Quantum teleportation Bashny.Net

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Quantum teleportation is not teleporting physical objects, not energy, and state. But in this case the condition is transmitted in such a way that in the classic view, it can be done. As a rule, to transmit information about an object requires a large number of comprehensive measurement. But they are destroying the quantum state, and we have no way of re-measuring it. Quantum teleportation is used to transmit, to transfer a certain state with a minimum of information about him not "looking" at it, not measuring, thereby not disturbing.

Qubits
The qubit is the state that is transmitted by quantum teleportation. The quantum bit is in a superposition of two States. Classic as is, for example, either state 0 or state 1. Quantum is in a superposition, and, very importantly, while we did not measure, it will not be defined. Imagine that we had a qubit 30% 0 and 70% 1. If we measure, we can obtain both 0 and 1. In one measurement it is impossible to say anything. But if you make 100, 1000 of these same States over and over again to measure them, we can accurately characterize this condition and to understand that indeed there was a 30% 0 and 70% 1.

This is an example of information in the classical way. Having a large amount of data, the recipient can recreate the condition. However, quantum mechanics allows one to cook a lot of States. Let us imagine that we have only one, unique, and the second does not. Then the classic pass it will not happen. Physically, directly, this is not always possible. But in quantum mechanics we can use the effect of entanglement.

We also use the phenomenon of quantum nonlocality, that is, a phenomenon which is impossible in the usual world, so that here is a condition disappeared, and there appeared. And most interesting is that applied to the same quantum object exists theorem nechirvan. That is not possible to create a second identical state. It is necessary to destroy one to have another one.

Quantum entanglement
What is the effect of confusion? It is specially prepared with two States, two quantum object — the qubit. For simplicity, it is possible to take photons. If these photons are spread over a long distance, they will correlate with each other. What's the meaning of that? Let us imagine that we have one photon of blue and the other green. If we blew, I looked up and I was blue, you were green, and Vice versa. Or if you take a box of shoes, where there is a right and left Shoe, discreetly take them out and bag to carry one Shoe you and one for me. So I opened the bag, I looked around and right. So, you just left.

The quantum case is different because the condition, which came to me before measurement, not blue and not green, it is in a superposition of blue and green. After you've shared the shoes, the result is already predetermined. While the bags are, while they have not yet opened, but already precisely it is clear that there will be. And yet, quantum objects are measured, nothing is decided.

If you take no color, and polarization, i.e. the direction of oscillation of the electric field, there are two options: vertical and horizontal polarization and +45° — -45°. If you add together equal proportions of the horizontal and vertical, you get +45°, if we subtract one from the other, -45°. Now imagine that exactly one photon was sent to me and another to you. I have read that it is vertical. So, you have horizontal. Now imagine that I saw the vertical, and you saw it in a diagonal basis, that is looked — it +45° or -45°, you will see equal likelihood that any other outcome. But if I looked in a diagonal basis and saw +45°, then just know 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. Einstein so long protested against quantum mechanics because he thought that nature could not with velocity greater than the speed of light, convey status information. We can post the photons are very far away, for example, light-year, and open at the same time. And we still see this correlation.

But really, relativity theory is not violated, because the information with this effect we pass still can not. Measured either vertical or horizontal photon. But no one knows in advance what it will be. Despite the fact that it is impossible to transmit information faster than the speed of light, entanglement allows to implement the Protocol of quantum teleportation. What is it? Born tangled pair of photons. One is sent to the transmitter, the other to the receiver. The transmitter produces a joint measurement of the target photon, which it needs to send. And with probability ¼ he will get the result OK. It can inform the recipient and the recipient at this point learns that he is exactly the same as it was at the transmitter. And with probability ¾ he gets a different result — not that a failed measurement, but just a different result. But in any case, this is useful information that can be sent to the recipient. The recipient in three of the four cases have to produce additional rotate his qubit to obtain the parameter condition. That is, transmitted 2 bits of information, and you can teleport complex condition, which they not encode.

Quantum cryptography
One of the main fields of application of quantum teleportation is the so — called quantum cryptography. The idea of this technology lies in the fact that a single photon cannot be cloned. Therefore, we can transmit information in this single photon, and no one will be able to duplicate it. Moreover, any attempt by someone to learn something about the information state of a photon will change or be destroyed. Accordingly, any attempt to get this information to outsiders will be noticed. It is possible to use cryptography to protect the information. However, it is not transmitted useful information, and the key, which then classically is possible to securely transmit information.

This technology has one big disadvantage. The fact that, as we said earlier, create a copy of the photon is impossible. The normal signal in the optical fiber can be enhanced. For the quantum case amplify the signal, as the gain will be equivalent to some interceptor. In real life, in real transmission lines is limited by the distance to about 100 kilometers. In 2016, the Russian quantum center, a demonstration was held on the lines of Gazprombank, where he demonstrated quantum cryptography in 30 miles of fiber in urban environments.

In the laboratory we can demonstrate quantum teleportation at a distance of up to 327 kilometres. But, unfortunately, the distance is impractical, because the photons are being lost in the fiber and the speed obtained is very low. What to do? You can put an intermediate server to obtain information to decrypt, then encrypt again and pass it on. Do, for example, Chinese in the construction of its network quantum cryptography. The same approach used by the Americans.

Quantum teleportation in this case is a new method which allows to solve the problem of quantum cryptography and to increase the distance to thousands of kilometers. In this case, the photon that passes repeatedly teleports. On this task has many groups all over the world.

Quantum memory
Imagine a chain of teleportations. Each of the links a generator of entangled pairs, which have to create and distribute. It is not always successfully occur. Sometimes you have to wait until successfully will the next attempt of distribution pairs. And the qubit must be some place where he'll wait for teleportation. It is a quantum memory.

Quantum cryptography is a kind of relay 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 is implemented. Largely because the technology is constantly developing, including through the telecommunication standards.

The experiment in the lab
If you come to the laboratory of quantum communication, 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 lab of Alexander Lvov, where, in particular, make the teleportation, you will see the optical bench, which is stable on pneumothorax. That is, if this table, which weighs a ton, touch your finger, it will start to float and wiggle. This is done due to the fact that the technology that implements quantum protocols is very sensitive. If you put on hard feet and will walk around, then it will be all the fluctuations of the Desk. That is, open optics, a sufficiently large expensive lasers. Overall this is a fairly bulky equipment.

The initial state is prepared by laser. To prepare entangled States of the nonlinear crystal which is pumped by a pulsed or continuous laser. Due to nonlinear effects are born a pair of photons. Let us imagine that we have a photon energy two — ℏ(2ω), it is converted into two photons of energy one — ℏω+ ℏω. These photons are born only together, will be able to separate one photon, then the other. And they are connected (entangled) and exhibit non-classical correlations.

History and current research
So, in the case of quantum teleportation observed effect that in daily life we can not watch. But it was very beautiful, a fantastic image, which by the way has come to describe this phenomenon, so called because quantum teleportation. As already mentioned, there is no time, when the qubit is still there, and there he has already appeared. That is, first destroyed here, and only then there appears. It is precisely the teleportation.

Quantum teleportation was proposed theoretically in 1993 by a group of American scientists under the leadership of Charles Bennett — and then came this term. The first experimental implementation was conducted in 1997 two groups of physicists in Innsbruck and Rome. Gradually scientists have been able to pass state at an increasing distance from one meter to hundreds of kilometers or more.

Now people are trying to do experiments that may in the future become the basis for quantum repeaters. It is expected that after 5-10 years we will see the real quantum repeaters. Develops and direction of wealth transfer between objects of different nature, including in may 2016 was carried out hybrid quantum teleportation in Quantum center, in the laboratory of Alexander Lvov. The theory also does not stand still. In the same Quantum center under the leadership of Alexey Fedorov developed the teleportation Protocol is not in one direction, and bidirectional, to use one pair at once towards each other to teleport the state.

As part of our work on quantum cryptography creates a quantum distribution device and key, that is, we generate a key that can't be intercepted. And then the user can encrypt with this key information, using so-called one-time pad. New benefits of quantum technologies shall be disclosed in the next decade. Develops the creation of quantum sensors. Their essence is that due to quantum effects we can much more accurately measure, for example, magnetic field, temperature. That is, take the so-called NV-centers in diamonds are tiny diamonds have nitrogen defects, which behave quantum objects. They are very similar to frozen a single atom. Despite this defect, one can observe temperature changes, and inside single cells. That is, to measure not just the temperature under the mouse, and the temperature of the organelles inside the cell.


The Russian quantum center also has a project spin diode. The idea is that we can take the antenna and start very effective to collect energy from the background radio waves. Suffice it to recall how many Wi-Fi sources in cities now to understand that the energy of the radio waves around a lot. It can be used for wearable sensors (e.g., sensor blood sugar level). They need a constant energy supply: either a battery or a system that gathers energy, including from a mobile phone. That is, on the one hand, these problems can be solved with the existing circuitry with a certain quality, and on the other hand, it is possible to apply quantum technology to solve this problem even better, even more petite.

Quantum mechanics changed human life. Semiconductors, atomic bomb, atomic energy is all the objects that work because of it. The whole world is now struggling to begin to control quantum properties of single particles, including tangled. For example, in teleportation involves three particles: one pair and the target. But each of them is controlled separately. Individual control of elementary particles opens up new horizons for technology, including quantum computer.

Yury Kurochkin, candidate of physico-mathematical Sciences, head of laboratory of quantum communications of the Russian quantum center.

Source: geektimes.ru/company/postnauka/blog/281010/