and contribute to society.
In RQC, three new teams will be launched in thefiscal year 2023. We will further deepen collaboration
with researchers inside and outside of RIKEN, conductmultifaceted discussions through the gathering of researcherswith diverse ideas and expertise, nurture talents to lead thenext generation of quantum technology, and work together asa center to conduct research and development towards therealization of quantum computers.

RIKEN Center for Quantum Computing
The RIKEN Center for Quantum Computing (RQC) will broaden quantum technology’s potential by engaging in coherent researchand development that encompasses everything from hardware to software, and fundamental science through to applications, withthe aim of realizing quantum computers as innovative information processing units based on the principles of quantum mechanics.
The RQC is consisted of 15 research teams in total. It is promoting the development of quantum computers with various
physics-related teams that cover the superconducting method, the optical method, the semiconductor method and so on, as well
as research and development by theoretical teams that cover quantum algorithms, quantum computation theory and other fields.

Director:
Yasunobu Nakamura
Deputy director: Akira Furusawa, Shinichi Yorozu

RIKEN Center for Quantum Computing
RQC Advisory Council

Superconducting Quantum Electronics Research Team: Yasunobu Nakamura
Superconducting Quantum Simulation Research Team: Jaw-Shen Tsai
Superconducting Quantum Electronics Joint Research Unit: Eisuke Abe
Superconducting Quantum Computing System Research Unit: Yutaka Tabuchi
Hybrid Quantum Circuits Research Team: Atsushi Noguchi
Optical Quantum Computing Research Team: Akira Furusawa
Quantum Many-Body Dynamics Research Team: Takeshi Fukuhara
Floating-Electron-Based Quantum Information RIKEN Hakubi Research Team: Erika Kawakami
Semiconductor Quantum Information Device Research Team: Seigo Tarucha
Semiconductor Quantum Information Device Theory Research Team: Daniel Loss
Quantum Computing Theory Research Team: Keisuke Fujii
Quantum Information Physics Theory Research Team: Franco Nori
Quantum Computational Science Research Team: Seiji Yunoki
Analytical Quantum Complexity RIKEN Hakubi Research Team: Tomotaka Kuwahara
RIKEN RQC-FUJITSU Collaboration Center: Yasunobu Nakamura
Office of the Center Director: Shinichi Yorozu

Superconductivity

Optics

Atoms

Electrons

Semiconductor

TheoryAdministration

5

RIKEN Center for Quantum Computing (Continued)

BudgetGovernment funding for operations

9%
393

11%
500

Total

External funds

3,417

President’s Discretionary

5% 243¥4,55375%

Government subsidies for facilities

million

As of Mar 31, 2023

Members

PI

8%
Students etc.

2% 4Indefinite-term employee

31%

Fixed-term employee

17

27%

Total

57

210

members64(Women: 24,
Non-Japanese:52)

32%

Visiting Scientist etc.

68

As of Mar 31, 2023

Publications
Superconducting

20
1

Total

Theory

61

103

16

publications

5

HybridOptics

Semiconductor
As of Mar 31, 2023 (RIKEN RARS System)

6

RQC Colloquiums and RQC Seminars
FY2022 RQC Colloquium
At the RQC, we regularly invite renowned researchers and hold RQC Colloquiums. RQC Colloquiums were held 10 times in fiscal
2022, and vigorous discussions took place as a result of making the events known to other centers as well, not just the RQC.
No.

Date

Speaker

Affiliation

Title

1st

Apr. 20, 2022

Prof. John Martinis

UC Santa Barbara

Building a Quantum Computer

2nd

May 18, 2022

Prof. Andrew Dzurak

UNSW Sydney

Silicon-based quantum computing: The path from thelaboratory to industrial manufacture

3rdJun. 15, 2022

Prof. Adam Kaufman

JILA

Quantum science with microscopically controlled arrays of
two-electron atoms

4thJul. 13, 2022

Prof. Isaac Chuang

MIT

From spin physics to quantum algorithms

5thSep. 7, 2022

Dr. Zachary Vernon

XanaduFault-tolerant photonic quantum computing
Measuring the quantum state of superconducting qubits

6thOct. 5, 2022

Prof. Alexandre Blais

Institut Quantique,
Université de Sherbrooke

7th

Nov. 2, 2022

Prof. Jonathan HomeETH Zürich

Scaling quantum computing with trapped ions

8thDec. 7, 2022

Dr. James ClarkeIntel

From a Grain of Sand to a |Quantum> Bit of Information

9th

Jan. 11, 2023

Laboratoire Charles Fabry,Prof. Antoine Browaeys
Institut d’Optique, CNRS

Exploring the many-body problem with assembled atoms

10th

Mar. 15, 2023

Prof. Peter van Loock

Optical quantum information processing

University of Mainz

FY2022 RQC Seminar
Additionally, at the RQC we also hold RQC Seminars, which each PI organizes independently. RQC Seminars were held 28 times
in fiscal 2022, and vigorous discussions that went beyond the team framework were held, with the aim of making breakthroughs
related to the research and development of quantum computers.
No.

Date

9th

Jun. 8, 2022

10th

Jul. 29, 2022

11th

Aug. 16, 2022

SpeakerDr. Tomotaka
Kuwahara

Affiliation

No.

Date

RIKEN RQC

23rd

Dec. 20, 2022 Prof. Christian Flindt

Aalto University

Prof. Junichiro Kono

Rice University

24th

Jan. 20, 2023

Dr. Vittorio Vitale

ICTP

Prof. Anton Frisk
KockumCHALMERS UTS

25thJan. 31, 2023

Dr. Xianjing Zhou

RIKEN RQC

12th

Aug. 26, 2022 Prof. Ievgen Arkhipov Palacky University

26th

Feb. 13, 2023

Ms. Polina Kofman

NU Karazin

13th

Sep. 1, 2022

Lancaster University

27th

Feb. 21, 2023

Dr. Enrico Russo

Messina Univ.

14th

Sep. 12, 2022 Dr. Dan Gunlycke

U.S. Naval Research
Lab.

28th

Feb. 22, 2023

Prof. Kay Brandner

Univ. of Nottingham

15thOct. 28, 2022

Dr. Dany
Lachance-Quirion

Nord Quantique

29thMar. 8, 2023

16th

Nov. 1, 2022

Prof. Yueh-Nan Chen

30th

Mar. 14, 2023

17th

Nov. 4, 2022

Dr. Po-Chen Kuo

31stMar. 16, 2023

18th

Nov. 10, 2022

Prof. Stephen
Hughes

19th

Nov. 14, 2022 Prof. Yosuke Nakata

20th
21st

Prof. Henning
Schomerus

National Cheng
Kung Univ.
National Cheng
Kung Univ.

SpeakerDr. MarcelloDalmonte
Prof. Fabio
Marchesoni
Prof. Pulak Kumar
Ghosh

Affiliation

ICTP
INFN Perugia
Presidency Univ.

Queen’s University

32nd Mar. 17, 2023

Dr. Kaoru Mizuta

RIKEN RQC

Osaka University

33rdMar. 22, 2023

Mr. Alberto Mercurio

DST

Nov. 18, 2022 Dr. Alessandro Ferreri Forschungszentrum

34thMar. 24, 2023

Mr. Fabio Mauceri

DST

Mr. Zane Marius
Nov. 22, 2022
Rossi

MIT Quanta Group

35th

Mar. 30, 2023

Dr. Vyacheslav Misko VUB

NUS

36th

Mar. 31, 2023

Dr. Masanori Hanada Univ. of Surrey

22nd Nov. 25, 2022 Prof. Valerio Scarani

7Collaborations
International[Europe][North America]■Aalto University■Argonne National Lab■Chalmers University of Technology■Intel Corporation■Copenhagen University■Massachusetts Institute of Technology (MIT)■Delft University of Technology (TU Delft)■Nord Quantique■Interuniversity Microelectronics Centre (IMEC)■University of Notre Dame
■Johannes Gutenberg University Mainz
etc.
■Palacky University■Qutech■University of Basel
[Asia, Oceania]■Walther Meissner Institute (WMI)■Center for Axion and Precision Physics Research,

etc.

Institute for Basic Science■Electronics and Telecommunications Research Institute(ETRI)■Hunan Normal University■Moscow Institute of Physics and Technology(MIPT)■National Tsing Hua University (NTHU)■Taiwan Semiconductor Research Institute (TSRI)■The University of New South Wales(UNSW)■University of Technology Sydney(UTS)

etc.

National[Research Institute]■National Astronomical Observatory of Japan(NAOJ)■National Institute of Advanced Industrial
Science and Technology (AIST)■National Institute of Information andCommunications Technology (NICT)
etc.

[National University Corporations]■Keio University■Nagoya University■Okinawa Institute of Science and Technology
Graduate University (OIST)■Osaka University■Shizuoka University■The University of Tokyo■Tohoku University■Tokyo Institute of Technology■Tokyo Medical and Dental University■Tokyo University of Scienceetc.
8[Private business]■Fujitsu Ltd.
■Hitachi.Ltd.
■Mitsubishi Electric Corporation■Nikon Corporation■Nippon Electric Company, Ltd.
■Nippon Telegraph and Telephon
Corporation (NTT)■TOSHIBA CORPORATION
etc.

Press Releases
April 1, 2022
A Quantum Error Correction Methodology toward Lattice
Surgery
Superconducting Quantum Computing System Research Unit

April 26, 2022
It takes three to tangle: long-range quantum entanglement
needs three-way interaction
Analytical Quantum Complexity RIKEN Hakubi Research Team

August 25, 2022
Researchers demonstrate error correction in a silicon qubit system
Semiconductor Quantum Information Device Research Team

October 6, 2022
Local variational quantum compilation of a large-scale
Hamiltonian dynamics
Quantum Computing Theory Research Team

October 31, 2022
Quantum arbitrary waveform generator
Optical Quantum Computing Research Team

March 6, 2023
43-GHz real-time optical quantum signal detection for
ultrafast quantum computation
Toward super quantum computers with optical high-speed
communication technologies
Optical Quantum Computing Research Team

9

Awards
April 20, 2022
Dr. Seigo Tarucha awarded Commendation by Ministerof Education, Culture, Sports, Science and Technology
Dr. Seigo Tarucha, Team Leader, Semiconductor Quantum Information
Device Research Team, has been awarded the Commendation for Scienceand Technology (Research Category) by the Minister of Education, Culture,
Sports, Science and Technology (2022). This Commendation is awarded tohonor individuals who have achieved outstanding results in research anddevelopment, promotion of understanding, and other areas related to scienceand technology. Dr. Seigo Tarucha was awarded the Commendation for his
work: “Research on the Physics of Semiconductor Quantum Information and
its Applications in Quantum Computing.”

August 28, 2022
Mr. Ivan Grytsenko awarded ULT2022 Best Poster Award
Mr. Ivan Grytsenko, Technical Scientist for the Floating-Electron-Based Quantum Information RIKEN Hakubi Research Team, hasbeen awarded the ULT2022 Best Poster Award (International Conference on Ultra Low Temperature Physics 2022). This Award isadministered by the IUPAP (International Union of Pure and Applied Physics), the Japan World Exposition 1970 Commemorative
Fund, and the Inoue Foundation for Science, and it was awarded to Mr. Grytsenko for his work: “Using a cryogenic tunable
resonance circuit to image-charge detection of surface electrons on He II.”

November 21, 2022
Dr. Franco Nori selected as Clarivate Highly Cited
Researcher 2022
Dr. Franco Nori, Team Leader at the Quantum Information Physics Theory
Research Team, has been selected as a Clarivate Highly Cited Researcher
2022, as announced by Clarivate Analytics. Highly Cited Researchers are
selected in each field of research, as the authors of papers that rank in the
top 1% based on number of citations in Clarivate Analytics’ Essential Science
Indicators database.

December 15, 2022
Dr. Keisuke Fujii awarded the Japan Society for the
Promotion of Science (JSPS) Prize
Dr. Keisuke Fujii, Team Leader, Quantum Computing Theory Research Team,has been awarded the 19th (FY2022) JSPS (Japan Society for the Promotion of
Science) Prize. This Prize is awarded to young researchers aged under 45 in all
fields of the humanities, social sciences and natural sciences, who are recognized
as having achieved particularly outstanding academic results through papers,
books, and other research achievements published in academic journals andsimilar publications inside and outside Japan. Dr. Fujii was awarded the JSPS
Prize for his work: “Pioneering Research on Quantum Computing Theory and
Quantum Software for the Realization of Quantum Computers.”

10January 1, 2023
Dr. Franco Nori awarded PQE-2023 Lamb Award
Dr. Franco Nori, Team Leader, Quantum Information Physics Theory Research
Team, has been awarded the Lamb Award at the PQE-2023, which was
hosted by the Physics of Quantum Electronics (PQE) conference. The Lamb
Award is presented annually for outstanding contributions to the field oflaser science and quantum optics. Dr. Nori received the award for his work:“Pioneering contributions to quantum optics, quantum electronics, andquantum information.”

March 3, 2023
Dr. Akira Furusawa awarded Incentive Award in the
38th Telecommunications Advancement Foundation
Award (Telecom System Technology Award)
Dr. Akira Furusawa, Deputy Director of RQC, has been awarded an Honorable
Mention in the 38th Telecommunications Advancement Foundation
Award (Telecom System Technology Award). This award is granted by the
Telecommunications Advancement Foundation, and is given to research
papers and works related to information and telecommunications that havemade significant contributions and achievements to the advancement,
development, and standardization of information and telecommunications
from technical and engineering perspectives. The name of the paper that won
the award is: “Fabrication of low-loss quasi-single-mode PPLN waveguideand its application to a modularized broadband high-level squeezer.”

March 13, 2023
Dr. Yasunobu Nakamura and Dr. Jaw-Shen Tsai awarded a Japan Academy Prize
Dr. Yasunobu Nakamura, Director of RQC,
and Dr. Jaw-Shen Tsai, Team Leader,Superconducting Quantum Simulation
Research Team, have been awarded aJapan Academy Prize. Each year, up tonine Japan Academy Prizes are awarded
to persons who have achieved notable
research landmarks or who have authoredparticularly outstanding academic papers
or books. The title of Dr. Nakamura and Dr.
Tsai’s work is: “Pioneering Research Workson Superconducting Qubits and their
Quantum Control (Joint Research).”11

Quantum Technology Innovation Hubs

As the core organization of the 10 Quantum Innovation Hubs (QIH), RIKEN brings
all the hubs together as one to accelerate the social implementation of quantum
technology
QIH were established as hubs for streamlining the engagement of industry, government and academia in everything from basic
research on quantum technology through to technology verification, intellectual property management and human resources
development, from the perspective of securing and strengthening Japan’s international competitiveness, on the basis ofgovernment strategies known as the National Quantum Technology Innovation Strategy (January 2020) and the Vision of Quantum
Future Society (April 2022).
As the core organization of the QIH, RIKEN performs a headquarters function that strives for coordination among the 10 hubs.
Additionally, within the 10 domestic hubs, RIKEN is active as a quantum computing development hub that aims to realize andestablish quantum computer systems.

Quantum Technology Innovation Hubs

Global quantum Industrial Support(provisional) (AIST)

Quantum computing development(RIKEN)

Quantum software(Osaka University)

Quantum computer applications(UTokyo & Business alliance)

Quantum sensing(Tokyo Tech)

Quantum
Technology
Innovation Hubs
(Headquarters : RIKEN)

Quantum security(NICT)

Quantum technology international
Collaboration(OIST)

12

Quantum life science andfunctionalization(QST)

Quantum material(NIMS)

Quantum solution(Tohoku University)Subcommittees
Quantum Technology Innovation Promotion Committee

International Cooperation
SubcommitteeIntellectual Property /Standardization
Subcommittee

Industry-GovernmentAcademia Collaboration
Subcommittee

Human Resources
Development
Subcommittee

The positioning of the respective subcommittees
With an eye on the social implementation of quantum technology, the QIH established and operate the Quantum Technology
Innovation Promotion Committee as a meeting framework for undertaking joint recommendations and promotions. Foursubcommittees where the hubs exchange views on challenges, and share awareness and strategies, are established beneath thePromotion Committee.
• International Cooperation Subcommittee: Holds international symposiums, and promotes international collaborations such as
international joint research•Intellectual Property / Standardization Subcommittee: Shares strategies between the hubs on intellectual property andinternational standardization• Industry-Government-Academia Collaboration Subcommittee: Promotes industry-government-academia collaborations aimed
at the social implementation of quantum technology• Human Resources Development Subcommittee: Strengthens entries by young researchers, and human resources developmentthat transcends institutions and research fields

Pick up Activities
Quantum Innovation 2022, an international symposium
concerning quantum science and technology innovation,
was held on November 28-30, 2022 as one part of the QIH
activities. This Symposium, which was held mainly online,
served as a valuable platform for disseminating information.
Over the three-day period it featured approximately 91
lectures, two panel discussions, and sessions for young
participants with 64 presentations by young researchers,
and attracted remote participation from approximately 50
countries and a total of approximately 1,100 individuals.

Quantum Innovation 2022, November 28-30, 2022 (at the broadcastingvenue)

13

Quantum Technology Innovation Hubs (Continued)
In addition, the QIH are also focusing on outreach activities to promote the training and retention of personnel who will carry
quantum computing and quantum technology into the future, and partnerships with the industrial community.

夏休み科学イベント

ようこそ ふしぎな『量子』の世界へ
はじめの一歩！

学んでみよう。量子コンピュータ

5階ワークスにて、
実験ショー（1日2回）
工作教室（1日3回/定員各10名
/整理券配布）を開催

2022年

土
木 ‐13日○
8月11日○



10：0016：50

Images showing Welcome to the mysterious world of Quantum, a summer vacation science event heldon August 11-13, 2022, at the Science Museum

科学技術館 5階特設会場

東京都千代田区北の丸公園２番１号

※ご入館には事前のネット予約が必要です。
ご予約はこちらから。▶
主催：文部科学省委託事業「光・量子飛躍フラッグシッププログラム(QLEAP)」量子情報処理技術領域
共催：国立研究開発法人理化学研究所 量子コンピュータ研究センター
協力：科学技術館
お問い合わせ：hq_qleap_qc@ml.riken.jp
理化学研究所QLEAP HQ 夏休み科学イベント事務局
Exhibition at Nano tech 2023, which
was held on February 1-3, 2023, at
Tokyo Big Sight

Toshio Tonouchi (Ph.D.), CoordinatorSelected Publications
1 Toshio Tonouchi et al., “A fast method of verifying network routing with back-trace header space analysis”, IEEE/
IFIP IM 2015
2 CS Hong, Toshio Tonouchi ed., Internet for Changing Business and New Computing Services: 12th Asia-Pacific
Network Operations and Management Symposium, APNOMS 2009, LNCS 5787
3 Yoshinori Watanabe et al. “UTRAN O&M Support System with Statistical Fault Identification and Customizable
Rule Sets”, NOMS 2008
4 Nicholas Damianou, Naranker Dulay, Emil Lupu, Morris Sloman, Toshio Tonouchi, “Tools for Domain-Based
Policy Management of Distributed Systems”, IEEE/IFIP NOMS 2002
5 Toshio Tonouchi et al., “An Implementation of OSI Management Q3 Agent Platform for Subscriber Networks”,
IEEE Int Conf on Communication (ICC) 1997

Brief resume
1990
1992
1992
1999
2004
2008
2011
2018
2020
2021
2022

B.S. in Information Science, The University of Tokyo
M.S. in Information Science, The University of Tokyo
Researcher, C&C Systems Research Laboratories, NEC Corporation
Visiting Researcher, Imperial College
Principal Researcher, Internet System Research Laboratories, NEC Corporation
Ph.D. (Information Science), Osaka University
Senior Principal Researcher, Service Platform Research Laboratories, NEC Corporation
Director, Council for Science, Technology and Innovation, Cabinet Office
Deputy Manager, Planning Office for the Quantum Computing Center
Research Administrator, Office of the Center Director, RIKEN Center for Quantum Computing
Coordinator, Office of the Center Director, RIKEN Center for Quantum Computing (-present)

Dr. Toshio Tonouchi is angaged in work of the head quaters of QIH as a coordinator.

14

RQC FY2022 Pick Up Topics
RIKEN Open Campus
RIKEN Wako Open campus was held on April 23, 2022. The RQC implemented a variety of content to inform the public about
quantum computer research and development.

Science lecture by Dr. Yorozu.

Lab tour by Dr. Kawakami

Virtual Lab tour by Dr. Abe and Dr. Fukuhara.

Youtube video released by Dr. Tabuchi

Launch of and Opening Ceremony for Japan’s superconducting quantum computing
cloud service
On March 27, 2023, RIKEN installed and launched the cloud service of a domestically-built superconducting quantum computer,which was made by a joint-research group composed of Japanese universities, research institutes and businesses, for external
parties. In addition, that same day, RIKEN also held an Opening Ceremony for members of industry, government and academia
related to the project, and introduced the research and conducted a tour of the quantum computer hardware. The event wascovered by a wide range of media, including newspapers and TV broadcasters.

Commemorative Photo of Participants at the Opening Ceremony

Tour of the Quantum Computer

YouTube Video Release
In order to promote understanding of quantum computers, RIKEN
collaborated with the famous Japanese YouTuber Ichiken to make avideo introducing superconducting quantum computer research anddevelopment, in cooperation with the Superconducting Quantum
Simulation Research Team, and released it on YouTube.

15

Superconductivity

Superconducting Quantum Electronics Research Team
Keywords: Quantum computing, Superconducting circuits, Josephson junction, Microwave quantum optics, Circuit quantum
electrodynamics

Research Outline
Our team, in collaboration with other teams in RQC as well as
collaborators outside RIKEN, is conducting research and developmenton superconducting quantum circuits for quantum computing and otherquantum technologies. Currently, our focus is on the development of asuperconducting quantum computing platform with integrated qubits. Fora scalable implementation of qubits on a chip, we design and fabricate a
two-dimensional array of fixed-frequency transmon qubits on a Si wafer
with superconducting through-silicon vias. The control and readout ports
for the all-microwave architecture are brought vertically from the backsideof the chip as an array of coaxial cables and connected to the chip withspring contacts. In parallel, we are setting up dilution refrigerators, control
hardware and software.
We also investigate various phenomena and techniques in microwave
quantum optics, where superconducting qubits strongly coupled to aresonator and/or a waveguide are used as a tool for controlling microwave
photons stored or transmitted. The topics include microwave photon
emitters/receivers, non-reciprocal devices using Josephson junctions,nonlinear microwave quantum optics, and parametric amplification. They
can be both research targets and tools for other experiments.
Through those activities, we deepen our understanding further andmaster superconducting quantum electronics, which we believe leads us
to next breakthroughs.

Schematics of the package for an integrated
superconducting-qubit circuit

Photograph of a 64-qubit chip

Yasunobu Nakamura (Ph.D.), RQC Director, Team Leader
Selected Publications
1 S. Kono, K. Koshino, Y. Tabuchi, A. Noguchi, and Y. Nakamura, “Quantum non-demolition detection of anitinerant microwave photon”, Nature Physics 14, 546 (2018).
2 O. Astafiev, A. M. Zagoskin, A. A. Abdumalikov, Jr., Yu. A. Pashkin, T. Yamamoto, K. Inomata, Y. Nakamura, andJ.S. Tsai, “Resonance fluorescence of a single artificial atom”, Science 327, 840 (2010).
3 T. Yamamoto, K. Inomata, M. Watanabe, K. Matsuba, T. Miyazaki, W. D. Oliver, Y. Nakamura, and J. S. Tsai,“Flux-driven Josephson parametric amplifier”, Appl. Phys. Lett. 93, 042510 (2008).
4 I. Chiorescu, Y. Nakamura, C.J.P.M. Harmans and J.E. Mooij, “Coherent quantum dynamics of asuperconducting flux-qubit”, Science 299, 1869 (2003).
5 Y. Nakamura, Yu. A. Pashkin, and J. S. Tsai, “Coherent control of macroscopic quantum states in a singleCooper-pair box”, Nature 398, 786 (1999).

Brief resume
1992
1997
2001
2001
2002
2005
2012
2014
2020
2021
2022

16

Researcher, Fundamental Research Laboratories, NEC Corporation
Senior Researcher, Fundamental Research Laboratories, NEC Corporation
Principal Researcher, Fundamental Research Laboratories, NEC Corporation (-2005)
Visiting Researcher, Department of Applied Physics, Delft University of Technology (-2002)
Researcher, Frontier Research System, RIKEN (-2013)
Research Fellow, Fundamental and Environmental Research Laboratories, NEC Corporation (-2012)
Professor, Research Center of Advanced Science and Technology, The University of Tokyo (-2022)
Team leader, RIKEN Center for Emergent Matter Science
Group Director, RIKEN Center for Emergent Matter Science
Director, RIKEN Center for Quantum Computing (-present)
Professor, Department of Applied Physics, Graduate School of Engineering, The University of Tokyo (-present)Recent Achievements
Demonstration of NISQ algorithms on a superconducting quantum computing chip
We have developed and been testing 16- and 64-qubit
chips. As for the control and readout, we have achieveda single-qubit and two-qubit gate fidelities of 99.96% and99.1%, respectively, and a readout fidelity of 99.0% as thebest values. For the all-microwave architecture with fixedfrequency transmon qubits, frequency collisions between
neighboring qubits and resonators are a critical issue. We
are currently testing local laser annealing of individual qubits
to reduce the deviations of the qubit frequencies from thetargeted values.
We have implemented small-scale NISQ applications bypartially using one of the 16-qubit chips. The examples are
the subspace-search variable quantum eigensolver (SSVQE)
and subspace variational quantum simulator (SVQS).

(a) Unit-cell structure of our chip. A larger chip can be constructedby tiling the unit cell. (b) Energy levels of the ground and first-excitedstates of a hydrogen molecule obtained by SSVQE as a function of the
distance between two atoms.

K. Heya et al., “Subspace variational quantum simulator”, Phys. Rev. Research 5, 023078 (2023).

Fast, high-fidelity readout and reset of superconducting qubits
Fast, high-fidelity readout and reset of a qubit are criticalfor improving the performance of superconducting quantum
computers towards fault-tolerant quantum computing.
We invented an “intrinsic Purcell filter” and applied it to
the dispersive readout of a transmon qubit. The compact
notch-type filter implemented by using the internal mode
structure of a readout resonator suppresses qubit relaxation
through the resonator, while enabling fast readout of thequbit strongly coupled. We demonstrated qubit readout witha 40-ns readout pulse and 99.1% assignment fidelity in a
3D implementation. Later with a planar device, we further
improved the numbers to 36 ns and 99.6%, respectively.
The circuit also works for microwave-controlled fast
unconditional reset of the qubit.
Y. Sunada et al., “Fast readout and reset of a superconducting
qubit coupled to a resonator with an intrinsic Purcell filter”,
Phys. Rev. Appl. 17, 044016 (2022). Editors’ Suggestion(a)-(c) Mode structures at the relevant frequencies. The mode at the qubit
frequency has a node at the output coupling. (d) Schematic of the device
implemented in a 3D cavity.

Core members(Research Scientist) Shuhei Tamate(Research Scientist) Alexander Badrutdinov(Special Postdoctoral Researcher) Chung Wai Sandbo Chang(Postdoctoral Researcher) Zhiguang Yan(Postdoctoral Researcher) Rui Li(Postdoctoral Researcher) Chih-Chiao Hung
(Postdoctoral Researcher) Zhiling Wang

(Postdoctoral Researcher) Shiyu Wang
(Visiting Researcher) Ryo Sasaki(Visiting Researcher) Peter Anthony Spring(Senior Technical Staff) Koichi Kusuyama(Technical Staff I) Koh-ichi Nittoh(Technical Staff I) Laszlo Szikszai(Technical Staff I) Harumi Hayakawa

17

Superconductivity

Superconducting Quantum Simulation Research Team
Keywords: Superconductivity, Josephson effect, Macroscopic quantum coherence, Superconducting qubit, Superconducting quantum
information processing

Research Outline
We are conducting research aimed at realizing superconducting quantum computersand quantum simulators. Here, one-way quantum computers and gate-model quantum
computers are considered. Superconducting qubit possesses high degree of freedoms
in the circuit design and ability to local control as well as readout quantum states.
We proposed an integration method for superconducting qubit circuits that canbe easily packaged using conventional planar wiring and are working on a prototype ofsuch a quantum chip [Image 1]. This is a quasi-two-dimensional network that connects
qubits using local 3D wire crossing utilizing superconducting air bridges [Image 2].
Using this circuit method, we prototyped a 100-bit superconducting computer andconfirmed that it fits on a 20mm square chip.
Operations of superconducting computers require periodic initialization of qubits.
We have proposed a new circuit scheme that initializes superconducting qubits athigh speed and has succeeded in prototyping.
This circuit scheme is realized by connecting
a superconducting resonator with a variableattenuation rate to a qubit.
Quantum computer using Bosonic code
has an ability to correct errors relatively easily.
It protects quantum information from errors by
utilizing the infinite degrees of freedom of theresonator. We are conducting research on Kerr
parametric oscillators (KPO) to realize the cat code
Image 2. Photograph of wireusing superconducting circuits, which is one of thecrossing with airbridge in thepractical bosonic codes.
pseudo 2D network

Image 1. Qubits connected bypseudo-2D network

Jaw-Shen Tsai (Ph.D.), Team Leader
Selected Publications
1 H. Mukai, K. Sakata, S.J. Devitt, R. Wang, Y. Zhou, Y. Nakajima, and J.S. Tsai, “Pseudo-2D superconducting
quantum coupling circuit for the surface code: proposal and preliminary tests”, New Journal of Physics, 22,043013 (2020)
2 A. O. Niskanen, K. Harrabi, F. Yoshihara, Y. Nakamura, S. Lloyd and J. S. Tsai, “Quantum Coherent Tunable
Coupling of Superconducting Qubits”, Science, 316, 723 (2007)
3 T. Yamamoto, Yu. Y. Pashkin, O. Astafiev, Y. Nakamura, and J. S. Tsai, “Demonstration of conditional gate
operation using superconducting charge qubits”, Nature, 425, 941 (2003)
4 Yu. A. Pashkin, T. Yamamoto, O. Astafiev, Y. Nakamura, D. V. Averin and J. S. Tsai, “Quantum oscillations in two
coupled charge qubits”, Nature, 421, 823 (2003)
5 Y. Nakamura, Yu. A. Pashkin, J. S. Tsai, “Coherent Control of Macroscopic Quantum States in a Single-Cooperpair Box”, Nature, 398, 786 (1999)

Brief resume
1975
1983
1983
2001
2001
2012
2012
2014
2015
2022

18

Bachelor of Arts degree in Physics at University of California at Berkeley
Ph.D. State University of New York at Stony Brook
Research Scientist, Microelectronics Research Laboratories, NEC
Fellow, Nano Electronics Research Laboratories, NEC
Team Leader, Macroscopic Quantum Coherence Team, RIKENGroup Director, Single Quantum Dynamics Research Group, RIKENTeam Leader, Macroscopic Quantum Coherence Research Team, RIKENTeam Leader, Superconducting Quantum Simulation Research Team, RIKEN
Professor, Department of Physics, Tokyo University of Science (-present)
Professor, Research Institute for Science and Technology, Tokyo University of Science (-present)Recent Achievements
100-bit superconducting quantum computer

Image 1. Photograph of a 100-bit superconducting quantum computer.
Conventional flat wiring can be used for wiring to each qubit.

Image 2. Partial magnified view of the 100-bit chip. Quantum bits
and air bridges are visible.

We proposed a new integration scheme for superconducting qubit circuits that can be easily packaged using conventional planar
wiring and are working on a prototype of such a quantum chip [Image 1]. This is a quasi-two-dimensional network that connects
qubits using local 3D wire crossing utilizing superconducting air bridges [Image 2]. Using this circuit method, we prototyped a
100-bit superconducting computer and confirmed that it fits on a 20mm square chip.
In this circuit, all the qubits are located on the periphery of the chip, which means that conventional planar microwave
interconnect technology can be applied, eliminating the need for advanced 3D interconnect technology. The qubits are coupled
by a network of alternating crossed resonators, and nearest neighbor junctions are realized.

High-speed initialization of superconducting qubit
Operations of superconducting computers require periodic
initialization of qubits. We have proposed a new circuit
scheme that initializes superconducting qubits at high speed
and has succeeded in prototyping. This circuit scheme isrealized by connecting a superconducting resonator with avariable attenuation rate to a qubit.
The Q factor of the resonator is set to a high value during
normal operation of the qubit, so that the lifetime of the qubit
is not affected by the resonator. And during initialization of
the qubit, the Q factor of the resonator is set to a low value,
allowing for fast initialization. This is achieved by connecting
a SINIS junction to the resonator. In such a circuit, applying
a bias voltage to the SINIS junction induces photon-assisted
tunneling, which causes a drastic reduction in the Q-value ofthe resonator.
In our experiments, fast initialization in 180 ns with 99.5%
accuracy was achieved.

Fast initialization circuit photo. Superconducting qubits are connected
to a superconducting resonator containing a SINIS junction.

Core members(Special Postdoctoral Researcher) Hiroto Mukai

19

Superconductivity

Superconducting Quantum Electronics Joint Research Unit
Keywords: Superconducting quantum circuit, Quantum computing, Quantum technology, Microwave engineering, Quantum
entanglement

Research Outline
We develop scalable, multi-qubit quantum computers, in collaboration with the Superconducting Quantum Electronics ResearchTeam. We design a superconducting quantum circuit consisting of Josephson junctions, microwave resonators, transmission
lines, filter circuits and so on to implement superconducting qubits and functionalities for coherent controls and non-demolition
measurements of their quantum states on a single chip. We integrate a chip, a device package for connecting the chip and coaxial
cables, cryogenic microwave components such as Josephson parametric amplifiers, a dilution refrigerator to realize the ultracold
environment, and room-temperature electronics for qubit control into a single hardware operating as an intermediate-scalequantum computer with 50 –150 qubits. We evaluate control fidelities for single- and two-qubit gates, initialization, and readout
and aim to improve them. At the same time, we explore the potential of the system for NISQ (noisy intermediate-scale quantum)
applications. We also work on developing element technologies necessary for further scaling up the number of available qubits,
such as packing more microwave cables and components in a limited space, and realizing quantum control across different chips.
Ultimately, we aim to pave the way for realizing a system capable of quantum error correction, and to bring a quantum computerthat executes computations intractable with classical computers closer to reality.

Left: Wiring of measurement lines inside of adilution refrigerator
Right: Inside of a dilution refrigerator (left) and a64-qubit chip mounted on a sample package(right)Eisuke Abe (D.Sc.), Unit Leader
Selected Publications
1 K. Sasaki, H. Watanabe, H. Sumiya, K. M. Itoh, and E. Abe, “Detection and control of single proton spins in athin layer of diamond grown by chemical vapor deposition”, Applied Physics Letters 117, 114002 (2020).
2 S. Ishizu, K. Sasaki, D. Misonou, T. Teraji, K. M. Itoh, and E. Abe, “Spin coherence and depths of single nitrogenvacancy centers created by ion implantation into diamond via screening masks”, Journal of Applied Physics 127,
244502 (2020).
3 D. Misonou, K. Sasaki, S. Ishizu, Y. Monnai, K. M. Itoh, and E. Abe, “Construction and operation of a tabletop
system for nanoscale magnetometry with single nitrogen-vacancy centers in diamond”, AIP Advances 10,
025206 (2020).
4 R. Sakano, A. Oguri, Y. Nishikawa, and E. Abe, “Bell-state correlations of quasiparticle pairs in the nonlinear
current of a local Fermi liquid”, Physical Review B 99, 155106 (2019).
5 E. Abe and K. Sasaki, “Tutorial: Magnetic resonance with nitrogen-vacancy centers in diamond—microwave
engineering, materials science, and magnetometry”, Journal of Applied Physics 123, 161101 (2018).

Brief resume
2005
2006
2006
2010
2011
2012
2013
2015
2016
2019
2021

20

Research Fellow DC2, Japan Society for the Promotion of Science
D. Sci., Keio University
Research Associate, The Institute for Solid State Physics, The University of Tokyo
Postdoctoral Research Assistant, Department of Materials, University of Oxford
Specially-Appointed Researcher, Institute for Nano Quantum Information Electronics, The University of Tokyo
Specially-Appointed Researcher, Principles of Informatics Research Division, National Institute of Informatics
Research Scientist, Center for Emergent Matter Science, RIKEN
Project Lecturer, Faculty of Science and Technology, Keio University
Project Associate Professor, Keio Advanced Research Centers, Keio University
Unit Leader, Center for Emergent Matter Science, RIKEN
Unit Leader, RIKEN Center for Quantum Computing (-present)Recent Achievements
Detection and control of single proton nuclear spins with a solid-state quantum
sensorControl of solid-state qubits including superconducting qubits
often requires high-frequency signals ranging from a few to a
few tens of GHz, making microwave engineering a common
technology platform. In this study, we apply microwave
technologies to a single nitrogen-vacancy (NV) centers,
which are solid-state qubits operating at room temperatures
and attract attention as quantum sensors, and demonstratea high-sensitivity detection of a single proton nuclear spin.
Proton nuclear spins are the most important detection targets
for magnetic resonance imaging (MRI) and molecular structure
analysis by nuclear magnetic resonance (NMR). Conventional
NMR is limited by the low detection sensitivity. We successfully
detected and controlled single proton nuclear spins by using
an electronic spin of a single NV center in diamond as aquantum sensor. Furthermore, we obtained the information on
the position of the nuclear spin and observed Rabi oscillations
and free precessions of the single nuclear spin. These results
pave the way for “single molecular structure analysis”, in which
the individual positions of nuclear spins constituting a single
molecule are determined to infer the molecular structure.

Control of a single proton spin with a quantum sensor. The horizontal
axis is the operation time, and the vertical axis is the sensor spinstate. The data shows a coherent rotation of a nuclear spin.
©American Institute of Physics
https://doi.org/10.1063/5.0016196

Bell-state correlation between quasiparticles excited in solid-state quantum devices
A crucial point that differentiates quantum mechanics fromclassical mechanics is the existence of quantum entanglement,
nonlocal correlation among physical quantities. Entanglement
is also thought to play an important role in understanding
the origin of computational power of quantum computers.
Creation of multi-qubit entanglement in solid-state qubits
such as superconducting or spin qubits has thus been an
ongoing research theme. Apart from qubits, in solid-state
devices, quasiparticles can create nontrivial correlations, with
Cooper pairs in superconductors forming spin-singlets as aprime example. In this work, we theoretically studied quantum
correlation between quasiparticles excited in quantum dots in
the Kondo regime, by deriving a special form of Bell’s inequality
for current cross correlation. We found that, in specific regimesof interaction parameters, quasiparticles can have correlations
beyond the bound defined by the so-called hidden variable
theory, thus exhibiting entanglement. This is an interesting
result in that the Kondo effect, a central theme in condensed
matter physics, is viewed from the perspective of quantum
information science.

Relation between the strength of antiferromagnetic interaction andcurrent correlation. The region describable by the hidden variable
theory (HVT) and that only describable by quantum mechanics (QM)
are shown.
©American Physical Society
https://doi.org/10.1103/PhysRevB.99.155106

21

Superconductivity

Superconducting Quantum Computing System Research Unit
Keywords: Superconducting quantum computers, System in Package (SIP), Heterogeneous integration

Research Outline
Our unit pursues the realization of practical quantum computers. We study a quantum mechanical system that integrates qubits,
readout circuits, wiring, control electronics, cooling units, signal processing circuits, etc., to exploit and maximize a quantummechanical feature in superconducting quantum chips. Each component has strengths and weaknesses, and the combination of
those gives trade-offs. We establish a design method that harmonizes the elements to expand the performance and scalability ofa quantum computing system.
For example, we explore scalable arrangements of qubits, inter-qubit wirings, and control lines in a realistic three-dimensional
space as device design. The error-correction mechanism in fault-tolerant quantum computation demands continuous refresh (orupdate) operation to superconducting qubits, losing room for time division multiplexing to simplify the device structure. Whereas
the surface code is extendable in a two-dimensional plane for redundancy and scalability, only one more dimension remains to
introduce the control and readout lines to the qubit chip. We seek possibility in stacked module systems that integrate qubits,
control, and readout circuitry in a few substrates with essential scalability toward fault-tolerant quantum computation. Furthermore,
the structure brings heterogeneous integration where various signal processing circuits, e.g., optical interconnects, single flux
quantum circuits, etc., are organized in a single module.
Top I/O

4K

Co -Decoder

Analog

10 mK

Vertical coax
Interposer

DigitalQubit Controller

DigitalRT

Interposer& Readout

QEC Decoder
Readout + A/D

Quantum Chip
Micro-bumps

Quantum Chip
D/A

Qubit

Thru silicon vias

Op-DEC Circuits

Bottom I/OStacked module systems

Implementation of stacked module systems

Yutaka Tabuchi (Ph.D.), Unit Leader
Selected Publications
1 Y. Ueno, M. Kondo, M. Tanaka, Y. Suzuki, Y. Tabuchi “QULATIS: A Quantum Error Correction Methodology
toward Lattice Surgery,” 28th IEEE International Symposium on High-Performance Computer Architecture(HPCA), pp.274-287 (2022).
2 Y. Ueno, M. Kondo, M. Tanaka, Y. Suzuki, Y. Tabuchi “QECOOL: On-Line Quantum Error Correction with aSuperconducting Decoder for Surface Code,” 58th IEEE/ACM Design Automation Conference (DAC), pp.451456 (2021).
3 D. Lachance-Quirion, S. Wolski, Y. Tabuchi, S. Kono, K. Usami, Y. Nakamura. “Entanglement-based single-shotdetection of a single magnon with a superconducting qubit,” Science, 367, pp.425-428 (2020).
4 Y. Tabuchi, S. Ishino, A. Noguchi, T. Ishikawa, R. Yamazaki, K. Usami, Y. Nakamura. “Coherent coupling between
a ferromagnetic magnon and a superconducting qubit,” Science, 348, pp.405-408 (2015).
5 Y. Tabuchi, S. Ishino, T. Ishikawa, R. Yamazaki, K. Usami, Y. Nakamura. “Hybridizing ferromagnetic magnons andmicrowave photons in the quantum limit,” Physical Review Letters, 113, p.083603 (2014).

Brief resume
2012
2015
2017
2020
2021

22

Postdoctoral Researcher, RCAST, University of Tokyo
JSPS Research Fellowship for Young Scientists
Associate Professor, RCAST, University of Tokyo
Unit Leader, Center for Emergent Matter of Science, Riken
Unit Leader, RIKEN Center for Quantum Computing (-present)Recent Achievements
Scalable quantum computers: from control electronics to decoding circuits:
Control electronics
Superconducting quantum computers consist of many elements, such as qubits, physical gates, device and device structures(including packaging), refrigerators, analog microwave front-ends, signal processors and controllers, quantum error correction
codes, decoders, and logical operations. Each element has to be scalable and integrable to accommodate million of physicalqubits for enjoyable application with fault tolerance under an assumption of a code distance and an error per gate of around 30
and 0.1%, respectively.
Here we study analog front-end circuits and back-end controller circuits. In controller circuits, many challenges remain toscale the system toward the goal. For example, the quantum operations to physical qubits demand precise phase synchronization
among controller channels since phase coherence of the microwave signals is an inevitable assumption for multi-qubit gates. Here,
we have demonstrated in a conference [Takefumi Miyoshi et al., “FPL Demo: A Flexible and Scalable Quantum-Classical Interface
based on FPGAs,” 22nd International Conference on Field Programmable Logic & Applications (31, Aug, 2022)] the scalable
realization of control electronics. For instance, we have shown that a clock synchronization mechanism similar to IEEE1588
guarantees the exact timing alignment in multi-controller operations for phase coherency. Even though we have validated our
method with 50-qubit-sized systems, the approach is scalable and can be a concrete path toward the goal.

Scalable quantum computers: from control electronics to decoding circuits:
Decoding circuits
In connection with the previous achievement, we study decoding circuits dedicated to ultra-fast operation in cryogenic temperature.
Decoding circuits are signal-processing circuits of measurement outcomes for fault-tolerant quantum computation. In a paper [Y.
Ueno et al., “QECOOL: On-Line Quantum Error Correction with a Superconducting Decoder for Surface Code,” DAC2021], wehave organized an “online” decoding circuit and evaluated it in a single flux quantum digital logic platform. The word online means
processing faster enough for physical operations, and the measurement outcomes are discardable after the use, leading to drastic
wiring reduction from low temperature to room temperature. We have extended the decoder in a paper [Y. Ueno et al., “QULATIS:
A Quantum Error Correction Methodology toward Lattice Surgery,” HPCA2022] to the one capable of merge-and-split operations
in lattice surgery with specialized care at the logical boundary. While this result is a proof-of-principle type of research, concrete
evaluation of back-end circuits may recast a different organization of quantum computers having scalability.
An implementation of fault-tolerant
quantum computers
A bunch of wiringbeing bottlekeck
for scale up

Decoderat room temp.

Off-load “software decoding” todedicated hardware at LT
Reduction of # of wirings
between LT and RT

Refrigerator

Refrigerator

Syndrome bits

Syndrome
extraction

Syndrome
extraction

Low temp.
20 mK-4K

Quantumbits

Low temp.
20 mK-4K

Decoder
at low temp.

Quantum
bits

Syndrome bits

Organizations of superconducting quantum computers (left: conventional, right: our proposal)

Core members(Technical Staff) Bunpei Masaoka(Technical Staff) Miyuki Ozawa

23

Superconductivity, Electrons

Hybrid Quantum Circuits Research Team
Keywords: Hybrid quantum systems, Microwave quantum optics, Electron trap, Quantum manipulations

Research Outline
A superconducting circuit is not merely a circuit without electrical resistance; with the Josephson junction, various quantum
functions such as qubits and parametric circuits can be realized. In particular, our team has recently succeeded in fabricatingsuperconducting qubits with an extremely long lifetime. In addition, taking advantage of the designability of superconducting
circuits, we have been investigating of various kinds of quantum gates and their fidelity improvement. Furthermore, we are interested
in research and development of hybrid quantum systems that combine such high-performance superconducting circuits withother quantum systems, such as microwave resonators and trapped electrons. They can be ultra-long llifetime quantum systems.
By observing and controlling these systems with superconducting circuits, we are trying to establish quantum control technologythat greatly surpasses existing performance. The realization of ultimate quantum technologies, such as fault-tolerant quantum
computers, will depend on the availability of high-precision quantum control that is far beyond the current state-of-the-art. To
address these issues, we aim to realize bosonic quantum error-correcting codes based on superconducting circuits, and quantum
manipulations of trapped electron in the vacuum. Furthermore, we aim to develop new quantum fundamental technologies throughthe coexistence and collaboration of quantum systems.

Transmon qubits made from TiN electrode. We are fabricating thequbit with state-of-the-art.

Wavefunction of the GKP qubit encoded in the Harmonic oscillator.
We simulate the stabilizing measurement from the vacuum state.

Atsushi Noguchi (Ph.D.), Team Leader
Selected Publications
1 S. Shirai, Y. Okubo, K. Matsuura, A. Osada, Y. Nakamura, and A. Noguchi, “All-microwave manipulation ofsuperconducting qubits with a fixed-frequency transmon coupler”, arXiv:2302.06930.
2 M. Shigefuji, A. Osada, M. Yabuno, S. Miki, H. Terai, and A. Noguchi, “Efficient low-energy single-electron
detection using a large-area superconducting microstrip”, arXiv:2301.11212.
3 A. Osada, K. Taniguchi, M. Shigefuji, and A. Noguchi, “Feasibility study on ground-state cooling and singlephonon readout of trapped electrons using hybrid quantum systems”, Phys. Rev. Research 4, 033245 (2022).
4 A. Noguchi, A. Osada, S. Masuda, S. Kono, K. Heya, S. Piotr Wolski, H. Takahashi, T. Sugiyama, D. LachanceQuirion, and Y. Nakamura, “Fast parametric two-qubit gates with suppressed residual interaction using aparity-violated superconducting qubit”. Phys. Rev. A 102, 062408 (2020).
5 A. Noguchi, R. Yamazaki, Y. Tabuchi, and Y. Nakamura, “Single-photon quantum regime of artificial radiationpressure on a surface acoustic wave resonator”, Nat. Commun. 11, 1183 (2020).

Brief resume
2013
2014
2015
2015
2016
2019
2020
2020
2021
2022

24

Postdoc researcher, Osaka University
Postdoc researcher, RCAST, The University of Tokyo
Postdoc researcher, RCAST, The University of Tokyo
Project Associate, RCAST, The University of Tokyo (-2018)
Researcher, JST PREST (-2019)
Associate Professor, Graduate School of Arts and Science, The University of Tokyo (-present)
Fellow, Inamori Research Institute for Science (-present)
Team leader, CEMS (-2021)
Team leader, RQC (-present)
Researcher, JST PREST (-present)Recent Achievements
All-microwave manipulation of superconducting qubits with a fixed-frequency
transmon coupler
The performance of a quantum computer is strongly dependent
on the accuracy of quantum manipulations. Among the circuits
used as superconducting qubits, the highest-performance
circuit is a fixed-frequency transmon qubit. This circuit does
not have an external tuning knob of the frequency, making it
robust against noise. However, it is known that the inability
to change the frequency limits the accuracy of the quantum
gates due to the unwanted residual interactions that occur
even when the gate is not active. In this work, we proposeda circuit in which two fixed-frequency qubits are connected
through a fixed-frequency qubit coupler, and developed a newquantum gate for this circuit. This method not only reduces theresidual interactions, but also has the feature that the residualinteractions do not deteriorate the gate accuracy.
S. Shirai, Y. Okubo, K. Matsuura, A. Osada, Y. Nakamura,and A. Noguchi, arXiv:2302.06930.

(a) Pulse sequence of the calibration experiment for the gate. (b)The result of the calibration experiment (c) Controlled Z gate. (d)Interleaved randomized benchmarking.

Efficient low-energy single-electron detection using a large-area superconducting
microstrip
Trapped electrons in cryogenic Paul traps are expected to be high fidelity quantum
system. However, because of the difficulty of cooling the trapped electrons and thehigh voltage in the microwave frequency range required for their trapping, electron traps
have not been studied extensively. In recent years, there have been reports of electron
trap at room temperature, in which captured electrons are emitted from a trap andhit an electrode while being accelerated, which is observed as an amplified electricalsignal. However, such electron detectors have been difficult to use in low-temperatureenvironments due to heating problems. In this study, we focused on electron detection
with superconducting wires, which are used in superconducting photon detectors(SNSPD). The superconducting electron detector successfully detected electrons ofabout 15 eV. The electron energy dependence of the detection efficiency suggests thatthe detection of electrons cab be modeled by the vortex model of SNSPD. A deeper
understanding of this mechanism will lead to the development of a device that canlower-energy electrons.

Signal from a single electronCore members(Postdoctoral Researcher) Yusuke Tominaga

25

Optics

Optical Quantum Computing Research Team
Keywords: Quantum information science, Quantum optics

Research Outline
Quantum computers use interference coming from the wave nature of quantum mechanics to surpass classical computers.
There are two types of waves, standing waves and traveling waves, and the feature of our method using light is that it handles
traveling waves. Whereas most other methods deal with standing waves, where short decoherence times matter, our method
has no decoherence problems because quantum states are generated one after another as traveling light pulses, which arethen destroyed by measurement. In our method, quantum teleportation is repeated to pass quantum information to successively
generated optical pulses. The huge quantum entanglement for repeated quantum teleportation is called a cluster state, which
can be considered as a quantum look-up table containing all input-output relations as a superposition. In our method, this canbe created on a large scale in a compact optical system by the time-domain multiplexing technique. The great advantage of our
method is high-speed computation. The bandwidth of optical parametric amplifiers that generate quantum light can be as highas 10 THz. Although teleportation is a speed bottleneck, when combined with 5G technology, it is possible to realize quantum
computers with very fast clocks of tens of gigahertz. Furthermore, when combined with all-optical teleportation in the future, asuper-fast quantum computer that effectively utilizes the bandwidth of 10 THz can be expected.

A model representing the structure of two-dimensional clusteredstates that can be created in the quantum state of light.

Optical system for generating large cluster states. Interferometers
are made using optical fibers.

Akira Furusawa (Ph.D.), RQC Deputy Director, Team Leader
Selected Publications
1 K. Takase, A. Kawasaki, B. K. Jeong, T. Kashiwazaki, T. Kazama, K. Enbutsu, K. Watanabe, T. Umeki, S. Miki,
H. Terai, M. Yabuno, F. China, W. Asavanant, M. Endo, J. Yoshikawa, and A. Furusawa “Quantum arbitrary
waveform generator”, Science Advances, 8, eadd4019 (2022).
2 K. Fukui, S. Takeda, M. Endo, W. Asavanant, J. Yoshikawa, P. van Loock, and A. Furusawa “Efficient
backcasting search for optical quantum state synthesis”, Phys. Rev. Lett., 128, 240503 (2022).
3 K. Takase, A. Kawasaki, B. K. Jeong, M. Endo, T. Kashiwazaki, T. Kazama, K. Enbutsu, K. Watanabe, T. Umeki,
S. Miki, H. Terai, M. Yabuno, F. China, W. Asavanant, J. Yoshikawa, and A. Furusawa “Generation of Schrödinger
cat states with Wigner negativity using continuous-wave low-loss waveguide optical parametric amplifier”, Optics
Express, 30, 14161-14171 (2022).
4 T. Sonoyama, W. Asavanant, K. Fukui, M. Endo, J. Yoshikawa, and A. Furusawa “Analysis of optical quantum
state preparation using photon detectors in the finite-temporal-resolution regime”, Phys. Rev. A, 105, 043714(2022).
5 W. Asavanant, Y. Shiozawa, S. Yokoyama, B. Charoensombutamon, H. Emura, R. N. Alexander, S. Takeda, N.
C. Menicucci, H. Yonezawa, and A. Furusawa “Generation of time-domain-multiplexed two-dimensional cluster
state”, Science, 366, 373 (2019).

Brief resume
1991 Ph.D. in Physical Chemistry, The University of Tokyo
1986 Research staff member of Nikon Corporation (-2000)
1988 Visiting faculty member at Research Center for Advanced Science and Technology (RCAST), The Universityof Tokyo
1996 Visiting faculty member at California Institute of Technology
2000 Associate Professor of Applied Physics, The University of Tokyo
2007 Professor of Applied Physics, The University of Tokyo (-present)
2021 Deputy Director of the RIKEN Center for Quantum Computing / Team Leader of the Optical Quantum
Computing Research Team (-present)

26

Recent Achievements
Generation of time-domain-multiplexed two-dimensional cluster state
Cluster states are important entangled states that serve as
universal resources in our quantum computers based on
quantum teleportation. They contain arbitrary input-output
relations as a superposition and can be shrunk to a specificinput-output relation by measurement depending on thedesired computation. In our previous work, one-dimensional
clustered states were generated on a large scale by timedomain multiplexing, which enable a computation on a single
input. By using dual time-domain multiplexing, we havesucceeded in extending the cluster states to two dimensional
structures. This has enabled universal quantum computationthat handles multiple inputs.

Two-dimensional cluster states by time-domain multiplexing.
A, B: Extension from one-dimensional to two-dimensional
to accommodate multiple inputs. C: Setup to generate
two-dimensional cluster states. D, E: Structure of generated twodimensional cluster states.
©W. Asavanant et al., Science 366, 373 (2019). DOI: 10.1126/
science.aay2645.

Quantum arbitrary waveform generator
Arbitrary waveform generators (AWG), which generate arbitrary
waveform optical pulses, have been studied only for classicallight. For application to quantum technology, a new lightsource, quantum arbitrary waveform generator (Q-AWG), which
generates arbitrary quantum states with arbitrary waveform
optical pulses, will be required in the future. We have proposeda method to achieve this by using quantum entanglement. We
have also succeeded in experimentally generating quantum
light with a special waveform, which has been difficult. This isan achievement that will lead to a versatile “ultimate quantum
light source,” and is expected to contribute to various quantum
technologies, including optical quantum computers.
Concept of quantum arbitrary waveform generator. A: Twoconfiguration items of a quantum arbitrary waveform generator,
waveform and quantum state. B: Setup where the output of
the quantum arbitrary waveform generator is used as input to aquantum computer.
©K. Takase et al., Science Advances 8, eadd4019 (2022). DOI:
10.1126/sciadv.add4019.

Core members(Research Scientist) Jun-ichi Yoshikawa(Postdoctoral Researcher) Atsushi Sakaguchi

27

AtomsQuantum Many-Body Dynamics Research Team
Keywords: Quantum simulation, Quantum dynamics, Cold atom, Optical lattice

Research Outline
Deepening our knowledge of quantum many-body systems contributes to discover and understand various phenomena and todevelop new technologies. Quantum simulation, which experimentally unveil the quantum many-body systems of interest by using
other quantum many-body systems with excellent controllability, has been attracting attention. Our team aims to conduct quantum
simulation with ultracold atoms in optical lattices. Optical lattice systems provide ideal platforms for studying important issues incondensed matter physics, such as high-temperature superconductivity and quantum frustration. The systems are also suitablefor investigating non-equilibrium dynamics in quantum many-body systems due to less dissipation and decoherence.
We especially focus on the physics in frustrated spin systems. Various quantum phases and novel quantum states such as
quantum spin liquids emerge in frustrated spin systems, but systematic understanding of these phenomena has not yet beendeveloped. Furthermore, the existence of quantum phases that have not yet been discovered experimentally has been pointed out.
As a platform for investigating these issues, we have constructed a geometrically frustrated triangular lattice and loaded a quantum
gas into it. We also implemented a quantum gas microscope, which enables us to detect ultracold atoms in optical lattices at thesingle-atom level, and thus to microscopically observe quantum correlations and dynamics of the system. With this experimental
system, we will elucidate frustrated spin systems and explore unknown quantum many-body phenomena and quantum phases.

Schematic of experimental setup. A high-numerical-aperture (NA)
objective allows to observe atomic ensembles in triangular opticallattices at the single-atom level.

Single atom detection of ultracold gases in triangular optical lattices

Takeshi Fukuhara (D.Sci.), Team Leader
Selected Publications
1 R. Yamamoto, H. Ozawa, D. C. Nak, I. Nakamura, and T. Fukuhara, “Single-site-resolved imaging of ultracold
atoms in a triangular optical lattice”, New J. Phys. 22, 123028 (2020).
2 F. Schäfer, T. Fukuhara, S. Sugawa, Y. Takasu, and Y. Takahashi, “Tools for quantum simulation with ultracold
atoms in optical lattices”, Nat. Rev. Phys., 2, 411 (2020).
3 D. Yamamoto, T. Fukuhara, and I. Danshita, “Frustrated quantum magnetism with Bose gases in triangular
optical lattices at negative absolute temperatures”, Commun. Phys., 3, 56 (2020).
4 I. Nakamura, A. Kanemura, T. Nakaso, R. Yamamoto, and T. Fukuhara, “Non-standard trajectories found bymachine learning for evaporative cooling of 87Rb atoms”, Opt. Express, 27, 20435 (2019).
5 T. Fukuhara, S. Hild, J. Zeiher, P. Schauß, I. Bloch, M. Endres, and C. Gross, “Spatially Resolved Detection of aSpin-Entanglement Wave in a Bose-Hubbard Chain”, Phys. Rev. Lett. 115, 035302 (2015).

Brief resume
2009
2009
2010
2014

D. Sci., Kyoto University
Researcher, ERATO Ueda Macroscopic Quantum Control Project, Japan Science and Technology Agency
Postdoctoral researcher, Max Planck Institute of Quantum Optics, Germany
Unit Leader, Quantum Many-Body Dynamics Research Unit, Cross-Divisional Materials Research Program,
RIKEN Center for Emergent Matter Science
2021 Unit Leader, Quantum Many-Body Dynamics Research Unit, RIKEN Center for Quantum Computing (RQC)
2022 Team Leader, Quantum Many-Body Dynamics Research Team, RIKEN Center for Quantum Computing(RQC) (-present)28

Recent Achievements
Quantum gas microscopy of Mott transition in a triangular optical lattice
Quantum phase transition from superfluid to Mott insulator
occurs when a quantum gas is loaded into a triangular opticallattice and the depth of the optical lattice is increased. Quantumgas microscope allows us to observe real-space distributionsof quantum gases at the single-atom level. We observed anincrease in the filling at each lattice site as the system goes tothe Mott insulating state. In addition, we can obtain information
on phase coherence by implementing the time-of-flight method
to the quantum gas microscope. The Mott insulating state
has been observed also from the disappearance of the phasecoherence. The superfluid-Mott insulator quantum phasetransition is the most representative phenomenon in the opticallattice systems, and therefore the observation of the transition
is an important step towards quantum simulations of strongly
correlated quantum many-body systems.

Quantum gas microscope enables us to access both phasecoherence (upper right) and real space distributions (lower left) ofquantum gas in optical lattices. Lower right image is average data
for the phase coherence.

Quantum gas in frustrated triangular optical lattice
By mapping the phase of Bose-Einstein condensates (BECs)
in optical lattices as spin, the XY spin model is realized. Since
the coupling between spins corresponds to tunneling between
lattice sites, ferromagnetic spin-spin coupling is usuallyrealized. By modulating the phase of the optical lattice, we canchange the sign and magnitude of the effective tunneling, andthus adjust the spin-spin coupling parameters. We realized
XY spin model on the triangular lattice by loading BEC into a
triangular optical lattice and confirmed various phases in thisspin model by observing measuring interference patterns via
time-of-flight measurement. A geometrical frustration inducedby the negative tunneling leads to two chiral modes. We also
observed that one chiral mode appears randomly in eachexperiment, due to spontaneous symmetry breaking.

Interference patterns correspond to various phase in the XY spinmodel on the triangular lattice

H. Ozawa et al., in preparation.

Core members(Research Scientist) Ryuta Yamamoto(Postdoctoral Researcher) Hideki Ozawa

29

Electrons

Floating-Electron-Based Quantum Information RIKEN Hakubi
Research Team
Keywords: Quantum technology, Quantum computer, Quantum bit, Two-dimensional electron system, Microwave

Research Outline
Our team is working on the application of electrons floating on liquid helium toquantum information. This physical system has a high potential for providing an
ideal platform on which to realize a quantum computer, since it is free of impurities
and defects. The quantized states normal to the liquid helium surface are called
Rydberg states. The Rydberg-ground state and the Rydberg-1st-excited state arelocated 10 nm and 30 nm away from the liquid helium surface, respectively. The
Rydberg state of different electrons can be coupled via the long-range Coulombinteraction, which allows us to place electrons at a moderate distance while
keeping a considerable interaction between them to realize a two-qubit gate.
We are also working on the development of cryogenic microwave sources
for large-scale quantum computation. In most cases, qubits are placed at low
temperature and microwaves are sent to control and read out the qubits’ states.
For a small-scale quantum computer that is presently existing, we use thickcables that connect microwave generators at room temperature and qubits at
low temperature. However, it is difficult to prepare a so high number of such thickcables inside a cryogenic refrigerator as to be required for large-scale quantum
computation. In order to overcome this circumstance, we propose to develop
small-sized and low-power consumption microwave generators which function at
low temperature and place them inside the cryogenic refrigerator.

Experimental apparatus called “cell” to store
liquid helium

Erika Kawakami (Ph.D.), RIKEN Hakubi Team Leader
Selected Publications
1 E. Kawakami, A. Elarabi, and D. Konstantinov “Relaxation of the excited Rydberg States of Surface Electrons on
Liquid Helium”, Phys. Rev. Lett., 126, 106802 (2021).
2 A. Elarabi, E. Kawakami, and D. Konstantinov “Cryogenic amplification of image-charge detection for readout of
quantum states of electrons on liquid helium”, J. of Low Temp. Phys. 202, 456 (2021).
3 E. Kawakami, A. Elarabi, and D. Konstantinov “Image-Charge Detection of the Rydberg States of Surface
Electrons on Liquid Helium”, Phys. Rev. Lett., 123 086801 (2019).
4 E. Kawakami, T. Jullien, P. Scarlino, D. R. Ward, D. E. Savage, M. G. Lagally, Viatcheslav Dobrovitski, MarkFriesen, S. N. Coppersmith, M. A. Eriksson, and L. M. K. Vandersypen, “Gate fidelity and coherence of anelectron spin in a Si/SiGe quantum dot with micromagnet”, Proc. Natl. Acad. Sci., 113, 42, 11738 (2016).
5 E. Kawakami, P. Scarlino, D. R. Ward, F. R. Braakman, D. E. Savage, M. G. Lagally, Mark Friesen, S. N.
Coppersmith, M. A. Eriksson, and L. M. K. Vandersypen, “Electrical control of a long-lived spin qubit in a Si/SiGequantum dot”, Nat. Nanotechnol., 9, 666-670 (2014).

Brief resume
2016
2016
2017
2020

30

Ph.D., Delft University of Technology, The Netherlands
Postdoctoral researcher, Okinawa Institute of Science and Technology
PRESTO, Japan Science and Technology Agency (-present)
Team Leader, Floating-Electron-Based Quantum Information RIKEN Hakubi Research Team (-present)Recent Achievements
Hybrid Rydberg-spin qubit of electrons on helium
We propose a new way to realize qubits: a hybrid qubit of the
Rydberg state and the spin state of electrons floating on thesurface of liquid helium. An artificially introduced interaction
between the Rydberg state and the spin state allows us totransfer the qubit state between the Rydberg and spin states.
In this way, we can benefit from both the long coherence timeof the spin state and the long-range interaction of the Rydberg
state in the course of qubit operation.
The interaction between the Rydberg state and the spinstate is artificially introduced by a ferromagnet which is placednear the electron. We theoretically showed that the introduced
magnetic field gradient mixes the spin state and the orbital
state and shortens the spin relaxation time to 50 ms but does
not degrade the qubit fidelity. We estimated the single-qubit
gate and the two-qubit gate fidelities to be > 99.9999% and~99%, respectively.

A ferromagnetic pillar of which the tip is rounded and of the diameteris 100 nm is shown in blue and the outer electrodes are shown ingray. A single electron is trapped on top of the pillar. The thicknessof liquid 4He (light blue) above the center electrode is set to 140 nm.
Two neighboring electrons A and B are separated by d = 0.88 μm andare used as qubit A and qubit B, respectively. In the case shown here,
electron A’s Rydberg transition is on resonance with the MW applied
through a waveguide. Electron A goes back and forth between the
Rydberg-ground state and the Rydberg-1st-excited state.

Cryogenic microwave source for qubit read-out
One of the key features required to realize a fault-tolerant
scalable quantum computer is the integration of reliable andenergy-efficient electronics for qubit control and readout.
Recently, qubit control electronics have been successfully
integrated using cryogenic CMOS technology andsuperconducting Josephson junctions. Here, we focus on
the development of readout electronics using tunnel-diodeoscillator circuits. Compared to cryogenic CMOS devices andsuperconducting Josephson junction circuits, the tunnel-diodecircuits have lower power dissipation (~1μW). Furthermore, we
propose a new idea to increase the qubit readout fidelity using
the synchronization of two oscillator circuits.

Spectrum of the signal generated from the home-made cryogenic
microwave source. The signal frequency is around 107.3 MHz.

Core members(Technical Scientist) Ivan Grytsenko(Postdoctorial Researcher) Asher Jennings(Technical Staff) Hiromi Itoh

(SPDR) Xianjing Zhou
(Research Part Timer I) Mohan Rajesh(Research Part Timer I) Oleksiy Rybalko

31Semiconductor

Semiconductor Quantum Information Device Research Team
Keywords: Quantum computer, Semiconductor, Quantum bit, Quantum dot, Electron spin

Research Outline
We perform research and development to apply semiconductor electron (or hole) spins as units (qubits) of quantum information toquantum computing. Studies on semiconductor quantum computing have been motivated by advantages of long coherence time,compatibility with existing semiconductor device integration technology and capability of high-temperature (> 1 Kelvin) operation.
To date we have achieved various kinds of major quantum operations, including single qubit and two-qubit gates, initialization
and readout with high enough fidelities exceeding fault tolerant thresholds using spin qubits in Si quantum dots. Based on theseachievements we are now aiming to build up basic technologies of constructing medium to large scale quantum computers in Si.
In this line we will develop relevant quantum logic calculation methods, advanced quantum architectures, qubit devices that havecompatibility with semiconductor device integration technology.

Picture of a laboratory

Image of a one-dimensiional spin qubit device

Tarucha Seigo (D.Eng.), Dr.
Selected Publications
1 K. Takeda, A. Noiri, T. Nakajima, T. Kobayashi, and S. Tarucha, “Quantum error correction with silicon spin
qubits”, Nature, 608, 682-686 (2022).
2 M. Tadokoro, T. Nakajima, T. Kobayashi, K. Takeda, A. Noiri, K. Tomari, J. Yoneda, S. Tarucha, and T. Kodera,“Designs for a two-dimensional Si quantum dot array with spin qubit addressability”, Sci. Rep., 11, 19406(2021).
3 T. Nakajima, Y. Kojima, Y. Uehara, A. Noiri, K. Takeda, T. Kobayashi, and S. Tarucha, “Real-time feedback control
of charge sensing for quantum dot qubits”, Phys. Rev. Applied, 15, L031003 (2021).
4 K. Takeda, A. Noiri, J. Yoneda, T. Nakajima, and S. Tarucha, “Resonantly driven singlet-triplet spin qubit insilicon”, Phys. Rev. Lett., 124, 117701-1 – 5 (2020).
5 A.Noiri, K. Takeda, J. Yoneda, T. Nakajima, T. Kodera, and S. Tarucha,” Radio-frequency detected fast charge
sensing in undoped silicon quantum dots”, Nano Lett., 20, 947 (2020).

Brief resume
1978
1986
1990
1998
2004
2013
2013
2018
2019
2020

32

Basic Research Laboratories of Nippon Tel. & Tel. Corp.
Dr of Engineering
Research group leader, NTT Basic Research Laboratory
Professor, Department of Physics, University of Tokyo
Professor, Department of Applied Physics, University of Tokyo
Division Director, Quantum Information Electronics Division, RIKEN Center for Emergent Matter Science(-present)Group Director, Quantum Functional System Research Group, Division Director, Quantum Information
Electronics Division, RIKEN Center for Emergent Matter Science (-present)
Deputy Director, RIKEN Center for Emergent Matter Science
Guest Professor, Department of Physics, Tokyo University of Science (-present)
Team Leader, Semiconductor Quantum Information Device Research Team, RIKEN Center for Emergent
Matter Science(-present)Recent Achievements
High-fidelity universal quantum gate set using electron spin qubits in silicon

2qubit gate benchmark.pptx

High fidelity (> 99%) quantum gate set is an important ingredient for fault-tolerant quantum computing, but not achieved in silicon(Si) spin qubits until recently. Here we exceed the limit to achieve the fault tolerant condition.
We use a Si quantum dot (QD) device (Fig. 1) made in an isotopically enriched Si/SiGe quantum well and equipped with amicro-magnet which is utilized to speed up the single-qubit rotation. We realize a fast CNOT gate, the most important two-qubit
gate with use of the large exchange coupling and fast spin rotation, and obtain the fidelity of 99.8% and 99.5% for the single andtwo-qubit gate, respectively (Fig. 2).
F
,y
tli
e
id
f
e
c
n
e
u
q
e
S

Number of Clifford gates, n
Fig.1 Silicon spin qubit device chip used in this work.

Fig.2 Two-qubit gate fidelity characterization by Clifford-based
randomized benchmarking. The gate fidelity is obtained from the decayof the sequence fidelity as a function of number of Clifford gates.

Real-time feedback control of single-electron
charge sensor
Readout of spin qubits relies on spin-to-charge conversion followed bycharge sensing using a sensor quantum dot (QD). However, a charge
sensor is prone to electrical disturbances. We develop a real-time feedback
control system that allows for fast and reliable charge sensing.
The feedback controller is implemented on a field-programmable gatearray (Fig. 1). The conductance of an on-chip charge sensor placed next toa Si double QD (DQD) is probed by the rf reflectometry and processed in the
feedback controller so that the sensor is automatically gate-tuned to keep
the conductance constant. A DQD charge stability diagram taken without
the feedback reveals only a fraction of the DQD charge transitions because
the sensor is occasionally detuned from its sensitive point. This effect iscompensated by the feedback and we observe clear charge transitions.

Fig.1 Schematic of the feedback control system for aquantum-dot charge sensor.

Core members(Research Scientist) Takashi Kobayashi(Postdoctoral Researcher) Chien-Yuan Chang

33

Semiconductor, Theory

Semiconductor Quantum Information Device Theory Research
Team
Keywords: Quantum dots, Spin-based quantum information science, Qubit, Spin-orbit interaction, Quantum information processing

Research Outline
Our team is working on the theory of a spin-based quantum
computer. We design its CMOS-compatible components
deriving from Si and Ge gated quantum dots. We focus on
spin qubits that can be manipulated by electric fields throughvarious spin-orbit interactions. Using advanced band-structure
models, we investigate the properties of holes and electrons
confined in low-dimensional geometries. We search for optimal
setups and ways of protecting the qubits from noise. We
analyze perspective qubit interconnects which would allow
assembling a large number of qubits into networks. Our
ultimate goal is to identify fast, small, and scalable elements ofthe future quantum computer.

Spin-based quantum computing uses the spin of an electron in asolid to represent a quantum bit.

An array of quantum dots envisioned to realize a quantum processor.

Daniel Loss (Ph.D.), Team Leader
Selected Publications
1 P. Stano and D. Loss, “Review of performance metrics of spin qubits in gated semiconducting nanostructures,”,Nat. Rev. Phys. 4, 672 (2022).
2 A. Gutierrez-Rubio, J. S. Rojas-Arias, J. Yoneda, S. Tarucha, D. Loss, and P. Stano, “Bayesian estimation ofcorrelation functions,” Phys. Rev. Research 4, 043166 (2022).
3 O. Malkoc, P. Stano, and D. Loss, “Charge-noise induced dephasing in silicon hole-spin qubits”, Phys. Rev. Lett.
129, 247701 (2022).
4 T. Nakajima, A. Noiri, J. Yoneda, M. R. Delbecq, P. Stano, T. Otsuka, K. Takeda, S. Amaha, G. Allison, K.
Kawasaki, A. Ludwig, A. D. Wieck, D. Loss, S. Tarucha, “Quantum non-demolition measurement of an electron
spin qubit”, Nature Nanotechnology 14, 555 (2019).
5 D. Loss, D. DiVincenzo, “Quantum computation with quantum dots”, Phys. Rev. A 57, 120 (1998).

Brief resume
1985
1985
1989
1991
1993
1996
2012
2013

Ph.D. in Theoretical Physics, University of Zurich, Switzerland
Postdoctoral Research Associate, University of Zurich, Switzerland
Postdoctoral Research Fellow, University of Illinois at Urbana-Champaign, USA
Research Scientist, IBM T. J. Watson Research Center, USA
Assistant/Associate Professor, Simon Fraser University, Canada
Professor, Department of Physics, University of Basel, Switzerland (-present)
Team Leader, Emergent Quantum System Research Team, RIKENTeam Leader, Quantum System Theory Research Team, Quantum Information Electronics Division, RIKEN
Center for Emergent Matter Science (-present)
2021 Team Leader, Semiconductor Quantum Information Device Theory Research Team, RIKEN Center for
Quantum Computing (-present)34

Recent Achievements
Bayesian estimation of correlation functions
Correlation is intimately related to causality and prediction, and correlation functions are ubiquitous in different theories andformalisms. They also play a major role in the study of noise, which limits current state-of-the-art quantum devices. With thegoal of understanding the noise origin and properties, we have constructed a Bayesian theory of estimation of correlations of
stochastic variables sampled at regular intervals. Our results allow one to better understand statistical noise fluctuations, assess
the correlations between two variables, and postulate parametric models of spectra, which can be further tested. We have also
proposed a new method to numerically generate correlated noise with a given spectrum.

Estimated probability distributions of correlations parameters. The plot is in the complex plane, withcorrelation strength (sk) as the radial coordinate and correlation phase (ϕk) as the polar angle. The
color contours enclose 75, 50, 25, 10, and 5%-probability regions. Different colors correspond todifferent correlation degree and phase in the signal.
A. Gutierrez-Rubio, J. S. Rojas-Arias, J. Yoneda, S. Tarucha, D. Loss, P. Stano, “Bayesianestimation of correlation functions”, Phys. Rev. Research 4, 043166 (2022).

The field of spin qubits is vast. There is a host of variants on thesample material and structure, device design, or qubit encoding.
While this versatility in the qubit types is beneficial for overcoming
possible roadblocks, it also makes comparison of different spin
qubits difficult. To overcome this difficulty, we have collectedvalues of selected performance characteristics of semiconductor
spin qubits defined in electrically controlled nanostructures. The
characteristics are envisioned to serve as a community source
for the values of figures of merit with agreed-on definitions
allowing comparison of different qubit platforms. We include
characteristics on the qubit coherence, speed, fidelity, and thequbit-size of multi-qubit devices.

III-V
T2*

Ge
T2Echo

28

Si
DynD
T2

Si

Si:X

28

Si:X

C

T2Rabi

Coherence time [ns]Review of performance metrics ofspin qubits in gated semiconducting
nanostructures

106

103

100
2005

2010

2015

2020

Publication dateSpin coherence times according to the publication date. The pointcolor shows the device material. The point symbol shows thecoherence type as given in the legend.
P. Stano and D. Loss, “Review of performance metrics of spin
qubits in gated semiconducting nanostructures”, Nature Reviews
Physics 4, 672 (2022).

Core members(Postdoctoral Researcher) Juan Rojas-Arias

35

Theory

Quantum Computing Theory Research Team
Keywords: Quantum computing, Quantum information science, Quantum machine learning, Quantum error correction

Research Outline
Quantum computing is revolutionizing technology, and Quantum Computing Theory Research team is at the forefront of thistransformation. Our focus is on developing quantum computing theory and software essential for realizing quantum computers,
designing new quantum algorithms, and analyzing their performance.
Our team is working on near-term technologies to harness the power of quantum computers on scales achievable now andin the near future. We explore applications in fundamental physics, quantum chemistry, and quantum machine learning while
addressing quantum computer architectures optimization of quantum circuits. Another target is designing large-scale, fault-tolerant
quantum computers being equipped with quantum error correction, which capable of complex calculations with high reliability.
Embracing interdisciplinary research, we foster connections between quantum information science and fields like fundamental
physics, quantum chemistry, machine learning, and high-performance computing. We aim to open up new scientific frontiers withquantum computers or through the lens of quantum information science. This collaborative approach drives advancements inquantum computing and its real-world applications, positioning RQC as a key player in shaping the future of quantum technology.

A quantum circuit for quantum error correction (Left), fault-tolerant
quantum computing using the surface code (Right).

Quantum Circuit Learning: A supervised machine learning usingparameterized quantum circuits.

Keisuke Fujii (Ph.D.), Team Leader
Selected Publications
1 K. Mizuta, Y. O. Nakagawa, K. Mitarai, and K. Fujii, “Local variational quantum compilation of a large-scale
Hamiltonian dynamics” PRX Quantum 3, 040302 (2022).
2 K. Fujii, K. Mizuta, H. Ueda, K. Mitarai, W. Mizukami, and Y. O. Nakagawa, “Deep Variational Quantum
Eigensolver: a divide-and-conquer method for solving a larger problem with smaller size quantum computers”
PRX Quantum 3, 010346 (2021).
3 K. Mitarai, M. Negoro, M. Kitagawa and K. Fujii, “Quantum Circuit Learning”, Phys. Rev. A, 98, 032309 (2018).
4 K. Fujii and K. Nakajima, “Harnessing Disordered-Ensemble Quantum Dynamics for Machine Learning”, Phys.
Rev. Applied 8, 24030 (2017).
5 K. Fujii, M. Negoro, N. Imoto, and M. Kitagawa, “Measurement-Free Topological Protection Using Dissipative
Feedback”, Phys. Rev. X 4, 041039 (2014).

Brief resume
2011
2011
2013
2016
2017
2019
2020
2020

36

Ph.D. (Engineering), Kyoto University
Postdoc, Osaka University
Program-Specific Assistant Professor, Kyoto University
Assistant Professor, The University of Tokyo
Program-Specific Associate Professor, Kyoto University
Professor, Graduate School of Engineering Science, Osaka University (-present)
Deputy Director, Center for Quantum Information and Quantum Biology, Osaka University(-present)
Team Leader, Quantum Computation Theory Research Team, RIKEN (-present)Recent Achievements
An efficient compiling of quantum algorithm for large-scale quantum many-bodysystems
Quantum simulation, crucial for understanding complex quantum systems, relies on implementing time-evolution operators on
quantum circuits. The standard method, Trotterization, demands a massive number of gates for accuracy. We have proposed
the Local Variational Quantum Compilation (LVQC) algorithm,
enabling accurate and efficient compilation of time-evolution
operators in large-scale quantum systems through optimization
with smaller-sized systems. Utilizing a subsystem cost function
and the Lieb-Robinson bound, LVQC can work with limitedsize quantum computers or classical simulators. This technique
can efficiently construct time-evolution operators for various
systems, even with finite-, short-, and long-ranged interactions.
By using LVQC, we have successfully compressed the depthof time-evolution operators up to 40 qubits, showcasing
its potential for designing large-scale quantum circuits and©K. Mizuta et al., “Local variational quantum compilation of a largescale Hamiltonian dynamics”, PRX Quantum 3, 040302 (2022).
applications of intermediate-scale quantum devices.

Reducing overhead for FTQC by using quantum noise mitigation
In the early stages of fault-tolerant quantum computing (FTQC), limited scalability of quantum devices and insufficient computational
power of classical decoding units create challenges. We have developed an efficient FTQC architecture that integrates quantum
error correction and quantum error mitigation to effectively increase the code distance and T-gate count at the cost of constantsampling overheads. This can reduce the required number of physical qubits by 80% to 45% in various quantum computing
regimes. When the achievable code distance is up to about 11, the scheme enables executing 10^3 times more logical operations.
This breakthrough could significantly reduce computational overheads and accelerate the arrival of the FTQC era.

© Y. Suzuki et al., “Quantum Error Mitigation as a Universal Error Reduction Technique: Applications
from the NISQ to the Fault-Tolerant Quantum Computing Eras”, PRX Quantum 3, 010345 (2022).

Core members(Postdoctorial Researcher) Kaoru Mizuta(Postdoctorial Researcher) Tatsuhiko Ikeda(Postdoctorial Researcher) Koji Inui

37

Theory

Quantum Information Physics Theory Research Team
Keywords: Quantum Physics, Quantum optics, Quantum information processing and quantum computing, Artificial Intelligence,
Machine learning, Software for quantum physics, Superconducting qubits

Research Outline
Our research group performs interdisciplinary studies at the interface between quantum computing, quantum information
processing, superconducting quantum circuitry for quantum computing, photonics, quantum optics, atomic physics, nanomechanics, nanoscience, mesoscopics, computational physics, and condensed matter physics.
We developed the QuTiP software used worldwide for quantum information processing, quantum optics, and quantum open
systems. We are also using techniques from AI and Machine Learning to solve computationally hard problems. The Web of
Science has listed our research work as Highly Cited for the past six years (from 2017 to 2022). Less than 0.1% of researchers
reach this milestone.
We have published more than 30 papers in collaboration with various companies (NEC, Hitachi, Toshiba, NTT, IBM, etc.).
Currently, we are conducting joint research with NTT Research laboratories with the goal of solving difficult computational problems.

Our software QuTiP is widely used by very many research groups,
and it has been downloaded more than one million times.

Quantum cat states play an important role in quantum computing
and we have obtained interesting results in this area.

Franco Nori (Ph.D.), Team Leader
Selected Publications
1 W. Qin, A. Miranowicz, F. Nori, “Beating the 3 dB Limit for Intracavity Squeezing and Its Application to
Nondemolition Qubit Readout”, Phys. Rev. Lett. 129, 123602 (2022).
2 W. Qin, A. Miranowicz, H. Jing, F. Nori, “Generating Long-Lived Macroscopically Distinct Superposition States in
Atomic Ensembles”, Phys. Rev. Lett. 127, 093602 (2021).
3 Y. Nomura, N. Yoshioka, F. Nori, “Purifying Deep Boltzmann Machines for Thermal Quantum States”, Phys. Rev.
Lett. 127, 060601 (2021).
4 X. Wang, T. Liu, A.F. Kockum, H.R. Li, F. Nori, “Tunable Chiral Bound States with Giant Atoms”, Phys. Rev. Lett.
126, 043602 (2021).
5 Y.H. Chen, W. Qin, X. Wang, A. Miranowicz, F. Nori, “Shortcuts to Adiabaticity for the Quantum Rabi Model:
Efficient Generation of Giant Entangled Cat States via Parametric Amplification”, Phys. Rev. Lett. 126, 023602(2021).

Brief resume
1982 Conicit Fellow and Graduate Research Assistant; Physics Department. Also at the Materials ResearchLaboratory; University of Illinois, USA
1987 Postdoctoral Research Fellow, Institute for Theoretical Physics, University of California, Santa Barbara, USA
1990 Assistant Professor, Associate Professor, Full Professor and Research Scientist, Department of Physics,
University of Michigan, Ann Arbor, USA. (-present)
2002 Team Leader, Frontier Research System and, afterwards, Advanced Science Institute, RIKEN, Saitama,
Japan.
2013 Concurrent positions as: Group Director of the Quantum Condensed Matter Research Group, CEMS, andalso Team Leader at iTHES (Interdisciplinary Theoretical Sciences). RIKEN
2013 Chief Scientist. Theoretical Quantum Physics Laboratory, Cluster for Pioneering Research, RIKEN, Japan.
(-present)
2020 Team Leader for the Quantum Information Physics Theory Research Team, Quantum Computing Center,
RIKEN, Japan. (-present)

38

Recent Achievements
Quantum Information with quantum cat states
Quantum cat states play a very important role in quantum science and technology, and we have obtained several interesting resultsin this area. Our recent results in this area can be found in our publications, including these four examples:
Y.H. Kang et al., Nonadiabatic geometric quantum computation with cat-state qubits via invariant-based reverse engineering,
Phys. Rev. Research 4, 013233 (2022).
Z.Y. Zhou et al., Enhancing dissipative cat-state generation via nonequilibrium pump fields, Phys. Rev. A 106, 023714 (2022).
Y.H. Chen et al., Fault-Tolerant Multiqubit Geometric Entangling Gates Using Photonic Cat-State Qubits, Phys. Rev. Applied 18,
024076 (2022).
W. Qin, A. Miranowicz, F. Nori, Beating the 3 dB Limit for Intracavity Squeezing and Its Application to Nondemolition Qubit
Readout, Phys. Rev. Lett. 129, 123602 (2022).
Superconducting quantum circuit for implementing our proposal. The circuit consists of a SQUID array(black), a shunting capacitor (black), a flux bias line (purple), and an ac gate voltage (red).
©APS. Reference: Y.H. Kang et al., Nonadiabatic geometric quantum computation with cat-state qubits
via invariant-based reverse engineering, Phys. Rev. Research 4, 013233 (2022).

General results in the field of ultrastrong light-matter coupling.
We have obtained several general results in the field of ultrastrong light-matter coupling, including these recent results:
V. Macri et al., Revealing higher-order light and matter energy exchanges using quantum trajectories in ultrastrong coupling, Phys.
Rev. A 105, 023720 (2022).
W. Salmon et al., Gauge-independent emission spectra and quantum correlations in the ultrastrong coupling regime of open
system, cavity-QED, Nanophotonics 11, pp. 1573 (2022).
A. Mercurio et al., Regimes of cavity QED under incoherent excitation: From weak to deep strong coupling, Phys. Rev. Research
4, 023048 (2022).
Y.H. Chen et al., Enhanced-Fidelity Ultrafast Geometric Quantum Computation Using Strong Classical Drives, Phys. Rev. Applied
18, 064059 (2022).
V. Macri et al., Spontaneous scattering of Raman photons from cavity-QED systems in the ultrastrong coupling regime, Phys. Rev.
Lett., 129, 273602 (2022).
L.B. Fan et al., Quantum coherent control of a single molecular-polariton rotation, Phys. Rev. Lett., 130, 043604 (2023).
Logarithmic 2D plots of the cavity emission spectra Sc(ω)
for values of η reaching the Ultra-Strong Coupling and
Deep-Strong-Coupling regimes obtained using an effective
qubit temperature Tq.
©APS. Reference: A. Mercurio, V. Macri, C. Gustin, S.
Hughes, S. Savasta, F. Nori, Regimes of cavity QED underincoherent excitation: From weak to deep strong coupling,
Phys. Rev. Research 4, 023048 (2022).

Core members(Research Scientist) Clemens Gneiting(Postdoctoral Researcher) Yuran Zhang

39

Theory

Quantum Computational Science Research Team
Keywords: Quantum many-body systems, Quantum dynamics, Quantum information physics, Tensor network, High performancecomputingResearch Outline
Our main interest is to propose quantum-classical hybrid algorithms for simulating quantum many-body systems. We also analyze
quantum dynamics of quantum computing based on quantum information. For these purposes, we develop quantum simulations
for quantum computing using classical computers. We are also interested in quantum-classical hybrid systems for future highperformance computing.

0e f swap

1

eZZ
2

eZZ
eZZ

3

eZZ
power neZZeZZ

+ EFD + EST =

k=0

(p)
cn,k Ŝ 2m

∆τ
2

n−2k

+ EFD + EST∆τ−i∆τ ĤB

e−i 2 ĤA e

O(∆2τ ) errorĤ 1 ≈

1
+ −

+

1

-2

1

Ĥ 2 ≈

−

+ −

+ −

+

-1

3

-3

1

+

−

+ −

Ĥ 3 ≈+..
.

Ĥ n ≈i∆τ∆τ− Û −Û∆τ
22
i2 
∆τ
 Û
2
∆2τ 


i3 
∆τ
 Û
2
∆3τ
n

cn,k Û
k=02∆τ
− 2Iˆ + Û −
2

3

− 3Û
∆τ
2

Û(∆τ )





2

∆τ
∆τ
∆τ
+ 3Û −− Û −
2
2
2

n−2k

=

∆τ
∆τ
in
Û
− Û −
2
∆nτ
2

n





3

(p)
(∆τ ) =≈ Ŝ 2m

m−1
D(p)
+1
2m = 2(NΓ − 1)pO(∆2m+1) errorτ
due to the 2mth order Suzuki-Trotter decomposition

eZZ

Schematic figure for quantum power methods=

e f swap

RZ (θk )Quantum circuits for VQE calculations of Hubbard model

Seiji Yunoki (Ph.D.), Team Leader
Selected Publications
1 Q. Xie, K. Seki, and S. Yunoki, “Variational counterdiabatic driving of the Hubbard model for ground-state
preparation”, Phys. Rev. B 106, 155153 (2022).
2 K. Seki and S. Yunoki, “Energy-filtered random-phase states as microcanonical thermal pure quantum states”,
Phys. Rev. B 106, 155111 (2022).
3 K. Seki, Y. Otsuka, and S. Yunoki, “Gutzwiller wave function on a quantum computer using a discrete HubbardStratonovich transformation”, Phys. Rev. B 105, 155119 (2022).
4 K. Seki and S. Yunoki, “Spatial, spin, and charge symmetry projections for a Fermi-Hubbard model on aquantum computer”, Phys. Rev. A 105, 032419 (2022).
5 K. Seki and S. Yunoki, “Quantum Power Method by a Superposition of Time-Evolved States”, PRX Quantum 2,010333 (2021).

Brief resume
1996
1996
1999
2001
2006
2008
2010
2012
2017
2018
2021

40

∆τ

e−i 2 ĤA

due to the central finite-difference schemeeZZe f swap

X

n

n

+

−

−

∆τ
in
∆τ
(p)
− Ŝ 2m−
Ŝ (p)
2∆nτ 2m 2

Symmetric Suzuki-Trotter decompositionĤ 0 = Iˆ

-1++ EFD =

1 time t/∆τ

1

−

n

in
∆τ
dn Û(t)∆τ=− Û −
Û22
dtn t=0 ∆nτ

−

eZZ

16 |0

H

e f swap

15 |0

H

=

=

e f swapXe f swap

14 |0

13 |0

H

e f swapXe f swap

12 |0

11 |0

e f swapXe f swap

10 |0

e f swapHe f swapXe f swap

9 |0He f swap

8 |0XĤ n = in

Central finite difference-10

e f swap

7 |0He f swap

6 |0X
e f swap

5 |0

H

×D

e f swap

4|0e f swap

3 |0X

e f swap

2 |0He f swap

1 |0

Ph.D. (Engineering), Nagoya University
Postdoc, National High Magnetic Field Laboratory (USA)
Postdoc, Groningen University (The Netherlands)
Postdoc, SISSA (Italy)
Long-Term Visiting Scientist/Research Assistant Professor, Oak Ridge National Laboratory & University ofTennessee
Associate Chief Scientist, Computational Condensed Matter Physics Laboratory, RIKENTeam Leader, Computational Materials Science Research Team, Advanced Institute of Computational
Science, RIKENTeam Leader, Computational Quantum Matter Research Team, RIKEN Center for Emergent Matter Science
Chief Scientist, Computational Condensed Matter Physics Laboratory, RIKENTeam Leader, Computational Materials Science Research Team, RIKEN Center for Computational Science
Team Leader, Quantum Computational Science Research Team, Riken Center for Quantum Computing

Recent Achievements
Variational counterdiabatic driving of the Hubbard model for ground-state preparation
1.0

Timp

Tadi

l= 2
l= 3

UA
l= 1

adiabatic

0.8
0.6

FTT

Finding the ground state of a quantum many-body system by unitary time
evolution is a challenging task and an important application of quantum
computation to the study of quantum many-body systems. We apply thevariational counterdiabatic driving (CD) protocol to a one-dimensional twocomponent Fermi-Hubbard model and report the details of obtaining its
ground state. The CD protocol introduces an adiabatic gauge potential (AGP)
into the driving process, which suppresses the transition to the excited state,
thereby enabling fast driving of quantum states. We first show that the set ofthe optimal variational parameters in an approximate AGP consisting of nested
commutators of initial and final Hamiltonians is given as the solution vectorof a system of linear equations whose coefficients are given by the square
Frobenius norm of these commutators. The variational CD protocol up to
the third driving order is then applied to the one-dimensional Fermi-Hubbard
model and it is confirmed that the ground-state fidelity is systematically
improved with respect to the driving order and the driving duration. Our
results demonstrate the usefulness of the variational CD protocol to the
Fermi-Hubbard model and permit a possible route towards fast ground-state
preparation for many-body systems.

0.4
0.2
0.0 − 2
10

impulse10− 1intermediate
100

T

101

102

Dependence of the ground-state fidelity (vertical axis)of the Hubbard model on the driving time (horizontalaxis). Results are shown for the case without driving
assistance (unassisted, UA) and for driving orders
l=1,2,3 with variational AGP.
©Qing Xie, Kazuhiro Seki, and Seiji Yunoki, Phys.
Rev. B 106, 155153 (2022) “Copyright (2022) by theAmerican Physical Society.”

Q. Xie, K. Seki, and S. Yunoki, “Variational counterdiabatic driving of the
Hubbard model for ground-state preparation”, Phys. Rev. B 106, 155153(2022).

Quantum-classical hybrid computing for microcanonical ensembles
In condensed matter physics, not only the properties of particular states suchas the ground state of a quantum many-body system but also the propertiesof statistical mixture of many quantum states are important. In this study, we
propose a quantum-classical hybrid method for microcanonical ensembles.
The method is based on the idea that a pure state with a given energy as an
approximate expectation value is obtained by the Fourier transform of a timeevolved random state, and the inverse of the time parameter corresponding
to the cutoff of the time integration range in the Fourier transform sets thewidth of the energy shell of the microcanonical ensemble. Thermodynamic
quantities such as entropy and temperature are computed from the traceof the time evolution operator through random quantum circuits basedon unitary designs and the Fourier representation of the Gaussian. Fromnumerical simulations for the one-dimensional spin-1/2 Heisenberg model,
we confirmed that a significant difference in the statistical error appears
depending on the order of the unitary design and that the method is mostefficient for the target energy around which the dense distribution of energy
eigenstates is found.

Quantum circuit for preparing a time-evolved
random state. The random state is prepared in thefirst three parts (blue, orange, and red boxes). In thetime-evolution operator part (green box), we assume
the one-dimensional Heisenberg model underperiodic boundary conditions.
©Kazuhiro Seki and Seiji Yunoki, Phys. Rev. B 106,
155111 (2022) “Copyright (2022) by the American
Physical Society.”

K. Seki and S. Yunoki, “Energy-filtered random-phase states as microcanonicalthermal pure quantum states”, Phys. Rev. B 106, 155111 (2022).

Core members(Research Scientist) Kazuhiro Seki

(Postdoctoral Researcher) Xie Qing

41

Theory

Analytical Quantum Complexity RIKEN Hakubi Research Team
Keywords: Quantum entanglement, Hamiltonian complexity, Quantum many-body systems, Quantum computation, Quantum
simulation

Research Outline
The field of revealing the computational complexity ofquantum many-body system simulations when using
a classical computer (or quantum computer) is called
Hamiltonian Complexity. In particular, from the viewpoint of
quantum supremacy of quantum computers and quantum
algorithms employing NISQ (i.e., a small quantum computer
without error correction), Hamiltonian complexity has
become one of the most important research topics in thefield of quantum information. Our research aims to solve
open mathematical problems in Hamiltonian complexity.
The critical concept in the study of Hamiltonian
complexity is quantum entanglement, an exotic property
arising from the superposition principle of quantum
mechanics. The Nobel Prize in Physics for 2022 was
Schematic figure for quantum power methods
awarded to Alain Aspect, John Clauser, and Anton Zeilinger,
who experimentally verify the quantum entanglement.
Theoretical advances in quantum entanglement have opened up a series of new fields, including the complete classificationof quantum phases, the development of classical and quantum algorithms to simulate quantum matter, and fault-torelant quantum
computation. Among these, macroscopic length-scale quantum entanglement (long-range entanglement) is a key resource topresenting truly nontrivial quantum effects.
It is generally known that quantum entanglement only survive at ultra-low temperatures close to absolute zero. Since quantum
entanglement plays a crucial role in quantum computation, one of the key open problems is to elucidate to what extent quantum
entanglement survives in a finite temperature environment. One of the main goals of Hamiltonian complexity is to characterize
quantum entanglement quantitatively and qualitatively in quantum many-body systems.

Research Theme• Entanglement area law• Lieb-Robinson bound in interacting bosons• Quantum Markov property at arbitrary temperatures

Tomotaka Kuwahara (Ph.D), RIKEN Hakubi Team Leader
Selected Publications
1 T. Kuwahara, K. Saito, “Exponential Clustering of Bipartite Quantum Entanglement at Arbitrary Temperatures,”Physical Review X, 12, 021022 (2022)
2 A. Anshu, S. Arunachalam, T. Kuwahara, M. Soreimanifar (alphabet order), “Sample-efficient learning of quantum
many-body systems,” Nature Physics, 17, 931–935 (2021), Featured in News&Views
3 T. Kuwahara, A. M. Alhambra, and A. Anshu, “Improved thermal area law and quasi-linear time algorithm forquantum Gibbs states,” Physical Review X, 11, 11047 (2021)
4 T. Kuwahara, K. Saito, “Area law of noncritical ground states in 1D long-range interacting systems,” Nature
Communications, 11 4478 (2020)
5 T. Kuwahara, K. Saito, “Strictly Linear Light Cones in Long-Range Interacting Systems of Arbitrary Dimensions,”
Physical Review X, 10, 031010 (2020), Featured in Physics

Brief resume
2010 Bachelor in Applied Physics, The Faculty of Engineering, Department of Applied Physics, The University ofTokyo2012 Master in Physics, Faculty of Science, Department of Physics, The University of Tokyo
2015 Ph.D. in Physics, Faculty of Science, Department of Physics, The University of Tokyo
2015 JSPS Research Fellowship for Young Scientists (JSPS PD), The University of Tokyo
2016 Assistant Professor, Advanced Institute for Materials Research (AIMR), Tohoku University,
2017 Research scientist, Center for Advanced Intelligence Project, RIKEN
2021 Sakigake Researcher, Japan Science and Technology Agency
2022 RIKEN Hakubi team leader, RIKEN Cluster for Pioneering Research / RIKEN Center for Quantum
Computing, RIKEN

42

Recent Achievements
Clustering theorem of quantum entanglement
We have shown that in the thermal equilibrium state of a quantum many-body system, there is generally no “quantum
entanglement” over long distances. We proved that beyond a distance of 1/T (T: temperature), the standard “bi-partite” quantum
entanglement decays exponentially in distance. This result is quite general in the sense it is applied to all possible quantum manybody systems in any dimension and at any temperature. It shows that long-range entanglement survives only in the form of a
special “more than tri-partite” quantum entanglement at finite temperatures. This supports previous observations that long-range
entanglement at finite temperatures occurs as a topological order associated with tri-partite quantum entanglement. Our result is
expected to provide many clues for quantum computation, including
quantum machine learning, and to contribute to the classificationof quantum entanglement involved in various quantum phenomena(superconductivity, superfluidity, spin liquid, etc.) observed at finite
temperature.

Optimal Bosonic Lieb-Robinson boundWe have established the resolution to the Lieb-Robinson bound ininteracting boson systems, which has been a long-standing problem
in quantum many-body physics and quantum information theory.
T. Kuwahara and K. Saito, “Exponential clustering of bipartite
A fundamental principle of many-body physics is causality, i.e.,quantum entanglement at arbitrary temperatures”, Physicala strict prohibition of information propagation outside the light cone.
Review X, 12, 021022 (2022)
However, in non-relativistic systems, it is often unclear whether sucha light cone can be well defined. In the famous work by Lieb andRobinson, the amount of information is proved to be restricted in
the effective light cone, which is characterized by the so-called “Lieb-Robinson bound.” So far, the Lieb-Robinson bound is a
crucial concept in accessing the precision error of various quantum simulation algorithms and mathematical structure of quantum
entanglement at low (or zero) temperatures.
Our findings are twofold. As the first main result, we haveidentified the optimal light cone for interacting bosons andestablished a gate complexity to simulate the dynamics with anefficiency guarantee. Second, contrary to conventional expectations,
our results have unraveled a stark difference between the bosons
and fermions regarding information propagation. In detail, we havedemonstrated an acceleration of entanglement propagation as acharacteristic feature of bosonic systems. A simple protocol should
allow the experimental observation for such acceleration.
T. Kuwahara, T. Van Vu and K. Saito, “Optimal light coneand digital quantum simulation of interacting bosons,”
arXiv:2206.14736

Core members(Research Scientist) Yusuke Kimura(Postdoctoral Researcher) Tan Van Vu(Part timer) Hideaki Nishikawa

43

Superconductivity

RIKEN RQC-FUJITSU Collaboration Center
Keywords: Quantum computer, Superconducting qubit, Error mitigation technology, Error correction technology, Quantum application

Research Outline
Our group, which was established on April 1, 2021, conducts research anddevelopment (R&D) to realize quantum computers for practical use. We integrate
RIKEN’s advanced quantum computer technology using superconducting
circuits with FUJITSU’s computing technology and knowledge of quantum
technology applications based on customer perspectives.
Specifically, we develop hardware and software technologies that will
enable large-scale quantum computers with 1,000 qubits. In addition, wedevelop quantum applications using the quantum computers developed. In
terms of research on hardware, we conduct R&D of fundamental technologies,
such as the improvement of uniformity in qubit manufacturing, the reduction ofOpening of the “RIKEN RQC-Fujitsu Collaboration
the size and noise of peripheral and wiring components, and the development
Center”
of low-temperature packaging technology. Moreover, we integrate thehardware technologies above and develop a prototype superconducting quantum computer. In terms of software research, wedevelop middleware and a cloud computing system necessary for operating quantum computers and develop algorithms forquantum applications. We also verify the usefulness of error mitigation technologies in practical applications by executing quantum
algorithms that integrate such mitigation technologies with quantum chemistry calculations on a prototype superconducting
quantum computer. At the same time, we conduct basic experiments for quantum error detection and correction to identify issues
and improve technologies for realizing quantum error correction.
We work together with various research institutions and companies to advance science and technology using quantum
computers, and bring about innovations to realize a more sustainable world.

Shintaro Sato (Ph.D.), Deputy Director*
Selected Publications
1 T. Takahashi, N. Kouma, Y. Doi, S. Sato, S Tamate, and Y Nakamura “Uniformity improvement of Josephsonjunction resistance by considering sidewall deposition during shadow evaporation for large-scale integration of
qubits”, Jpn. J. Appl. Phys., 62 SC1002 (2023)
2 T. Kurita, M. Morita, H. Ohshima, and S. Sato “Pauli String Partitioning Algorithm with the Ising Model for
Simultaneous Measurements”, J. Phys. Chem. A, 127, 4, 1068–1080 (2023)
3 J. Fujisaki, H. Oshima, S. Sato, and K. Fujii “Practical and scalable decoder for topological quantum error
correction with an Ising machine”, Physical Review Research 4, 043086 (2022)
4 J. Yamaguchi, H. Hayashi, H. Jippo, A. Shiotari, M. Ohtomo, M. Sakakura, N. Hieda, N.Aratani, M. Ohfuchi,
Y. Sugimoto, H. Yamada, and S. Sato “Small bandgap in atomically precise 17-atom-wide armchair-edged
graphene nanoribbons.” Commun. Mater. 1, 36 (2020)
5 H. Hayashi, J. Yamaguchi, H. Jippo, R. Hayashi, N. Aratani, M. Ohfuchi, S. Sato, and H. Yamada “Experimental
and Theoretical Investigations of Surface-Assisted Graphene Nanoribbon Synthesis Featuring Carbon–Fluorine
Bond Cleavage.” ACS Nano 11, 6204 (2017)

Brief resume
1990
1990
2001
2001
2002
2006
2007
2010
2014
2018
2018
2020
2021
2021
2022

MS in Science and Engineering (Physics), University of Tsukuba
Ushio Inc. (until 1997)
Ph.D. in Mechanical Engineering, University of Minnesota, USA
Electronic Devices Business Unit, Fujitsu Limited
Researcher, Nanotechnology Research Center, Fujitsu Laboratories Ltd.
Researcher (Senior Researcher from 2007), Semiconductor Leading-Edge Technology Inc. (concurrent position
until 2010)
Research Manager, Nanotechnology Research Center, Fujitsu Laboratories Ltd.
Group Leader, Green Nanoelectronics Research Center, The National Institute of Advanced Industrial Scienceand Technology (AIST) (Sent from Fujitsu until 2014)
Research Manager, Functional Devices Division, Devices and Materials Laboratory, Fujitsu Laboratories Ltd.
Project Director, Next-Generation Materials Project, Devices and Materials Laboratory, Fujitsu Laboratories Ltd.
Fellow, The Japan Society of Applied Physics
Project Director, Quantum Computing Project, ICT Systems Laboratory, Fujitsu Laboratories Ltd.
Head of Quantum Computing Research Center, Fujitsu Research, Fujitsu Limited
Deputy Director, RIKEN RQC-Fujitsu Collaboration Center (-present)
Head of Quantum Laboratory, Fujitsu Research, Fujitsu Limited (-present)*The current Director is Dr. Yasunobu Nakamura

44

Recent Achievements
Uniformity improvement of Josephson-junction resistance for large-scale integrationof qubits
The uniformity of Josephson-junction (JJ) characteristics is crucial in wafer-scale superconducting qubit integration. However, theJJs with a structure of Al/AlOx/Al have variations in resistance at room temperature. One of the reasons for the variations is thatJJs are fabricated by using a method called “angled shadow evaporation”. In this method, a wafer is inclined with respect to theincident axis, casing position-dependent variations of Al deposition. To overcome the issue, our group has clarified the mechanismof the inhomogeneities from a detailed analysis of the evaporation process and introduced a two-step shadow evaporation
method to reduce the variations. As a result, the coefficient of variations (CV) across a 3-inch wafer decreases from 6.7% to 4.5%,
achieving 1.1% in a chip with an area of 10 mm × 10 mm.
(T. Takahashi et al., Jpn. J. Appl. Phys., 62 SC1002 (2023))(a) Schematic of the resist pattern and the first and the second metal layers in the
Manhattan-style JJ fabrication process (b) Junction structure© Copyright 2023 The Japan Society of Applied Physics

Bar chart of the CV values of junction resistances
fabricated with the conventional and improved
methods© Copyright 2023 The Japan Society of Applied
Physics

Development of Characterization Methods for Implementation Accuracies of
Quantum Operations
Further improvement of implementation accuracies of quantum operations is one of the major challenges for realizing a practicalquantum computer. Characterization of implementation accuracies of quantum operations is a fundamental technique for accuracy
improvements. Current standard characterization methods have several advantages and disadvantages, and a new method that
overcomes the disadvantages is necessary. Our group works on theoretical research toward improving the efficiency and reliabilityof a characterization method class, called “Quantum Tomography”. Recently, we have developed a new tomographic method forquantum gates, which uses error amplification circuits, and we have theoretically proved that its efficiency at the data-processing
part is better than the known method called Gate-Set Tomography.

Conceptual diagram of an error amplification circuit

45

Administration

Office of the Center Director
Keywords: Management of RQC, Head quarter of Quantum technology innovation hubs

OutlineThe Office of the Center Director performs managementtasks related to the overall operation of the Center, including
budget management, research space management, surveys
on technology trends, intellectual property and standardization
trends, information dissemination, and the organization of
events.
Based on the government’s Quantum Technology
Innovation Strategy, RIKEN has been positioned as a coreorganization that coordinates the 10 quantum technology
innovation hubs. We are responsible for the management of
this core organization. In addition, we are engaged in managing
and promoting MEXT Q-LEAP project (Quantum information
technology) as the head quarter.
I am also working on research and development ofsuperconducting integrated systems for fault tolerant quantum
computers as one of the project performers of the Moonshot
Research and Development Program of the Cabinet Office.

Members of Office of the Center Director

Core members(Coordinator) Toshio Tonouchi(Research Administrator) Koji Ikado(Research Administrative Support Staff) Kimiko Kowashi(Temporary Employee) Masanobu Arai
(Temporary Employee) Haruyuki Iwabuchi

Shinichi Yorozu (Ph.D.), RQC Deputy Director, Director of Office of the Center Director
Selected Publications
1 Y. Iwasaki, R. Sawada, V. Stanev, M. Ishida, A. Kirihara, Y. Omori, H. Someya, I. Takeuchi, E. Saitoh, S.
Yorozu, Identification of advanced spin-driven thermoelectric materials via interpretable machine learning, npj
Computational Materials volume 5, Article number: 103 (2019)
2 A. Kirihara, M. Ishida, R. Yuge, K. Ihara, Y. Iwasaki, R. Sawada, H. Someya, R. Iguchi, K. Uchida, E. Saitoh, S.
Yorozu, Annealing-temperature-dependent voltage-sign reversal in all-oxide spin Seebeck devices using RuO2,
2018 J. Phys. D: Appl. Phys. 51 154002
3 K. Uchida, H. Adachi, T. Kikkawa, A. Kirihara, M. Ishida, S. Yorozu, S. Maekawa, E. Saitoh, Thermoelectric
Generation Based on Spin Seebeck Effects, Proceedings of the IEEE (Volume: 104, Issue: 10, Oct. 2016) 1946 –1973
4 K. Takemoto, Y. Nambu, T. Miyazawa, Y. Sakuma, T. Yamamoto, S. Yorozu, Y. Arakawa, Quantum key
distribution over 120 km using ultrahigh purity single-photon source and superconducting single-photon
detectors, Scientific Reports 5, Article number: 14383 (2015)
5 A. Kirihara, K. Uchida, Y. Kajiwara, M. Ishida, Y. Nakamura, T. Manako, E. Saitoh, S. Yorozu, Spin-current-driven
thermoelectric coating, Nature Materials, Vol. 11, pp. 686–689 (2012)

Brief resume
1993
1993
1997
2005
2015
2018
2019
2021

46

Ph.D. in Applied Physics, The University Tokyo
Researcher, Fundamental Research Laboratories, NEC Corporation
Visiting Researcher, State University of New York at Stony Brook
Senior Manager, Fundamental and Environmental Research Laboratories, NEC Corporation
Deputy General Manager, Smart Energy Research Laboratories, NEC Corporation
Executive Chief Engineer, System Platform Research Laboratories, NEC Corporation
Coordinator, RIKEN Center for Emergent Matter Science
Deputy Director, RIKEN Center for Quantum Computing (-present)

RESEARCH HIGHLIGHTS

Realizing fault-tolerant quantum computers
A proposal to use cat states promises to make more quantum computers less prone to errors
Category: Applied Physical Sciences Field: Quantum computingError correction in quantum computers could be simplified
by a new protocol proposed by an all-RIKEN team basedon ‘cat states’1. It could cut the computing resources needed
to fix errors to the same level as conventional computers,
making quantum computers cheaper and more compact.
Quantum computers are looming ever larger on thehorizon of computing. They have already demonstrated theability to outperform traditional computers for certain kindsof calculations. But they are more prone to errors thanconventional computers.
Since traditional computers are based on bits that areeither 0 or 1, the only error they are susceptible to is when a
bit accidentally flips from 0 to 1 or vice versa.
But quantum computers use qubits, which can be in asuperposition of two states. When the states are depicted
on a sphere, the angle between the two states is known as
the qubit’s phase. This phase can also be flipped in quantum
computers. They thus need more computing resources tocorrect for this additional source of error.
An attractive way to sidestep this problem is to use
qubits based on so-called cat states. These states are named
after Schrödinger’s hypothetical cat, which is simultaneously
dead and alive until observed. By analogy, cat states aresuperpositions of two states with opposite phase.
Unlike other qubits, cat-state qubits cannot undergo
phase flips, so that engineers making quantum computers
based on them need only worry about bit flips—just like inconventional computers. Researchers are now exploring how
to use these cat-state qubits to perform computations.
Now, Ye-Hong Chen and four co-workers, all at the
RIKEN Center for Quantum Computing, have theoreticallydemonstrated a way to use cat states to realize fault-tolerant
gates for connecting multiple qubits in a process known asentanglement.
“Conventional computers can only process data one bit
at a time, but entanglement allows quantum computers toprocess a lot of data simultaneously,” explains Chen. “Thegates can rapidly generate entangled cat states with highaccuracy.”
The team showed that such fault-tolerant quantum gates
could be used to implement a quantum search algorithm witha high efficiency. The algorithm will allow databases to be
searched faster than is currently possible using conventional
computers.
“Let us assume that you are searching for one key thatwill open a box among 100 keys. On average, you would
need to try 50 keys using a conventional search algorithm
to identify the one key that opens that box,” says Chen. “But© Rhoeo/iStock/Getty Images

In a thought experiment, Schrödinger conjectured a cat could be
both alive and dead until observed if a quantum event such as the
decay of a radioactive particle (right) would trigger an event thatwill kill the cat. Now, by using cat states, RIKEN researchers haveproposed a scheme for realizing fault-tolerant gates to entangle
multiple qubits.

with the quantum search algorithm the average is only 10
attempts,” says Chen.
The team is now exploring how to develop other usefulquantum algorithms based on fault-tolerant quantum codes.
Reference
1. Chen, Y.-H., Stassi, R., Qin, W., Miranowicz, A. & Nori, F.
Fault-tolerant multiqubit geometric entangling gates using
photonic cat-state qubits. Physical Review Applied 18,
024076 (2022).
RIKEN Research Spring 2023

These articles are edited versions of RIKEN Research Highlights.

47

RESEARCH HIGHLIGHTS

Nanoscale heat engines
A nanoscale device offers insights into how single electrons interact with vibrations in thepresence of a temperature gradientCategory: Applied Physical Sciences Field: Quantum heat engines

R

IKEN physicists have fabricated a nanoscale ‘heat
engine’ that uses a property of electrons known as spin
as the effective working medium1. It is promising for exploring
the development of spintronic heat engines capable ofharvesting waste heat from devices.
Heat engines convert a heat difference into more usefulforms of energy as heat flows from warmer to cooler regionsof electronic devices. Reducing them to the nanoscale would
enable waste heat generated to be converted back into
electrical energy and thus improve efficiency.
One way to create nanoscale heat engines is to use tiny
crystals of semiconductors known as quantum dots (Fig. 1).
Somewhere in the range of 10 to 100 nanometers in diameter,a quantum dot can trap one or a few electrons.
All heat engines are driven by a difference in temperature—
one end of the heat engine needs to be hotter than the otherfor the heat engine to work. In quantum systems, there are
two different temperatures: the temperature of the atoms (orlattice temperature) and that of the electrons.
Previous heat engines based on quantum dots have usedreservoirs of electrons at different temperatures. However, togain a better understanding of the underlying thermodynamics
it is desirable to create a heat engine that operates on lattice
temperature. But generating a lattice temperature gradientacross a few hundred nanometers is technically challenging.
Now, Seigo Tarucha at the RIKEN Center for Emergent
Materials Science and his colleagues have succeeded indoing this in their quantum-dot heat engine.
In their device, electrons are confined using electric fields
generated at surface metal electrodes on a gallium arsenide
surface. The device had two interlinked quantum dots and abuilt-in charge sensor to passively monitor what was goingon within the double quantum dot (Fig. 1). A third quantum
dot was used to control the double quantum dot’s thermal
environment; effectively, it acted as a local heater.
The researchers anticipate the device will greatly
contribute to our understanding of the fundamental physics
of thermoelectric devices. “The results give valuable insights
into developing spintronic heat engines,” says Tarucha. “Inparticular, this setup will provide an experimental platform
for studying the thermodynamics of cooperative spin–charge
systems.”
The next challenge will be to introduce on-demand controlof heat flow in the spin–charge cooperative system. “We’re
now developing a technique to rapidly switch the heat flow,”
explains Tarucha. “This will provide a new platform to study thephysics and apply for development of spintronic heat engines.”48

A double quantum dot device enables the study of how the spinand charge of an electron influences thermodynamic action in thepresence of a lattice temperature gradient. Lattice vibrations known
as phonons (three red squiggles) come from the red quantum dot on
the right, which acts as a local heater. They can cause the spin on of
the two electrons (yellow dots) to flip, which is detected by the green
quantum dot, which acts as a charge sensor.
Reprinted, with permission, from Ref. 1. Copyright 2022 by theAmerican Physical Society.

Reference
1. Kuroyama, K., Matsuo, S., Muramoto, J., Yabunaka,
S., Valentin, S. R., Ludwig, A., Wieck, A. D., Tokura, Y.
& Tarucha, S. Real-time observation of charge-spin
cooperative dynamics driven by a nonequilibrium phonon
environment. Physical Review Letters 129, 095901 (2022).
(doi: 10.1103/PhysRevLett.129.095901)
RIKEN Research Spring 2023

These articles are edited versions of RIKEN Research Highlights.

Observing field-theory physics in the bathtub
An important concept in quantum physics known as spin can be seen in water waves
Category: Exploratory Physical Sciences Field: Wave physics

Water waves can be used to visualize fundamental
concepts, such as spin angular momentum, that arise
in relativistic field theory, RIKEN physicists have shown1.
This will help to provide new insights into very different wave
systems.
First introduced nearly a century ago, the concept of spin
angular momentum, or spin, is critically important in quantum
physics and underpins the emerging fields of spintronics andquantum computing. In high school physics, the spin of anelectron is usually described as the electron spinning on its
axis, similar to a spinning top. But a fuller description of spin
is more abstract and doesn’t yield itself to simple pictures.
Now, Konstantin Bliokh of the RIKEN Theoretical
Quantum Physics Laboratory and his co-workers haveshown that spin can appear as small circular motions of waterparticles in water waves (Fig.).

“We were surprised that our collaborators from theAustralian National University were able to observe this effectin experiments so readily,” says Bliokh. “Similar phenomena
in optics and acoustics tend to be too tiny to observe, butwith water waves, everything is a few millimeters in size andyou can observe it with your eyes. That’s the beauty of thisexperiment.”
It was also unexpected because the concept of spin
comes from the mathematics that describes relativistic field
theory, and does not apply directly to water waves. But the
researchers were able to show that there is a mathematicalconnection between water waves and formal theory for spin
angular momentum. As is often the case in physics, diverse
phenomena that appear to be totally unrelated can beconnected by common mathematics.
“It’s nice to gain a unified picture of different wave systems
and see the parallels between them,” says Bliokh. “Thisapproach illuminates the physics behind different phenomena
and could be very fruitful for the future development ofdifferent fields.” He notes that insights could flow both ways
and that we could learn more about fluid dynamics from theconnection.
Bliokh also considers that the demonstration could be
helpful for teaching quantum field theory. “Quantities likespin density are derived in a very abstract way. It appears
in some equations, but you observe totally different things inexperiments,” says Bliokh. “For the first time, we have directly
observed spin density in water waves. So it’s really a platform
for visualizing properties that are hidden in quantum field
theory.”The team is now exploring how field theory can be used
to gain new insights into other types of classical waves.
Reference
1. Bliokh, K. Y., Punzmann, H., Xia, H., Nori, F. & Shats, M.
Field theory spin and momentum in water waves. Science
Advances 8, eabm1295 (2022). (doi: 10.1126/sciadv.
abm1295)
RIKEN Web Research News Apr.18, 2022

Reprinted with permission of AAAS from Ref. 1.
© K. Y. Bliokh et al., some rights reserved; exclusive licensee AAAS.
Distributed under a CC BY-NC 4.0 License [http://creativecommons.
org/licenses/by-nc/4.0/]
Researchers at RIKEN have used water waves to generate small
circulations of water particles (pink arrows) that generate spin
angular momentum of water waves, a classical counterpart of thespin of quantum wave fields.

These articles are edited versions of RIKEN Research Highlights.

49

RESEARCH HIGHLIGHTS

Applying the latest computing technologies toquantum gravity
Emerging technologies are promising for helping physicists develop a theory that unites
quantum physics and gravity
Category: Exploratory Physical Sciences Field: Quantum gravity

I

KEN physicists have put quantum computing and deeplearning through their paces and found that they are
powerful tools for gleaning insights into new theories ofquantum gravity1. They could thus help solve one of the mostformidable challenges in modern physics—developing atheory of gravity that jives with quantum physics.
When Einstein nutted out his theory of general relativity
in 1915, his only tools were pen and paper. The same
was true of the pioneers of quantum theory. But the next
major breakthrough in theoretical physics could be madewith help from emerging technologies such as quantum
computers and machine learning, Enrico Rinaldi of RIKEN
Theoretical Quantum Physics Laboratory thinks. “I believethese technologies are poised to transform the way we dotheoretical physics,” he says.
Both general relativity and quantum physics are incredibly
successful at describing different aspects of physical reality.
The only snag is that they are incompatible with each other.
This keeps theoretical physicists up at night because it meansthat our understanding of reality is incomplete, and there is amore comprehensive theory waiting to be discovered.
There has been a sustained push to derive a quantum
theory of gravity. But while many theories have been
proposed, it is notoriously difficult to perform even thesimplest calculations based on them.
Now, Rinaldi and his co-workers have explored whether
quantum computing and deep learning could aid theoreticalphysicists with these calculations.
They applied the two techniques to problems inholographic duality—theories that relate particles in one
system to gravity in a system that has an additional dimension,much like a two-dimensional hologram can create a threedimensional image. For the purposes of the trial, they picked
problems that could be solved by conventional techniques sothey could assess the accuracy of the methods.
Both techniques performed well. Of the two, deeplearning had the upper hand. “The deep-learning method had
the best scaling with resources,” notes Rinaldi.
But quantum computing also showed impressive
performance. “The biggest surprise was how well the quantumcomputing algorithm performed. We just took what wasavailable—so we didn’t know if it would work,” says Rinaldi.
“We were surprised how accurate the answers it gave were.”
The team didn’t use a real quantum computer since
the technology is still in its infancy; instead they simulated a50© 2022 Enrico Rinaldi/University of Michigan, RIKEN and A. Silvestri
RIKEN researchers have used quantum computing (represented bylines, squares and circles in the top right of the image) and deeplearning (depicted by graph points in the bottom left of the image) toperform calculations using quantum gravity theories.

quantum computer on a conventional computer.
Rinaldi is encouraged by the recent progress being madein developing theories of quantum gravity. “I really think we’regetting closer,” he says. “There has been a lot of progress inthe past two years.”

Reference
1. Rinaldi, E., Han, X., Hassan, M., Feng, Y., Nori, F.,
McGuigan, M. & Hanada, M. Matrix-model simulations
using quantum computing, deep learning, and latticeMonte Carlo. PRX Quantum 3, 010324 (2022). (doi:
10.1103/PRXQuantum.3.010324)
RIKEN Web Research News Apr.11, 2022

These articles are edited versions of RIKEN Research Highlights.

Protecting quantum matter with light
Quantum light can help create longer lasting quantum states useful for information processing
Category: Exploratory Physical Sciences Field: Quantum materials

Quantum optics might offer a way to make a class of
exotic materials known as topological materials evenmore robust against defects, theoretical physicists at RIKEN
have shown1. This finding could benefit the developmentof quantum computers and other emerging quantum
technologies.
Topological materials are exciting because local defects
and imperfections in them don’t affect their properties. But
they are not immune to larger-scale, non-local disorder within
the material.
Quantum optics is the study of how individual ‘particles’of light, or photons, interact with matter. Its theory tells
us that this interaction can be altered by controlling theelectromagnetic properties of the matter’s environment.
Coupling between light and matter has been employed
to detect and manipulate topological matter. However, it
is an open question how topological matter is modified byquantum light.
Now, in a theoretical study, Wei Nie and Franco Nori fromthe RIKEN Theoretical Quantum Physics Laboratory and theircolleagues from France and China have discovered that the
topology can be protected in vacuum electromagnetic fields.
“Our work pinpoints the novel properties of topologicallight–matter coupling,” says Nori. “This may allow us tomanipulate topological matter with quantum light.”
By combining ideas from both condensed-matterphysics and quantum optics, the researchers found that lightemission from the topological electronic states changes withthe strength of the light–matter coupling in a counterintuitive
way. Light can be emitted in a weak-coupling regime,
whereas strong light–matter coupling inhibits emission. It also
increases the quantum coherence—the length of time thelight can maintain its quantum state.
This is counterintuitive because stronger coupling to
the environment usually reduces coherence: the symmetries
protecting topological matter are supposed to be broken by theelectromagnetic environment. “Instead, our work shows thatfor a topological emitter array coupling to a one-dimensional
electromagnetic environment, the chiral symmetry that
protects the system can be preserved,” explains Nori.
This suggests an approach to tune quantum coherence
without fiddling with the coupling between the system andthe environment. Topological matter’s robustness to local
disorder makes it of great interest for quantum computation.
However, non-local disorder remains a problem and limits thequantum coherence that is vital for all quantum technologies.
The improved coherence offered by taking advantage ofstrong coupling can therefore be employed for better quantum

Background vector created by macrovector - www.freepik.com[https://www.freepik.com/vectors/background].
Chiral-symmetry protected topological states can be preserved
through strong coupling to its electromagnetic environment.

information storage.
“Our work shows that topological light–matter couplingis an important resource for quantum optics and condensedmatter physics,” says Nie. “The topological features of matter
give rise to novel quantum optical phenomena, which areuseful for quantum computation and quantum technologies.”
Reference
1. Nie, W., Antezza, M., Liu, Y.-X. & Nori, F. Dissipative
topological phase transition with strong systemenvironment coupling. Physical Review Letters 127,
250402 (2021). (doi: 10.1103/PhysRevLett.127.250402)
RIKEN Research Fall 2022 (p23)

These articles are edited versions of RIKEN Research Highlights.

51

RESEARCH HIGHLIGHTS / PEOPLE

Quantum reboot gets a speed boost
Simulations suggest a new technique for resetting ‘qubits’ in a quantum computer withoutharming themCategory: Applied Physical Sciences Field: Quantum heat enginesR

ebooting a quantum computer is a tricky process thatcan damage its parts, but now two RIKEN physicists
have proposed a fast and controllable way to hit reset1.
Conventional computers process information stored as
bits that take a value of zero or one. The potential power ofquantum computers lies in their ability to process ‘qubits’ that
can take a value of zero or one—or be some fuzzy mix of both
simultaneously.
“However, to reuse the same circuit for multiple
operations, you have to force the qubits back to zero fast,”
says Jaw Shen Tsai, a quantum physicist at the RIKEN Centerfor Quantum Computing. But that is easier said than done.
One of the best current ways to hit reset for qubits built
from tiny superconductors is to link the qubit to a photon—aparticle of light—in a tiny device called a resonator. The qubit
transfers its energy to the resonator, after which the photon in
the resonator decays, releasing its energy to the environment.
This process causes the qubit state to drop back to the groundstate (zero). The trouble with this method is that permanent
entanglement to a decaying photon rapidly degrades thequbit’s quality, so that it rapidly ceases to be useful for future
operations. “It’s bad for the qubit, whose lifetime becomesshort,” says Tsai.
Now, Tsai and his RIKEN colleague Teruaki Yoshioka
have devised a simulation to help find a better way of resetting
the qubit, without harming it.
Based on their calculations, the pair proposed building aresonator that can be controlled using an additional junction
made by sandwiching a superconducting material withan insulator, a normal metal, another insulator and anothersuperconductor. This layered junction is controlled by applying
a voltage. While the qubit operation is being carried out, theset-up is tuned so that the photon cannot decay. Only when

the operation has been completed do the physicists change
the voltage, allowing the photon to release energy. “Thisadjustable resonator is the key to our proposal,” says Tsai.
The best current lab record for resetting a qubit is 280
nanoseconds, with 99.0% fidelity. “Our simulations suggest
we could reset the qubit in 80 nanoseconds, with 99.0%
fidelity,” says Yoshioka.
The team is now testing this set-up, which is held at low
temperatures using a dilution refrigerator (Fig.), with promising
results. “This device should be very useful if we can implement
it in a quantum circuit,” Tsai says.
Reference
1. Yoshioka, T. & Tsai, J. S. Fast unconditional initialization
for superconducting qubit and resonator using quantumcircuit refrigerator. Applied Physics Letters 119, 124003(2021). (doi: 10.1063/5.0057894)
RIKEN Research Summer 2022 (p8)© 2022 RIKEN Center for Quantum Computing
A photograph of a dilution refrigerator that houses qubits.

52

These articles are edited versions of RIKEN Research Highlights.

Looking for rules within quantum complexity
Tomotaka Kuwahara
Team Leader, Analytical Quantum Complexity RIKEN
Hakubi Research Team, RIKEN Center for Quantum Computing

Please briefly describe yourcurrent research.
My team aims to solve the mostimportant unsolved problems in the fieldof quantum information.
To do this, I look at Hamiltonian
complexity. The Hamilton of a systemis the sum of its kinetic energy (energyof motion) and its potential energy(energy of position). Hamiltonian
complexity is the study of the universal
structural constraints of a system ofmany interacting quantum particles—
quantum many-body systems—due tothese energies.
Currently, my aim is to characterize
via an information-theoretic lensthe intrinsic complexity of quantum
many-body systems and apply theseunderstandings to the quantification,
storage and communication of digital
information.

What excites you the mostabout your current research?
Well, statements on Hamiltonian
complexity are universal and can be
written in mathematically precise ways.
These laws may resolve great mysteries
about how the world works.
Physicists are also very keen
to figure out how to solve quantum
many-body problems using quantum/
classical hybrid computers. However,understanding the computational
complexity of a quantum many-bodysystem is equivalent to completely
clarifying the information-theoretic
structure of the quantum many-bodysystem.

How did you become
interested in your current
field of research?
I was initially puzzled about why ourliving world is so complicated despite
having such simple fundamental laws.
This, I discovered, may boil down to theessential difference between one-bodyand many-body problems. However,realistic quantum many-body systems
are thought to be ‘not too complicated’.
That is, realistic many-body systems aremuch simpler than typical theoreticalones. So I became interested in why ourworld has a moderate level of complexity
in terms of physics.

What do you think has
been the most interesting
discovery in the last few
years?
Recently, the ‘no low energy trivial state
conjecture’, a previously unprovenidea posed in 2014, has been solved.

It had been a fundamental unresolved
obstacle to applying a theorem, called
the PCP theorem, to the quantum realm.
The PCP theorem is a cornerstone ofconventional computing, helping us tounderstand algorithmic complexity andthe conditions of near-optimal solutions
for optimization. This finding inspired
me to work towards solving famous
unsolved conjectures.

What other goals do you
have at RIKEN and in yourlife?
I aim to lead a team that will increase thepresence of Japanese researchers at theworld’s most prestigious international
conferences on quantum physics, suchas the Quantum Information Processing
conference and the Theory of Quantum
Computation, Communication and
Cryptography conference. Since I am a
Christian, my ultimate goal is to follow in
the footsteps of great Christian physicist
predecessors like Isaac Newton,
Michael Faraday, and Blaise Pascal.
This always encourages me: “For I cando everything through Christ, who gives
me strength” (Philippians 4:13).

These articles are edited versions of
RIKEN Research Highlights.

53

RESEARCH HIGHLIGHTS / PEOPLE

Modeling quantum environments
Neill Lambert
Senior Research Scientist, Theoretical Quantum Physics Laboratory, RIKEN Cluster for Pioneering Research

How and when did you join
RIKEN?
I joined RIKEN in 2008 after spending
several years at the University of Tokyo
as a Japan Society for the Promotionof Science fellow. Initially I came togive a seminar in RIKEN, and thenapplied for a postdoc position after
enjoying the interactions I had with Dr.
Franco Nori’s group (theTheoretical
Quantum
Physics
Laboratory
within the RIKEN
Cluster for Pioneering
Research).

Please describe your role.

I study open quantum systems, or
how a large environment affects the
behavior of small quantum systems,
particularly in situations where parts ofthe environment itself must be modelledquantum mechanically. This research
could be important to understanding
how to control noise within quantum
computers or how thermodynamics
behave in quantum regimes. Day today, I mainly work on developing new
methodologies for this field, conduct
numerical simulations, and assist
with developing and administering a
popular open-source software package
for simulating the dynamics of open
quantum systems created by ourgroup – QuTiP (the Quantum Toolbox
in Python).

What are some of thetechnologies that you
use?
Recently, because of several
numerically demanding projects,
I have been using the excellent
computing resources available in
RIKEN, in particular the Hokusai
BigWaterFall supercomputer.

What excites you
the most about yourcurrent research?

What do you think has been
the most interesting recent
discovery in your field?
The development of small-scalequantum computers by the likes of IBM
and Google has truly changed the field
I work in. As a theorist, it is exciting that
I can access real devices, through a
cloud computing service, and conduct
my own experiments.

How did you become
interested in your current
research field?
As an undergraduate student, I did anexciting quantum physics course taught
by Professor Tobias Brandes at UMIST(now The University of Manchester) in
the UK. He would eventually become
my Ph.D. supervisor.

What has been your mostmemorable experience at
RIKEN?
I have very much enjoyed theopportunity to supervise and traininternational students. One student
recently contacted me and told me hehad found a tenured university positionin his home country, which made mevery happy.

The question of whether
quantum effects can beobserved with large objects isfascinating. Some of my recent
work helps us to unravel part of thisissue, like how large collections ofsmall quantum systems might start
to appear classical, which also helps
make practical simulation methods
and tools more efficient.
These articles are edited versions of RIKEN
Research Highlights.

54

RIKEN Center for Quantum Computing
2-1 Hirosawa, Wako, Saitama 351-0198 Japan
Email: rqc_info@ml.riken.jp
https://rqc.riken.jp