Quantum human teleportation. What is quantum teleportation? Physicist answers

Quantum teleportation is the transfer of a quantum state over a distance. It is difficult to explain it separately; this can only be done in conjunction with all quantum physics. In his lecture, held as part of the “Lecture 2035” at VDNH, Alexander Lvovsky, a professor at the Faculty of Physics at the University of Calgary (Canada), a member of the Canadian Institute of Higher Studies, tried to talk in simple language about the principles of quantum teleportation and quantum cryptography. Lenta.ru publishes excerpts from his speech.

Key to the lock

Cryptography is the art of communicating in a secure manner over an insecure channel. That is, you have a certain line that can be tapped, and you need to send a secret message over it that no one else can read.

Let's imagine that, say, if Alice and Bob have a so-called secret key, namely a secret sequence of zeros and ones that no one else has, they can encrypt a message with that key by using an exclusive OR operation so that the zero matches with zero, and one with one. Such an encrypted message can already be transmitted over an open channel. If someone intercepts it, it's not a big deal, because no one can read it except Bob, who has a copy of the secret key.

In any cryptography, in any communication, the most expensive resource is a random sequence of zeros and ones, which is owned only by two communicating. But in most cases, public key cryptography is used. Let's say you buy something with a credit card from an online store using the secure HTTPS protocol. Through it, your computer talks to some server with which it has never communicated before, and it did not have the opportunity to exchange a secret key with this server.

The secret of this dialogue is ensured by solving a complex mathematical problem, in particular, factorization. It is easy to multiply two prime numbers, but if their product has already been given, then it is difficult to find two factors. If the number is large enough, it will require a conventional computer to calculate for many years.

However, if this computer is not ordinary, but quantum, it will solve such a problem easily. When it is finally invented, the above widely used method will be rendered useless, which is expected to have disastrous consequences for society.

If you remember, in the first Harry Potter book, the main character had to go through security to get to the Philosopher's Stone. Here is something similar: the one who installed the protection will easily get through it. Harry had a very difficult time, but in the end he overcame it.

This example illustrates public key cryptography very well. Someone who doesn't know it could, in principle, be able to decipher the messages, but it would be very difficult for them and potentially take many years. Public key cryptography does not provide absolute security.

Quantum cryptography

All this explains the need for quantum cryptography. She gives us the best of both worlds. There is a one-time pad method, which is reliable, but, on the other hand, requires an “expensive” secret key. In order for Alice to communicate with Bob, she must send him a courier with a suitcase full of disks with such keys. He will gradually use them up, since each of them can only be used once. On the other hand, we have the public key method, which is “cheap” but does not provide absolute reliability.

Image: Science Museum / Globallookpress.com

Quantum cryptography, on the one hand, is “cheap”; it allows the secure transmission of a key through a channel that can be hacked, and on the other hand, it guarantees secrecy thanks to the fundamental laws of physics. Its meaning is to encode information in the quantum state of individual photons.

In accordance with the postulates of quantum physics, the quantum state at the moment when it is attempted to be measured is destroyed and changed. Thus, if there is some spy on the line between Alice and Bob, trying to eavesdrop or spy, he will inevitably change the state of the photons, the communicaters will notice that the line is being tapped, stop communication and take action.

Unlike many other quantum technologies, quantum cryptography is commercial and not science fiction. There are already companies producing servers that connect to a regular fiber optic line, with the help of which you can carry out secure communication.

How does a polarizing beam splitter work?

Light is a transverse electromagnetic wave, oscillating not along, but across. This property is called polarization, and it is present even in individual photons. They can be used to encode information. For example, a horizontal photon is a zero, and a vertical photon is a one (the same is true for photons with a polarization of plus 45 degrees and minus 45 degrees).

Alice has encoded the information in this way, and Bob needs to accept it. For this, a special device is used - a polarizing beam splitter, a cube consisting of two prisms glued together. It transmits horizontally polarized flow and reflects vertically polarized flow, due to which information is decoded. If the horizontal photon is zero and the vertical photon is one, then in the case of a logical zero one detector will click, and in the case of a one the other will click.

But what happens if we send a diagonal photon? Then the famous quantum randomness begins to play a role. It is impossible to say whether such a photon will pass through or be reflected - with a 50 percent probability it will do either one or the other. It is impossible in principle to predict his behavior. Moreover, this property underlies commercial random number generators.

What should we do if we have the task of distinguishing polarizations of plus 45 degrees and minus 45 degrees? You need to rotate the beam splitter around the beam axis. Then the law of quantum randomness will apply to photons with horizontal and vertical polarization. This property is fundamental. We cannot ask the question of what polarization this photon has.

Photo: Science Museum / Globallookpress.com

Principle of quantum cryptography

What is the idea behind quantum cryptography? Suppose Alice sends Bob a photon, which she encodes either horizontally-vertically or diagonally. Bob also flips a coin, randomly deciding whether his basis will be horizontal-vertical or diagonal. If their encoding methods match, Bob will receive the data that Alice sent, but if not, then some kind of nonsense. They carry out this operation many thousands of times, and then “call each other” via an open channel and inform each other in what basis they made the transfer - we can assume that this information is now available to anyone. Next, Bob and Alice will be able to weed out events in which the bases were different, and leave those in which they were the same (there will be about half of them).

Let's say some spy has broken into the line and wants to eavesdrop on messages, but he also needs to measure the information on some basis. Let's imagine that it coincided for Alice and Bob, but not for the spy. In a situation where the data was sent in a horizontal-vertical basis, and the eavesdropper measured the transmission in a diagonal basis, he will receive a random value and forward some arbitrary photon to Bob, since he does not know what it should be. This way his intervention will be noticed.

The biggest problem with quantum cryptography is loss. Even the best and most modern optical fiber produces 50 percent losses for every 10-12 kilometers of cable. Let's say we send our secret key from Moscow to St. Petersburg - 750 kilometers, and only one out of a billion billion photons will reach the goal. All this makes the technology completely impractical. This is why modern quantum cryptography only works at a distance of about 100 kilometers. Theoretically, it is known how to solve this problem - with the help of quantum repeaters, but their implementation requires quantum teleportation.

Photo: Perry Mastrovito / Globallookpress.com

Quantum entanglement

The scientific definition of quantum entanglement is a delocalized state of superposition. It sounds complicated, but a simple example can be given. Suppose we have two photons: horizontal and vertical, whose quantum states are interdependent. We send one of them to Alice and the other to Bob, who makes measurements on a polarization beam splitter.

When these measurements are made in the usual horizontal-vertical basis, it is clear that the result will be correlated. If Alice noticed a horizontal photon, then the second one, naturally, will be vertical, and vice versa. This can be imagined more simply: we have a blue and a red ball, without looking we seal each of them in an envelope and send it to two recipients - if one receives a red one, the second will definitely receive a blue one.

But in the case of quantum entanglement, the matter does not stop there. This correlation takes place not only in the horizontal-vertical basis, but also in any other. For example, if Alice and Bob simultaneously rotate their beam splitters by 45 degrees, they will again have a perfect match.

This is a very strange quantum phenomenon. Let's say that Alice somehow rotated her beam splitter and detected some photon with polarization α that passed through it. If Bob measures his photon in the same basis, he will find a polarization of 90 degrees +α.

So at the beginning we have a state of entanglement: Alice's photon is completely uncertain and Bob's photon is completely uncertain. When Alice measured her photon and found some value, it is now known exactly what photon Bob has, no matter how far away he is. This effect has been repeatedly confirmed by experiments; it is not fantasy.

Quantum teleportation

Let's say Alice has a certain photon with polarization α, which she does not yet know, that is, it is in an unknown state. There is no direct channel between her and Bob. If there was a channel, then Alice would be able to register the state of the photon and convey this information to Bob. But it is impossible to know the quantum state in one measurement, so this method is not suitable. However, between Alice and Bob there is a prearranged entangled pair of photons. Due to this, it is possible to force Bob’s photon to accept the initial state of Alice’s photon, then “calling” via a conventional telephone line.

Here is a classic (albeit very distant analogue) of all this. Alice and Bob each receive a ball in an envelope - blue or red. Alice wants to send Bob information about what hers is. To do this, she needs to “call” Bob and compare the balls, telling him “I have the same one” or “we have different ones.” If someone overhears this line, it will not help them to know their color.

How does it all work? We have an entangled state and a photon that we want to teleport. Alice must make an appropriate measurement of the original teleported photon and ask what state the other one is in. She randomly receives one of four possible answers. As a result of the remote cooking effect, it turns out that after this measurement, depending on the result, Bob's photon went into a certain state. Before that, he was entangled with Alice's photon, being in an indeterminate state.

Alice tells Bob over the phone what the result of her measurements was. If its result, say, turned out to be ψ-, then Bob knows that his photon was automatically transformed into this state. If Alice reported that her measurement gave the result ψ+, then Bob's photon assumed polarization -α. At the end of the teleportation experiment, Bob ends up with a copy of Alice's original photon, and her photon and the information about it are destroyed in the process.

Teleportation technology

Now we can teleport the polarization of photons and some states of atoms. But when they write that scientists have learned to teleport atoms, this is a lie, because atoms have a lot of quantum states, an infinite number. At best, we figured out how to teleport a couple of them.

My favorite question is when will human teleportation happen? The answer is never. Let's say we have Captain Picard from Star Trek who needs to be teleported to the surface of a planet from a ship. To do this, as we already know, we need to make a couple more of the same Picards, bring them into an entangled state, which includes all its possible states (sober, drunk, sleeping, smoking - absolutely everything) and take measurements on both. It is clear how difficult and unrealistic this is.

Quantum teleportation is an interesting but laboratory phenomenon. It won't come down to teleportation of living beings (at least in the near future). However, it can be used in practice to create quantum repeaters to transmit information over long distances.

A group of scientists from the Chinese Academy of Sciences conducted a satellite experiment on the transfer of quantum states between pairs of entangled photons (so-called quantum teleportation) over a record distance of more than 1200 km.

The phenomenon (or entanglement) occurs when the states of two or more particles are interdependent (correlated), which can be separated to arbitrarily large distances, but at the same time they continue to “feel” each other. Measuring the parameter of one particle leads to instantaneous destruction of the entangled state of another, which is difficult to imagine without understanding the principles of quantum mechanics, especially since particles (this was specially shown in experiments on violation of the so-called Bell inequalities) do not have any hidden parameters in which information about the state of the “companion” would be stored, and at the same time, an instantaneous change in state does not lead to a violation of the principle of causality and does not allow useful information to be transmitted in this way.

To transmit real information, it is additionally necessary to involve particles moving at a speed not exceeding light speed. For example, photons that have a common progenitor can act as entangled particles, and, say, their spin is used as a dependent parameter.

Not only scientists involved in fundamental physics, but also engineers designing secure communications are showing interest in transmitting the states of entangled particles over increasingly long distances and under the most extreme conditions. It is believed that the phenomenon of particle entanglement will provide us with, in principle, unhackable communication channels in the future. “Protection” in this case will be the inevitable notification of the conversation participants that a third party has intervened in their communication.

Evidence of this will be the inviolable laws of physics - the irreversible collapse of the wave function.

Prototypes of devices for implementing such secure quantum communication have already been created, but ideas are also emerging to compromise the operation of all these “absolutely secure channels,” for example, through reversible weak quantum measurements, so it is still unclear whether quantum cryptography will be able to leave the prototype testing stage without whether all developments will turn out to be doomed in advance and unsuitable for practical use.

Another point: the transmission of entangled states has so far only been carried out over distances not exceeding 100 km, due to the loss of photons in the optical fiber or in the air, since the probability that at least some of the photons will reach the detector becomes vanishingly small. From time to time, reports appear about the next achievement along this path, but it is not yet possible to cover the entire globe with such a connection.

So, earlier this month, Canadian physicists announced successful attempts to communicate via a secure quantum channel with an aircraft, but it was only 3-10 km from the transmitter.

The so-called quantum repeater protocol is recognized as one of the ways to radically improve signal propagation, but its practical value remains in question due to the need to solve a number of complex technical issues.

Another approach is precisely the use of satellite technology, since the satellite can remain in line of sight to different very distant places on Earth at the same time. The main advantage of this approach would be that most of the photon path would be in a virtual vacuum, with almost zero absorption and no decoherence.

To demonstrate the feasibility of satellite experiments, Chinese experts conducted preliminary ground tests that demonstrated successful bidirectional propagation of entangled photon pairs through an open medium at distances of 600 m, 13 and 102 km with an effective channel loss of 80 dB. Experiments have also been carried out on the transfer of quantum states on moving platforms under conditions of high loss and turbulence.

After detailed feasibility studies with the participation of Austrian scientists, a $100 million satellite was developed and launched on August 16, 2016 from the Jiuquan Satellite Launch Center in the Gobi Desert using a Long March 2D launch vehicle into an orbit at an altitude of 500 km.

The satellite was named “Mo Tzu” in honor of the ancient Chinese philosopher of the 5th century BC, the founder of Moism (the doctrine of universal love and state consequentialism). For several centuries in China, Mohism successfully competed with Confucianism until the latter was adopted as the state ideology.

The Mozi mission is supported by three ground stations: Delinghe (Qinghai Province), Nanshan in Urumqi (Xinjiang) and the GaoMeiGu Observatory (GMG) in Lijiang (Yunnan Province). The distance between Delinghe and Lijian is 1203 km. The distance between the orbiting satellite and these ground stations ranges from 500-2000 km.

Because entangled photons cannot be simply “amplified” like classical signals, new techniques had to be developed to reduce attenuation in transmission links between Earth and satellites. To achieve the required communication efficiency, it was necessary to simultaneously achieve minimal beam divergence and high-speed and high-precision targeting of detectors.

Having developed an ultra-luminous cosmic source of two-photon entanglement and high-precision APT (acquiring, pointing, and tracking) technology, the team established “quantum coupling” between pairs of photons separated by 1203 km, scientists conducted the so-called Bell test to test locality violations (the ability to instantly influence the state of a remote particles) and obtained a result with a statistical significance of four sigma (standard deviations).

Diagram of the photon source on the satellite. The thickness of the KTiOPO4 (PPKTP) crystal is 15 mm. A pair of off-axis concave mirrors focuses the pump laser (PL) at the center of the PPKTP crystal. The output of a Sagnac interferometer uses two dichromatic mirrors (DM) and filters to separate signal photons from the pump laser. Two additional mirrors (PI), remotely controlled from the ground, are used to finely adjust the beam direction for optimal beam collection efficiency. QWP - quarter-wave phase section; HWP - half-wave phase section; PBS - polarizing beam splitter.

Compared to previous methods using the most common commercial telecommunications fibers, the efficiency of the satellite connection was many orders of magnitude higher, which, according to the study authors, opens the way to practical applications previously unavailable on Earth.

Quantum teleportation- this is teleportation not of physical objects, not of energy, but of states. But in this case, states are transmitted in a way that is impossible to do in the classical representation. As a rule, transmitting information about an object requires a large number of comprehensive measurements. But they destroy the quantum state, and we have no way to measure it again. Quantum teleportation is used to transmit and transfer a certain state, having minimal information about it, without “looking” into it, without measuring it, and thereby without disturbing it.

Qubits

A qubit is the state that is transferred during quantum teleportation. A quantum bit is in a superposition of two states. The classical state, for example, is either in state 0 or state 1. The quantum state is in a superposition, and, very importantly, until we measure it, it will not be defined. Let's imagine that we had a qubit that was 30% - 0 and 70% - 1. If we measure it, we can get both 0 and 1. You can't say anything with one measurement. But if we prepare 100, 1000 such identical states and measure them over and over again, we can quite accurately characterize this state and understand that there really were 30% - 0 and 70% - 1.

This is an example of obtaining information in the classical way. Having received a large amount of data, the recipient can recreate this state. However, quantum mechanics makes it possible not to prepare many states. Let's imagine that we have only one, unique one, and there is no other one. Then it will no longer be possible to convey it in the classics. Physically, directly, this is also not always possible. And in quantum mechanics we can use the entanglement effect.

We also use the phenomenon of quantum nonlocality, that is, a phenomenon that is impossible in the world we are accustomed to, so that this state disappears here and appears there. Moreover, the most interesting thing is that in relation to the same quantum objects there is a theorem about non-cloning. That is, it is impossible to create a second identical state. One must be destroyed in order for another to appear.

Quantum entanglement

What is the entanglement effect? These are two states prepared in a special way, two quantum objects - qubits. For simplicity, we can take photons. If these photons are separated over a large distance, they will correlate with each other. What does it mean? Let's imagine that we have one photon that is blue and another that is green. If we took them apart, looked at them and I found blue, that means yours turned out to be green, and vice versa. Or if you take a box of shoes containing a right and a left shoe, quietly take them out and in a bag take one shoe to you and the other to me. So I opened the bag, I look: I have the right one. So, you definitely have the left one.

The quantum case is different in that the state that came to me before the measurement is neither blue nor green - it is a superposition of blue and green. Once you've separated the boots, the outcome is already predetermined. While the bags are being carried, they have not yet been opened, but it is already clear what will be there. Until quantum objects are measured, nothing has been decided yet.

If we take not color, but polarization, that is, the direction of oscillations of the electric field, we can distinguish two options: vertical and horizontal polarization and +45° - -45°. If you add together the horizontal and vertical in equal proportions, you get +45°, if you subtract one from the other, then -45°. Now let’s imagine that in exactly the same way one photon got to me, and the other to you. I looked: it is vertical. So yours is horizontal. Now let’s imagine that I saw a vertical one, and you looked at it in a diagonal basis, that is, if you looked at it - it’s +45° or -45°, you will see with equal probability one or the other outcome. But if I looked at the diagonal basis and saw +45°, then I know for sure that you have -45°.

Einstein-Podolsky-Rosen paradox

Quantum entanglement is associated with the fundamental properties of quantum mechanics and the so-called Einstein-Podolsky-Rosen paradox. Einstein protested against quantum mechanics for so long because he believed that nature could not transmit information about a state at a speed faster than the speed of light. We can spread the photons very far, for example, by a light year, and open them at the same time. And we will still see this correlation.

But in fact, this does not violate the theory of relativity, because we still cannot transmit information using this effect. Either a vertical or horizontal photon is measured. But it is not known in advance what exactly it will be. Despite the fact that it is impossible to transmit information faster than the speed of light, entanglement makes it possible to implement a quantum teleportation protocol. What is it? An entangled pair of photons is born. One goes to the transmitter, the other to the receiver. The transmitter makes a joint measurement of the target photon that it must transmit. And with a ¼ probability he will get the result OK. He can communicate this to the receiver, and the receiver at that moment knows that he has exactly the same condition as the transmitter had. And with a probability of ¾ he gets a different result - not just an unsuccessful measurement, but simply a different result. But in any case, this is useful information that can be conveyed to the recipient. In three out of four cases, the recipient must perform an additional rotation of his qubit to obtain the transmitted state. That is, 2 bits of information are transmitted, and with their help you can teleport a complex state that cannot be encoded with them.

Quantum cryptography

One of the main areas of application of quantum teleportation is the so-called quantum cryptography. The idea behind this technology is that a single photon cannot be cloned. Therefore, we can transmit information in this single photon, and no one can duplicate it. Moreover, with any attempt by someone to find out something about this information, the state of the photon will change or be destroyed. Accordingly, any attempt to obtain this information by outsiders will be noticed. This can be used in cryptography and information protection. True, it is not useful information that is transmitted, but a key, which then classically makes it possible to transmit information absolutely reliably.

This technology has one big drawback. The fact is that, as we said earlier, it is impossible to create a copy of a photon. A normal signal in an optical fiber can be amplified. For the quantum case, it is impossible to amplify the signal, since the amplification will be equivalent to some kind of interceptor. In real life, on real lines, transmission is limited to a distance of approximately 100 kilometers. In 2016, the Russian Quantum Center conducted a demonstration on Gazprombank lines, where they showed quantum cryptography on 30 kilometers of fiber in an urban environment.

In the laboratory, we are able to demonstrate quantum teleportation at distances of up to 327 kilometers. But, unfortunately, long distances are impractical because photons are lost in the fiber and the speed is very low. What to do? You can install an intermediate server that will receive information, decrypt it, then encrypt it again and transmit it further. This is what the Chinese do, for example, when building their quantum cryptography network. The Americans use the same approach.

Quantum teleportation in this case is a new method that allows you to solve the problem of quantum cryptography and increase the distance to thousands of kilometers. And in this case, the same photon that is transmitted is teleported many times. Many groups around the world are working on this task.

Quantum memory

Let's imagine a chain of teleportations. Each of the links has a generator of entangled pairs, which must create and distribute them. This doesn't always happen successfully. Sometimes you need to wait until the next attempt to distribute pairs is successful. And the qubit must have some place where it will wait for teleportation. This is quantum memory.

In quantum cryptography, it is a kind of way station. Such stations are called quantum repeaters, and they are now one of the main areas for research and experimentation. This is a popular topic; in the early 2010s, repeaters were a very distant prospect, but now the task looks feasible. Largely because technology is constantly evolving, including due to telecommunications standards.

Progress of the experiment in the laboratory

If you come to the quantum communications laboratory, you will see a lot of electronics and fiber optics. All optics are standard, telecommunications, lasers are in small standard boxes - chips. If you go into the laboratory Alexander Lvovsky, where, in particular, they do teleportation, then you will see an optical table that is stabilized on pneumatic supports. That is, if you touch this table, which weighs a ton, with your finger, it will begin to float and sway. This is done because the technology that implements quantum protocols is very sensitive. If you stand on hard legs and walk around, then it will all be due to the vibrations of the table. That is, these are open optics, fairly large expensive lasers. In general, this is quite bulky equipment.

The initial state is prepared by laser. To prepare entangled states, a nonlinear crystal is used, which is pumped by a pulsed or continuous laser. Due to nonlinear effects, pairs of photons are born. Let's imagine that we have a photon of energy two - ℏ(2ω), it is converted into two photons of energy one - ℏω+ ℏω. These photons are born only together; first one photon cannot separate, then the other. And they are connected (entangled) and exhibit non-classical correlations.

History and current research

So, in the case of quantum teleportation, an effect is observed that we cannot observe in everyday life. But there was a very beautiful, fantastic image, which was just right for describing this phenomenon, which is why it was called so - quantum teleportation. As already mentioned, there is no moment in time when a qubit still exists here, but there it has already appeared. That is, it is first destroyed here, and only then appears there. This is the same teleportation.

Quantum teleportation was proposed theoretically in 1993 by a group of American scientists led by Charles Bennett - that’s when the term appeared. The first experimental implementation was carried out in 1997 by two groups of physicists in Innsbruck and Rome. Gradually, scientists were able to transmit states over increasingly greater distances - from one meter to hundreds of kilometers or more.

Now people are trying to do experiments that may become the basis for quantum repeaters in the future. It is expected that in 5–10 years we will see real quantum repeaters. The direction of state transfer between objects of different nature is also developing, including in May 2016, hybrid quantum teleportation was carried out at the Quantum Center, in the laboratory of Alexander Lvovsky. The theory also does not stand still. In the same Quantum Center, under the leadership of Alexei Fedorov, a teleportation protocol is being developed not in one direction, but bidirectional, so that with the help of one pair, states can be teleported simultaneously towards each other.

Our work on quantum cryptography creates a quantum distribution and key device, meaning we generate a key that cannot be intercepted. And then the user can encrypt information with this key, using the so-called one-time pad. New advantages of quantum technologies should be revealed in the next decade. The creation of quantum sensors is developing. Their essence is that due to quantum effects we can measure, for example, magnetic field and temperature much more accurately. That is, the so-called NV centers in diamonds are taken - these are tiny diamonds, they have nitrogen defects that behave like quantum objects. They are very similar to a frozen single atom. Looking at this defect, one can observe temperature changes, even inside a single cell. That is, measure not just the temperature under the arm, but the temperature of the organelle inside the cell.

The Russian Quantum Center also has a spin diode project. The idea is that we can take an antenna and start harvesting energy from background radio waves very efficiently. It is enough to remember how many Wi-Fi sources there are now in cities to understand that there is a lot of radio wave energy around. It can be used for wearable sensors (for example, a blood sugar sensor). They require constant energy supply: either a battery or a system that collects energy, including from a mobile phone. That is, on the one hand, these problems can be solved with the existing element base with a certain quality, and on the other hand, quantum technologies can be applied and this problem can be solved even better, even more miniaturized.

Quantum mechanics has greatly changed human life. Semiconductors, the atomic bomb, nuclear energy - these are all objects that work thanks to it. The whole world is now struggling to begin to control the quantum properties of single particles, including entangled ones. For example, teleportation involves three particles: one pair and a target one. But each of them is managed separately. Individual control of elementary particles opens new horizons for technology, including a quantum computer.

Yuri Kurochkin, Candidate of Physical and Mathematical Sciences, Head of the Quantum Communications Laboratory of the Russian Quantum Center.

Tags:

Add tags

On the website of the journal Nature, on August 9, Chinese scientists were published who managed to achieve quantum teleportation over a distance of about 97 km. This is a new record, although in arXiv.org since May 17 there has been one that has not yet been published anywhere by another group, which reports successful experiments on teleportation over a distance of about 143 km.

Despite the fact that the phenomenon of quantum teleportation has been studied for quite some time, people far from science have no understanding of what it is. I will try to dispel some myths associated with this part of science.

Myth 1: Quantum teleportation theoretically allows you to teleport any object.

In fact, during quantum teleportation, it is not physical objects that are transmitted, but some information recorded using the quantum states of objects. Typically this state is photon polarization. As is known, a photon can have two different polarizations: for example, horizontal and vertical. They can be used as carriers of bit information: say, 0 will correspond to horizontal polarization, and 1 to vertical. Then the transfer of the state of one photon to another will ensure the transfer of information.

In the case of quantum teleportation, data transfer occurs as follows. First, a pair of so-called linked photons is created. This means that their states turn out to be connected in a sense: if one has a horizontal polarization when measured, then the other will always have a vertical polarization and vice versa, and both options occur with the same probability. Then these photons are carried away: one remains at the source of the message, and the other is carried away by its receiver.

When a source wants to transmit its message, it couples its photon with another photon whose state (that is, polarization) is precisely known, and then measures the polarization of both of its photons. At this moment, the state of the photon located at the receiver changes in a consistent manner. By measuring its polarization and learning through other communication channels the results of measurements of source photons, the receiver can accurately determine which bit of information was transmitted.

Myth 2: Quantum teleportation can transmit information faster than the speed of light.

Indeed, according to modern concepts, the transfer of states between linked photons occurs instantly, thus, one may get the feeling that information is transmitted instantly. This, however, is not the case, since although the state was transmitted, it can be read by deciphering the message only after transmitting additional information about what the polarizations of the two photons located at the source are. This additional information is transmitted via classical communication channels and its transmission speed cannot exceed the speed of light.

Myth 3: It turns out that quantum teleportation is completely uninteresting.

Of course, in practice it turns out that the process of quantum teleportation may not be as exciting as its name might suggest, but it can have important practical applications. First of all, this is a secure data transfer. It is always possible to intercept a message sent through classical communication channels, but only the one who has the second linked photon can use it. Everyone else simply won’t be able to read the message. Unfortunately, the real use of this effect is still far away; at this stage, only scientific experiments are underway, requiring quite complex equipment.

If you are interested in this topic, you may also be interested in reading about what

Alexander Lvovsky, a professor at the Faculty of Physics at the University of Calgary (Canada), a member of the Canadian Institute of Advanced Studies, tried to talk in simple language about the principles of quantum teleportation and quantum cryptography.

Key to the lock

In any cryptography, in any communication, the most expensive resource is a random sequence of zeros and ones, which is owned only by two communicating. But in most cases, public key cryptography is used. Let's say you buy something with a credit card in an online store using the secure HTTPS protocol. Through it, your computer talks to some server with which it has never communicated before, and it did not have the opportunity to exchange a secret key with this server.

The secret of this dialogue is ensured by solving a complex mathematical problem, in particular, factorization. It’s easy to multiply two prime numbers, but if you’ve already been given the task of finding their product, finding two factors, then it’s difficult. If the number is large enough, it will require a conventional computer to calculate for many years.

Quantum cryptography

How does a polarizing beam splitter work?

Principle of quantum cryptography

Quantum entanglement

Thus, there are four possible outcomes of events (conventionally, the recipients have blue balls, red balls, red and blue, or blue and red). They are interesting because they form a basis. If we have two photons with an unknown polarization, then we can “ask them a question” in which of these states they are in and get an answer. But if at least one of them turns out to be entangled with some other photon, then the effect of remote preparation will occur, and the third, remote photon will be “prepared” in a certain state. This is what quantum teleportation is based on.

Teleportation technology