Quantum information transfer will become more reliable. More than love

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

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 (as part of the European 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 at Moscow State University and the Foundation for Advanced Research tested an automatic quantum communication system 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.

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 the advent of 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 of the solar system, and protecting this entire flow - such problems cannot be solved by a new communication standard alone.

Quantum entanglement

Today, quantum communications are used, for example, in the banking industry, where special security conditions are required. Companies Id Quantique, MagiQ, Smart Quantum already offer ready-made cryptosystems. Quantum technologies for ensuring 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 at Moscow State University and the Foundation for Advanced Research tested an automatic quantum communication system 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 recent years, the issue of information security in the field of quantum communications has been increasingly discussed. 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 areas for the development of global communications.

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 entanglement 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.

Drawing by D. Bell in the manuscript of his 1980 paper. On the left is written in French "Mr. Bertleman's Socks and the Nature of Reality." Above the left leg it is written: “pink”, above the right leg: “not pink”.

Quantum teleportation research facility at the University of Tokyo.

Visualization of the process of quantum teleportation of qubits. On the left is a transmitter, on the right is a receiver, between which information about the quantum state of qubits is transmitted using entangled photons.

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 temperatures close to absolute zero.

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 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 the basic concepts of 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, the physical meaning of 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. To transfer information, you can teleport qubits - the smallest elements for storing information in a quantum computer.

The development of experimental quantum physics in recent decades has led to interesting results. Abstract ideas are gradually found practical use. In the field of quantum optics, this is, first of all, the creation of a quantum computer and telecommunications based on quantum cryptography - the technology closest to implementation.

Modern optical communication lines do not guarantee the confidentiality of transmitted information, since millions of photons move along fiber optic lines, largely duplicating each other, and some of them can be intercepted unnoticed by the recipient.

Quantum cryptography uses single photons as information carriers, so if they are intercepted, they will not reach the recipient, which will immediately become a signal that espionage is taking place.

To conceal the interception, the spy must measure the photon's quantum state (polarization or phase) and send a "duplicate" to the recipient. But according to the laws of quantum mechanics, this is impossible, since any measurement made changes the state of the photon, that is, it does not make it possible to create its “clone”.

This circumstance guarantees complete secrecy of data transmission, so such systems are gradually beginning to be used in the world by secret services and banking networks.

The first quantum cryptography protocol was invented by American scientists Charles Bennett and Jill Brassard in 1984, which is why it is called BB84. Five years later they created such a system in research center IBM, placing the transmitter and receiver in a light-proof casing at a distance of only 30 cm from each other. The system was controlled from personal computer and allowed the exchange of a secret key over the air channel (without cable) at a speed of 10 bit/s.

Very slowly and very close, but it was the first step.

The essence of the BB84 protocol is the transmission of photons with polarization in four possible directions. Two directions are vertical-horizontal and two diagonal (at angles of plus or minus 45 degrees). The sender and recipient agree that, say, vertical polarization and polarization at an angle of plus 45 degrees correspond to logical zero, and horizontal polarization and minus 45 degrees correspond to one. Then the sender sends to the recipient a sequence of single photons, randomly polarized in one of these directions, and the recipient, through an open communication channel, reports in which coordinate system (polarizations) he measured the received rays, but does not report the result of his measurements. Since each photon can be either a zero or a one, for the interceptor this open information useless. The sender reports whether the coordinate system for each photon is correct. Then they write down the matching sequence, which becomes a ready-made binary code for them - the secret key to decrypt the data. Now all encrypted data can be transmitted over open networks.

The invention aroused great interest throughout the world.

Coding of photons by polarization is used in experimental atmospheric communication links, since when radiation propagates through the atmosphere, the polarization of the radiation will change slightly, and to suppress solar or moonlight spectral, spatial and temporal filters are used. In the first experimental installation in 1992, the distance between the transmitter and receiver (the length of the quantum channel) was only 30 cm, in 2001 it was already almost 2 km. A year later, key transmission was demonstrated abroad over distances exceeding the effective thickness of the atmosphere - 10 km and 23 km. In 2007, the key was transmitted to 144 km, and in 2008, the reflected single-photon signal from a laser pulse from a satellite was recorded on Earth.

To generate single photons, highly attenuated radiation from semiconductor lasers is used. But you can also use sources of single photons - single-photon emitters on quantum dots, developed at the Institute of Semiconductor Physics. A. V. Rzhanova SB RAS. These are semiconductor structures that make it possible to emit radiation from only one quantum dot. Since transmission secrecy requires no more than one photon in each laser pulse, the photodetectors of the receiving node are subject to high requirements. They must have a sufficiently high registration probability (more than 10%), low noise and a high counting rate.

Avalanche photodiodes can serve as single-photon detectors, which differ from conventional ones in the amplification of electrical pulses: in conventional photodiodes, no more than one electron is born per incident photon, and in avalanche photodiodes - thousands. When the voltage on the photodiode exceeds a certain threshold and a photon hits it, an avalanche multiplication of charge carriers occurs. The higher the voltage above the threshold, the greater the probability of recording a photon, but also the stronger the noise.

To remove these noises, they (the detectors) must be cooled to minus 50 degrees Celsius with a special semiconductor microrefrigerator.

But superconducting detectors made from a set of nanowires about 50 nm thick can also be used. Such structures are in a transition regime from conducting to superconducting. The passage of one photon through this detector and its absorption is enough to heat up the nanowires and change the current through them. The incoming photon is detected by the change in current. Superconducting detectors are much less noisy than avalanche photodiodes. Foreign experiments with superconducting detectors have demonstrated maximum range quantum key transmission - 250 km compared to 150 km when using avalanche photodiodes. The main limiting factor for the serial use of superconducting detectors is the need for their deep cooling using expensive helium cryostats.

The range and speed of information transmission are limited by the capabilities of fiber optic communication lines, the efficiency of detectors and their noise level.

The maximum range of information transmission using quantum cryptography technology over optical fiber is about 150 kilometers, but at this distance the transmission speed will be only about 10 bits per second, and at fifty kilometers - about 10 kbits per second.

Therefore, quantum communication lines are only of high value for transmitting sensitive data.

For fiber optic communication lines they are used various ways coding quantum states of photons. Some of the first cryptosystems worked on the basis of polarization coding, just like for the BB84 protocol. However, in conventional optical fiber the polarization of photons is greatly distorted, so phase encoding is the most popular.

Modern commercial quantum fiber optic cryptosystems use two-pass optical design and phase encoding of photons. This system was first used by Swiss scientists in 2002. In her scheme, photons pass through a quantum channel (an optical fiber tens of kilometers long) twice - first in the form of a multiphoton laser pulse from the receiver to the transmitter, and then at the transmitter side they are reflected from the so-called Faraday mirror, attenuated to the level of single photons and sent back through the quantum channel to the receiver. A Faraday mirror “rotates” the polarization (direction) of reflected photons by 90 degrees due to the Faraday effect (rotation of polarization) in a special magneto-optical glass placed in a magnetic field. And on the way back to the receiver, all polarization and phase distortions of photons in the quantum channel undergo reverse changes, that is, they are automatically compensated. The technology does not require setting up a quantum channel and allows you to work with standard fiber optic communication lines.

Today, just such an experimental communication line in Russia has been created at the Novosibirsk Institute of Semiconductor Physics, where it is currently being tested and fine-tuned with a quantum channel 25 km long (it is planned to increase its length to 100 km).

A special feature of the created system is the use of specially designed high-speed controllers that control its setup and operation in automatic mode. Only a few of these systems have been developed in the world, and the technology for their implementation is not disclosed, so the only way to introduce quantum communication lines in our country is our own domestic development.

Prepared by Maria Rogovaya (Novosibirsk)

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 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.