2023

RIKEN Center for
Quantum Computing

Annual Report

1

RIKEN

Contents
Director’s Greetings................................................................................................................................4
Overview of RIKEN Center for Quantum Computing............................................................................5
Collaborations......................................................................................................................................10
Press Releases.....................................................................................................................................11
Awards..................................................................................................................................................12
Overview of Quantum Technology Innovation Hubs..........................................................................14
RQC FY2023 Pick Up Topics...............................................................................................................17
Team Outlines and Achievements• Superconducting Quantum Electronics Research Team.................................................................18• Superconducting Quantum Simulation Research Team..................................................................20• Superconducting Quantum Electronics Joint Research Unit..........................................................22• Superconducting Quantum Computing System Research Unit......................................................24• Hybrid Quantum Circuits Research Team........................................................................................26• Optical Quantum Computing Research Team.................................................................................28• Optical Quantum Control Research Team.......................................................................................30• Quantum Many-Body Dynamics Research Team............................................................................32• Floating-Electron-Based Quantum Information RIKEN Hakubi Research Team.............................34• Semiconductor Quantum Information Device Research Team........................................................36• Semiconductor Quantum Information Device Theory Research Team............................................38• Quantum Computing Theory Research Team.................................................................................40• Quantum Information Physics Theory Research Team....................................................................42• Quantum Computational Science Research Team..........................................................................44• Quantum Computer Architecture Research Team...........................................................................46• Analytical Quantum Complexity RIKEN Hakubi Research Team.....................................................48• Mathematical Quantum Information RIKEN Hakubi Research Team..............................................50• RIKEN RQC-FUJITSU Collaboration Center...................................................................................52• Office of the Center Director............................................................................................................54
Research Highlights / Features...........................................................................................................55
Publication List.....................................................................................................................................60

Director’s Greetings

T

he RIKEN Center for Quantum Computing (RQC) wasestablished in April 2021. Our mission is to develop
quantum computers and pioneer the frontier of quantum
information science. We engage in full-stack research anddevelopment encompassing layers of science and technology
from hardware to software and basic science to applications.
Next year, in 2025, we will mark the centennial of thetheory of quantum mechanics, a milestone in the history ofscience. Quantum mechanics, which has evolved over a
century through the collective efforts of numerous researchers,
is the most fundamental theory of physics and has contributed
widely to the development of science and technology in
general. Quantum information science, which emerged in thelate 20th century, has also attracted attention for exploiting new
possibilities by applying the principles of quantum mechanics
to information science.
The global research and development of quantum
computers are rapidly gaining momentum, with new ideas
and technologies constantly emerging. At RQC, we areat the forefront of this dynamic landscape, conducting
research on various quantum computing platforms, such as
superconducting, optical, and semiconducting. In FY2024,
we are set to establish two new research teams on cold-atom
platforms, a testament to our commitment to accelerate ourresearch and development. We foster a culture of mutual
discovery and learning, generating new ideas through synergy
by pursuing multiple approaches simultaneously. RQC also
houses other experimental research teams developing basic
technology for quantum information processing through novel
techniques and several theoretical teams covering quantum
information theory, quantum computing theory, quantum
algorithms, quantum architecture, quantum software, etc. Our
diverse talents cooperate on various topics, from basic science
to applications and from experiments to theories, striving for4

breakthrough research every day.
Following the release of “RQC-A,” the first Japan-built
superconducting quantum computer, RQC started operating
a cloud service in FY2023. In addition, a second unit was
unveiled at the RIKEN RQC-FUJITSU Collaboration Center. We
also collaborated on releasing a third machine at the Centerfor Quantum Information and Quantum Biology at Osaka
University. While technology is still evolving, we hope to amass
the wisdom of many people through open innovation for future
development.
Furthermore, RQC plays a pivotal role as the headquarters
and the quantum computer development hub, at the core of
all eleven Quantum Technology Innovation Hubs under the
Quantum Technology and Innovation Strategy promoted by
the Government of Japan. We spearhead advancements andcollaborations in Japanese quantum technology research anddevelopment. Through the activities connecting participants inacademia, industry, and the government, as well as by sharing
knowledge across diverse subfields and tightening lateralcollaborations, we accelerate the innovation cycles, promote
the development of science and technology, and contribute tosocial values.
In FY2023, we launched three new research teams in RQC
as a part of our growth plan to establish a critical mass. The
fusion of quantum information science with rapidly evolving
cutting-edge technologies, such as artificial intelligence,high-performance computers, semiconductors, and opticalcommunication, is also emerging as an essential topic. We
will further strengthen collaborations among researchers insideand outside RIKEN, encourage interdisciplinary discussions
among researchers with diverse backgrounds and expertise,and nurture talents to lead the next generation of quantum
science and technology, along with our persistent research anddevelopment towards realizing quantum computers.

Overview of 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 will launch three new teams in fiscal 2023, giving it a lineup of 18 research teams in total. It is promoting thedevelopment of quantum computers with various physics-related teams that cover the superconducting method, the opticalmethod, 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
Optical Quantum Control Research Team: Hidehiro Yonezawa
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
Quantum Computer Architecture Research Team: Hayato Goto
Analytical Quantum Complexity RIKEN Hakubi Research Team: Tomotaka Kuwahara
Mathematical Quantum Information RIKEN Hakubi Research Team: Bartosz Regula
RIKEN RQC-FUJITSU Collaboration Center: Yasunobu Nakamura
Office of the Center Director: Shinichi Yorozu
Superconductivity

Optics

Atoms

Electrons

Semiconductor

TheoryAdministration
5Overview of RIKEN Center for Quantum Computing (Continued)

Budget (FY2023)
Government funding for operations

18%
663

4% 143

Total

External funds

65%
2,326

¥3,597 13
million% 466

Government subsidies
for facilities

President’s Discretionary

As of Mar 31, 2024

Members (FY2023)

Student trainees etc.

PIs

8%

21%

4% 9

Indefinite-term employees

24%

Fixed-term employees

20

54

Total

253

members60(Women: 23,
Non-Japanese:57)
Visiting scientists etc.

43%
110

As of Mar 31, 2024

6

RQC Colloquiums and RQC Seminars
FY2023 RQC Colloquium
At the RQC, we regularly invite renowned researchers and hold RQC Colloquiums. RQC Colloquiums were held 12 times in fiscal
2023, 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

11

April 19, 2023
(online)Prof. Michel Devoret

Yale University

Error correction of a logical quantum bit

12

May 17, 2023
(online)Prof. Scott Aaronson

University of Texas atAustinHow much information is in a quantum state?

13

June 21, 2023

Prof. Kenji Ohmori

Institute for Molecular
Science

Ultrafast quantum simulation and quantum computing
with ultracold atom arrays at quantum speed limit

14

July 7, 2023

Prof. Jacob Taylor

National Instituteof Standards andTechnology

Exploring fundamental physics with quantum information
science

15

July 21, 2023

Prof. Ulrik Lund Andersen

Technical University ofDenmark

Advancements in continuous variable quantum
information technology

16

September 13, 2023 Prof. Takao Aoki

Waseda University

Nanofiber cavity quantum electrodynamics systems for
distributed quantum computing

17

October 25, 2023

Prof. Mio Murao

The University of Tokyo

Fully-quantum learning: Higher-order quantum
algorithms for comparison and inversion of unknown
unitary operations

18

November 21, 2023
(online)Prof. Charles Marcus

University of Washington

New views of a classic (but not classical) system theJosephson Junction Array

19

December 27, 2023

Prof. Kihwan Kim

Tsinghua University

Scalable and programmable phononic network using
vibrational modes of trapped ions20

January 31, 2024

Prof. Seth Lloyd

Massachusetts Institute of
Quantum machine learning
Technology

21

February 14, 2024

Prof. Hidetoshi Nishimori

Tokyo Institute ofTechnology

Quantum simulations of problems in statistical physics

22

March 19, 2024

Prof. Eric Lutz

University of Stuttgart

Converting quantum statistics into work

7

FY2023 RQC Seminar
Additionally, at the RQC we also hold RQC Seminars, which each PI organizes independently. RQC Seminars were held 72 times
in fiscal 2023, 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

Speaker

Affiliation

No.

Date

Speaker

Affiliation

37

April 3, 2023

Prof. Mauro
Antezza

University of Montpellier

62

July 20, 2023

Mr. Daniele
Lamberto

Università degli Studi diMessina38

April 10, 2023

Dr. Fabrizio
Minganti

Ecole PolytechniqueFédérale de Lausanne

63

July 21, 2023

Prof. Britton
Plourde

Syracuse University

39

April 12, 2023

Mr. Yi-Te Huang

National Cheng Kung
University

64

July 24, 2023

Mr. Po-Rong Lai

National Cheng Kung
University

40

April 14, 2023

Dr. Onur Oktay

University of Surrey

65

July 13, 2023

Prof. Hou Ian

University of Macau

41

April 17, 2023

Dr. Carlos
Autonomous University
Sánchez Muñoz of Madrid

66

July 27, 2023

Dr. Hirofumi
Yanagisawa

Shizuoka University

42

April 20, 2023

Prof. Kyungwon
Seoul National University
An

67

July 28, 2023

CNRS, CRHEA,
Université Nice Côted’Azur43

May 1, 2023

Prof. Wolfgang
Lorenzon

Dr. Antoine
ReserbatPlantey

University of Michigan
68

July 28, 2023

Dr. Reiko
YamadaThe Institute of Photonic
Sciences

May 9, 2023

Prof. Xuedong
Hu

University of Buffalo
69

July 31, 2023

Dr. Martin
Physical Review Letters
Rodriguez-Vega

May 11, 2023

Prof. Oleksandr
Dobrovolskiy

University of Vienna
70

Dr. Daria
Smirnova

Australian National
University

August 4,
2023

Prof. Adam
Miranowicz

Adam Mickiewicz
University

71

August 7,
2023

Prof. Enectali
FigueroaFelicianoNorthwestern University

72

August 8,
2023

Ms. Zara Yu

Massachusetts Instituteof Technology

73

August 9,
2023

Prof. Xin Wang

Xi'an Jiaotong University

44
45
46

May 15, 2023

47

May 19, 2023

Ms. Therese
Karmstrand

Chalmers University ofTechnology

48

May 25, 2023

Dr. Bartosz
RegulaRIKEN RQC

49

May 26, 2023

Dr. Takuya
Okuda

The University of Tokyo

50

June 2, 2023

Mr. Yuma
Kawaguchi

The City College of New
York

74

August 10,2023

Mr. Harvey Cao

Imperial College London

Okinawa Institute of
Science and Technology
Graduate University

75

August 18,
2023

Prof. Masaki
TezukaKyoto University

June 5, 2023

Dr. Josephine
Dias

76

August 14,
2023

Dr. Vanja Maric

Université Paris-Saclay

77

August 21,
2023

Prof. Mark
Mitchison

Trinity College Dublin

78

August 22,
2023

Mr. Isaac Layton

University of College
Longon79

August 29,
2023

Mr. Donghoon
Kim

KAIST

80

August 31,
2023

Prof. Hong-Bin
Chen

National Cheng Kung
University

81

September 4,
2023

Dr. Roberto
Stassi

Messina University

82

September 7,
2023

Prof. Valentin
Freilikher

Bar-Ilan University

83

September
12, 2023

Dr. Éric Giguère

ISC Applied Systems

51
52

June 14, 2023 Dr. Hayato Goto RIKEN RQC
Mr.Vaibhav
Gautam53

June 16, 2023

54

Dr. FredrikJune 23, 2023
BrangeAalto University

55

June 27, 2023 Prof. Chia-Yi Ju

National Sun Yat-Sen
University

56

Dr. Yosuke
June 28, 2023
Ueno

57

Prof. Guang-Yin National Chung Hsing
June 30, 2023
Chen
University

58

July 11, 2023

Prof. Rainer
Dumke

Nanyang Technological
University, National
University of Singapore

59

July 12, 2023

Prof. Omar Di
StefanoUniversità degli Studi diMessina84

September
14, 2023

Prof. Martin
KlieschHamburg University ofTechnology

60

July 14, 2023

Prof. Salvatore
Savasta

Università degli Studi diMessina85

September
15, 2023

Prof. Mauricio
Gutiérrez

University of Costa Rica

61

July 18, 2023

Dr. Steve
Brierley

Riverlane86

October 12,
2023

Dr. Gustav
Andersson

The University of
Chicago8

University of Surrey

RIKEN RQC

No.

Date

Speaker

Affiliation

87

October 24,
2023

Prof. Hideo
Kosaka,Dr. Hodaka
Kurokawa

Yokohama National
University

88

October 26,
2023

Prof. Igor
Aronson

The Pennsylvania State
University

89

October 27,
2023

Dr. Huan-Yu Ku

National Cheng Kung
University

90

October 30,
2023

Priv.-Doz. Dr.
Hans Huebl

Bavarian Academyof Sciences andHumanities

91

October 30,
2023

Prof. Stephen
A. LyonPrinceton University andEeroQ Corp

92

October 31,
2023

Mr. Feng-Jui
ChanNational Cheng Kung
University

93

November 1,
2023

Prof. Yueh-Nan
Chen

National Cheng Kung
University

94

November 9,2023

Dr. Takahiro
TsunodaYale University

95

November 13,
Mr. Zhiyu Jiang
2023

96

November 17, Mr. Juan Román
University of Zaragoza
2023
Roche

97

November 20, Prof. Simone De University of2023
LiberatoSouthampton

98

November 22, Dr. Suguru
2023
Endo

NTT

99

December 4,
2023

NTT

100

December 14, Dr. Maja
2023
Cassidy

University of New South
Wales

101

Mr. Dolev
December 15, Bluvstein,
2023
Mr. Marcin
Kalinowski$6

Harvard University

102

December 15, Dr. Emanuele
2023
Mendicelli

York University

103

December 28, Dr. Anton Frisk
2023
KockumChalmers University ofTechnology

104

January 9,
2024

Dr. Shingo Kono

Ecole PolytechniqueFédérale de Lausanne

105

January 24,
2024

Dr. Satoya ImaiQSTAR, INO-CNR, and
LENS

106

February 2,
2024

Mr. Siddhant
SinghDelft University ofTechnology

107

March 18,
2024

Prof. Ludovico
Lami

QuSoft / University of
Amsterdam

108

March 21,2024

Prof. MarioBerta

RWTH Aachen
University

Dr. Edwin Ng

Hokkaido University9Collaborations
International[Europe][North America]■Aalto University■Argonne National Lab■Chalmers University of Technology■Intel Corporation■Delft University of Technology (TU Delft)■Massachusetts Institute of Technology (MIT)■Interuniversity Microelectronics Centre (imec)■Nord Quantique■Johannes Gutenberg University Mainz■Perimeter Institute for Theoretical Physics■Moscow Institute of Physics and Technology■University of Notre Dame
(MIPT)
etc.
■Palacky University■Qutech■Swiss Federal Institute of Technology in[Asia, Oceania]
Lausanne (EPFL)■Australian National University■University of Amsterdam■Center for Axion and Precision Physics Research,■University of Basel
Institute for Basic Science■University of Tübingen■Griffith University■Walther Meissner Institute (WMI)■Hunan Normal University■National Tsing Hua University (NTHU)
etc.
■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 andTechnology (AIST)■National Institute of Information and Communications
Technology (NICT)
etc.

[National University Corporations]■International Christian University■Keio University■Kyoto 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.
10[Private business]■Fujitsu Ltd.
■Hitachi, Ltd.
■Mitsubishi Electric Corporation■NEC Corporation■Nikon Corporation■Nippon Telegraph andTelephon Corporation (NTT)■TOSHIBA CORPORATION
etc.

Press Releases
June 1, 2023
Feedback system could help to correct errors inquantum computers-Quantum circuit can reset quantum bits carried byelectron spins in siliconSemiconductor Quantum Information Device Research Team

June 30, 2023
Invention of the small cross-talk quantum gate for superconducting
quantum computer
Hybrid Quantum Circuits Research Team

July 12, 2023
Nonlinear feedforward enabling quantum computation
Optical Quantum Computing Research Team

January 19, 2024
Logical states for fault-tolerant quantum computation
with propagating light
Optical Quantum Computing Research Team

March 29, 2024
Speed limit of quantum information propagation-theoretical discovery in interacting boson systems andapplications to quantum computationAnalytical Quantum Complexity RIKEN Hakubi Research Team11

Awards
April 7, 2023
Researcher Tatsuhiko Ikeda awarded Commendation by Minister of Education, Culture,
Sports, Science and Technology
Researcher Tatsuhiko Ikeda of the Quantum Computing Theory Research Team received the Minister of Education, Culture,
Sports, Science and Technology (MEXT) Prize for Young Scientists in the field of science and technology for the fiscal year 2023.
This prize is awarded to honor individuals who have achieved outstanding results in research and development, promotion ofunderstanding, and other areas related to science and technology. Mr. Ikeda was awarded the prize for his work: “Analysis of
Optical Phenomena using Floquet Theory and Research on New Phenomena Exploration.”

September 5, 2023
Dr. Yasunobu Nakamura and Dr. Jaw-Shen Tsai awarded Hattori Hokokai Foundation’s
Hattori Hoko Award
Dr. Yasunobu Nakamura, Director of
RQC, and Dr. Jaw-Shen Tsai, Team
Leader,Superconducting
Quantum
Simulation Research Team, have been
awarded the 2023 (93rd) Hattori Hokokai
Foundation’s Hattori Hoko Award. Thisaward is presented to researchers who
have achieved outstanding research
results in the field of engineering, and Dr.
Nakamura and Dr. Tsai received the award
for their work: “Pioneering Research on
Superconducting Qubit Circuits towardrealizing Quantum Computers.”

July 18, 2023
Dr. Franco Nori selected for Academia Europaea
Dr. Franco Nori, Team Leader, Quantum Information Physics Theory Research
Team, has been selected for Academia Europaea. Academia Europaea
members are selected once a year as a result of recommendations andscreening by members. Dr. Nori was selected based on his influential researchachievements and large amount of important joint research with researchersin Europe.

12October 10, 2023
Dr. Yasunobu Nakamura and Dr. Jaw-Shen Tsai awarded C&C Prize
Dr. Yasunobu Nakamura, Director of RQC, and Dr. Jaw-Shen Tsai, Team Leader, Superconducting Quantum Simulation Research
Team, have been awarded the 2023 C&C Prize. This prize is awarded to individuals who have undertaken pioneering work
and/or research in the fields of information processing technology, communications technology, electronic devices technology,
and technological fields that integrate those technologies and have made remarkable contributions to social-scientific research
activities that bring about progress in those fields. Dr. Nakamura and Dr. Tsai received the prize for their work: “Contributing to
the Field of Quantum Information Technology, including the Development of Superconducting Qubits and Quantum Computers.”

October 19, 2023
RIKEN JRA student Shang Cheng awarded the Beijing Academy of Quantum Information
Sciences’ Best Poster Award
RIKEN JRA student Shang Cheng of the Analytical Quantum Complexity RIKEN Hakubi Research Team has been awarded
the Beijing Academy of Quantum Information Sciences’ Best Poster Award. Mr. Shang was presented with the award at the
5th International Symposium on Quantum Physics and Quantum Information Sciences (QPQIS-2023) for his poster entitled“Equivalence between operator spreading and information propagation.”

November 15, 2023
Dr. Franco Nori selected as Clarivate Highly Cited Researcher 2023
Dr. Franco Nori, Team Leader, Quantum Information Physics Theory Research Team, has been selected as a Clarivate Highly Cited
Researcher 2023. Clarivate 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.

March 8, 2024
Dr. Franco Nori selected as the 2024 recipient of the Charles Hard Townes Medal (Optica)
Dr. Franco Nori, Team Leader, Quantum Information Physics Theory Research Team, has been selected as the 2024 recipient of
the Charles Hard Townes Medal (Optica). Dr. Nori received the medal for his many fundamental contributions to quantum optics,
quantum information processing, and quantum circuits, and for his track record of developing key quantum software tools.

13Overview of Quantum Technology Innovation Hubs

As the core organization of the eleven Quantum Innovation Hubs (QIH), RIKEN brings
all the hubs together as one to accelerate the social implementation of quantum
technology.
Quantum Innovation Hubs (hereinafter QIH) were established as hubs for industry, government and academia to engage in a
streamlined manner in everything from basic research on quantum technology through to technology verification, intellectual property
management and human resources development. They are securing and strengthening Japan’s international competitiveness, on
the basis of government strategies known as the Quantum Technology and Innovation Strategy (January 2020), the Vision of
Quantum Future Society (April 2022) and the Strategy of Quantum Future Industry Development (April 2023).
As the core organization of the QIH, RIKEN performs a headquarters function that strives for coordination among the elevenhubs. At the same time, as one of the eleven hubs RIKEN is a quantum computation pioneering hub that aims to realize andestablish quantum computer systems.

Quantum Technology Innovation Hubs

14

Activities

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, were established beneaththe Quantum Technology Innovation Promotion 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 and international
standardization.
• Industry-Government-Academia Collaboration Subcommittee: Promotes industry-government-academia collaborations aimed
at the social implementation of quantum technology.
• Human Resources Development Subcommittee: Promotes an increase in the number of young researchers in the quantum fieldand strengthens human resource development across institutions and research fields.

Pick up Activities
Quantum Innovation 2023, an international symposium concerning quantum science and technology innovation, was held in Tokyo
on November 15-17, 2023 as one part of the QIH activities. With things returning to normal following the COVID-19 pandemic, thistime the symposium was held in a face-to-face format for the first time. Over the three-day period it featured 132 lectures, threepanel discussions and 140 poster presentations. More than 600 people from 14 participating countries registered to take part, and543 people attended. Along with serving as a valuable forum for disseminating information, the symposium included a reception that
also served as an opportunity
for international exchange.
In addition, of the poster
presentations that were given
by students, outstanding
poster awards were presented
for 15 presentations with thegoal of nurturing young talent.

15

Overview of Quantum Technology Innovation Hubs (Continued)
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.

Summer vacation science event “What are Quantum Computers?”
held at the Science Museum from August 11 (Fri.) to 13 (Sun.), 2023

nano tech 2024 exhibition at Tokyo Big Sight from January 31
to February 2, 2024

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.

16

RQC FY2023 Pick Up Topics“Call for quantum computer development supporters – RIKEN RQC” crowdfunding held
Crowdfunding was carried out at RQCfrom April 7, 2023 to May 31, 2023. We
were able to reach our monetary target
thanks to the support of a large numberof individuals, and the contributions arebeing utilized to strengthen our quantumrelated human resources development
and outreach activities (for making ourresearch results known).

Examples of thank you gifts: Left: Acrylic stand commemorating the launch of Japan’s firstquantum computing cloud service Right: Replica of a superconducting qubit chip

Nickname and logo decided for Japan’s first homemade quantum computer
Following an open call for suggestions for a nickname for the first Japanese built
quantum computer available through the cloud, on 27 March, 2023 we settled on“A” (in Japanese “EI”) as the nickname. Furthermore, with the aim of making “A” feel
more familiar to a larger number of people, we came up with alogo expressing the notable characteristics that “A” possesses.

Logo for “A”

Superconducting quantum computer “A”

Unveiling and press conference for Japan-made quantum computer available
through the cloud held at the RIKEN RQC-FUJITSU Collaboration Center
A new superconducting quantum
computer was developed based on thedevelopment expertise accumulated with“A”, Japan’s first homemade quantum
computer, and it began being operated
on Fujitsu’s hybrid quantum computing
platform on 5 October, 2023 at the RIKEN
RQC-FUJITSU Collaboration Center. In
addition, a press conference was held on
the same day and the event was coveredby a wide range of media, including
newspapers and TV broadcasters.

Commemorative Photo at the Press
Conference

Commemorative Photo at the Tour of the
Quantum ComputerAwarded the Prime Minister Award, the highest honor, at the 53rd Japan Industrial
Technology Awards
On March 15, 2024 a joint researchgroup that included RIKEN, Fujitsu andothers won the Prime Minister Award,
the highest honor, at the 53rd Japan
Industrial Technology Awards, which
was hosted by Nikkan Kogyo Shimbun,
Ltd., for a high-performance computing
platform that utilizes a superconducting
quantum computer.

Commemorative Photo at the Award Ceremony

Receiving the Award

17

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
2007
2010
2012
2014
2020
2021
2022

18

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)
Research Fellow, Nanoelectronics Research Laboratories, NEC Corporation
Research Fellow, Green Innovation 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
Dispersive readout of a superconducting qubit using a nonlinear Purcell filter
Residual noise photons in a readout resonator become a major source ofdephasing for a superconducting qubit when the resonator is optimized fora fast, high-fidelity dispersive readout. Here, we propose and demonstrate
a nonlinear Purcell filter that suppresses such an undesirable dephasing
process without sacrificing the readout performance. When a readout
pulse is applied, the filter automatically reduces the effective linewidth of
the readout resonator, increasing the sensitivity of the qubit to the input
field. The noise tolerance of the device we have fabricated is shown to beenhanced by a factor of 3 relative to a device with a linear filter. A readout
fidelity of 99.4% is achieved using a 40-ns readout pulse.
Y. Sunada et al., “Photon-noise-tolerant dispersive readout of asuperconducting qubit using a nonlinear Purcell filter”, PRX Quantum 5,010307 (2024).
Qubit readout circuit containing a nonlinear Purcell
filter. (a) Photograph of the entire chip. (b) Distributedelement circuit model.

Single-qubit-gate robustness optimization
against residual ZZ interactions between
superconducting qubits
Overcoming the issue of qubit-frequency fluctuations is essential to realizing
stable and practical quantum computing with solid-state qubits. Static ZZ
interaction, which causes a frequency shift of a qubit depending on thestate of neighboring qubits, is one of the major obstacles to integrating
fixed-frequency transmon qubits. Here we propose and experimentallydemonstrate ZZ-interaction-free single-qubit-gate operations on asuperconducting transmon qubit by utilizing a semi-analytically optimized
pulse based on a perturbative analysis. The gate is designed to be robust
against slow qubit-frequency fluctuations. Our result paves the way for an
efficient approach to overcoming the issue of ZZ interaction without any
additional hardware overhead.
S. Watanabe et al., “ZZ-interaction-free single-qubit-gate optimization insuperconducting qubits”, Phys. Rev. A 109, 012616 (2024).
(a) Optimized single-qubit-gate drive waveform.
(b) Error rate per gate versus drive detuning.

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) Peter Anthony Spring(Senior Technical Staff) Koichi Kusuyama(Technical Staff I) Laszlo Szikszai(Technical Staff I) Harumi Hayakawa(Technical Staff I) Machie Kaito

19

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 computers and 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.
Bosonic code, which is relatively easy to correct errors in quantum computers, protects quantum information from errors bytaking advantage of the infinite number of degrees of freedom of the resonator. We are conducting research on Cat Qubit, which
realizes Cat code, which is one of the practical bosonic codes, using a car parametric oscillator (KPO) using superconducting
circuits. By generating a cat state with a two-dimensional KPO circuit (Fig 1 and 2) and evaluating the fidelity using a quantum
tomography method, we succeeded in realizing a universal quantum gate that can operate 1-bit and 2-bit gates.
In the 2-bit circuit, we used the unique properties of KPO to generate an entangled cat state in two ways. The first method is
to convert the entangled state (bell state) based on the Fock state to the entangled cat state. In the second method, a iSWAP
gate is added to two independent cat states to create an entangled cat state.
These results indicate that the planar superconducting KPO circuit successfully
developed in this study has the potential to become a new platform of scalable
quantum information processing.

Fig 1. Chip photo of 2D KPO circuit. It contains two coupled
KPOs.

Fig 2. Schematic diagram and equivalent circuitof a 1-bit KPO. Ancilla transmon was used forreadout.

Jaw-Shen Tsai (Ph.D.), Team Leader
Selected Publications
1 Daisuke Iyama, Takahiko Kamiya, Shiori Fujii, Hiroto Mukai, Yu Zhou, Toshiaki Nagase, Akiyoshi Tomonaga, Rui
Wang, Jiao-Jiao Xue, Shohei Watabe, Sangil Kwon & Jaw-Shen Tsai, “Observation and manipulation of quantum
interference in a superconducting Kerr parametric oscillator”, Nature Communications, 15:86 (2024), doi.
org/10.1038/s41467-023-44496-1
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

20

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 (-present)
Professor, Department of Physics, Tokyo University of Science (-present)
Professor, Research Institute for Science and Technology, Tokyo University of Science (-present)Recent Achievements
Realization of the cat state in superconducting KPO
and its one-qubit manipulation
We have fabricated KPO consisting of 10 DC SQUIDs. After a Fock state wasprepared, the intensity of the two-photon pump was slowly increased from 0 to
generate cat states, and the quantum interference of KPO was confirmed by Wigner
tomography.
We performed quantum gate operations in the cat state. The X/2 gate was
embodied by applying a single quantum drive, and the Z/2 gate was embodied bymodulating the frequency of the pump. In the case of the KPO cat state, the pumpis the reference phase, so when observing the Rabi oscillation by the X-gate, thephase (ϕ d) and frequency detuning (Δ d) of the drive must be matched. Quantum
process tomography was used to determine the fidelity after performing X/2 andZ/2, which were 0.844 and 0.794, respectively (Fig 3).

Fig 3. X Gate and Y Gate process
topographies of superconducting KPO cat
qubit and their Rabi oscillation

Entanglement generation and 2-qubit gate operation
with superconducting KPO qubit
Using two KPOs, we showed that the entangled Bell-Fock state is converted one-toone to the entangled cat state (Bell-Cat state). In a 2-mode Wigner tomography, theinterference pattern was observed. It indicates that the two cat states are entangled
rather than simply correlated.
In addition, we succeeded in directly entangling two cat states by iSWAP gate
operation (Fig 4). This research achievement is significant for it realizes a universal
gate set of gate operation with planar KPO circuits, and it indicates that it is a new
scalable quantum information processing platform.

Integration of 5 qubit using resonator-network

Fig 4. Rabi oscillation caused by the energy
exchange of the two cat states. The gate
time of iSWAP is 275 ns.

Our team has integrated qubits based on the proposed pseudo-twodimensional architecture. As a demonstration, we have integrated five transmon
superconducting qubits using resonator-network coupling. In the pseudo-twodimensional architecture, all the control wiring of the qubits can be arranged in aplanar form. This requires the use of a crossed resonator network with air bridges(Fig 5).
Characterization of this chip showed a lifetime of about 40 us. We will continue
to improve the performance of the integrated qubit chip. Since a single qubit chipfor testing has a lifetime of over 100 us using essentially the same fabrication
process, the lifetime of the integrated chip will be improved by clarifying thedifference between the two.
Fig 5. The chip is 10 mm square. The two
resonators in the center intersect using air
bridges.

Core members(Special Postdoctoral Researcher) Hiroto Mukai(Postdoctoral Researcher) Hang Xue

21

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 theSuperconducting Quantum Electronics Research Team. 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 asingle 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-temperatureelectronics for qubit control into a single hardware operating as an intermediate-scalequantum computer with 50–150 qubits. We have built a 64-qubit superconducting
quantum computer named “A”, and are currently setting up a system with more than
100 qubits. We aim to improve the fabrication yield and uniformity of qubit chips as well
as extending the coherence times of the qubits. We also evaluate control fidelities for
single- and two-qubit gates, initialization, and readout and aim to improve them. At thesame time, we explore the potential of the system for NISQ (noisy intermediate-scalequantum) applications, implement a proof-of-principle experiment of quantum error
correction, and simulate many-body quantum systems, using our quantum computer.
We also work on developing element technologies necessary for further scaling up thenumber of available qubits, such as packing more microwave cables and components ina limited space, and realizing quantum control across different chips. Ultimately, we aim
to pave the way for realizing a system capable of large-scale quantum error correction,and to bring a quantum computer that executes computations intractable with classicalcomputers closer to reality.

64-qubit superconducting quantum
computer “A”

Eisuke Abe (D.Sc.), Unit Leader
Selected Publications
1 K. Sasaki and E. Abe, “Suppression of Pulsed Dynamic Nuclear Polarization by Many-Body Spin Dynamics”,
Physical Review Letters 132, 106904 (2024).
2 E. Abe, “Superconducting route to quantum computing”, Proceeding of 2023 International Conference on
Simulation of Semiconductor Processes and Devices (SISPAD), P.1–4 (2023).
3 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).
4 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).
5 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).

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

22

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
Suppression of pulsed dynamic nuclear polarization by many-body spin dynamics
The initialization of qubits is the first step of quantum control
in any physical system. Nuclear spin systems, with weakinteractions with their environments and small Zeeman splitting
under the magnetic fields, exhibit notoriously small initialization
fidelities (spin polarization). Recently, pulsed dynamic nuclear
polarization (DNP) methods, in which the electron-nuclear spin
interaction is engineered via a microwave pulse sequence toimprove the nuclear polarization, has attracted attention. In
this work, we reveal that, in a system with multiple nuclear
spins with negligible spin-spin interactions and longitudinalrelaxation, there exists a mechanism that suppresses thepolarization achievable by pulsed DNP via higher-order nuclear
spin dynamics mediated by an electron spin, using analyticaland numerical calculations. The formation of a dark state isa well-known phenomenon whereby nuclear polarization issuppressed by a many-body effect, but we discuss under acertain condition higher-order effect cooperatively suppress thepolarization. This work highlights the importance of taking thehigher-order many-body effect into account when designing
control sequences of nuclear spin systems.

Numerical simulation of nuclear spin polarization as a function of thehyperfine interaction strength (A⊥) and the number of nuclear spins(Nnuc). The increase in both the hyperfine interaction strength andthe number of nuclear spins results in suppressed polarization. The
upper (lower) figure takes (does not take) into account the formationof the nuclear spin dark state.
©American Physical Society
https://doi.org/10.1103/PhysRevLett.132.106904

Detection and control of single proton nuclear spins with a solid-state quantum sensor
Control 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 single nitrogen-vacancy (NV) centers, which
are solid-state qubits operating at room temperatures andattract attention as quantum sensors, and demonstrate a highsensitivity detection of a single proton nuclear spin. Proton
nuclear spins are the most important detection targets formagnetic 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 spin state.
The data shows a coherent rotation of a nuclear spin. The inset is a
confocal microscope image of the sample, with the arrow indicating
the emission of the quantum sensor used for the measurement.
©American Institute of Physics
https://doi.org/10.1063/5.0016196

23

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.

Stacked 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

24

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:
Quantum circuits
We study chip stacking techniques for superconducting circuits to improve the scalability of qubit devices. The chip-stacking
method secures the available footprint area for the circuits, enabling us to integrate devices with a higher density. We havedemonstrated a vertical signal waveguide as an analog interface between substrates [Bunpei Masaoka et al., “Vertical signaltransmission in stacked substrates for superconducting quantum circuits,” The 36th International Symposium on superconductivity(2023)]. We have designed the vertical waveguidethrough the substrate using a pseudo-coaxial structure
consisting of a center- and ground-electrode vias anddemonstrated a broadband signal transmission from
2-10 GHz with an insertion loss of less than 2 dB. The
established technology makes it possible to integrate(a) A schematic illustration of vertical coax structure. (b) A photographsuperconducting devices by connecting inner-layerof the device structure. (c) Transmission coefficient obtained through thecircuits to the through-substrate waveguide.
stacked chips.

Scalable quantum computers: from control electronics to decoding circuits:
Signal processing circuits and architecture
The bandwidth restriction between cryogenic and roomtemperature electronics is one of the critical bottlenecks
in superconducting noisy intermediate-scale quantum
computers. We have demonstrated an algorithm-awaresystem-level optimization to address the bottleneck
issue [Yosuke Ueno et al., Inter-Temperature Bandwidth
Reduction in Cryogenic QAOA Machines, IEEE
Computer Architecture Letters 23, 6-9 (2023)]. We focus
on the quantum approximate optimization algorithm asa target algorithm. We design digital circuits that countthe coincidence of the qubit outcome using single-flux
quantum logic operated at cryogenic temperature. We
show in our detailed analysis of the circuit exponential
bandwidth reduction and suppressions of heat inflow
and peripheral power consumption of inter-temperature
cables, highlighting the scalability of superconducting
quantum computers.

Baseline system design of a superconducting quantum computer (left) andproposed system design (right).
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(Technical Staff) Bunpei Masaoka(Technical Staff) Miyuki Ozawa

25

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.

The Schrödinger’s cat state generated by the backactionof the quantum measurement.

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”, Phys. Rev. Lett. 130, 260601 (2023).
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

26

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 accuracyof 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 theaccuracy of the quantum gates due to the unwanted residual interactions that occur
even when the gate is not active. In this work, we proposed a circuit in which two
fixed-frequency qubits are connected through a fixed-frequency qubit coupler, anddeveloped a new quantum gate for this circuit. This method not only reduces theresidual interactions, but also has the feature that the residual interactions do not
deteriorate the gate accuracy. We also have investigated a structure in which many
qubits can be integrated with a small number of wires, taking advantage of thefeature that all quantum gates are possible only by driving couplers.
S. Shirai, Y. Okubo, K. Matsuura, A. Osada, Y. Nakamura, and A. Noguchi,
Phys. Rev. Lett. 130, 260601 (2023).

Integrated-circuit architecture with frequencyfixed couplers and multiplexed wirings.

Quantum manipulation of the microwave photon and its back action
The operation of correcting errors in quantum circuits and extending the lifetime of quantum states is called the quantum error
correction. In particular, by using microwave photons in a resonator, which has a larger degree of freedom than a qubit, quantum
error correction can be realized with less overhead, leading to higher-performance quantum chips. Such research has been
conducted using 3D microwave resonators due to their long lifetime, but from the viewpoint of scalability, it is desirable to realize
such research in 2D circuits. Our team aims to improve the performance of 2D resonators to realize quantum microwave photon
systems with both high integration and high performance.
In this fiscal year, we fabricated a lumped element microwave resonator with a high-performance titanium nitride superconducting
film and developed a two-dimensional resonator with a Q-value exceeding 1,000,000. We have succeeded in developing a quantum
chip by coupling this microwave resonatorand the cubic transmon developed byour team. Using this chip, we realized aquantum measurement called modular
measurement, which is important forquantum error correction using microwave
photons. Furthermore, we succeededin generating a macroscopic quantum
state called Schrödinger’s cat state in
the resonator by controlling the quantum
state of microwave photons through theWe have demonstrated a modular
High-Q microwave resonator with lumpedmeasurement of the resonator with theelement circuits.
reaction of the measurement.
superconducting qubit and manipulated
it by the back action of the measurement.

Core members(Special Postdoctoral Researcher) Ryo Sasaki(Postdoctoral Researcher) Yusuke Tominaga

27

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 S. Konno, W. Asavanant, F. Hanamura, H. Nagayoshi, K. Fukui, A. Sakaguchi, R. Ide, F. China, M. Yabuno, S.
Miki, H. Terai, K. Takase, M. Endo, P. Marek, R. Filip, P. van Loock, and A. Furusawa, “Logical states for faulttolerant quantum computation with propagating light”, Science 383, 6680 (2024).
2 A. Sakaguchi, S. Konno, F. Hanamura, W. Asavanant, K. Takase, H. Ogawa, P. Marek, R. Filip, J. Yoshikawa,
E. Huntington, H. Yonezawa, and A. Furusawa, “Nonlinear feedforward enabling quantum computation” Nature
Communications 14, 3817 (2023).
3 F. Hanamura, W. Asavanant, S. Kikura, M. Mishima, S. Miki, H. Terai, M. Yabuno, F. China, K. Fukui, M. Endo,and A. Furusawa, “Single-shot single-mode optical two-parameter displacement estimation beyond classicallimit”, Phys. Rev. Lett. 131, 230801 (2023).
4 A. Inoue, T. Kashiwazaki, T. Yamashima, N. Takanashi, T. Kazama, K. Enbutsu, K. Watanabe, T. Umeki, M.
Endo, and A. Furusawa, “Toward a multi-core ultra-fast optical quantum processor: 43-GHz bandwidth realtime amplitude measurement of 5-dB squeezed light using modularized optical parametric amplifier with 5G
technology”, Appl. Phys. Lett. 122, 104001 (2023).
5 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).

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)28

Recent Achievements
Nonlinear feedforward enabling quantum computation
Measurement-based quantum computer using time-domain multiplexing of propagating light is promising from the viewpointof scalability. In this approach, fault-tolerance and universality of quantum computation are achieved via appropriate quantum
state preparation and electro-optic feedforward of measurement results. Linear feedforward is an established technology, andvarious Gaussian operations have been realized using it. On the other
hand, nonlinear feedforward, which enables deterministic non-Gaussian
operations, had not been realized. We realized for the first time a fast andflexible nonlinear feedforward and further demonstrated non-Gaussian
measurements by combining it with non-Gaussian ancillary states. This
achievement will lead to high-speed and universal quantum computation
using propagating light.

Diagram of nonlinear measurement using nonlinear feedforward. The
measurement at the bottom is adaptively changed, according to nonlinear
calculations on measurement results at the top.

Virtual quantum state expressing nonlinear measurement.
Parabolic structure indicates quadratic nonlinearity, andinterference structure shows nonclassicality.

©A. Sakaguchi et al. Nat. Commun. 14, 3817 (2023).

©A. Sakaguchi et al. Nat. Commun. 14, 3817 (2023).

Logical states for fault-tolerant quantum computation with propagating light
A promising approach to realize fault-tolerant quantum computer in bosonic quantum system such as light is to utilize GottesmanKitaev-Preskill (GKP) qubits. GKP qubits are quantum states with a lattice-like distribution embedded in continuous-variablestate space of bosonic quantum system. Although it is very hard to generate GKP qubits in propagating light, it is known thatapproximate ones can be generated by interference of multiple Schrödinger cat states followed by homodyne measurements.
We for the first time succeeded in generating such approximate GKP qubits from interference of two Schrödinger cat states. We
expect improvement of quality by increasing the number of Schrödinger
cat states in the future.

Setup to generate optical GKP qubits from Schrödinger cat states.
©S. Konno et al., Science 383, 289 (2023).

Approximate GKP qubit state generated from interference of
two Schrödinger cat states and homodyne measurement.
©S. Konno et al., Science 383, 289 (2023).

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

29Optics

Optical Quantum Control Research Team
Keywords: Quantum computing, Quantum optics, Quantum control, Optical quantum computing, Quantum estimation

Research Outline
The goal of our team is to develop an optical quantum computer. In particular, we investigate optical quantum control technology
and measurement-based optical quantum computation. There are many advantages in an optical platform, including compatibility
with room temperature, high scalability, and applicability to communication technology. The key technology for optical quantum
information processing is quantum control. Quantum states often fluctuate due to environmental disturbances or measurements. It
is critical to develop effective control technology of quantum states, which may involve maintaining or manipulating quantum statesin noisy environment, or realising high-performance measurements through control of basic measurement devices. In addition,
quantum estimation is a core technique because it is the basis for quantum control technology.
We are investigating high-performance measurement and estimation techniques, control techniques based on them,measurement-based optical quantum computation, and related quantum optical technology. Figure shows quantum estimationof the dynamics of quantum states in an optical parametric oscillator. We have achieved high-precision estimation of quantum
states using a quantum-state smoothing technique, where both quantum filtering and retro-filtering are used to produce more
precise acausal estimation than conventional methods. In addition, we are also investigating coherent control and measurementbased control. Coherent control is a technique that does not involve measurements, leading to a faster and simpler system.
Measurement-based control is a technique based on measurement, which allows more complex processing than coherent
control. Through these investigations, our team aims to realise and improve optical quantum computers and related quantum
information technologies.

Estimation of quantum state of light

Hidehiro Yonezawa (Ph.D.), Team Leader
Selected Publications
1 S. Yokoyama, et al., “Feasibility study of a coherent feedback squeezer,” Phys. Rev. A 101, 033802 (2020).
2 S. Yokoyama, et al., “Characterization of entangling properties of quantum measurement via two-mode quantum
detector tomography using coherent state probes,” Opt. Express 27, 34416 (2019).
3 W. Asavanant, et al., “Generation of time-domain-multiplexed two-dimensional cluster state,” Science 366, 373(2019).
4 S. Yokoyama, et al., “Ultra-large-scale continuous-variable cluster states multiplexed in the time domain,” Nature
Photon. 7, 982 (2013).
5 H. Yonezawa, et al., “Quantum-Enhanced Optical-Phase Tracking,” Science 337, 1514 (2012).

Brief resume
2007
2007
2009
2013
2023

30

Ph.D. in Engineering, The University of Tokyo
Research associate, The University of Tokyo
Project Assistant Professor, The University of Tokyo
Senior lecturer, University of New South WalesTeam leader, Optical Quantum Control Research Team, RIKEN Center for Quantum Computing, RIKEN(-present)Recent Achievements
Development of measurement-based optical quantum computer
Our team is developing a measurement-based optical quantum computer in collaboration with optical quantum computation
research team. Our optical quantum computer consists of four squeezers, an optical interferometer with delay lines, and four
homodyne detection systems. The squeezers are NTT-made LN waveguide modules with 6 THz bandwidth, which enables
ultra-fast quantum computation in future. The optical interferometer utilises two different length optical delay lines, creating ultralarge-scale quantum entanglement with time multiplexing. The number of inputs of our current quantum computer, which are
determined by the length of the optical delay lines, is 102, while there is no limitation for computational steps. In the four homodyne
detector systems, we measure the entanglement and perform quantum computation by changing the measurement bases. We
have developed optical quantum computer hardware and verify the correlation in the ultra-large-scale quantum entanglement.

Schematic of optical quantum computer

Research on quantum state smoothing
Quantum smoothing is a method used in quantum estimation, where the estimate is calculated as a weighted average of the forward
and backward estimations. While real-time estimation is not possible with this method, it can achieve higher accuracy estimation in
problems where post-processing is allowed. So far, quantum smoothing has mainly been applied to parameter estimation, whereclassical smoothing techniques are almost directly applicable. In recent years, quantum smoothing technique has been extended
for quantum state estimation with quantum mechanically valid formulation, revealing that it is possible to estimate quantum
states more accurately through smoothing. Our team is conducting research with quantum optical techniques to apply quantum
smoothing to the estimation of quantum states inside an optical cavity. As an optical cavity interacts with environment, the internal
state of the cavity undergoes decoherence. By measuring the leakage from the optical cavity and applying quantum smoothing,accurate estimation is achieved. This leads to purer quantum state inside the optical cavity by suppression of the decoherence. In
this research we investigate the effectiveness of quantum smoothing in detail.

Core members(Senior Research Scientist) Shota Yokoyama

31

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.

Takeshi Fukuhara (D.Sci.), Team Leader
Selected Publications
1 H. Ozawa, R. Yamamoto, and T. Fukuhara, “Observation of chiral-mode domains in a frustrated XY model on
optical triangular lattices”, Phys. Rev. Res. 5, L042026 (2023).
2 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).
3 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).
4 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).
5 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).

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)

32

Recent Achievements
Observation of chiral-mode domains
By mapping the phase of a Bose-Einstein condensate (BEC) in an optical lattice as a spin, the XY spin model is realized. For
the BEC in a triangular optical lattice, the sign of the spin-spin coupling can be altered by modulating the phase of the opticallattice, which enables us to change the system from non-frustrated tofrustrated. If the change is performed slowly enough, the system canbe transferred to the ground state of the frustrated spin system, which
has two degenerate chiral modes. We observed that one chiral mode
appears randomly in each experiment, due to spontaneous symmetry
breaking. Further, we investigated nonequilibrium dynamics (relaxationand excitation) after the change. We observed that chiral mode domain
structures were formed when the system is changed relatively quickly.

(Upper) Images with two chiral modes steaming from spontaneous
symmetry breaking. (Lower) Images with chiral-mode domains.
Experimental data (left) and numerical simulation (right).
©Hideki Ozawa, Ryuta Yamamoto, and Takeshi Fukuhara, “Observationof chiral-mode domains in a frustrated XY model on optical triangularlattices”, Physical Review Research 5, L042026 (2023).

Observation of single atoms in an ultracold 85Rb gas
By loading bosonic atoms into an optical lattice and preparing a Mott
insulating state with unity filling, we can realize a quantum spin systemthat is governed by spin-1/2 Heisenberg model if we consider two
internal atomic states as pseudospin. In this mapping, the anisotropy ofthe spin-spin interaction can be controlled by changing the interatomic
interaction via a Feshbach resonance. Although, for quantum gas
microscope experiments, rubidium-87 atoms have been utilized, in thisstudy we performed experiments using rubidium-85 atoms which havea Feshbach resonance at a magnetic field of B ~ 155 Gauss. We cooled
rubidium-85 atoms down to a temperature of the order of micro Kelvin,and, for the first time in the world, succeeded in observing the atoms ina triangular lattice at the single-atom level. This system is expected to
be used for a quantum simulation of frustrated magnets that follow the
spin-1/2 triangular-lattice antiferromagnetic Heisenberg model.

Single atom detection of ultracold 85Rb gases

Core members(Research Scientist) Ryuta Yamamoto(Postdoctoral Researcher) Hideki Ozawa(Technical Staff I) Yoichiro Otsuka

33

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 A. Jennings, X. Zhou, I. Grytsenko, E. Kawakami, “Quantum computing using floating electrons on cryogenic
substrates: Potential And Challenges”, Appl. Phys. Lett. 124, 120501 (2024).
2 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).
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

34

Ph.D., Delft University of Technology, The Netherlands
Postdoctoral researcher, Okinawa Institute of Science and Technology
PRESTO, Japan Science and Technology Agency
RIKEN Hakubi Team Leader, Floating-Electron-Based Quantum Information RIKEN Hakubi Research Team(-present)Recent Achievements
Blueprint for quantum computing using 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. This method also offers a scalable
solution for increasing the number of qubits. An artificiallyintroduced interaction between the Rydberg state and the spinstate allows us to transfer the qubit state between the Rydberg
and spin states. In this way, we can benefit from both the longcoherence time of the spin state and the long-range interactionof the Rydberg state in the course of qubit operation.
The interaction between the Rydberg state and thespin state is artificially introduced by a ferromagnet which isplaced near the electron. We theoretically showed that the
introduced magnetic field gradient mixes the spin state andthe orbital state and shortens the spin relaxation time to 50
ms but does not degrade the qubit fidelity. We estimated thesingle-qubit gate and the two-qubit gate fidelities to be >99.9999% and ~99%, respectively.

Electrons (blue circles) are arranged in a two-dimensional arrayto facilitate a scalable quantum computer. Our goal is to integratequantum bits at bitline and wordline intersections, echoing the DRAM
method of classical computers.

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 and energyefficient electronics for qubit control and readout. Recently, qubit control electronics have been successfully integrated using
cryogenic CMOS technology and superconducting Josephson
junctions. Here, we focus on the development of readout
electronics using tunnel-diode oscillator circuits. Comparedto cryogenic CMOS devices and superconducting Josephson
junction circuits, the tunnel-diode circuits have lower power
dissipation (~1μW). Recently, we successfully measured thecharacteristics of a resonator placed at ultra-low temperaturesusing a tunnel diode oscillator that also operates under ultralow temperatures.

The upper figure shows the measurement resultsof a resonator placed in an ultra-low temperatureenvironment using a microwave oscillator also set
at ultra-low temperatures. The lower figure presents
the results of measuring the resonator at ultra-low
temperatures using a microwave oscillator placed atroom temperature, for comparison

Core members(Technical Scientist) Ivan Grytsenko(Postdoctorial Researcher) Asher Jennings(Research Part Timer I) Oleksiy Rybalko

35

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

Seigo Tarucha (D.Eng.), Team Leader
Selected Publications
1 T. Kobayashi, T. Nakajima, K. Takeda, A. Noiri, J. Yoneda, and S. Tarucha, “Feedback-based active reset of aspin qubit in silicon”, npj Quantum Information 9, 52 (2023).
2 J.S. Rojas-Arias, A. Noiri, P. Stano, P (Stano, P.), T. Nakajima, J. Yoneda, K. Takeda, T. Kobayashi, A. Sammak,
G. Scappucci, D. Loss, and S. Tarucha, “Spatial noise correlations beyond nearest neighbors in 28Si/Si-Ge spin
qubits”, Phys. Rev. Appl. 20, 5 (2023).
3 K. Takeda, A. Noiri, T. Nakajima, T. Kobayashi, and S. Tarucha, “Quantum error correction with silicon spin
qubits”, Nature, 608, 682-686 (2022).
4 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).
5 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).

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

36

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 spin readout in silicon qubits
It is difficult to directly detect spin orientation in a single shot measurement. Therefore, a combined method of spin to charge
information conversion and charge sensing is employed. For the spin-charge conversion spin dependent tunneling is often utilized.
However, this method is slow and not so accurate. Here we use spin blockade effect instead to significantly improve the speed
and accuracy of spin readout in silicon (Si) qubit devices.
We fabricate a Si quadruple quantum dot (QD) device made in Si/SiGe and use a double QD having two electron spins for the
spin readout experiment (Fig.1). When the two spins are antiparallel, either electron can move between the two dots, generating
double occupation (2,0) while when they are parallel, the double occupancy does not appear, according to Pauli exclusion. We
improve the charge sensor design for detecting the electron occupancy either (2,0) or (1,1) (Fig.2), and in addition the sensitivity of
the charge sensor and have finally raised visibility of the charge sensor signal for distinguishing the spin state up to 99.6% (highest
in the world), and shortened the measurement time as well.

Fig.1 Two qubit device

Fig. 2 Spin readout

Feedback-based active reset of a spin qubit in silicon
Feedback preparation of qubits is a sought-after technique for important quantum information protocols such as fault-tolerant
quantum error correction. Such a preparation scheme is implemented for silicon spin qubits recently, but the preparation fidelity islimited by the qubit readout fidelity. We have developed an advanced feedback protocol to improve the preparation fidelity.
We incorporate a cumulative readout technique consisting of multiple quantum non-demolition (QND) measurements of aqubit to a feedback control system (Fig.1). The control pulse is conditioned according to the cumulative readout result, whichenables the preparation fidelity to exceed the readout fidelity of the single measurement. We have achieved the preparation fidelity
higher than 98% and expected further improvements with higher readout fidelity and short measurement time.

Fig.1 Schematic of the feedback quantum control based on multiple
measurement results.

Fig.2 An example of qubit operation after the active reset. The
preparation fidelity can be estimated from the visibility of the oscillations.

Core members(Research Scientist) Takashi Kobayashi(Technical staff I) Reiko Kuroda

37

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 J. S. Rojas-Arias, A. Noiri, P. Stano, T. Nakajima, J. Yoneda, K. Takeda, T. Kobayashi, A. Sammak, G.
Scappucci, D. Loss, and S. Tarucha, “Spatial noise correlations beyond nearest neighbors in 28Si/Si-Ge spin
qubits”, Phys. Rev. Appl. 20, 054024 (2023).
2 J. Yoneda, J. S. Rojas-Arias, P. Stano, K. Takeda, A. Noiri, T. Nakajima, D. Loss, and S. Tarucha, “Noisecorrelation spectrum for a pair of spin qubits in silicon”, Nat. Phys. 19, 1793 (2023).
3 P. Stano and D. Loss, “Review of performance metrics of spin qubits in gated semiconducting nanostructures,”,Nat. Rev. Phys. 4, 672 (2022).
4 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).
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)

38

Recent Achievements
Noise-correlation spectrum for a pair of spin qubits in silicon
One of the main advantages of spin qubits is their reduced size, in the order of tens of nanometers, a great feature for scalability.
However, at the same time, the compactness of the system implies that the qubits share the same local environment and could
be subject to correlated noise, which could be a big obstacle in the implementation of an error-corrected quantum computer.
In collaboration with experimentalists, we quantified for the first time the noise correlations between two neighboring spin qubits
and found that, indeed, the noise can be strongly correlated between qubits and with exchange interaction. We devised a simple
theoretical model for the effect of charge noise on the qubits andreached quantitative agreement with the measurements without any
fitting parameters, proving that the model encapsulates the main
physics behind decoherence in the device and that we understand theorigin of the noise correlations.

Normalized noise cross-correlation amplitude (a) and phase (b) for a pair ofneighboring spin qubits. Two regimes are found: one with completely out-ofphase correlations at low frequencies, and another one with fully in-phasecorrelations at higher frequencies. This is the first quantification of qubit-noisecorrelations in spin qubits.
©J. Yoneda, J. S. Rojas-Arias, P. Stano, K. Takeda, A. Noiri, T. Nakajima, D. Loss,and S. Tarucha, “Noise-correlation spectrum for a pair of spin qubits in silicon”,
Nat. Phys. 19, 1793 (2023).

Scaling of charge noise correlations with interqubit distance
Knowing that noise in spin qubits is correlated, the natural question that arises is: “What is the range of the noise correlations?”. The
question is not trivial and is of the outmost importance for quantum error correction. To answer it, we modeled charge noise as theeffect of charge two-level fluctuators that act on the qubits via Coulomb interaction. With the model we fitted the noise cross-spectraof real devices reaching quantitative agreement. Finally, we performed Monte-Carlo simulations to quantify the average correlation
amplitude dependence on interqubit separation. We found a power-law decay which is steeper in the presence of screening from themetallic gates on the surface of the device. Our work suggests that it ispreferable to fabricate devices with a high density of metallic gates tosuppress noise correlations and provides an expectation on their range.

Average noise cross-correlation amplitude as a function of interqubit distance.
Pink (gray) shows the results in the presence (absence) of screening from themetallic gates. Circles and squares show two different types of averaging.
©J. S. Rojas-Arias, A. Noiri, P. Stano, T. Nakajima, J. Yoneda, K. Takeda, T.
Kobayashi, A. Sammak, G. Scappucci, D. Loss, and S. Tarucha, “Spatial noisecorrelations beyond nearest neighbors in 28Si/Si-Ge spin qubits”, Phys. Rev. Appl.
20, 054024 (2023).

Core members(Postdoctoral Researcher) Juan Rojas-Arias

39

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 Y. Akahoshi, K. Maruyama, H. Oshima, S. Sato, and K. Fujii, “Partially fault-tolerant quantum computing
architecture with error-corrected clifford gates and space-time efficient analog rotations” PRX Quantum 5,010337 (2024).
2 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).
3 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).
4 K. Mitarai, M. Negoro, M. Kitagawa and K. Fujii, “Quantum Circuit Learning”, Phys. Rev. A, 98, 032309 (2018).
5 K. Fujii and K. Nakajima, “Harnessing Disordered-Ensemble Quantum Dynamics for Machine Learning”, Phys.
Rev. Applied 8, 24030 (2017).

Brief resume
2011
2011
2013
2016
2017
2019
2020
2020

40

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
A new concept quantum computer architecture using partial quantum error correction
Computational power of the current quantum computers with tens to hundreds qubits is limited by the problem of noise. Therefore,
the realization of a fault-tolerant quantum computer (FTQC) with quantum error correction is necessary to execute complex quantum
algorithms with theoretically proven quantum speedup. It is estimated that a fault-tolerant quantum computer requires the orderof one million qubits, a gap of four orders of magnitude between NISQ and FTQC. In this study, we propose a new architecture
for early fault-tolerant quantum computation (earlyFTQC), in which Clifford operations, which are frequently required in quantum
algorithms, are protected by quantum
error correction, while continuous
rotation gates, which usually require
much overhead for quantum error
correction, are executed with error
detection. With an error probability of
10-4, even 10,000 qubits can performtasks that are difficult to simulate evenon a supercomputer with guaranteed
accuracy. This architecture is a newmilestone for the evolution of quantum
computers from the NISQ era to FTQC
in a sustainable manner, and alsoenables early application of quantum
A conceptual design of a new quantum computer architecture for early FTQC.
computers to real-world problems.

Optimal quantum algorithm for simulating time-dependent Hamiltonian dynamics
Since Richard Feynman proposed the concept of a quantum computer bysaying, “if you want to make a simulation of nature, you’d better make it(computer) quantum mechanical,” the simulation of quantum many-bodysystems has been one of the most promising applications of quantum
computers. In particular, a new type of algorithm called the quantum singularvalue transformation algorithm has recently emerged, which is the optimalquantum algorithm for the time-independent simulation of Hamiltonian
dynamics. In this study, by applying Floquet theory to time-dependent
Hamiltonians, we construct a quantum algorithm that optimally simulates
the time evolution under a time-dependent Hamiltonian. The results enable
faster and more accurate simulations of complex phenomena such as
nonequilibrium phenomena and chemical reactions.

Conceptual diagram of the effective Hamiltonian when
the time-dependent Hamiltonian is described in a
Floquet-Hilbert space

Core members(Research Scientist) Tatsuhiko Ikeda(Special Postdoctoral Researcher) Yasushi Yoneta

41

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 seven years (from 2017 to 2023). 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.
More than 1.5 Million downloads!
Our quantum software (QuTiP) is used by more groups
than the ones by Google, Microsoft, D-Wave, etc.

Our software QuTiP is widely used by very many research groups,
and it has been downloaded more than 1.5 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)42

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(JSPS Postdoctoral Researcher) Paul MENCZEL

43

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

44

∆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(-present)
Chief Scientist, Computational Condensed Matter Physics Laboratory, RIKEN (-present)
Team Leader, Computational Materials Science Research Team, RIKEN Center for Computational Science(-present)
Team Leader, Quantum Computational Science Research Team, Riken Center for Quantum Computing(-present)Recent Achievements
Simulating Floquet scrambling circuits on trapped-ion quantum computers
Complex quantum many-body dynamics spread initiallylocalized quantum information across the entire system.
Information scrambling refers to such a process, whose
simulation is one of the promising applications of quantum
computing. We demonstrate the Hayden-Preskill recovery
protocol and the interferometric protocol for calculating out-oftime-ordered correlators to study the scrambling property ofa Floquet dynamics based on a one-dimensional kicked-Isingmodel on a 20-qubit trapped-ion quantum processor. We
experimentally confirm the growth of signals in the HaydenPreskill recovery protocol and the decay of out-of-time-orderedcorrelators at late times. These results imply that the Floquet
quantum circuit indeed exhibits information scrambling.
The figure shows the out-of-time-ordered correlators for various
positions δ (figure legends) of butterfly operators as a function of
Floquet cycles.

Topologically distinct ground states of a Su-Schrieffer-Heeger model described byparametrized quantum circuits
The Su-Schrieffer-Heeger (SSH) model exhibits two
topologically distinct ground states depending on the relative
magnitude of hopping parameters within and between unit
cells. We conducted a classical simulation of the ground stateof the SSH model using a variational quantum circuit in theform of the Quantum Alternating Operator Ansatz (QAOA)
to investigate the quantum resources required to prepare
topologically trivial or non-trivial ground states. The results
revealed the following: (i) When the initial state and final state
belong to the same topological phase, the variational energy
approaches the exact ground state energy exponentially withrespect to circuit depth. (ii) When the initial state and final state
belong to different topological phases, a circuit depth equal toa quarter of the system size is always required. Furthermore,
we calculated the topological order parameter using an ion trap
quantum computer for an 18-site system and exper-imentally
validated the topological phase transition.

The figure shows the dependence of the polarization PR on thedepth of the variational quantum circuit M for an 18-qubit system.
It demonstrates the transition from a topologically trivial initial state(M=0) to a nontrivial final state (M=4).

Core members(Research Scientist) Kazuhiro Seki

(Postdoctoral Researcher) Qing Xie

45

Theory

Quantum Computer Architecture Research Team
Keywords: Quantum computer, Quantum error correction, Fault-tolerant quantum computation, Physical implementation

Research Outline
We are theoretically studying the total design of quantum computers, from the approaches to fault-tolerant quantum computation(FTQC) to their physical implementations, namely, quantum computer architecture. At present, various physical implementationsof quantum computers, such as superconducting circuits, neutral atoms, ions, light, semiconductor quantum dots, and spins insolids, are under development. Since different implementations have different types of errors and different connectivity, it is effective
to develop an approach to FTQC dedicated to each implementation. Also, since early quantum computers will play a role similar
to ASIC or an accelerator, they should be designed for specific quantum algorithms. Moreover, since conventional computers play
important roles such as control of physical systems, decoding in error correction, and quantum-classical hybrid implementationsof algorithms, quantum-classical cooperative system design is required. Thus, the research on quantum computer architecture
requires to consider all aspects of quantum computers.
The requirement of large resource overheads for FTQC is a central problem at present. To solve this problem, we focus
on high-rate codes. Conventional approaches to FTQC use the encoding of a single logical qubit into many physical qubits,
leading to the resource problem. Thus high-rate codes encoding many logical logical qubits at once have recently attracted
much attention. However, FTQC with them has not been established. We aim at solving the resource problem by developing
FTQC with high-rate codes.

Quantum computer architecture

Conventional single-logical-qubit encoding (Left), high-rate code
(Right).

Hayato Goto (Ph.D.), Team Leader
Selected Publications
1 H. Goto, “Many-hypercube codes: High-rate quantum error-correcting codes for high-performance fault-tolerant
quantum computation”, arXiv:2403.16054 (2024).
2 H. Goto, Y. Ho, and T. Kanao, “Measurement-free fault-tolerant logical-zero-state encoding of the distance-threenine-qubit surface code in a one-dimensional qubit array”, Phys. Rev. Research, 5, 043137 (2023).
3 H. Goto, “Minimizing resource overheads for fault-tolerant preparation of encoded states of the Steane code”,
Sci. Rep., 6, 19578 (2016).
4 H. Goto, “Step-by-step magic state encoding for efficient fault-tolerant quantum computation”, Sci. Rep., 4,7501 (2014).
5 H. Goto and H. Uchikawa, “Fault-tolerant quantum computation with a soft-decision decoder for error correction
and detection by teleportation”, Sci. Rep., 3, 2044 (2013).

Brief resume
2003
2007
2016
2020
2023
2023

46

Researcher, Toshiba Corporation
Ph.D. (Science), The University of Tokyo
Senior Research Scientist, Toshiba Corporation
Fellow, Toshiba Corporation
Senior Fellow, Toshiba Corporation (-present)
Team Leader, Quantum Computer Architecture Research Team, RIKEN Center for Quantum Computing(-present)Recent Achievements
High-performance fault-tolerant quantum computing with the proposed high-rate
codes named “many-hypercube codes”
In conventional fault-tolerant quantum computation (FTQC), a single logical qubit is encoded into many physical qubits. For
instance, the distance-d surface code encodes a logical qubit into d 2 physical qubits, where the distance represents the code
size and distance-d codes can correct, in principle, qubit errors less than d/2. In the case of the surface code, the encoding rate
r is 1/ d 2, which asymptotically becomes zero as the code size increases. This is the major reason for huge resource overhead
in FTQC. Thus recently, high-rate codes with constant rates, such as quantum LDPC (low-density parity check) codes, haveattracted attention. For example, 4%-rate quantum LDPC codes have been well studied for FTQC. However, the quantum LDPC
codes have the issues that their rates are still low and moreover it is difficult for them to perform logical gates in parallel. Hence, the“high-performance FTQC” realizing both high encoding rates and parallelizability of logical gates has been difficult so far.
We propose the [[6L,4L,2L]] quantum code obtained by concatenating the [[6,4,2]] code (distance-two quantum code encoding
4 logical qubits into 6 physical qubits), which is one of the simplest quantum codes. The structure of this code can be expressedby 4L hypercubes. Therefore we call it “many-hypercube code.” The encoding rate of this code is (4/6)L, which asymptoticallyapproaches zero, but as high as 30% and 20% for the distance 8 and 16 (L=3, 4), respectively. We have developed a highperformance decoder for the high-rate quantum code. Moreover, developing fault-tolerant encoders dedicated to this code, wehave shown by numerical simulation that the error threshold for logical controlled-NOT gates is as high as 1%. Also, we haveproposed the methods to perform logical gates necessary for universal quantum computation in parallel. Thus, our proposed
many-hypercube code will allow us to realize the high-performance FTQC.
H. Goto, “Many-hypercube codes: High-rate quantum error-correcting codes for high-performance fault-tolerant quantum
computation”, arXiv:2403.16054 (2024).

Comparison between many-hypercube code and surface code (code distance is 8)

Core members(Research Scientist) Ryota Nakai

47

Theory

Analytical Quantum Complexity RIKEN Hakubi Research Team
Keywords: Quantum Computing, Quantum Entanglement, Hamiltonian Complexity, Quantum many-body physics, Quantum Algorithm

Research Outline
Over the past two decades, quantum computation within
the realm of quantum information theory remained largely
theoretical, with doubts cast on its practical applicability.
This changed recently, especially in 2019, when Google’s
53-qubit “Sycamore” quantum computer demonstrated“quantum supremacy” by performing specific calculations
much faster than conventional computers. However, many
issues still need resolution for practical use.
To meet these challenges, researchers worldwide areworking on developing more useful algorithms, particularly
in quantum many-body theory. In the realm of quantum
physics, invisible particles like electrons and atoms play themain roles. Their interactions determine the properties ofmatter. These collections of particles, known as “quantum
Schematic figure for quantum Hamiltonian
many-body systems,” are central to understanding
phenomena from electron movement to superconductivity
and the development of quantum computers. This theory, which deals with the interactions of numerous quantum particles, canlead to unexpected phenomena, making it difficult to calculate. This area of study is known as Hamiltonian complexity.
Hamiltonian complexity research focuses on when and how quantum Hamiltonians can be efficiently simulated. Significantly,
many problems in quantum computing belong to a class known as QMA (the quantum version of NP), which can be reduced toquantum many-body problems. Therefore, developing algorithms to solve quantum many-body problems is key to addressing all
problems in the QMA class.
Hamiltonian complexity is a research area between computer science and physics with many mathematically defined unsolved
problems. Our laboratory explores solutions to these mathematical challenges, having already achieved partial or complete
solutions to significant issues such as the quantum entanglement boundary law conjecture, the quantum Markov conjecture, theline-ar-light-cone problem in the Lieb-Robinson bounds, quantum Hamiltonian learning, and long-range quantum entanglement at
finite temperatures. Our goal is to further develop our research findings and tackle new unsolved problems.

Tomotaka Kuwahara (Ph.D), RIKEN Hakubi Team Leader
Selected Publications
1 Tomotaka Kuwahara, Tan Van Vu, Keiji Saito, “Effective light cone and digital quantum simulation of interacting
bosons” Nature Communications 15(1) 2520 (2024).
2 T. Kuwahara, K. Saito, “Exponential Clustering of Bipartite Quantum Entanglement at Arbitrary Temperatures,”Physical Review X, 12, 021022 (2022).
3 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.
4 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).
5 T. Kuwahara, K. Saito, “Area law of noncritical ground states in 1D long-range interacting systems,” Nature
Communications, 11 4478 (2020).

Brief resume
2015
2016
2017
2021
2022

48

JSPS Research Fellowship for Young Scientists (JSPS PD), The University of Tokyo
Assistant Professor, Advanced Institute for Materials Research (AIMR), Tohoku University,
Research scientist, Center for Advanced Intelligence Project, RIKEN
Sakigake Researcher, Japan Science and Technology Agency (-present)
RIKEN Hakubi team leader, RIKEN Cluster for Pioneering Research / RIKEN Center for Quantum
Computing, RIKEN (-present)Recent Achievements
Partial resolution of quantum Markov expectation in quantum many-body systems.
One of the most fundamental properties of classicalprobability theory is Markovianity, which prescribes conditional
independence between the random variables. A multivariate
probability distribution satisfying Markov property is called
the Markov network. The celebrated Hammersley-Clifford
theorem connects the classical Markovianity to the manybody physics, which guarantees the equivalence of Markov
networks and Gibbs distributions. Whether a quantum versionof this holds is called the quantum Markov conjecture, which
is mathematically described as “the exponential decay of theconditional mutual information in an arbitrary quantum Gibbs
state.” In this study, we provide proof of the exponential decayof the mutual information content among small subsystems.
This result shows in general that all quantum Gibbs states areat least Pairwise Markov networks. This result was accepted
for a Long Plenary Talk at Quantum Information Processing
2024 (QIP2024). Only 4 submissions are chosen from over1000 submissions (<0.4%).

We prove that quantum states at finite temperature generally
satisfy quantum Markov property. In other words, we show thatthe conditional mutual information decays exponentially with the
distance. Previous results only prove this for high temperature
regimes above a temperature threshold, and we aim to extend thisto all temperature regimes.

Clustering theorem in 1D long-range interacting systems at arbitrary tempera
One of the most fundamental theorems in statistical mechanics
is the absence of phase transitions in one-dimensional
systems. The extension to quantum systems was achieved ina famous 1969 paper by Hujihiro Araki, but this result imposes
the condition that the interaction length is finite. Recently,
an extension of this result to long-range interacting systems
was discussed, but it became clear that there exists anessential problem of divergence of the complex Lieb-Robinson
bound. The absence of phase transitions in long-range
interacting systems has been a fundamental open question.
In collaboration with Yusuke Kimura of our team, we tackled
this problem and succeeded for the first time in extending the
decay theorem (Clustering theorem) for two-body correlations.
On the other hand, to solve the non-existence of phasetransitions in long-range interacting systems, it is necessary
to investigate the zero point of the partition function, and weplan to continue our analysis in the next fiscal year and beyond.
This work was selected as an Outstanding poster at Theoryof Quantum Computation, Communication and Cryptography2024 (TQC2024).

The zero point of the partition function characterises quantum manybody systems with finite temperatures. The complex zero points
means that the partition function becomes zero when the inverse
temperature is extended to complex planes, and it is known thatthe zero-free region has smaller complexity in the computer science
sense. We aim to establish a universal theorem on the distribution
structure of zero points, in particular, the non-existence of phasetransitions in one-dimensional long-range interacting systems.

Core members(Research Scientist) Yusuke Kimura(Postdoctoral Researcher) Donghoon Kim(Postdoctoral Researcher) Tan Van Vu(RIKEN JRA student) Shang Cheng(Part Timer) Hideaki Nishikawa

49

Theory

Mathematical Quantum Information RIKEN Hakubi Research Team
Keywords: Quantum information, Quantum resource theories, Quantum Shannon theory, Mathematical physics

Research Outline
Our group studies the mathematical underpinnings of quantum information theory, with a particular focus on the investigationof the mathematical structure of quantum resources — physical phenomena that underlie practical advantages of quantum
technologies in areas such as communication and computation.
We aim to develop technical frameworks that help address the fundamental questions of how to quantify, manipulate, andtake advantage of physical resources in quantum information and communication tasks. Our approach is to establish a solidand rigorous mathematical foundation which can be directly
used to study a variety of physical settings, allowing for broad
applications and generalisations. In addition to advancing thefrontiers of knowledge on the fundamental laws governing
quantum systems, we hope to provide insight into the physicallyachievable limits of the advantages of quantum resources,
which can then find use in benchmarking practical quantum
technologies. We will directly apply our methods to shed lighton the properties of resources such as quantum entanglement,
quantum coherence, magic-state quantum computation, as
well as the dynamical quantum resources of quantum channels.
Beyond that, we are broadly interested in the mathematicalproblems of quantum information theory, e.g. the properties
and applications of entropic quantities, the characterisation
There are many different quantum resources that can fuel theof operational tasks such as quantum hypothesis testing, andpractical applications of quantum technologies. Understanding
convex optimisation problems, which can be encountered intheir properties is thus an important problem, but their complex
almost every area of quantum information.
mathematical structure often makes this very difficult. (Schematic
figure adapted from Phys. Rev. X 9, 031053 (2019).)

Bartosz Regula (Ph.D.), RIKEN Hakubi Team Leader
Selected Publications
1 B. Regula and L. Lami, “Reversibility of quantum resources through probabilistic protocols”, Nat. Commun. 15,3096 (2024)
2 L. Lami and B. Regula, “No second law of entanglement manipulation after all”, Nat. Phys. 19, 184–189 (2023)
3 B. Regula, “Probabilistic transformations of quantum resources”, Phys. Rev. Lett. 128, 110505 (2022)
4 B. Regula and R. Takagi, “Fundamental limitations on distillation of quantum channel resources”, Nat. Commun.
12, 4411 (2021)
5 J. R. Seddon, B. Regula, H. Pashayan, Y. Ouyang, and E. T. Campbell, “Quantifying quantum speedups:
improved classical simulation from tighter magic monotones”, PRX Quantum 2, 010345 (2021)

Brief resume
2018
2019
2021
2023

50

Ph.D. Mathematics, University of Nottingham, UK
Presidential Postdoctoral Fellow, Nanyang Technological University, Singapore
JSPS Postdoctoral Research Fellow, University of Tokyo, JapanRIKEN Hakubi Team Leader, Mathematical Quantum Information RIKEN Hakubi Research Team, RIKEN,
Japan (-present)Recent Achievements
A probabilistic “second law” of quantum resources
The second law of thermodynamics is a fundamental law of physics. It tells us that the thermodynamic transformations ofphysical systems are governed by a single function: the entropy. We show that a similar result can be obtained not only inthermodynamics, but also for general quantum phenomena that find use as resources in physical settings — in particular, forquantum entanglement, whose dynamics are notoriously hard to characterise exactly.
We showed that the transformations of quantum systems can be described by a single entropic measure of resources, and alltransformations can be made asymptotically reversible in a suitable probabilistic setting. The result simplifies the characterisationof the transformations of quantum resources and the understanding of their limitations. The price to pay in our approach is thatreversibility, and thus the second law, is only achievable with some probability.

Schematic illustration of probabilistic reversibility of quantum entanglement. Transformations
between many copies of entangled quantum states (here represented by red and green
shapes) are made reversible by using probabilistic protocols, with the transformation rate
given by the regularised relative entropy of entanglement. This leads to a consistent andunique notion of an entropic measure of entanglement.
©B. Regula and L. Lami, “Reversibility of quantum resources through probabilistic protocols”,
Nature Communications 15, 3096 (2024)

Virtual distillation of quantum resources
Distillation (purification) is central to the practical use of quantum resources in noisy settings often encountered in quantum
communication and computation. Conventionally, distillation requires using some restricted 'free' operations to convert a noisy
state into one that approximates a desired pure state. We propose to relax this setting by only requiring the approximation of themeasurement statistics of a target pure state. This allows us to integrate the power of classical postprocessing to enhance thecapabilities of free quantum operations.
We show that this extended scenario of virtual resource distillation provides considerable advantages over standard
approaches, allowing for the purification of noisy states from which no resources can be distilled conventionally. Our methods
can be applied to various settings such as the theories of entanglement, coherence, or non-stabiliser quantum computation.
⇤

d

(a)

d0

Tr " d 0

⇡

d

⇤

"

d0
d

k ⌦<

00

Tr " d 0"_+_˜Tr " ⇤(d)Tr " d 00"

Tr "k ⌦<

k ⌦<

⇡

"

(b)

⇤+

"

Tr "k ⌦<

Comparison of conventional and virtual resource distillation. (a) Conventional resource distillation employs a free operation Λ to map a noisy stateρ into a state that approximates (many copies of) the target state ψ. (b) Virtual distillation approximates the measurement outcomes of copies ofψ by using a virtual operation, that is, a linear combination of free operations.
©X. Yuan, B. Regula, R. Takagi, and M. Gu, “Virtual quantum resource distillation”, Physical Review Letters 132, 050203 (2024)

Core members(Postdoctoral Researcher) Hayato Arai

51

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 Y. Akahoshi, K. Maruyama, H. Oshima, S. Sato, K. Fujii, “Partially Fault-tolerant Quantum Computing
Architecture with Error-corrected Clifford Gates and Space-time Efficient Analog Rotations,” PRX Quantum 5,010337 (2024)
2 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)
3 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)
4 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)
5 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)

Brief resume
1990
1990
2001
2001
2002
2006
2007
2010
2014
2018
2018
2020
2021
2021
2022
2023
*Director is Dr. Yasunobu Nakamura

52

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
Fellow, Head of Quantum Laboratory, Fujitsu Research, Fujitsu Limited (-present)Recent Achievements
Development of superconducting quantum computer, paving the way for platform forhybrid quantum computing
Our group developed a new 64 qubit superconducting quantum computer at the RIKEN RQC-Fujitsu Collaboration Center. The
new quantum computer leverages the technology developed by RIKEN and a consortium of joint research partners including
Fujitsu for Japan’s first superconducting quantum computer, which was first revealed to the public in March 2023. Accompanying
this announcement, we further revealed the launch of a platform for hybrid quantum computing, which combines the computing
power of the newly developed 64 qubit superconducting quantum computer with one of the world’s largest 40 qubit quantum
computer simulators developed by Fujitsu. Fujitsu and RIKEN provided the new platform to companies and research institutions
that are conducting joint research with Fujitsu and RIKEN from October 5, 2023. The new hybrid platform enables easy comparisonof calculation results of noisy intermediate-scale quantum (NISQ) computers against error-free results from quantum simulators,
contributing to accelerated research in areas including performance evaluation of error mitigation algorithms in quantum applications.

Quantum computer developed at the RIKEN
RQC-Fujitsu Collaboration Center

Overview of the new platform for hybrid quantum computing

Improvement of Characterization Method toward Achievement of Quantum
Operations with Higher Accuracy
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 research toward improving the efficiency and reliability of acharacterization method class, called “Quantum Tomography”. Specifically, we focus on reduction of experimental and numericalcosts of a method called “Gate-Set Tomography” (GST).
Analytical method of error amplification circuits plays an important role at cost reduction of GST. We developed a new
tomographic method for quantum gates, which is a cost reduction of GST, but it had a problem that it was not applicable toan important class of quantum gates, 180-degree-rotation gates. Recently, we have improved the analytical method of error
amplification, and the cost-reduced method has become applicable to 180-degree-rotation gates.

Conceptual diagram of an error amplification quantum circuit

53Administration

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

OutlineWe are responsible for the management of the Quantum Computing Research Center’s overall operations, including research ontechnology, intellectual property, and standardization trends, information dissemination, and the organization of events.
Based on the government’s Quantum Technology Innovation Strategy, RIKEN is positioned as the core organizationthat coordinates the 11 domestic quantum technology innovation centers and their activities from basic research to social
implementation of quantum technology. We are responsible for the management of this core organization. Furthermore, as thehead quarter of Flagship Program for the Q-LEAP by MEXT, we
are engaged in promotion activities for the quantum technology.
We are also in charge of the operation and management of
the superconducting quantum computer “A,” which has been
opened to the public in March 2023.
I am also involved in the following research and development.
• A project leader of “Building and operation of a domesticallydeveloped quantum computer testbed environment” on
CSTI-SIP program “Promoting the application of advanced
quantum technology platforms to social issues”.
• A project leader of “Design and fabrication of hybrid chipsof qubits and peripheral electronics” on “Development ofIntegration Technologies for Superconducting Quantum
Circuits” of Moonshot Goal6 projects.
Members of Office of the Center Director

Core members(Coordinator) Toshio Tonouchi(Research Administrator) Koji Ikado(Research Administrator) Satoshi Tomita(Research Administrative Support Staff) Kimiko Kowashi(Temporary Employee) Masanobu Arai
(Temporary Employee) Haruyuki Iwabuchi(Administrative Part-time Worker I) Minako Yamaguchi

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
2015
2018
2019
2021

54

Ph.D. in Applied Physics, The University Tokyo
Researcher, Fundamental Research Laboratories, NEC Corporation
Visiting Researcher, State University of New York at Stony Brook
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 / Features

Simplifying quantum computation
A novel protocol for quantum computers could reproduce the complex dynamics of quantum materials
Category: Exploratory Physical Sciences Field: Quantum computersAquantum-computational algorithm that could be used
to efficiently and accurately calculate atomic-levelinteractions in complex materials has been developed by RIKEN
researchers1. It has the potential to bring an unprecedented
level of understanding to condensed-matter physics andquantum chemistry—an application of quantum computers
first proposed by brilliant physicist Richard Feynman in 1981.
Quantum computers bring the promise of enhanced
number-crunching power and the ability to crack problems thatare out of the reach of conventional computers.
Qubits, the building blocks of quantum computers, areessentially tiny systems—nanocrystals or superconducting
circuits, for example—governed by the laws of quantum
physics. Unlike bits used in conventional computers, which
can be either one or zero, qubits can have multiple values
simultaneously. It is this property of qubits that gives quantum
computers their advantage in terms of speed.
An unconventional way of computation also requires a
new perspective on how to efficiently process data in order totackle problems too difficult for conventional computers.
One notable example of this is the so-called time-evolution
operator. “Time-evolution operators are huge grids of numbersthat describe the complex behaviors of quantum materials,”
explains Kaoru Mizuta of the RIKEN Center for Quantum
Computing. “They’re of great importance because they givequantum computers a very practical application—better
understanding quantum chemistry and the physics of solids.”
The prototype quantum computers demonstrated to datehave achieved time-evolution operators using a relatively simple
technique called Trotterization. But Trotterization is thought tobe unsuitable for the quantum computers of the future because
it requires a huge number of quantum gates and thus a lotof computational time. Consequently, researchers have been
striving to create quantum algorithms for accurate quantum
simulations that use fewer quantum gates.
Now, Mizuta, working with colleagues from across Japan,
has proposed a much more efficient and practical algorithm. A
hybrid of quantum and classical methods, it can compile timeevolution operators at a lower computational cost, enabling
it to be executed on small quantum computers, or evenconventional ones.
“We have established a new protocol for constructing
quantum circuits that efficiently and accurately reproduce timeevolution operators on quantum computers,” explains Mizuta.
“By combining small quantum algorithms with the fundamental
laws of quantum dynamics, our protocol succeeds in designing
quantum circuits for replicating large-scale quantum materials,
but with simpler quantum computers.”

Figure: An illustration showing the two states of a cuprate hightemperature superconductor. A new protocol for constructing
quantum circuits could help with calculations on quantum materials
such as superconductors.
© US DEPARTMENT OF ENERGY/SCIENCE PHOTO LIBRARY[https://www.sciencephoto.com/media/1075086/view]

Mizuta and his team next intend to clarify how the timeevolution operators optimized by their method can be applied
to various quantum algorithms that can compute the propertiesof quantum materials. “We anticipate that this work will
demonstrate the potential of using smaller quantum computers
to study physics and chemistry.”

Related content:
Quantum computing and deep learning could help solve themysteries of quantum gravity[https://www.riken.jp/en/news_pubs/research_news/
rr/20220411_1/index.html]
Scientists achieve key elements for fault-tolerant quantum
computation in silicon spin qubits[https://www.riken.jp/en/news_pubs/research_news/
pr/2022/20220120_1/index.html]
Quantum entanglement of three spin qubits demonstrated
in silicon[https://www.riken.jp/en/news_pubs/research_news/
rr/20210903_1/index.html]
Reference
1. Mizuta, K., Nakagawa, Y. O., Mitarai, K. & Fujii, K.
Local variational quantum compilation of large-scale
Hamiltonian dynamics. PRX Quantum 3, 040302 (2022).
(doi: 10.1103/PRXQuantum.3.040302)
RIKEN Research Summer 2023 (p.21)
These articles are edited versions of RIKEN Research Highlights.

55

Research Highlights / Features

Feedback system could fix quantum-computing errors
Quantum circuit can reset quantum bits carried by electron spins in silicon
Category: Applied Physical Sciences Field: Quantum computingA

method to reset the data held in silicon-based,
quantum-computing devices has been developed by
RIKEN researchers1. This could help to enhance the reliabilityof quantum computers by forming a crucial part of an errorcorrection system.
Quantum computers promise to dramatically increase
our computing power, but these highly sensitive devices areprone to errors.
Data in quantum computers is encoded by the quantum
states of particles, such as the spin of an electron. These
quantum bits (qubits) can be linked together through aphenomenon called entanglement, and then process
information through a combination, or superposition, of
their quantum states. This enables quantum computers to
perform certain complex calculations much more rapidly thanconventional computers.
Electrical confinement in silicon formed by metallic
electrodes, known as quantum dots, is a promising candidate
to host an electron-spin qubit. But quantum states can bedelicate, and the qubits often acquire errors that need to becorrected.
To help address this problem, RIKEN researchers havedevised a way to rapidly and accurately read a spin qubit,
and then reset it. This kind of control system will be essential
to build fault-tolerant quantum computers based on siliconquantum dots.
One challenge the team faced is that a qubit loses
correlation between the readout result and the qubit state
after reading. To overcome this, the team used a pair ofsilicon quantum dots to accommodate two entangled qubits.
While one qubit carried data, the other served as an auxiliary.
By using a charge sensor to read the auxiliary qubit, theresearchers could estimate the state of the data qubit without
destroying its information—a method known as a quantum
non-demolition (QND) measurement. Depending on theoutcome of the measurement, the system could then delivera microwave pulse that resets the data qubit.
The team constructed a quantum circuit to repeatedly
perform the QND measurements, producing a single
microwave pulse to reset the data qubit with cumulatively
improved reliability. “Since the quantity measured by aquantum non-demolition measurement is not disturbed by
the measurement, it can be measured again and again toestimate the data-qubit state more accurately,” says Takashi
Kobayashi of the RIKEN Center for Quantum Computing.
The whole reset process currently takes about 60
microseconds at least, which might be too slow for a practicalquantum computer. To shorten this time, the researchers

56

suggest adding a second auxiliary qubit, which would allow
them to use a faster measurement method.
“The time for the protocol might be reduced to a few
microseconds,” says Kobayashi. “This would pave the way tofeedback-based quantum error correction.”

Related content:
Researchers demonstrate error correction in a silicon qubit system
https://www.riken.jp/en/news_pubs/research_news/
pr/2022/20220825_1/index.html
Two-electron qubit points the way to scaling up quantum
computers
https://www.riken.jp/en/news_pubs/research_news/
rr/20200619_1/
Scientists succeed in measuring electron spin qubit without
demolishing it
https://www.riken.jp/en/news_pubs/research_news/
pr/2020/20200303_2/
Faster technique for resetting quantum circuits proposed
https://www.riken.jp/en/news_pubs/research_news/
rr/20220121_1/index.html

Conceptual illustration of a computer microchip with a grid ofquantum spin states. RIKEN researchers have shown how aquantum circuit can reset quantum bits carried by electron spins insilicon.
https://www.sciencephoto.com/media/948603/view© ELLA MARU STUDIO/SCIENCE PHOTO LIBRARY

Reference
1. Kobayashi, T., Nakajima, T., Takeda, K., Noiri, A.,
Yoneda, J. & Tarucha, S. Feedback-based active resetof a spin qubit in silicon. npj Quantum Information 9, 52(2023). doi:10.1038/s41534-023-00719-3
RIKEN Research Winter 2023 (p.11)
These articles are edited versions of RIKEN Research Highlights.

Machine learning realizes better quantum error
correction
Harnessing machine learning helps to correct errors that develop in quantum computers
Category: Applied Physical Sciences Field: Quantum computingIn a step toward realizing practical quantum computers,
RIKEN researchers have demonstrated that machinelearning can be used to perform error correction for quantum
computers1.
Classical computers perform operations using bits, which
can take the values 0 and 1. In contrast, quantum computers
use qubits, which can assume any superposition of these
two states. When combined with quantum entanglement—aquantum way of linking distant objects—qubits enable
quantum computers to perform entirely new operations.
This gives them a potential advantage over conventional
computers for some tasks, such as large-scale searches,
optimization and cryptography.
The main hurdle to realizing quantum computers forpractical applications is the extremely fragile nature of quantum
superpositions. Tiny perturbations from the environment cangenerate errors that rapidly destroy quantum superpositions.
To tackle this problem, sophisticated methods forquantum error correction have been developed. While theycan, in theory, neutralize the effect of errors, they often greatly
increase device complexity, which itself is error prone and thus
potentially even increases exposure to errors. Consequently,
full-fledged error correction has remained elusive.
Now, a team led by Franco Nori of the RIKEN Center for
Quantum Computing (RQC) has leveraged machine learning
in a search for error-correction schemes that minimize thedevice overhead while maintaining good error-correction
performance.
The team adopted an autonomous approach toquantum error correction, where a cleverly designed,
artificial environment replaces the need to perform frequent
measurements to detect errors.
They also looked at ‘bosonic qubit encodings’, whichare, for instance, available and utilized in some of the currently
most promising and widespread quantum-computing
machines based on superconducting circuits.
Finding high-performing candidates in the vast search
space of bosonic qubit encodings was a complex optimization
task. The team addressed this problem by using reinforcement
learning, an advanced machine learning method in which anagent explores a possibly abstract environment to learn andoptimize its action policy.
Using this approach, the team found a surprisingly
simple, approximate qubit encoding that could not only greatly
reduce the device complexity compared to other proposed
encodings, but that also outperformed its competitors interms of its ability to correct errors.
“Our work not only demonstrates the potential for
deploying machine learning towards quantum error correction,
but it may also bring us a step closer to the successful
implementation of quantum error correction in experiments,”
says Yexiong Zeng, also of the RQC.
“Machine learning can play a pivotal role in addressing
large-scale quantum computation and optimization
challenges,” says Nori. “Currently, we are actively involved ina number of projects that integrate machine learning, artificial
neural networks, quantum error correction, and quantum fault
tolerance.”

Reference
1. Zeng, Y., Zhou, Z.-Y., Rinaldi, E., Gneiting, C. & Nori,
F. Approximate autonomous quantum error correction
with reinforcement learning. Physical Review Letters 131,
050601 (2023).
RIKEN Research Winter 2023 (p.19)

These articles are edited versions of RIKEN Research Highlights.

57

Research Highlights / Features

Quantum computers start to measure up
An alternative approach to quantum computers that stores and processes information as light
could enable reprogrammable systems.
Category: Applied Physical Sciences Field: Quantum computingT

he race to develop quantum computers has really heatedup over the past few years. State-of-the-art systems
can now run simple algorithms using dozens of qubits—orquantum bits—which are the building blocks of quantum
computers.
Much of this success has been achieved in so-calledgate-based quantum computers. These computers use
physical components, most notably superconducting circuits,
to host and control the qubits. This approach is quite similar
to conventional, device-based classical computers. The
two computing architectures are thus relatively compatible
and could be used together. Furthermore, future quantum
computers could be fabricated by harnessing the technologies
used to fabricate conventional computers.
But the Optical Quantum Computing Research Team at
the RIKEN Center for Quantum Computing has been taking
a very different approach. Instead of optimizing gate-based
quantum computers, Atsushi Sakaguchi, Jun-ichi Yoshikawa
and Team Leader Akira Furusawa have been developing
measurement-based quantum computing.

Atsushi Sakaguchi and his team are exploring the possibility of using
light to produce quantum computers that are measurement based
rather than gate based.

MEASUREMENT-BASED COMPUTING
Measurement-based quantum computers process information
in a complex quantum state known as a cluster state, which
consists of three (or more) qubits linked together by a nonclassical phenomenon called entanglement. Entanglement iswhen the properties of two or more quantum particles remain
linked, even when separated by vast distances.
Measurement-based quantum computers work bymaking a measurement on the first qubit in the cluster
state. The outcome of this measurement determines what
measurement to perform on the second entangled qubit,a process called feedforward. This then determines how tomeasure the third. In this way, any quantum gate or circuit canbe implemented through the appropriate choice of the seriesof measurements.
Measurement-based schemes are very efficient whenused on optical quantum computers, since it’s easy to entangle
a large number of quantum states in an optical system. This
makes a measurement-based quantum computer potentiallymore scalable than a gate-based quantum computer. For
the latter, qubits need to be precisely fabricated and tuned
for uniformity and physically connected to each other. Theseissues are automatically solved by using a measurementbased optical quantum computer.
Importantly, measurement-based quantum computation
offers programmability in optical systems. “We can change
58

Gate-based quantum computers are becoming more common.
But the Optical Quantum Computing Research Team at the
RIKEN Center for Quantum Computing has been developing
measurementbased quantum computing, with digital circuitry forelectrical-optical control (pictured). Measurementbased systems arepotentially more scaleable than gatebased quantum computing.

the operation by just changing the measurement,” says
Sakaguchi. “This is much easier than changing the hardware,
as gated-based systems require in optical systems.”
But feedforward is essential. “Feedforward is a control
methodology in which we feed the measurement results toa different part of the system as a form of control,” explains
Sakaguchi. “In measurement-based quantum computation,
feedforward is used to compensate for the inherent
randomness in quantum measurements. Without feedforward
operations, measurement-based quantum computation
becomes probabilistic, while practical quantum computing
will need to be deterministic.”
The Optical Quantum Computing Research Teamand their co-workers—from The University of Tokyo,
Palacký University in the Czech Republic, the Australian
National University and the University of New South Wales,
Australia—have now demonstrated a more advanced formof feedforward: nonlinear feedforward. Nonlinear feedforward
is required to implement the full range of potential gates in
optics-based quantum computers.
“We’ve now experimentally demonstrated nonlinear
quadrature measurement using a new nonlinear
feedforward technology,” explains Sakaguchi. “This typeof measurement had previously been a barrier to realizing
universal quantum operations in optical measurementbased quantum computation.”“But the key message is that, although superconducting
circuit-based approaches may be more popular, opticalsystems are a promising candidate for quantum-computer
hardware,” he adds.
Reference
For a full list of references, check the online version of thisarticle: www.riken.jp/en/news_pubs/research_news/
RIKEN Research Winter 2023 (p.26)

OPTICAL COMPUTERS
Optical quantum computers use qubits made of wave
packets of light. At another institution, some of the current
RIKEN team had previously constructed the large opticalcluster states needed for measurement-based quantum
computation. Linear feedforward has also been achieved toconstruct simple gate operations, but more advanced gatesneed nonlinear feedforward.
A theory for practical implementation of nonlinear
quadrature measurement was proposed in 2016. But thisapproach presented two major practical difficulties: generatinga special ancillary state (which the team achieved in 2021)
and performing a nonlinear feedforward operation.
The team overcame the latter challenge with complex
optics, special electro-optic materials and ultrafast electronics.
To do this they exploited digital memories, in which the desired
nonlinear functions were precomputed and recorded in thememory. “After the measurement, we transformed the opticalsignal into an electrical one,” explains Sakaguchi. “In linear
feedforward, we just amplify or attenuate that signal, but weneeded to do much more complex processing for nonlinear
feedforward.”
The key advantages of this nonlinear feedforward
technique are its speed and flexibility. The process needs tobe fast enough that the output can be synchronized with theoptical quantum state.
“Now that we have shown that we can perform nonlinear
feedforward, we want to apply it to actual measurementbased quantum computation and quantum error correction
using our previously developed system,” says Sakaguchi.
“And we hope to be able to increase the higher speed of
our nonlinear feedforward for high-speed optical quantum
computation.”

This feature looks at the work of ATSUSHISAKAGUCHI
Atsushi Sakaguchi completed his PhD under the supervision of
Akira Furusawa at the University of Tokyo in 2021. He then joined
his current team, the Optical Quantum Computing Research Teamat the RIKEN Center for Quantum Computing. Here he works on
experimental quantum optics and quantum information research.
He is currently collaborating on research into classical operations forquantum computing.
These articles are edited versions of RIKEN Research Highlights.

59

Publication List
Data for FY2023
Title

Authors

ArticleBlack hole graviton and quantum gravity

Y. KimuraPhysica Scripta 99, 045024 (2024).

Effective light cone and digital quantum simulation ofinteracting bosons

T. Kuwahara, T. V. Vu, and K. Saito

Nature Communications 15, 2520
(2024).

Kekule valence bond order in the Hubbard model on thehoneycomb lattice with possible lattice distortions forgrapheneY. Otsuka and S. Yunoki

Physical Review B 109, 115131(2024).

Quantum computing using floating electrons on cryogenic
substrates: Potential And Challenges

A. Jennings, I. Grytsenko, and E. Kawakami

Applied Physics Letters Perspective
124, 120501 (2024).

Realizing quantum optics in structured environments withgiant atoms

X. Wang, H. B. Zhu, T. Liu, and F. Nori

Physical Review Research 6, 013279(2024).

Emergent parallel transport and curvature in Hermitian and
C. Y. Ju, A. Miranowicz, Y. N. Chen, G. Y. Chen, and F. Nori
non-Hermitian quantum mechanics

Quantum 8, 1277 (2024).

Passive magnetic-free broadband optical isolator basedon unidirectional self-induced transparency

H. Wu, J. Tang, M. Chen, M. Xiao, Y. Lu, K. Xia, and F. Nori

Optics Express 32, 11010 (2024).

Proposal of ensemble qubits with two-atom decay

W. Qin, A. Miranowicz, and F. Nori

New Journal of Physics 26, 033006(2024).

Suppression of pulsed dynamic nuclear polarization by
many-body spin dynamics

K. Sasaki and E. Abe

Physical Review Letters 132, 106904(2024).

Partially fault-tolerant quantum computing architecturewith error-corrected Clifford gates and space-time efficient Y. Akahoshi, K. Maruyama, H. Oshima, S. Sato, and K. Fujii
analog rotations

PRX Quantum 5, 010337 (2024).

Control and readout of a transmon using a compact
superconducting resonator

J. Zotova, S. Sanduleanu, G. Fedorov, R. Wang, J. S. Tsai,and O. Astafiev

Applied Physics Letters 124, 102601(2024).

Experimental demonstration of a high-fidelity virtual twoqubit gate

A. P. Singh, K. Mitarai, Y. Suzuki, K. Heya, Y. Tabuchi, K. Fujii,
and Y. Nakamura

Physical Review Research 6, 013235(2024).

Dissipation, quantum coherence, and asymmetry of finitetime cross-correlations

T. V. Vu, V. T. Vo, and K. Saito

Physical Review Research 6, 013273(2024).

Topological temporally mode-locked laser

C. R. Leefmans, M. Parto, J. Williams, G. H. Y. Li, A. Dutt, F.
Nori, and A. Marandi

Nature Physics 20, 852 (2024).

Rapid single-shot parity spin readout in a silicon double
quantum dot with fidelity exceeding 99%

K. Takeda, A. Noiri, T. Nakajima, L. C. Camenzind, T.
Kobayashi, A. Sammak, G. Scappucci, and S. Tarucha

npj Quantum Information 10, 22(2024).

Tuning the initial phase to control the final state of a driven
qubit

P. O. Kofman, S. N. Shevchenko, and F. Nori

Physical Review A 109, 022409(2024).

Virtual quantum resource distillation

X. Yuan, B. Regula, R. Takagi, and M. Gu

Physical Review Letters 132, 050203(2024).

Tunable compact on-chip superconducting switch

J. Zotova, A. Semenov, R. Wang, Y. Zhou, O. Astafiev, andJ. S. Tsai

Physical Review Applied 21, 024059(2024).

Virtual quantum resource distillation: General framework
and applications

R. Takagi, X. Yuan, B. Regula, and M. Gu

Physical Review A 109, 022403(2024).

Water-wave vortices and skyrmions

D. A. Smirnova, F. Nori, and K. Y. Bliokh

Physical Review Letters 132, 054003(2024).

Quantum phase transitions in optomechanical systems

B. Wang, F. Nori, and Z. L. Xiang

Physical Review Letters 132, 053601(2024).

Identifying topology of leaky photonic lattices with machine
E. Smolina, L. Smirnov, D. Leykam, F. Nori, and D. Smirnova
learning

Nanophotonics 13, 271 (2024).

Photon-noise-tolerant dispersive readout of asuperconducting qubit using a nonlinear Purcell filter

Y. Sunada, K. Yuki, Z. Wang, T. Miyamura, J. Ilves, K.
Matsuura, P. A. Spring, S. Tamate, S. Kono, and Y. Nakamura

PRX Quantum 5, 010307 (2024).

Logical states for fault-tolerant quantum computation withpropagating light

S. Konno, W. Asavanant, F. Hanamura, H. Nagayoshi, K.
Fukui, A. Sakaguchi, R. Ide, F. China, M. Yabuno, S. Miki, H.
Terai, K. Takase, M. Endo, P. Marek, R. Filip, P. van Loock,and A. Furusawa

Science 383, 289 (2024).

ZZ-interaction-free single-qubit-gate optimization insuperconducting qubits

S. Watanabe, Y. Tabuchi, K. Heya, S. Tamate, and Y.
Nakamura

Physical Review A 109, 012616(2024).

Hamiltonian phase error in resonantly driven CNOT gate
above the fault-tolerant threshold

Y. H. Wu, L. Camenzind, A. Noiri, K. Takeda, T. Nakajima, T.
Kobayashi, C. Y. Chang, A. Sammak, G. Scappucci, H. S.
Goan, and S. Tarucha

npj Quantum Information 10, 8 (2024).

Sudden change of the photon output field marks phasetransitions in the quantum Rabi model

Y. H. Chen, Y. Qiu, A. Miranowicz, N. Lambert, W. Qin, R.
Stassi, Y. Xia, S. B. Zheng, and F. Nori

Communications Physics 7, 5 (2024).

D. Iyama, T. Kamiya, S. Fujii, H. Mukai, Y. Zhou, T. Nagase,Observation and manipulation of quantum interference in a
A. Tomonaga, R. Wang, J. J. Xue, S. Watabe, S. Kwon, andsuperconducting Kerr parametric oscillator
J. S. TsaiNature Communications 15, 86(2024).

Quantifying quantum coherence of multiple-charge states
in tunable Josephson junctions

J. He, D. Pan, M. Liu, Z. Lyu, Z. Jia, G. Yang, S. Zhu, G. Liu,
J. Shen, S. N. Shevchenko, F. Nori, J. Zhao, L. Lu, and F. Qu

npj Quantum Information 10, 1 (2024).

Ising model formulation for highly accurate topologicalcolor codes decoding

Y. Takada, Y. Takeuchi, and K. Fujii

Physical Review Research 6, 013092(2024).

60

TitleAuthors

ArticleRecursive quantum eigenvalue and singular-valuetransformation: Analytic construction of matrix signfunction by Newton iteration

K. Mizuta and K. Fujii

Physical Review Research 6, L012007(2024).

Simulation and performance analysis of quantum error
correction with a rotated surface code under a realistic
noise model

M. Katsuda, K. Mitarai, and K. Fujii

Physical Review Research 6, 013024(2024).

Inter-temperature bandwidth reduction in cryogenic QAOA Y. Ueno, Y. Tomida, T. Tanimoto, M. Tanaka, Y. Tabuchi, K.
machinesInoue, and H. Nakamura

IEEE Computer Architecture Letters23, 9 (2024).

Reply to: Gauge non-invariance due to material truncation
in ultrastrong-coupling quantum electrodynamics

O. D. Stefano, A. Settineri, V. Macrì, L. Garziano, R. Stassi, S.
Savasta, and F. Nori

Nature Physics 20, 379 (2024).

The tangled state of quantum hypothesis testing

M. Berta, F. G. S. L. Brandão, G. Gour, L. Lami, M. B. Plenio,
B. Regula, and M. Tomamichel

Nature Physics 20, 172 (2024).

Quantum error correction with an Ising machine undercircuit-level noise

J. Fujisaki, K. Maruyama, H. Oshima, S. Sato, T. Sakashita, Y.
Takeuchi, and K. Fujii

Physical Review Research 5, 043261(2023).

Phase engineering of anomalous Josephson effect derived S. Matsuo, T. Imoto, T. Yokoyama, Y. Sato, T. Lindemann, S.
from Andreev molecules
Gronin, G. Gardner, M. Manfra, and S. Tarucha

Science Advances 9, eadj3698 (2023).

Phase-dependent Andreev molecules andsuperconducting gap closing in coherently-coupled
Josephson junctions

S. Matsuo, T. Imoto, T. Yokoyama, Y. Sato, T. Lindemann, S.
Gronin, G. Gardner, S. Nakosai, Y. Tanaka, M. Manfra, and S.
TaruchaNature Communications 14, 8271
(2023).

Single-shot single-mode optical two-parameter
displacement estimation beyond classical limit

F. Hanamura, W. Asavanant, S. Kikura, M. Mishima, S. Miki,
H. Terai, M. Yabuno, F. China, K. Fukui, M. Endo, and A.
Furusawa

Physical Review Letters 131, 230801(2023).

Classically optimized variational quantum eigensolver withapplications to topological phases

K. N. Okada, K. Osaki, K. Mitarai, and K. Fujii

Physical Review Research 5, 043217(2023).

Higher-order singularities in phase-tracked
electromechanical oscillators

X. Zhou, X. Ren, D. Xiao, J. Zhang, R. Huang, Z. Li, X. Sun, X. Nature Communications 14, 7944
Wu, C. W. Qiu, F. Nori, and H. Jing
(2023).

Quantum field heat engine powered by phonon-photon
interactions

A. Ferreri, V. Macrì, F. K. Wilhelm, F. Nori, and D. E. Bruschi

Physical Review Research 5, 043274(2023).

Universal bounds on the performance of informationthermodynamic engine

T. Tanogami, T. V. Vu, and K. Saito

Physical Review Research 5, 043280(2023).

Variational generation of spin squeezing on onedimensional quantum devices with nearest-neighborinteractions

Z. H. Sun, Y. Y. Wang, Y. R. Zhang, F. Nori, and H. Fan

Physical Review Research 5, 043285(2023).

A fully pipelined architecture of quantum-classical interface T. Miyoshi, K. Koike, S. Morisaka, H. Shiomi, R. Matsuda, K.
for realizing fault-tolerant quantum computer
Ogawa, Y. Tabuchi, and M. Negoro

2023 IEEE International Conference on
Quantum Computing and Engineering(QCE) 2, 322 (2023).

Kondo QED: The Kondo effect and photon trapping in a
P. C. Kuo, N. Lambert, M. Cirio, Y. T. Huang, F. Nori, and Y.
two-impurity Anderson model ultrastrongly coupled to light N. Chen

Physical Review Research 5, 043177(2023).

Observation of chiral-mode domains in a frustrated XY
model on optical triangular lattices

H. Ozawa, R. Yamamoto, and T. Fukuhara

Physical Review Research 5, L042026(2023).

Superconducting route to quantum computing

E. AbeProceeding of 2023 International
Conference on Simulation of
Semiconductor Processes andDevices (SISPAD) (2023).

Optimization of quantum noise in space gravitational-wave
K. Tsuji, T. Ishikawa, K. Komori, K. Nagano, Y. Enomoto, Y.
antenna DECIGO with optical-spring quantum locking
Michimura, K. Umemura, R. Shimizu, B. Wu, S. Iwaguchi, Y.
considering mixture of vacuum fluctuations in homodyne
Kawasaki, A. Furusawa, and S. Kawamura
detection

Galaxies 11, 111 (2023).

Spatial noise correlations beyond nearest neighbors in
Si/Si-Ge spin qubits

J. S. Rojas-Arias, A. Noiri, P. Stano, T. Nakajima, J. Yoneda,
K. Takeda, T. Kobayashi, A. Sammak, G. Scappucci, D. Loss,and S. Tarucha

Physical Review Applied 20, 054024(2023).

Blueprint for quantum computing using electrons on
helium

E. Kawakami, J. Chen, M. Benito, and D. Konstantinov

Physical Review Applied 20, 054022(2023).

Selecting active matter according to motility in an
acoustofluidic setup: self-propelled particles and sperm
cells

V. R. Misko, L. Baraban, D. Makarov, T. Huang, P. Gelin,
I. Mateizel, K. Wouters, N. De Munck, F. Nori, and W. De
MalscheSoft Matter 19, 8635 (2023).

Unveiling and veiling an entangled light-matter quantum
state from the vacuum

R. Stassi, M. Cirio, K. Funo, J. Puebla, N. Lambert, and F. Nori

Physical Review Research 5, 043095(2023).

Fully solvable finite simplex lattices with open boundaries
in arbitrary dimensions

I. I. Arkhipov, A. Miranowicz, F. Nori, Ş. K. Özdemir, and F.
Minganti

Physical Review Research 5, 043092(2023).

An efficient Julia framework for hierarchical equations of
motion in open quantum systems

Y. T. Huang, P. C. Kuo, N. Lambert, M. Cirio, S. Cross, S. L.
Yang, F. Nori, and Y. N. Chen

Communications Physics 6, 313(2023).

Fast generation of Schrödinger cat states using a Kerrtunable superconducting resonator

X. L. He, Y. Lu, D. Q. Bao, H. Xue, W. B. Jiang, Z. Wang, A. F.
Roudsari, P. Delsing, J. S. Tsai, and Z. R. Lin

Nature Communications 14, 6358
(2023).

Gottesman-Kitaev-Preskill qubit synthesizer for
propagating light

K. Takase, K. Fukui, A. Kawasaki, W. Asavanant, M. Endo, J.
Yoshikawa, P. van Loock, and A. Furusawa

npj Quantum Information 9, 98 (2023).

Noise-correlation spectrum for a pair of spin qubits insiliconJ. Yoneda, J. S. Rojas-Arias, P. Stano, K. Takeda, A. Noiri, T.
Nakajima, D. Loss, and S. Tarucha

Nature Physics 19, 1793 (2023).

28

61

Publication List (Continued)
Title

Authors

Fabrication of superconducting quantum circuits on
S. Odate, N. Saito, K. Suzuki, H. Aoyama, K. Kusuyama, Y.
12-inch wafers using an ArF immersion lithography system Nakamura, and H. Tsukamoto

ArticleProc. SPIE 12652, UV and Higher
Energy Photonics: From Materials to
Applications 2023, 1265205 (2023).

Continuous-variable quantum computation with opticalquantum entanglement and quantum teleportation in time
domainW. Asavanant, A. Kawasaki, R. Ide, T. Suzuki, H. Brunel, B.
Proc. SPIE 12692, Quantum
Charoensombutamon, A. Sakaguchi, K. Fukui, T. Kashiwazaki,
Communications and Quantum
A. Inoue, T. Umeki, K. Takase, J. Yoshikawa, M. Endo, and A.
Imaging XXI, 1269202 (2023).
Furusawa

Active initialization experiment of a superconducting qubit
using a quantum circuit refrigerator

T. Yoshioka, H. Mukai, A. Tomonaga, S. Takada, Y. Okazaki,
N. Kaneko, S. Nakamura, and J. S. Tsai

Physical Review Applied 20, 044077(2023).

Dynamical nuclear spin polarization in a quantum dot with
an electron spin driven by electric dipole spin resonanceP. Stano, T. Nakajima, A. Noiri, S. Tarucha, and D. Loss

Physical Review B 108, 155306(2023).

Squeezed quantum hybrid system: Giant enhancement of
magnon-phonon-spin interactions

F. Nori

Science China Physics, Mechanics &Astronomy 66, 110331 (2023).

Hybrid spin and anomalous spin-momentum locking insurface elastic waves

C. Yang, D. Zhang, J. Zhao, W. Gao, W. Yuan, Y. Long, Y.
Pan, H. Chen, F. Nori, K. Y. Bliokh, Z. Zhong, and J. Ren

Physical Review Letters 131, 136102(2023).

Observation of a superradiant phase transition withemergent cat states

R. H. Zheng, W. Ning, Y. H. Chen, J. H. Lü, L. T. Shen, K.
Xu, Y. R. Zhang, D. Xu, H. Li, Y. Xia, F. Wu, Z. B. Yang, A.
Miranowicz, N. Lambert, D. Zheng, H. Fan, F. Nori, and S. B.
Zheng

Physical Review Letters 131, 113601(2023).

Non-Hermitian waveguide cavity QED with tunable atomic
mirrors

W. Nie, T. Shi, Y. Liu, and F. Nori

Physical Review Letters 131, 103602(2023).

On a gap in the proof of the generalised quantum Stein's
lemma and its consequences for the reversibility ofquantum resources

M. Berta, F. G. S. L. Brandão, G. Gour, L. Lami, M. B. Plenio,
B. Regula, and M. Tomamichel

Quantum 7, 1103 (2023).

Simulating Chern insulators on a superconducting
quantum processor

Z. C. Xiang, K. Huang, Y. R. Zhang, T. Liu, Y. H. Shi, C. L.
Deng, T. Liu, H. Li, G. H. Liang, Z. Y. Mei, H. Yu, G. Xue, Y.
Tian, X. Song, Z. B. Liu, K. Xu, D. Zheng, F. Nori, and H. Fan

Nature Communications 14, 5433
(2023).

Non-Gaussian-state generation with time-gated photon
detection

T. Sonoyama, K. Takahashi, B. Charoensombutamon, S.
Takasu, K. Hattori, D. Fukuda, K. Fukui, K. Takase, W.
Asavanant, J. Yoshikawa, M. Endo, and A. Furusawa

Physical Review Research 5, 033156(2023).

Roadmap on structured waves

K. Y. Bliokh, E. Karimi, M. J. Padgett, M. A. Alonso, M. R.
Dennis, A. Dudley, A. Forbes, S. Zahedpour, S. W. Hancock,
H. M. Milchberg, S. Rotter, F. Nori, Ş. K. Özdemir, N. Bender,
H. Cao, P. B. Corkum, C. Hernández-García, H. Ren, Y.
Kivshar, M. G. Silveirinha, N. Engheta, A. Rauschenbeutel,
P. Schneeweiss, J. Volz, D. Leykam, D. A. Smirnova, K.
Rong, B. Wang, E. Hasman, M. F. Picardi, A. V. Zayats, F.
J. Rodríguez-Fortuño, C. Yang, J. Ren, A. B. Khanikaev, A.
Alù, E. Brasselet, M. Shats, J. Verbeeck, P. Schattschneider,
D. Sarenac, D. G. Cory, D. A. Pushin, M. Birk, A. Gorlach, I.
Kaminer, F. Cardano, L. Marrucci, M. Krenn, and F. Marquardt

Journal of Optics 25, 103001 (2023).

High-cooperativity cavity magnon-polariton using a high-Q
K. Kato, R. Sasaki, K. Matsuura, K. Usami, and Y. Nakamura
dielectric resonator

Journal of Applied Physics 134,
083901 (2023).

Quantum microwave parametric interferometer

F. Kronowetter, F. Fesquet, M. Renger, K. Honasoge, Y. Nojiri,
K. Inomata, Y. Nakamura, A. Marx, R. Gross, and K. G.
Fedorov

Physical Review Applied 20, 024049(2023).

Quantum simulation of topological zero modes on a
41-qubit superconducting processor

Y. H. Shi, Y. Liu, Y. R. Zhang, Z. Xiang, K. Huang, T, Liu, Y.
Y. Wang, J. C. Zhang, C. L. Deng, G. H. Liang, Z. Y. Mei, H.
Li, T. M. Li, W. G. Ma, H. T. Liu, C. T. Chen, T. Liu, Y. Tian, X.
Song, S. P. Zhao, K. Xu, D. Zheng, F. Nori, and H. Fan

Physical Review Letters 131, 080401(2023).

Coherent-cluster-state generation in networks ofdegenerate optical parametric oscillators

Z. Y. Zhou, C. Gneiting, J. Q. You, and F. Nori

Physical Review A 108, 023704(2023).

Quantum-classical decomposition of Gaussian quantum
environments: A stochastic pseudomode model

S. Luo, N. Lambert, P. Liang, and M. Cirio

PRX Quantum 4, 030316 (2023).

Approximate autonomous quantum error correction withreinforcement learning

Y. Zeng, Z. Y. Zhou, E. Rinaldi, C. Gneiting, and F. Nori

Physical Review Letters 131, 050601(2023).

Optimal and nearly optimal simulation of multiperiodic
time-dependent Hamiltonians

K. Mizuta

Physical Review Research 5, 033067(2023).

Postselected quantum hypothesis testing

B. Regula, L. Lami, and M. M. WildeIEEE Transactions on Information
Theory 70, 3453 (2023).

A. K. Yi, S. Ahn, Ç. Kutlu, J. Kim, B. R. Ko, B. I. Ivanov,
H. Byun, A. F. van Loo, S. Park, J. Jeong, O. Kwon, Y.
Search for the Sagittarius tidal stream of axion dark matter Nakamura, S. V. Uchaikin, J. Choi, S. Lee, M. Lee, Y. C. Shin,
J. Kim, D. Lee, D. Ahn, S. Bae, J. Lee, Y. Kim, V. Gkika, K.
around 4.55 μeV
W. Lee, S. Oh, T. Seong, D. Kim, W. Chung, A. Matlashov, S.
Youn, and Y. K. Semertzidis

Physical Review D 108, L021304(2023).

Key observable for linear thermalization

Physical Review Research 5, 033037(2023).

62

Y. Chiba and A. Shimizu

TitleAuthors

ArticleProbing the symmetry breaking of a light–matter system
by an ancillary qubit

S. P. Wang, A. Ridolfo, T. Li, S. Savasta, F. Nori, Y. Nakamura,and J. Q. You

Nature Communications 14, 4397
(2023).

Nonlinear feedforward enabling quantum computation

A. Sakaguchi, S. Konno, F. Hanamura, W. Asavanant, K.
Takase, H. Ogawa, P. Marek, R. Filip, J. Yoshikawa, E.
Huntington, H. Yonezawa, and A. Furusawa

Nature Communications 14, 3817
(2023).

Pseudofermion method for the exact description offermionic environments: From single-molecule electronics
to the Kondo resonance

M. Cirio, N. Lambert, P. Liang, P. C. Kuo, Y. N. Chen, P.
Menczel, K. Funo, and F. Nori

Physical Review Research 5, 033011(2023).

Generalized Dicke model and gauge-invariant masterequations for two atoms in ultrastrongly-coupled cavity
quantum electrodynamics

K. Akbari, W. Salmon, F. Nori, and S. Hughes

Physical Review Research 5, 033002(2023).

Exotic quantum light-matter interactions in bilayer squarelattices

X. L. Dong, P. B. Li, J. Q. Chen, F. L. Li, and F. Nori

Physical Review B 108, 045407(2023).

Fast classical simulation of Hamiltonian dynamics bysimultaneous diagonalization using Clifford transformation
with parallel computation

Y. Kawase and K. Fujii

Computer Physics Communications
288, 108720 (2023).

All-microwave manipulation of superconducting qubits
with a fixed-frequency transmon coupler

S. Shirai, Y. Okubo, K. Matsuura, A. Osada, Y. Nakamura, and Physical Review Letters 130, 260601
A. Noguchi(2023).

Over-8-dB squeezed light generation by a broadband
waveguide optical parametric amplifier toward faulttolerant ultra-fast quantum computers

T. Kashiwazaki, T. Yamashima, K. Enbutsu, T. Kazama, A.
Inoue, K. Fukui, M. Endo, T. Umeki, and A. Furusawa

Feedback-based active reset of a spin qubit in silicon

T. Kobayashi, T. Nakajima, K. Takeda, A. Noiri, J. Yoneda, andnpj Quantum Information 9, 52 (2023).
S. Tarucha

Coherent dynamics of a photon-dressed qubit

M. P. Liul, C. H. Chien, C. Y. Chen, P. Y. Wen, J. C. Chen, Y.
H. Lin, S. N. Shevchenko, F. Nori, and I. C. Hoi

Physical Review B 107, 195441(2023).

Quantum biology: An overview

N. Lambert and F. Nori

in Encyclopedia of Condensed Matter
Physics, 2nd Edition (Elsevier 2023),
pp. 577-583.

Low-loss polarization control in fiber systems for quantum
computation

T. Nakamura, T. Nomura, M. Endo, R. He, T. Kashiwazaki, T.
Umeki, J. Yoshikawa, and A. Furusawa

Optics Express 31, 19236 (2023).

CAPP axion search experiments with quantum noise
limited amplifiers

S. V. Uchaikin, B. I. Ivanov, J. Kim, Ç. Kutlu, A. F. van Loo, Y.
Nakamura, S. Oh, V. Gkika, A. Matlashov, W. Chung, and Y.
K. Semertzidis

JPS Conference Proceedings 38,
011201 (2023).

Four-channel system for characterization of Josephsonparametric amplifiers

B. I. Ivanov, J. Kim, Ç. Kutlu, A. F. van Loo, Y. Nakamura, S.
V. Uchaikin, S. Oh, V. Gkika, A. Matlashov, W. Chung, and Y.
K. Semertzidis

JPS Conference Proceedings 38,
011200 (2023).

Josephson parametric amplifier in axion experiments

J. Kim, B. I. Ivanov, Ç. Kutlu, S. Park, A. F. van Loo, Y.
Nakamura, S. V. Uchaikin, S. Oh, V. Gkika, A. Matlashov, W.
Chung, and Y. K. Semertzidis

JPS Conference Proceedings 38,
011199 (2023).

Systematic approach for tuning flux-driven Josephsonparametric amplifiers for stochastic small signals

Ç. Kutlu, S. Ahn, S. V. Uchaikin, S. Lee, A. F. van Loo, Y.
Nakamura, S. Oh, and Y. K. Semertzidis

JPS Conference Proceedings 38,
011202 (2023).

Novel Union-Find-based decoders for scalable quantum
error correction on systolic arrays

M. J. Heer, E. Del Sozzo, K. Fujii, and K. Sano

2023 IEEE International Parallel andDistributed Processing Symposium
Workshops (IPDPSW), 524 (2023).

Fermionic quantum approximate optimization algorithm

T. Yoshioka, K. Sasada, Y. Nakano, and K. Fujii

Physical Review Research 5, 023071(2023).

Subspace variational quantum simulator

K. Heya, K. M. Nakanishi, K. Mitarai, Z. Yan, K. Zuo, Y. Suzuki, Physical Review Research 5, 023078
T. Sugiyama, S. Tamate, Y. Tabuchi, K. Fujii, and Y. Nakamura (2023).

Spectral stabilizability

T. Linowski, Ł. Rudnicki, and C. Gneiting

Physical Review A 107, 042218(2023).

Dynamically crossing diabolic points while encircling
exceptional curves: A programmable symmetricasymmetric multimode switch

I. I. Arkhipov, A. Miranowicz, F. Minganti, Ş. K. Özdemir, and
F. NoriNature Communications 14, 2076
(2023).

Half-metal and other fractional metal phases in doped AB
bilayer graphene

A. L. Rakhmanov, A. V. Rozhkov, A. O. Sboychakov, and F.
Nori

Physical Review B 107, 155112(2023).

Elucidation of spin-correlations, Fermi surface andpseudogap in a copper oxide superconductor

H. Kamimura, M. Araidai, K. Ishida, S. Matsuno, H. Sakata, K.
Condensed Matter 8, 33 (2023).
Sasaoka, K. Shiraishi, O. Sugino, J. S. Tsai, and K. Yamada

Non-Gaussian quantum state generation by multi-photon
subtraction at the telecommunication wavelength

M. Endo, R. He, T. Sonoyama, K. Takahashi, T. Kashiwazaki,
T. Umeki, S. Takasu, K. Hattori, D. Fukuda, K. Fukui, K.
Takase, W. Asavanant, P. Marek, R. Filip, and A. Furusawa

Optics Express 31, 12865 (2023).

Compact superconducting microwave resonators basedon Al-AlOx-Al capacitors

J. Zotova, R. Wang, A. Semenov, Y. Zhou, I. Khrapach, A.
Tomonaga, O. Astafiev, and J. S. Tsai

Physical Review Applied 19, 044067(2023).

Universal noise-precision relations in variational quantum
algorithms

K. Ito, W. Mizukami, and K. Fujii

Physical Review Research 5, 023025(2023).

Applied Physics Letters 122, 234003(2023).

63

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