The 83rd JSAP Autumn Meeting 2022 Highlighted Presentations
October 24, 2022

Keynote Speech: From Spin Glass to Quantum Annealing

World’s first commercial quantum computer and theNobel Prize in 2021 both have their origins in spin glass
research
Hidetoshi Nishimori (Specially Appointed Professor, Tokyo Institute of Technology)

Summary• Research and development of quantum computers is a very active field, and the firstmachine built in Japan expected to appear soon.
• While expectations from the society are growing, concerns exist over the tendency toseek short-term results. Prof. Hidetoshi Nishimori, an advocate of quantum annealing,
points out the need to make efforts to overcome the “quantum chasm”.
There exist two models of quantum computers, the quantum gate model (*1) and quantum annealing (*2).
Professor Nishimori and Tadashi Kadowaki first proposed the principle of quantum annealing in1998, which
has since been implemented as a real device.
Prof. Nishimori wrote a paper with Dr, Kadowaki entitled “Quantum Annealing in the Transverse Isingmodel,” in which he applied quantum mechanics to the problem of spin glass, which he has been working on
since his graduate school days, to search for the optimal state. In addition to quantum annealing, the theoryof spin glasses also is used to study the limit of error correction in quantum computers based on the gatemodel. . Also studies of the theory of spin glasses led Prof Giorgio Parisi to win the Nobel Prize in Physics
in 2021.
Quantum computers are still in the early stage of development in spite of an excessive amount ofexpectations from the society. This imparity may lead to marked disappointments and supports from thesociety may dry up, which may be called a “quantum chasm”(*3). It is important to prepare a way out it. InJapan, active research and development of quantum science and technology have started relatively
recently , For a sustainable development, it is crucial to support basic research and education.

Detail

Spin Glass Research Leads to Quantum Computers and Nobel Prize
The world’s first commercial quantum computer was developed by D-Wave Systems, Inc.,a Canadian company, in 2011. This quantum computer was developed specifically forsolving combinatorial optimization problems (*4) and was designed based on the conceptof quantum annealing proposed by Prof. Nishimori andDr. Kadowaki.
Prof. Nishimori has been studying spin glasses since his graduate school days, which led
him to propose quantum annealing. A spin is a phenomenon in which an electron has amagnetic moment owing to a property analogous to electromagnets. The state in which thedirection of spin is randomly frozen, like the positions of molecules in a glass, is called aspin glass. While studying the spin glass mechanism, Prof. Nishimori and his student
came up with the idea of applying the quantum mechanical concept of fluctuation tooptimization problems. The results of this research were published in 1998 in a paper
entitled “Quantum Annealing in a Transverse Field” This theory came to be recognized as
a computational technique with the potential to handle general combinatorial optimization
problems at high speed and with high accuracy.
The study of spin glass theory is an area of great interest in physics; in 2021, Prof, Giorgio
Parisi, who introduced a novel analytical approach to the theory, was awarded the Nobel
Prize in Physics “for the discovery of the interplay of disorder and fluctuations in physicalsystems from atomic to planetary scales”
Quantum computers are built mainly on two different models, quantum gate model andquantum annealing. Although the quantum gate model ideally allows the construction of asystem with great flexibility, it is difficult to suppress the errors. A theory of spin glass,which Prof. Nishimori proposed as a graduate student, has proven to be applicable to theanalysis of error correction schemes in the gate model, which is known to be a promisingsolution to the problem of error correction.
Quantum annealing: a step ahead as a working quantum computerQuantum annealing is a computational technique for combinatorial optimization problems
and quantum simulation applications. From the viewpoint of proximity to real applications,quantum annealing is currently running ahead of gate-model quantum computers because
the D-Wave device has frequently been used for proof-of-concept tests in close-to realworld settings.
Although many groups, including major IT companies such as Google and IBM, areworking on the development of gate-model quantum computers given the theoreticallywider applicability, many experts believe that it will take 10 years or more to build practicalsystems addressing the noise problem and other issues. The overwhelming power of gatemodel quantum computers for several specific tasks has been theoretically proven;however, many obstacles must be overcome before they reach the stage of practicalusefulness.
For a gate-model quantum computer to be able to solve practical social problems , it isconsidered necessary to build and precisely control a system with at least 1 million
quantum bits. However, achieving this under the present superconducting technology will
require cooling a space the size of a gymnasium to an ultra-low temperature of
approximately 10 mK (1 millikelvin ≈ -273 °C). This is obviously impossible. Achieving thetarget of practical gate-model quantum computer will require ground-breaking
developments in engineering .
As for quantum annealing, many proof-of-concept experiments have already been
conducted, and a good amount of results have been accumulated. However, future
development will depend on whether it is possible to solve real-world problems much more
efficiently than with classical computers under some criteria. This is a challenging toquantum annealing and, if unsuccessful in the short term, may lead to disappointments,leading to shrinking supports from the government and the society, which may be called a“quantum chasm”. It is necessary to be prepared for such a period before quantum
computers start to change the world in a massive scale.
In both gate model quantum computing and quantum annealing, Japan is plagued by anoverwhelming shortage of researchers in comparison with leading countries such as theUnited States and China. To promote world-leading research here in Japan, we need toinvest in education , by stimulating students to become interested in quantum scienceand technology at the undergraduate level. I hope that the Japan Society of Applied
Physics will adopt various measures to encourage young people to have a broader
perspective in this regard.
Annotations*1: Gate-model quantum computer
Conventional computers use digital units called bits that take values of either 0 or 1. A quantum computer performs
calculations by replacing bits with quantum bits or qubits that are in a superposition state of being both 0 and 1
simultaneously. Compared to conventional computers, gate-based quantum computers are theoretically known to havean overwhelmingly high calculation speed for some tasks.
*2: Quantum annealing
Quantum annealing is a computational technique that aims to perform combinatorial optimization problems with highspeed and high accuracy. It was first proposed theoretically by Dr. Tadashi Kadowaki and Prof. Hidetoshi Nishimori in1998.
*3: Quantum chasm
Both gate model and quantum annealing require many years of solid research and development before they can solve
real-worl problems efficiently , and during this period, they may continue to fail to meet the relatively excessive
expectations of the society. Full-scale practical applications will be reached only after overcoming this period of quantum
chasm .
*4: Combinatorial optimization problem
A combinatorial optimization problem is to find the best combination of discrete variables that satisfies the given
conditions.
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Figures
A paper by Prof. Nishimori et al. published in 1998 toward solving the spin glass ground-state search (a theoretical physics

problem), which was conceived purely out of academic interest.

Stages of development of quantum chemical technology