“A groundbreaking book written by one of the brightest young quantum computer developers.
“It shows the essence of quantum computers!”

The world's first quantum teleportation achieved, a master of optical quantum computing.
Professor Akira Furusawa of the University of Tokyo

“Professor Shuntaro Takeda, the author of this book, is a quantum physicist of light at the University of Tokyo.
We are researching quantum computers based on quantum optics.
The author begins this book by explaining the principles of quantum physics.
“We introduce the misconceptions surrounding quantum computers and the problems they can solve, and finally, we show how to build an actual quantum computer.”

Professor Kim Jae-wan, Vice President of the Graduate School of Advanced Sciences

Quantum computers: What exactly are they? Do they even exist?

Quantum computer.
I don't know for sure, but it seems like a machine with some incredible capabilities that could potentially unleash unimaginable abilities.
This is even more so because it has a name that is so confusing no matter how many times you hear it, called ‘quantum.’
So, some people think of quantum computers as some kind of future, all-purpose secret tool that only exists in movies or science fiction novels, but they are real, existing machines.

IBM began selling quantum computers in January 2019, and anyone reading this can even try one out for free on IBM's website.
Google also made a splash in October 2019 when it announced that “our quantum computer solved a problem that would take even the most advanced supercomputer 10,000 years to solve in just 200 seconds.”


When you hear stories like this, you might think, 'This incredible machine really exists! From now on, unimaginable things will happen!'
But this idea is not correct either.
Not yet, that is.
Existing quantum computers are merely miniature versions of the real quantum computers that will one day be available.


However, because of their enormous potential, quantum computers are attracting great attention worldwide.
Countries like Europe, the United States, and China are investing significant effort in developing quantum computers, even establishing national policies, and major IT companies like Google, IBM, Intel, and Microsoft are independently developing quantum computers.
Many people also have high expectations for quantum computers, and the media is churning out articles about them.


The problem is that the reality of quantum computers is not well known, and there is a lot of superficial or incorrect information out there.
If we understand the true nature of quantum computers, we can see how misleading it is to think that quantum computers can handle any problem, are unconditionally fast in calculations, and will be realized sooner or later.


Professor Shuntaro Takeda, a young quantum computer developer at the University of Tokyo, explains in "The Story of Quantum Computers for Beginners" what quantum computers are, how they work, and why they are fast in an easy-to-understand way, while trying to uncover the misunderstandings surrounding quantum computers and their true potential.
We also want to change people's 'unfounded' expectations about quantum computers into 'well-founded' expectations.


And while introducing the quantum computer development site, it vividly shows what an actual quantum computer device is like.
In order to convey the facts about quantum computers accurately, the author also introduces negative information that is rarely found in the media.
Readers will be able to easily understand the essence of quantum computers through the author's explanations.

Why are so many countries so eager to build quantum computers? It's because quantum computers have the potential to revolutionize the world.
The performance improvements of quantum computers do not mean that the performance of smartphones or personal computers will improve.
However, since most of the services and products around us today rely on computer performance, the quality of such services and products could dramatically improve as the performance of quantum computers improves.

For example, automobile engine development and aircraft fuselage shape design cannot be performed without computers.
Computers are also essential when developing therapeutic drugs.
This is because the new drug candidates are developed while calculating the effects and side effects of the new drug candidates using a computer.
Daily weather forecasts are calculated by computers based on atmospheric flow.
Not only that, but we are always receiving help from computers, such as when you open YouTube, videos recommended to you are displayed, or when a cleaning robot finds the optimal path to clean while avoiding obstacles.
Therefore, as computer performance improves, the quality of various services and products we have enjoyed so far will improve, dramatically enriching the world.
Having access to high-performance computers will advance science and technology, foster business growth, and contribute to national economic growth and security.
Therefore, both at the national and corporate levels, there is a desperate need to be the first to build a quantum computer.
--- p.22~23

The examples listed so far are just a small part of the future that quantum computers will bring.
When modern computers first appeared, no one could have imagined that they would change our lives to this extent.
When quantum computers become a reality, unprecedented uses and services will emerge, and the world will become even richer than it is today.

At this point, quantum computers are not yet at a usable level.
It will still take a lot of time and money before practical quantum computers appear.
However, the benefits of a completed quantum computer have already been theoretically proven, and their impact is unimaginable.
Therefore, I believe that quantum computer research is worth the time and expense.
--- p.47, 49

I've roughly explained the computational rules of quantum computers. Ultimately, how do quantum computer calculations differ from those of modern computers? As explained so far, modern computers represent information as bits 0 and 1 and perform calculations by converting these bits using logical operations.
Quantum computers represent information in a superposition of quantum bits and perform calculations by changing the superposition method through quantum logic operations.
At first glance, you might think that using superposition only slightly changes the calculation principle, but if you think about the specific calculations, the quality of the calculations is completely different.
--- p.115

In this way, quantum computers should be viewed not as simple parallel computing devices, but as ‘wave-based computing devices’ that derive answers by manipulating many waves.
It's easy to think of quantum computers as simply modern computers with quantum properties added, but the quality of the information they handle and the calculations they perform are completely different.
--- p.118~119

If a quantum computer can compute four patterns simultaneously, it might appear that addition is four times faster than on a conventional computer that can only compute one pattern at a time.
But in reality, that's not the case.
The reason is that there is no way to retrieve all the results of nested calculations.
As already explained, quantum bits have the property that when measured, the superposition is broken and they are determined to be one of the two.
If you measure the final calculation result without thinking, you will randomly select only one result among the four pattern calculations.
In that case, it would be better to do the addition using a normal computer.
The ability of quantum computers to perform superposition and parallel calculations is a crucial difference from modern computers.
However, since we cannot obtain the entire result of parallel calculation and only one result can be obtained at the end, the calculation does not become faster with parallel calculation alone.
To make good use of the results of parallel computation, it is necessary to make good use of the interference between waves when calculating by manipulating the wave set.
Only by making good use of superposition and interference can quantum computers truly demonstrate their capabilities and achieve faster calculations.
--- p.119, 121

While it's tempting to hope that quantum computers will be the saviors that solve difficult problems, there's one caveat.
The fact is that quantum computers cannot solve all problems quickly.
Moreover, the general theory of what types of problems can be solved with a quantum computer in terms of the number of calculations has not yet been revealed.
So far, researchers have only discovered, through tireless effort, a few problems that can be solved quickly by quantum computers.
In most other problems, we do not know how to take advantage of the properties of 'quantum', so there is no difference in the number of calculations between modern computers and quantum computers.
Such problems can be solved with modern computers without necessarily needing a quantum computer.
Therefore, quantum computers are more like supercomputers that are used only for specialized and limited purposes rather than everyday computers.
Moreover, there are many problems that are difficult to solve even with quantum computers.
Therefore, quantum computers are not the universal computers that many people expect.
--- p.138~139

To build a quantum computer, each and every quantum particle, such as an electron, atom, or photon, must be perfectly protected from all kinds of interference and controlled with extreme precision.
It's frustrating that there are no passing standards for quality.
We must pursue perfection by repeating the process of finding and eliminating the causes that interfere with or disrupt the quantum.
Quantum computer developers are working tirelessly every day.
I hope this gives you some idea of ​​how difficult it is to develop a quantum computer.
--- p.173~174

Research and development of quantum computers is like climbing a rugged mountain path that has just begun.
It's unclear how many more years it will take to perfect a high-performance quantum computer that could transform our lives.
However, quantum computers, which were considered a dream 20 to 30 years ago, are now taking shape to some extent through technological accumulation.
In the coming decades, progress will undoubtedly surpass expectations.
There is ample evidence that quantum computers are achievable if they solve each problem to a certain degree, and if they are realized, they will undoubtedly have the power to drastically change our lives.
You can build such exciting future devices yourself.
I believe that being able to continue to pursue such dreams and hopes is the true joy of research.

--- p.233