Why is the number of qubits “crap”, and quantum communication is valid only at short distances

We have witnessed the second quantum revolution, says Alexander Lvovsky, a professor of physics. He argues that mankind has learned to control quantum systems at a single level.

Then why does the quantum computer still not appear on everyone on the table and does not play music from “VKontakte” on an unbreakable quantum communication line? About this, as well as about the development of Russian science and the fact that “cooler” is the technology of blockchain or quanta, - the professor told personally.

Alexander Lvovsky - Doctor of Philosophy, Professor of the Faculty of Physics of the University of Calgary, member of the Scientific Council of the Russian Quantum Center (RCC), editor of Optics Express, popularizer of quantum science.

Interviewer: Ilya Lopatin
Respondent: Alexander Lvovsky

Today, a quantum computer can be bought for 10+ million dollars, which is done, for example, by Google. While such a computer is effective only in solving narrowly focused tasks. What do buyers expect to pay such a sum?

Google bought the “quantum computer” of the Canadian company D-Wave. This is not a universal quantum computer, but the so-called quantum annealer - “quantum annealing agent”. Many of my colleagues - theorists in the field of quantum cybernetics - believe that this device has no advantages over the classical one. Although Google and D-Wave show examples of specific tasks that can be solved faster with this machine than on a regular, classic computer, opponents argue that this is because classical algorithms are simply not optimal. In other words, an advantage is an illusion that arises due to the fact that we do not squeeze out of the classic "iron" everything that it is capable of.

In short, there are scientific disputes. We, ordinary people, will benefit from them anyway: either we will have quantum computers, or the classical algorithms will significantly improve.

In the meantime, these disputes have not been resolved, the question of whether this computer has a theoretical advantage over the classical one in principle, fades into the background. If using this device here and now you can solve a problem a hundred or a million times faster than on a classic computer, then why not solve it?

Investors are invested in many projects. A quantum computer project is a high risk high gain. It is clear that nothing can come out of this, but if it does, the exhaust will be enormous (thousands, or even tens of thousands of percent). Therefore, on average, according to expectations, the project turns out to be a winning, attractive for investment.

At Harvard, created a 51-qubit computer. Tell me, does this refer to the second quantum revolution or to the first one?

The second quantum revolution is the ability to control complex entangled quantum systems at the level of their individual components (ions, photons, atoms). This is the main difference from the first quantum revolution - the invention of macroscopic devices, which, although they use quantum physics as the basis of their work, are not capable of individual control. Transistors and lasers led to the emergence of integrated circuits, computers, the Internet, mobile communications - in general, they made the world as we know it now. And from the second quantum revolution, we expect even more!

The Harvard experiment is undoubtedly an element of the second quantum revolution. However, this is not a quantum computer, but a so-called quantum simulator. In other words, it is not a computing device, but a kind of physical system that allows you to emulate more complex physical systems, in particular, solid-state physics.


A device created at Harvard. The project was led by Mikhail Lukin and Markus Greiner from Harvard University, as well as Vladan Vuletic from MIT

What limits the increase in the number of qubits when creating quantum computers? Where do the numbers 5, 50, 2000 come from? Is the problem precisely the number of qubits?

Yes, everything is measured by the number of qubits ... Actually, this is crap, because it is not quantity that is important, but quality. Modern qubits are extremely "fragile": they interact with the outside world and lose their quantum information very quickly. With their help it is impossible to carry out even the simplest quantum calculations. To do this, you must first do what we call a logical qubit, which can support quantum information indefinitely by correcting errors. In order to make one logical qubit, dozens or even hundreds of physical ones are needed.

In classic computers, too, this is. There are constantly errors, and if they are not corrected, the information will disappear instantly. But since there are correction algorithms there, we see no errors. We see that information is stored indefinitely.

In quantum computing, such a technology is not yet available, because the quality of physical qubits is not enough to implement even a single logical one. Therefore, all the talk about the number of qubits should be taken carefully.

We regularly hear about the launch of a quantum communication line based on quantum entanglement. How applicable is it from a commercial point of view?

Most modern quantum communication lines are not based on quantum entanglements. This is good, because entanglement is a rather complex phenomenon: it is difficult to obtain and apply. Modern quantum coupling, as a rule, uses weak laser pulses — such that each individual pulse contains on average less than one photon. Sometimes one photon slips, then on it you can record some information and send it to some distance. In this case, you can be sure that if someone steals this photon, then he will not be able to reproduce it in the same quantum state. This is the basis for the secrecy of quantum communication.

The problem of modern quantum communication is that it acts only at short distances. Why? Because losses in fiber lines are about a factor of two for 10-15 km. This means that, for example, in the line from Moscow to St. Petersburg, only one of 10 ¹⁸ - billion billion - impulses will reach its destination. With such speed, of course, it is impossible to transmit reasonable amounts of information.

In the usual, classical communication line, this problem is also present, but it is solved with the help of repeaters or amplifiers. Every few tens of kilometers an amplifier is placed, which raises the level of the optical signal to the initial one. With the quantum line this can not be done, because such an amplifier will be indistinguishable from a spy: it changes the photons and does the same. Therefore, it is necessary to invent another technology - the so-called quantum follower. In order to realize it, quantum entanglement is needed. More precisely, two phenomena are quantum teleportation and quantum optical memory.

Why is the blockchain vulnerable to quantum technology?

The blockchain uses a cryptographic hash function: each subsequent block contains the hash function of the previous one, due to which it is impossible to change the information stored in one of the blocks without disturbing the integrity of the entire chain. A quantum computer can make the calculation of the hash function reversible, that is, it can pick up the change in the block so that the hash does not change.

There is one more vulnerability characteristic of cryptocurrency: digital signature. Anyone who has a quantum computer will be able to forge a digital signature, that is, for example, make a large money transfer on behalf of some billionaire, using its digital signature. Thus, as soon as a universal quantum computer is invented, Bitcoin, and indeed, in all modern cryptocurrencies, the cost will be reduced to zero.

Recently, we at the Russian Quantum Center have come up with a way to eliminate these vulnerabilities by applying quantum communication technology to blockchains. I think that, of course, blockchains and cryptocurrencies have a future - they will simply change and be synthesized with quantum technologies in the development process.

Suppose physicists are at an impasse in the study of the microworld. What will happen faster - the arrival of a person to understand that he doesn’t understand or invent anything further, or the impossibility to finance a global super-project that turns out to be 10 times more expensive than the hadron collider?

I believe that in the twentieth century, physics has gone far ahead compared with other sciences. The development of science has always been dictated by practical application. And now the level of knowledge of physics is many orders of magnitude higher than what can be used in practice. Can I learn more? Of course, but for this you really need to either build giant telescopes or build expensive accelerators. The question is - why? Idle curiosity?

Therefore, it seems to me that in the 21st century physics will slow down its development and yield to such sciences as cybernetics and biology, because in these sciences there are secrets right in front of our nose: how cells are made, how to treat diseases, heredity, etc. In technological matters - cybernetics at the junction with biology. How does the brain work, how do we think, how to make the car think like a person? These scientific tasks are extremely interesting and relevant, and most importantly - their solution allows developing real sold devices that will improve people's lives.



In one of your interviews, you noticed that the development of domestic scientists is hampered by isolation in research institutes. Is this statement true for foreign science? Or did it concern only science in Russia?

The Russian scientific system is the legacy of the Soviet, which was imprisoned under the cold war. In those days, it performed its functions well: Soviet science was quoted at the highest level in the world. And when the Cold War ended, the state support for science dramatically weakened, its funding decreased. As a result, Russian science inherited from the Soviet are not the best, and the worst features - in particular, its isolation. People have been doing the same thing for many years, interacting very poorly with the outside world and with world science. One of the reasons - the Russian scientist gets a permanent position at an early stage of his career. For example, a graduate student is guaranteed a workplace until the end of life. It turns out that it makes no sense to learn something new, to prove to the world that you are worth something - you can sit in this research institute, receive a small salary and live like an ordinary person.

In this regard, I prefer the approach of international science, where a person under 30-40 years old does not have a permanent position. As a result, he is motivated to keep searching, to work. If a scientist stops producing something new to the surface, he will simply end up in the garbage. Yes, such pressure is an unpleasant engine, but it is an engine not only of man, but of science itself.

In addition, in a foreign system, a person usually performs various stages of his career - before obtaining a permanent position - in different groups, universities, often in different countries. As a result, everyone is a carrier of unique experience and unique ideas that can be synthesized like no other.

Online education can be effective as an alternative source of knowledge?

I believe that online education is the future of education. I myself am now learning machine learning through online courses. The method works well - it is ready to witness efficiency itself.

The institute is a lecturer in the audience for 200 people, something broadcasts, and the students are sleeping. What's the point of this? There is a transfer of information in one direction, without feedback.

It is clear that without lectures can not do. But then why not find the best lecturer in the world who will read online not for 200 people, but for 20 thousand people. The listener can stop his lecture at any time, think it over; listen to the material when it is convenient when there is a mood. The role of the professor and teacher at the university will change: it will no longer be broadcasting, but an interactive education - seminars where the student can communicate directly with the teacher, learn from his experience and get answers to his questions.

Online education is effective in combination with full-time. I hope that the concept will change precisely in this direction: more interaction, and lectures - online.

What is your motivation to give public lectures?

This has to do with the topic of isolation you asked for. A scientist in general has a tendency to lock in his comfortable ivory tower - to explore what is interesting for him and two or three people around the world, to publish articles that only this couple of people will read. Stew in your own juice. And if we become such, stop communicating with the world, then what is the use of us? Therefore, I believe that our duty is to carry science to the masses. Not only for the sake of the altruistic goal of national enlightenment, but also in order not to forget to ask ourselves what benefits our work brings to progress and humanity.

What will be your popular science lecture at the Quantum Technology Conference on March 1?

In the first part of the lecture I will say a few words about the basics of quantum physics based on photons - elementary particles of light. And also about quantum paradoxes. I'm interested in this topic. It shows how amazing quantum physics is, how incredible changes in the concept of the world seem to have incredible consequences.

The second part of the presentation will be devoted to examples of quantum technologies: quantum computing, quantum cryptography, quantum chronometry and sensors.