The trouble of quantum teleportation
In the wake of the post Physics of the Impossible: Teleportation decided to write its own five kopecks. But with reference to communication systems.
So what we have. It is impossible to transmit information over any classical communication channels with a speed exceeding the speed of light. On the scale of the Earth, this may be enough, but for long-distance space communications, alas, no. 299792 km / s - of course, the value is huge, but not infinite. So the signal from the moon to earth will go one and a half seconds (this circumstance, by the way, spoiled a lot of blood to the operators of the Soviet lunar rovers), from Mars more than six minutes, and from Voyager, which was launched back in 1977 and is already a cockroach somewhere then on the outskirts of the solar system, at a distance of 15 billion kilometers from Earth, the signal will go more than 14 hours. In addition, the power of the radio signal decreases in proportion to the square of the distance traveled by it, because of which the weak emitters can be “lost” in the background cosmic noise very quickly.
But it is still too early to lower the paws because of the injustice of the laws of the universe. There is such a magic thing - quantum teleportation. The bottom line is that a certain amount of information (theoretically, arbitrarily large) can be instantly transmitted to any distance .
How it works. Suppose there is a certain particle (for example, a photon) which has a certain measurable quantum state (for example, polarization). Before the measurement, the quantum state (photon polarization) is in the field of probabilities, that is, it is not clearly defined. But after measuring, the whole variety of possible quantum states collapses into one, clearly defined.
In general, in quantum mechanics, observation plays a huge, almost mystical role. The properties of some objects in space and time can be changed simply by the mere fact of the presence of an observer. A vivid example is an experiment with two slots, I don’t know if this video was published before, but just in case, here:
')
But back to quantum teleportation. With one particle sorted out. Now add to the system the second photon associated with the first. This phenomenon is called quantum entanglement, when two or more particles are described by a single wave function. Moreover, if one measures the parameters of a single particle, the corresponding characteristics appear (exactly appear, not become known) in the second. At the same time, the distance that the particles are spaced apart does not matter. Moreover, time does not play a role either, which creates additional difficulties both in the implementation of the experiment and in the likely practical use of quantum teleportation.
The joke is that measurements separately do not make sense. Suppose we have two entangled particles. One in Moscow, the other in Yaroslavl. We measure the parameters of a particle located in Moscow. In this case, all silent as partisans. After some time in Yaroslavl, they also measure the parameters of their particle. The key point is that in Yaroslavl they are not aware of whether they have taken measurements in Moscow or not. Later, both groups of experimenters meet and share impressions. Of course, both groups had the same results. BUT! The trick is that from the point of view of quantum mechanics, it is impossible to answer the question, whose measurements have influenced the result. Yaroslavl or Moscow. Because time in this case does not matter. For definiteness, experimenters should exchange data via a classical communication channel, and they should signal simply the fact of the experiment, but not the results. In this case, the Sender and Recipient of information is determined accurately.
Let's repeat the same experiment with Moscow and Yaroslavl. This time measurements are carried out in Yaroslavl, after which exactly one bit of information is sent to the masquiches. "one". That symbolizes that measurements are taken. In Moscow, they also measure their particle and get the same result as in Yaroslavl. But this time it is clear that the Sender was Yaroslavl, and the Recipient was Moscow.
And now, about the evils of quantum teleportation.
The first trouble: as far as I know (although I'm not sure about it), it is impossible to predict the measurement results in advance. That is, if we take all the same polarization of the photon, then after the measurement we get just a random value, and we can not influence it in any way. Thus, with the help of quantum teleportation we transmit not meaningful data, but a set of random variables. For example, the photon polarization can be "+1" or "-1". Suppose "+1" we will have a logical 1 and "-1" - a logical zero. Type bits. And we want from Moscow to Yaroslavl to transmit a logical zero using quantum teleportation. Measure - bang! And when measuring we got a logical unit. Just chance so coincided. And in Yaroslavl, the received data is taken at face value and they think “Bah! logical unit came. Damn me, if I know what these stupid men from Moscow wanted to say. ”
The second trouble: as I wrote above, time does not matter. Thus, to obtain correct results, it is necessary to notify the Recipient that the Sender has already taken measurements. In this case, what is the point of stirring up all this quantum teleportation in general if a classical communication channel is still needed for data transmission? (although it is worth clarifying that a classic channel transmits a negligible amount of information, and a quantum channel theoretically, arbitrarily large, so that within computer networks, for example, there is a sense and some other, you want the speed of information exchange, say, 100 Gbit / s? And 100 Tbit / s? =)
The third misfortune: the second arises from the misfortune. Theoretically (again, I’m not sure about my conclusions) if the Sender and Recipient have synchronized clocks and they agreed in advance that at 2 o'clock in the afternoon the Sender will make measurements, then the Recipient can take measurements at 3 pm and get the correct result. But, in relation to space communications, there is also a problem here. Both the Sender and Recipient must belong to the form homo sapiens. Automatics are not suitable here, because any measurements made by automatics on a quantum particle still remain in the field of probabilities until they are seen by an Observer, that is, a person .
That such garbage going on in the world of quantum mechanics. However, everything is so confusing and interesting that scientists will never give up this area and still come up with how to use these and many other, in fact, magic quantum effects in practice.
So what we have. It is impossible to transmit information over any classical communication channels with a speed exceeding the speed of light. On the scale of the Earth, this may be enough, but for long-distance space communications, alas, no. 299792 km / s - of course, the value is huge, but not infinite. So the signal from the moon to earth will go one and a half seconds (this circumstance, by the way, spoiled a lot of blood to the operators of the Soviet lunar rovers), from Mars more than six minutes, and from Voyager, which was launched back in 1977 and is already a cockroach somewhere then on the outskirts of the solar system, at a distance of 15 billion kilometers from Earth, the signal will go more than 14 hours. In addition, the power of the radio signal decreases in proportion to the square of the distance traveled by it, because of which the weak emitters can be “lost” in the background cosmic noise very quickly.
But it is still too early to lower the paws because of the injustice of the laws of the universe. There is such a magic thing - quantum teleportation. The bottom line is that a certain amount of information (theoretically, arbitrarily large) can be instantly transmitted to any distance .
How it works. Suppose there is a certain particle (for example, a photon) which has a certain measurable quantum state (for example, polarization). Before the measurement, the quantum state (photon polarization) is in the field of probabilities, that is, it is not clearly defined. But after measuring, the whole variety of possible quantum states collapses into one, clearly defined.
In general, in quantum mechanics, observation plays a huge, almost mystical role. The properties of some objects in space and time can be changed simply by the mere fact of the presence of an observer. A vivid example is an experiment with two slots, I don’t know if this video was published before, but just in case, here:
')
But back to quantum teleportation. With one particle sorted out. Now add to the system the second photon associated with the first. This phenomenon is called quantum entanglement, when two or more particles are described by a single wave function. Moreover, if one measures the parameters of a single particle, the corresponding characteristics appear (exactly appear, not become known) in the second. At the same time, the distance that the particles are spaced apart does not matter. Moreover, time does not play a role either, which creates additional difficulties both in the implementation of the experiment and in the likely practical use of quantum teleportation.
The joke is that measurements separately do not make sense. Suppose we have two entangled particles. One in Moscow, the other in Yaroslavl. We measure the parameters of a particle located in Moscow. In this case, all silent as partisans. After some time in Yaroslavl, they also measure the parameters of their particle. The key point is that in Yaroslavl they are not aware of whether they have taken measurements in Moscow or not. Later, both groups of experimenters meet and share impressions. Of course, both groups had the same results. BUT! The trick is that from the point of view of quantum mechanics, it is impossible to answer the question, whose measurements have influenced the result. Yaroslavl or Moscow. Because time in this case does not matter. For definiteness, experimenters should exchange data via a classical communication channel, and they should signal simply the fact of the experiment, but not the results. In this case, the Sender and Recipient of information is determined accurately.
Let's repeat the same experiment with Moscow and Yaroslavl. This time measurements are carried out in Yaroslavl, after which exactly one bit of information is sent to the masquiches. "one". That symbolizes that measurements are taken. In Moscow, they also measure their particle and get the same result as in Yaroslavl. But this time it is clear that the Sender was Yaroslavl, and the Recipient was Moscow.
And now, about the evils of quantum teleportation.
The first trouble: as far as I know (although I'm not sure about it), it is impossible to predict the measurement results in advance. That is, if we take all the same polarization of the photon, then after the measurement we get just a random value, and we can not influence it in any way. Thus, with the help of quantum teleportation we transmit not meaningful data, but a set of random variables. For example, the photon polarization can be "+1" or "-1". Suppose "+1" we will have a logical 1 and "-1" - a logical zero. Type bits. And we want from Moscow to Yaroslavl to transmit a logical zero using quantum teleportation. Measure - bang! And when measuring we got a logical unit. Just chance so coincided. And in Yaroslavl, the received data is taken at face value and they think “Bah! logical unit came. Damn me, if I know what these stupid men from Moscow wanted to say. ”
The second trouble: as I wrote above, time does not matter. Thus, to obtain correct results, it is necessary to notify the Recipient that the Sender has already taken measurements. In this case, what is the point of stirring up all this quantum teleportation in general if a classical communication channel is still needed for data transmission? (although it is worth clarifying that a classic channel transmits a negligible amount of information, and a quantum channel theoretically, arbitrarily large, so that within computer networks, for example, there is a sense and some other, you want the speed of information exchange, say, 100 Gbit / s? And 100 Tbit / s? =)
The third misfortune: the second arises from the misfortune. Theoretically (again, I’m not sure about my conclusions) if the Sender and Recipient have synchronized clocks and they agreed in advance that at 2 o'clock in the afternoon the Sender will make measurements, then the Recipient can take measurements at 3 pm and get the correct result. But, in relation to space communications, there is also a problem here. Both the Sender and Recipient must belong to the form homo sapiens. Automatics are not suitable here, because any measurements made by automatics on a quantum particle still remain in the field of probabilities until they are seen by an Observer, that is, a person .
That such garbage going on in the world of quantum mechanics. However, everything is so confusing and interesting that scientists will never give up this area and still come up with how to use these and many other, in fact, magic quantum effects in practice.
Source: https://habr.com/ru/post/78842/
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