Quantum teleportation



    Quantum teleportation is the teleportation of states, not physical objects, not energy. But in this case, the states are transmitted in such a way that it cannot be done in the classical representation. As a rule, a large number of comprehensive measurements are required to transmit information about an object. But they destroy the quantum state, and we have no opportunity to re-measure it. Quantum teleportation is used to transfer, transfer some state, possessing minimal information about it, not “looking” at it, not measuring it and thus not disturbing it.

    Qubits


    A qubit is a state that is transmitted during quantum teleportation. The quantum bit is in a superposition of two states. The classical state is, for example, either in state 0 or state 1. The quantum state is in superposition, and, which is very important, until we measure it, it will not be defined. Imagine that we had a qubit at 30% - 0 and at 70% - 1. If we measure it, we can get both 0 and 1. We cannot say anything in one measurement. But if we prepare 100, 1000 of such identical states and measure them time after time, we can fairly accurately characterize this state and understand that there really was 30% - 0 and 70% - 1.

    This is an example of obtaining information in the classical way. Having received a large amount of data, the addressee can recreate this state. However, quantum mechanics allows not to prepare many states. Imagine that we have only one, unique, and there is no second one. Then in the classics pass it will not work. Physically, directly, this is also not always possible. And in quantum mechanics, we can use the effect of entanglement.

    We also use the phenomenon of quantum non-locality, that is, a phenomenon that is impossible in the world we are used to in order for this state to disappear and appear there. And the most interesting is that, as applied to the same quantum objects, there is a non-cloning theorem. That is, it is impossible to create a second identical state. It is necessary to destroy one in order to have another.

    Quantum confusion


    What is the effect of entanglement? These are two states specially prepared in a special way, two quantum objects - a qubit. For simplicity, you can take photons. If these photons are spaced apart over a long distance, they will correlate with each other. What does it mean? Imagine that one photon is blue and the other is green. If we smashed them, looked and turned out to be blue, it means that you turned green, and vice versa. Or if you take a box of shoes, where there is a right and left shoe, unnoticeably pull them out and in the bag carry one shoe to you, the other to me. So I opened the bag, I look: I have the right one. So you have exactly the left.

    The quantum case differs in that the state that came to me before the measurement is not blue and not green — it is in a superposition of blue and green. After you have divided the shoes, the result is already predetermined. While the bags are carried, they have not yet been opened, but it is already clear what will be there. In the meantime, quantum objects are not measured, nothing has yet been decided.

    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 put together in equal proportions horizontal and vertical, you get + 45 °, if you subtract one from the other, then -45 °. Now imagine that in the same way one photon came to me, and another to you. I looked: it is vertical. So you have a horizontal one. Now imagine that I saw a vertical, and you looked at it in a diagonal basis, that is, it looked - it is + 45 ° or -45 °, you will see with a similar probability this outcome. But if I looked at the diagonal basis and saw + 45 °, then I know for sure that you have -45 °.

    The paradox of Einstein - Podolsky - Rosen


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

    But in reality, the theory of relativity does not violate it, because with the help of this effect, we still cannot transmit information. Measured either vertical or horizontal photon. But it is not known in advance exactly what it will be. Despite the fact that it is impossible to transmit information faster than the speed of light, entanglement allows implementing the quantum teleportation protocol. What is it? A tangled 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, which it must transmit. And with probability ¼ he will get the result OK. He can inform the recipient about this, and the recipient at that moment finds out that he has exactly the same state as the transmitter did. And with probability ¾ he gets a different result - not that an unsuccessful measurement, but just another result. But in any case it is useful information that can be transferred to the recipient. In three out of four cases, the recipient must perform an additional rotation of his qubit in order to obtain the transmitted state. That is, 2 bits of information are transmitted, and with the help of them you can teleport a complex state that they cannot encode.

    Quantum cryptography


    One of the main applications of quantum teleportation is the so-called quantum cryptography. The idea of ​​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, in any attempt by someone to find out something about this information, the state of the photon will change or collapse. Accordingly, any attempt to get this information to outsiders will be noticed. This can be used in cryptography, in the protection of information. True, it is not the useful information that is transmitted, but the key, to which then it is already classically possible to transmit information absolutely reliably.

    This technology has one big disadvantage. The fact is that, as we have said before, it is impossible to create a copy of a photon. Normal signal in fiber can be enhanced. For the quantum case, the signal cannot be amplified, since the amplification will be equivalent to some interceptor. In real life, in real lines, transmission is limited to approximately 100 kilometers. In 2016, the Russian Quantum Center held a demonstration on the lines of Gazprombank, where they showed quantum cryptography on 30 kilometers of fiber in urban environments.

    In the laboratory, we are able to show quantum teleportation at a distance 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 put an intermediate server that will receive information, decrypt, then encrypt again and transfer further. So do, for example, the Chinese in the construction of their network of quantum cryptography. The same approach is used by the Americans.

    Quantum teleportation in this case is a new method that allows solving the problem of quantum cryptography and increasing the distance to thousands of kilometers. And in this case, the very photon that is transmitted is teleported many times. A lot of groups all over the world are working on this task.

    Quantum memory


    Imagine a teleportation chain. In each of the links there is a generator of entangled pairs, which must create and distribute them. This is not always successful. Sometimes you need to wait until another pair distribution attempt is successful. And the qubit should have some place where he will wait for teleportation. This is quantum memory.

    In quantum cryptography, this is a kind of intermediate 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. In many ways, because the technology is constantly evolving, including through telecommunications standards.

    The course 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 standard optics, telecommunications, lasers in small standard boxes - chips. If you go to the laboratory of Alexander Lvovsky , where, in particular, teleportation is done, then you will see an optical table that is stabilized on pneumatic supports. That is, if this table, which weighs a ton, is touched with a finger, then it will begin to float, sway. This is due to the fact that the technique that implements quantum protocols is very sensitive. If you put on tight legs and walk around, then it will all be according to the vibrations of the table. That is, it is open optics, rather large expensive lasers. In general, it is quite bulky equipment.

    The initial state is prepared by the laser. To prepare entangled states, a nonlinear crystal is used, which is pumped by a pulsed or cw laser. Due to non-linear effects, photon pairs are generated. 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; one photon cannot separate first, then the other. And they are connected (tangled) and show non-classical correlations.

    History and relevant research


    So, in the case of quantum teleportation, an effect is observed that we cannot observe in daily life. But on the other hand, it was a very beautiful, fantastic image, which was most appropriate for describing this phenomenon, which is why quantum teleportation was called that. As already mentioned, there is no point in time when the qubit still exists, and there it has already appeared. That is, first destroyed here, and only then appears there. This is the very teleportation.

    Quantum teleportation was proposed theoretically in 1993 by a group of American scientists under the leadership of Charles Bennett - then this term appeared. The first experimental implementation was carried out in 1997 by two groups of physicists at once in Innsbruck and Rome. Gradually, scientists were able to transfer states over an ever-increasing distance - from one meter to hundreds of kilometers or more.

    Now people are trying to do experiments, which, in the future, may become the basis for quantum repeaters. It is expected that after 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 a hybrid quantum teleportation was carried out in 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 Alexey Fedorov, the teleportation protocol is no longer developed in one direction, but bidirectional, in order to teleport states simultaneously with the help of one pair at a time to meet each other.

    As part of our work on quantum cryptography, a quantum distribution device and a key are created, that is, we generate a key that cannot be intercepted. And then the user can encrypt information with this key using the so-called one-time notebook. New advantages of quantum technologies should unfold in the coming decade. The development of quantum sensors is developing. Their essence is that due to quantum effects we can measure, for example, a magnetic field, temperature much more accurately. That is, the so-called NV-centers in diamonds are taken - these are tiny diamonds, they have nitrogen defects that quantum objects behave. They are very similar to the frozen single atom. Looking at this defect, it is possible to observe changes in temperature, and within a single cell. That is, to measure not just the temperature under the arm,


    The Russian quantum center also has a spin diode project. The idea is that we can take an antenna and begin to collect energy very efficiently from background radio waves. It is enough to remember how many Wi-Fi sources 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, for a blood sugar level sensor). They need a constant energy supply: either a battery or a system that collects energy, including from a mobile phone. That is, on the one hand, these tasks 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 miniature.

    Quantum mechanics have greatly changed human life. Semiconductors, the atomic bomb, atomic 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, three particles are involved in teleportation: one pair and target. But each of them is managed separately. Individual control of elementary particles opens up new horizons for technology, including a quantum computer.

    Yuri Kurochkin , Ph.D. in Physics and Mathematics, Head of the Quantum Communications Laboratory at the Russian Quantum Center.

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