Absolute protection: what quantum communications are and how they work. More than love

Russian and Czech-Slovak physicists have proposed a method for preserving the quantum entanglement of photons when passing through an amplifier or transmitting over a long distance.

Quantum entanglement or cohesion of particles is a phenomenon of connection between their quantum characteristics. It can arise from the birth of particles in one event or their interaction. This connection can be maintained even if the particles disperse over a long distance, which makes it possible to transmit information with their help. The fact is that if you measure the quantum characteristics of one of the bound particles, then the characteristics of the second automatically become known. The effect has no analogues in classical physics. It was experimentally proven in the 1970s and 80s, and has been actively studied in the last few decades. In the future, it may become the basis for a number of information technologies future.

A funny everyday analogy for this phenomenon was invented by one of its researchers, theoretical physicist John Bell. His colleague Reinhold Bertlmann suffered from absent-mindedness and often came to work in socks different color. It was impossible to predict these colors, but Bell joked that you only had to see the pink sock on Bertleman's left foot to deduce that he had a different color sock on his right foot without even seeing it.

One of the problems practical use The phenomenon of quantum entanglement is a disruption of communication when particles interact with the outside world. This can happen when the signal is amplified or transmitted over a long distance. These two factors can also act together, since in order to transmit a signal over a long distance it must be amplified. Therefore, photons, after passing through many kilometers of optical fiber, in most cases cease to be quantum entangled and turn into ordinary, unrelated quanta of light. To avoid communication breakdown in quantum computing experiments, it is necessary to use cooling to close to absolute zero temperatures

Physicists Sergei Filippov (MIPT and Russian Quantum Center in Skolkovo) and Mario Ziman (Masaryk University in Brno, Czech Republic, and Physics Institute in Bratislava, Slovakia) have found a way to preserve the quantum entanglement of photons when passing through an amplifier or, conversely, when transmitting over a long distance. Details published in an article (see also preprint) for the journal Physical Review A.

The essence of their proposal is that in order to transmit signals of a certain type, it is necessary that “the wave function of particles in the coordinate representation should not have the form of a Gaussian wave packet.” In this case, the probability of destruction of quantum entanglement becomes much lower.

The wave function is one of basic concepts quantum mechanics. It is used to describe the state of a quantum system. In particular, the phenomenon of quantum entanglement is described on the basis of ideas about general condition bound particles with a specific wave function. According to the Copenhagen interpretation of quantum mechanics physical meaning The wave function of a quantum object in coordinate representation is that the square of its modulus determines the probability of detecting the object at a given point. With its help you can also obtain information about momentum, energy or some other physical quantity object.

The Gaussian function is one of the most important mathematical functions, which has found application not only in physics, but also in many other sciences, including sociology and economics, which deal with probabilistic events and use statistical methods. Many processes in nature lead to this function during mathematical processing of observational results. Its graph looks like a bell-shaped curve.

Ordinary photons, which are now used in most experiments on quantum entanglement, are also described by a Gaussian function: the probability of finding a photon at a particular point, depending on the coordinates of the point, has a bell-shaped Gaussian shape. As the authors of the work showed, in this case it will not be possible to send entanglement far, even if the signal is very powerful.

The use of photons whose wave function has a different, non-Gaussian shape should significantly increase the number of entangled photon pairs reaching the recipient. However, this does not mean that the signal can be transmitted through an arbitrarily opaque medium or over an arbitrarily large distance - if the signal-to-noise ratio falls below a certain critical threshold, then the effect of quantum entanglement disappears in any case.

Physicists have already learned how to create entangled photons separated by several hundred kilometers, and have found several very promising applications for them. For example, to create a quantum computer. This direction seems promising due to the high speed and low power consumption of photonic devices.

Another direction is quantum cryptography, which makes it possible to create communication lines in which “eavesdropping” can always be detected. It is based on the fact that any observation of an object is an impact on it. And influencing a quantum object always changes its state. This means that an attempt to intercept a message must result in the destruction of the entanglement, which will be immediately known to the recipient.

In addition, quantum entanglement makes it possible to realize so-called quantum teleportation. It should not be confused with teleportation (transport in space) of objects and people from science fiction films. In the case of quantum teleportation, it is not the object itself that is transmitted over a distance, but information about its quantum state. The thing is that all quantum objects (photons, elementary particles), and with them, atoms of the same type are absolutely identical. Therefore, if an atom at the receiving point acquires a quantum state identical to the atom at the transmitting point, then this is equivalent to creating a copy of the atom at the receiving point. If it were possible to transfer the quantum state of all atoms of an object, then an ideal copy of it would appear at the receiving site. In order to transmit information, you can teleport qubits - smallest elements for storing information in a quantum computer.

Based on materials from the MIPT website

Sergey Kuznetsov

Editor

Quantum communication without unnecessary noise

Scientists at the Toshiba Research Center at the University of Cambridge seem to have made another breakthrough in quantum communications. The level of breakthrough is evidenced by the fact that their article was published in the top Nature. The authors of the article claim that they were able to transmit data encrypted using quantum key distribution (QKD) over regular commercial fiber over 550 kilometers with a “controlled noise level” - and this without the use of quantum repeaters. That is, they managed to exceed a certain limit of the ratio of the “thickness” of the channel and the data transmission distance.

To understand how important this is, let's understand what quantum key distribution is, which is discussed in the new work.

Usually, when it comes to quantum cryptography, we resort to three people - Alice and Bob, who want to communicate privately, and Eve, who wants to eavesdrop on them. There is Vernam's theorem, according to which Eve will never be able to read their correspondence if Alice and Bob share a key whose length is equal to the length of their messages. But knowing this, all good spies usually try to secretly copy the key at the very moment when Alice and Bob distribute it.

Here the quantum world comes to our aid, in which there is a ban on cloning (read: copying) an unknown quantum state. Yes, yes, we are talking about quantum entanglement here. Based on this, in 1984, Charles Bennett and Gilles Brassard proposed a quantum key distribution system by developing the BB84 protocol.

What does this mean in reality? In fact, Alice sends Bob individual photons, which have, for example, one of four types of polarization (vertical, horizontal and two diagonal).

For example, vertical and horizontal polarizations code for "zero" and "one" in one measurement method, and two diagonal polarizations code for "zero" and "one" in another measurement method. Bob then randomly chooses how to measure the state of the photon. Only if the method of preparing and measuring the photon coincide, Alice and Bob write the resulting bit into the secret encryption key. Instead of polarization, a change in the phase of the photon can be used.

But there are several fundamental problems. First, there is the problem of a device capable of sending single photons. In practice, commercial quantum communications links often use very weak laser pulses, although progress has also been made in the development of single-photon sources. And secondly, since signal transmission is carried out by individual photons, the problem of noise arises. Optical fiber heats up differently (thermal photons), can be bent differently, and so on.

Therefore, at the moment there are hardware-independent limits bandwidth quantum communication depending on distance. In practice, this is 1.26 megabits per second over a distance of 50 kilometers over a standard cable and - compare - 1.16 bits per hour (!) over a distance of 404 kilometers (symbolically) over a special cable with ultra-low data losses.

Here's an example: last August, Chinese researchers published in the same Nature results of an experiment on the implementation of quantum cryptography protocols between space and Earth. Then from the Mo Tzu satellite over a distance of 1200 kilometers more than 300 kilobytes of a secret key. This became possible because both the near-Earth space and the upper layers of the atmosphere make almost no noise. A typical 1,200-kilometer optical fiber would take about six billion years to transmit one bit of a sifted key.

To transmit a signal over a longer distance, quantum communication specialists are working on quantum repeaters. You might think that these are quantum repeaters, but in fact the principle of their operation is completely different.

We have already said that in quantum world it is impossible to clone a quantum state. But a conventional electromagnetic signal repeater (radio, for example) does exactly this: it receives the signal and reproduces it again. A quantum message cannot be treated this way. Therefore, a quantum repeater is more of an ordinary quantum computer that is capable of storing the original signal (qubit). However, for now, quantum repeaters in practice are a matter of the future.

Now let's return to the Cambridge article.

As we remember, Alice sends photons to Bob. That is, Alice has a laser, Bob has photon detectors. However, the authors suggest introducing Charlie, who is located in the middle, into the equation. Charlie is “outsourced”, the detectors are given to him. Both Alice and Bob generate phase-randomized optical fields that are combined at Charlie. Fields transmitted with the same random phase are “twins” and can be used to extract a quantum key.

In such a “twin field quantum key distribution” (TF-QKD) scheme, there is the same dependence of signal loss on distance, but due to this cunning move it is possible to maintain acceptable noise for another 550 kilometers. Truly a breakthrough!

The fact is that in the proposed scheme, the “noise” is a drift (creep) of the phase shift, which can be compensated if the Charlie station operates as a phase modulator, correcting the drift. This makes “noise-controlled” quantum communication possible over a distance of five hundred kilometers over conventional optical fiber, which was simply impossible without the use of quantum repeaters.

MOSCOW, June 16 - RIA Novosti. Scientists and engineers from the Russian Quantum Center have launched the country's first full-fledged quantum secure communication line. The first transfer of cryptographic information over a 30-kilometer commercial communication line connecting two Gazprombank buildings in Moscow took place on May 31, the RCC press service reports.

"This is a clear illustration of how basic science, the quantum physics brings visible technological fruits. And the quantum cryptographic line is only the first of them; we are developing other quantum technologies that will change people’s lives for the better,” said Ruslan Yunusov, CEO Russian Quantum Center.

The phenomenon of quantum entanglement is the basis of modern quantum technologies. This phenomenon, in particular, plays important role in secure quantum communication systems - such systems completely exclude the possibility of unnoticed “wiretapping” due to the fact that the laws of quantum mechanics prohibit “cloning” the state of light particles. Currently, quantum communication systems are being actively developed in Europe, China, and the USA.

Work on a quantum communication system at the Russian Quantum Center began in 2014 with the support of Gazprombank and the Russian Ministry of Education and Science. Investments in the project amount to about 450 million rubles.

Professor Alexander Lvovsky became the scientific director of the project. Later, to implement this project, the QRate company was created, headed by Yuri Kurochkin. Russia's first quantum secure communication channel was built between Gazprombank branches on Korovy Val and Novye Cheryomushki.

As Yunusov told RIA Novosti in November 2015, a distinctive feature of the Russian pilot project was that scientists do not use special communication lines manufactured and assembled specifically for transmitting secure information, as their colleagues in Switzerland, the USA and China do, but ordinary ones." city" fiber optic lines.

“It is fundamentally important that the channel was created on the basis of a standard telecommunications line, built from a regular fiber optic cable. This means that our technology can be widely used on existing networks without modifications,” explains Yuri Kurochkin, whose words are quoted by the RCC press service.

The total length of the line was 30.6 kilometers, the percentage of errors during key transmission did not exceed 5%, which is a very good indicator for a network in urban conditions. Gazprombank, which invested in this project, intends to subsequently use quantum communications in its work.

"The task of increasing the protection of banking communication channels, as well as electronic means payments from criminals is becoming increasingly important around the world. The introduction of advanced technologies implemented by the RCC makes it possible to counter the sophisticated techniques of cybercriminals with the highest achievements of science. Start practical application quantum inventions in the banking industry serves as the best confirmation of the importance of RCC at the forefront of science and technology,” added Dmitry Sauers, Deputy Chairman of the Board of Gazprombank.

Other organizations, including Sberbank, also showed interest in using RCC developments in the field of secure communications.

The telegraph “killed” pigeon mail. Radio replaced the wire telegraph. Radio, of course, has not disappeared anywhere, but other data transmission technologies have appeared - wired and wireless. Generations of communication standards replace each other very quickly: 10 years ago Mobile Internet was a luxury, and now we are waiting for 5G. In the near future, we will need fundamentally new technologies that will be no less superior to modern ones than radio telegraphs are to pigeons.

What this could be and how it will affect all mobile communications is under the cut.

Virtual reality, data exchange in a smart city using the Internet of things, receiving information from satellites and from settlements located on other planets solar system, and protecting this entire flow - such problems cannot be solved by a new communication standard alone.

Quantum entanglement

One of the main options for the evolution of communication awaiting us is the use of quantum effects. This technology will not eliminate, but may complement traditional types communication (although we cannot immediately reject the idea that a network based on quantum entanglement, theoretically, can displace other types of communication).

Quantum entanglement is the phenomenon of connection between quantum characteristics. The connection can be maintained even if the particles diverge over a long distance, since by measuring the quantum characteristics of one of the connected particles, we automatically know the characteristics of the second one. The first quantum cryptography protocol appeared back in 1984. Since then, many experimental and commercial systems have been created based on the phenomena of the quantum world.

(c) Chinese Academy of Sciences

Today, quantum communication is used, for example, in the banking industry, where compliance is required special conditions security. Companies Id Quantique, MagiQ, Smart Quantum already offer ready-made cryptosystems. Quantum technologies for security can be compared to nuclear weapons- this is almost absolute protection, which, however, implies serious implementation costs. If you transmit an encryption key using quantum entanglement, then intercepting it will not give attackers any valuable information - at the output they will simply receive a different set of numbers, because the state of the system in which an external observer is interfering changes.

Until recently, it was not possible to create a global perfect encryption system - after only a few tens of kilometers the transmitted signal faded. Many attempts have been made to increase this distance. This year, China launched the QSS (Quantum experiments at Space Scale) satellite, which should implement quantum key distribution schemes at a distance of more than 7,000 kilometers.

The satellite will generate two entangled photons and send them to Earth. If everything goes well, the distribution of the key using entangled particles will mark the beginning of the era of quantum communication. Dozens of such satellites could form the basis not only of a new quantum Internet on Earth, but also of quantum communications in space: for future settlements on the Moon and Mars, and for deep space communications with satellites heading beyond the solar system.

Quantum teleportation

With quantum teleportation, no material transfer of an object from point A to point B occurs - there is a transfer of “information”, not matter or energy. Teleportation is used for quantum communications, such as transferring secret information. We must understand that this is not information in the form we are familiar with. Simplifying the quantum teleportation model, we can say that it will allow us to generate a sequence of random numbers at both ends of the channel, that is, we will be able to create a encryption pad that cannot be intercepted. For the foreseeable future, this is the only thing that can be done using quantum teleportation.

For the first time in the world, photon teleportation took place in 1997. Two decades later, teleportation over fiber optic networks has become possible over tens of kilometers (within the framework of the European program in the field of quantum cryptography, the record was 144 kilometers). Theoretically, it is already possible to build a quantum network in the city. However, there is a significant difference between laboratory and real-world conditions. Fiber optic cable is subject to temperature changes, which changes its refractive index. Due to exposure to the sun, the phase of the photon may shift, which in certain protocols will lead to an error.

, Quantum Cryptography Laboratory.

Experiments are being conducted all over the world, including in Russia. Several years ago, the country's first quantum communication line appeared. It connected two buildings of ITMO University in St. Petersburg. In 2016, scientists from the Kazan Quantum Center KNITU-KAI and ITMO University launched the country's first multi-node quantum network, achieving a generation speed of sifted quantum sequences of 117 kbit/s on a 2.5-kilometer line.

This year, the first commercial communication line appeared - the Russian Quantum Center connected the offices of Gazprombank at a distance of 30 kilometers.

In the fall, physicists from the Laboratory of Quantum Optical Technologies of Moscow State University and the Foundation for Advanced Research tested automatic system quantum communication at a distance of 32 kilometers, between Noginsk and Pavlovsky Posad.

Taking into account the pace of creation of projects in the field of quantum computing and data transmission, in 5-10 years (according to the physicists themselves), quantum communication technology will finally leave the laboratories and become as common as mobile communications.

Possible disadvantages

IN last years the issue is increasingly being discussed information security in the field of quantum communications. It was previously believed that using quantum cryptography it was possible to transmit information in such a way that it could not be intercepted under any circumstances. It turned out that absolutely reliable systems do not exist: physicists from Sweden have demonstrated that, under certain conditions, quantum communication systems can be hacked thanks to some features in the preparation of a quantum cipher. In addition, physicists from the University of California have proposed a method of weak quantum measurements, which actually violates the observer principle and allows one to calculate the state of a quantum system from indirect data.

However, the presence of vulnerabilities is not a reason to abandon the very idea of ​​quantum communication. The race between attackers and developers (scientists) will continue at a fundamentally new level: using equipment with high computing power. Not every hacker can afford such equipment. In addition, quantum effects may make it possible to speed up data transfer. Entangled photons can transmit almost twice as much information per unit time if they are further encoded using the direction of polarization.

Quantum communication is not a panacea, but for now it remains one of the most promising directions development of global communications.

Despite the fact that this phenomenon is described by the theories of quantum mechanics and proven experimentally, many scientists are skeptical about it. This split in the scientific world has occurred since the dispute between Albert Einstein and Niels Bohr. Einstein said that quantum entanglement is an idea too absurd and has nothing to do with reality and observations. He called it "ghost interaction" ( spooky action), since this theory contradicted his statement about the irresistibility of the speed of light.

Today, scientists from Israel have experimentally proven that it is possible to create a pair of photons that have a quantum connection, even if they do not exist at the same time. That is, besides amazing fact that such a connection works even at a great distance (at least 13.8 billion light years), a time separation is also added. It turns out that the relationship between two particles is so strong that they can be separated by both time and space, and the quantum connection will still operate.

A quantum of light, also known as a photon (which is both a particle and a wave) can be polarized and, in fact, can take on two states: vertical and horizontal polarization. Entanglement occurs when there are paired photons, each of which can be either horizontally or vertically polarized. Their quantum connection manifests itself as follows: if you measure the state of one photon, you can confidently say that the state of its pair will be the opposite. That is, if a particle whose properties we can find out is vertically polarized, then a paired particle located at least at the other end of the Universe will be horizontally polarized, and vice versa.

Quantum optics specialist Eli Megidish and his colleague Hagai Eisenberg of the Hebrew University of Jerusalem created a quantum connection between two photons that did not exist at the same time.

They started with a scheme known as entanglement exchange ( entanglement swapping). To do this, they directed a laser beam twice at a special crystal to produce two pairs of photons. The resulting particles were designated by numbers: pair 1 and 2, pair 3 and 4. Initially, particles 1 and 4 did not have a quantum connection, but it should have appeared as soon as scientists established entanglement between photons 2 and 3.

The “projection measurement” of the properties of one of the particles causes the appearance of a certain state of it, as well as a change in the state of the paired particle to the opposite, as in the case of vertical and horizontal polarization. Thus, even if photons 2 and 3 were not initially entangled, through measurements, physicists gave one of them one of the two states, and its “partner” the opposite.

Any measurement causes photon entanglement, even if it destroys one of them. So, if we consider only the case in which particles 2 and 3 are in the same state, then photons 1 and 4 automatically turn out to be entangled after measurements. For a better understanding, you can give a simple example: if you have a chain of four links, then when its outer links are connected, the middle ones also become connected.

To create quantum entanglement between photons 1 and 4, which did not even exist at the same moment, Eisenberg and his colleagues first entangled photons from the pair 1 and 2, and then measured the polarization of photon 1 in the usual way. Then physicists “connected” particles 3 and 4 and made “projection measurements”. The last stage The researchers measured the polarization of photon 4. And even though photons 1 and 4 never coexisted, quantum entanglement still appeared between them, the scientists report in a preprint of the paper on arXiv.org.

Eisenberg says that even under the theory of relativity, where two observers moving with at different speeds, perceive the sequence of events in time differently, none of them will say that particles 1 and 4 from his experiment ever existed simultaneously.

"Our experiment shows that it is not entirely logical to consider quantum entanglement as some kind of real physical phenomenon. Since the two photons never existed at the same time, it is impossible to say that there was a connection between them at any point in time,” says Eisenberg.

University of Vienna physicist Anton Zeilinger believes that the experiment of his Israeli colleagues in Once again proves how unstable the concepts of quantum mechanics are. " Quantum effects have little in common with what we observe in real life every day," he says.

And yet, progress in the field of quantum mechanics can radically change life as we know it. For example, based on the research of Eisenberg and his colleagues, it will be possible to create an unbreakable hidden connection between two users located at a great distance from each other. The user at the other end of the “wire” will not need to wait while the information is transmitted: a change in the state of the opposite photon will instantly cause a change in its pairs. Zeilenger also hopes that such experiments can inspire the creators of quantum computers to improve the technology.