December 21, 2009 at 11:53The trouble of quantum teleportation
In the wake of the post Physics of the Impossible: Teleportation decided to write its five cents. But with reference to communication systems.
So what we have. It is impossible to transmit information with a speed exceeding the speed of light through any classical communication channels. On Earth’s scale, this may be enough, but alas, no for long-distance space communications. 299792 km / s - of course, the value is huge, but not infinite. So the signal from the Moon to the Earth will go one and a half seconds (this circumstance, incidentally, spoiled a lot of blood for the operators of the Soviet moon rovers), from Mars - more than six minutes, and from Voyager, which was launched back in 1977 and is now 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, due to which weak emitters can be "lost" in the background cosmic noise very quickly.
But it is too early to lower paws due to the injustice of the laws of the Universe. There is such a magical 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 some measurable quantum state (for example, polarization). Before the measurement, the quantum state (polarization of the photon) is in the probability field, that is, it is not clearly defined. But after the measurement, 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 you measure the parameters of one particle, the second appears (it appears, and not become known) the corresponding characteristics. In this case, the distance to which the particles are spaced does not play any role. Moreover, time does not play a role, which creates additional difficulties both in the implementation of the experiment and in the likely practical use of quantum teleportation.
The trick 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. At the same time, everyone is silent as partisans. After some time, the parameters of their particle are also measured in Yaroslavl. The key point is that in Yaroslavl they don’t know whether measurements were taken in Moscow or not. Later, both groups of experimenters meet and share their impressions. Of course, the results for both groups were the same. BUT! The trick is that from the point of view of quantum mechanics, it’s impossibleanswer the question whose measurements influenced the result. Yaroslavl or Moscow. Because time does not play a role in this case. For definiteness, experimenters should exchange data via the classical communication channel, and they should signal simply about the fact of the experiment, but not about the results. In this case, the Sender and the Recipient of information are precisely determined.
Let us repeat the same experiment with Moscow and Yaroslavl. This time the measurements are carried out in Yaroslavl, after which exactly one bit of information is sent to the Muscovites. "1". What symbolizes that the 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 troubles of quantum teleportation.
First trouble:as far as I know (though I'm not sure about this), it is impossible to predict the measurement results in advance. That is, if we take the same polarization of the photon, then after the measurement we get just a random variable, and we can not influence it in any way. Thus, using quantum teleportation, we transmit not meaningful data, but a set of random variables. For example, the polarization of a photon can be "+1" or "-1". Let's say "+1" we will have a logical 1 and "-1" - a logical zero. Like bits. And we want to transfer logical zero from Moscow to Yaroslavl using quantum teleportation. Measuring - bam! And when measuring, we got a logical unit. It’s just that the probability coincided. And in Yaroslavl, the data received is taken at face value and they think, “Ba! logical unit came. Damn me
The second misfortune: as I wrote above, time does not play a role. Thus, in order to get the correct results, it is necessary to notify the Recipient that the Sender has already taken measurements. In this case, what is the point of muddying this entire quantum teleportation if a classical communication channel is still required for data transfer? (True, it’s worth clarifying that an infinitesimally small amount of information is transmitted through a classical channel, and theoretically, arbitrarily large, through a quantum channel, so within computer networks, for example, there’s also some sense, you want the speed of information exchange, say, in 100 Gbit / s? And 100 Tbit / s? =) The
trouble is the third:follows from the misfortune of the second. Theoretically (again, here I am not sure of my conclusions) if the Sender and the Recipient have synchronized clocks and have agreed in advance that the Sender will take measurements at 2 pm, the Recipient can take measurements at 3 pm and get the correct result. But, with regard to space communications, there is also a problem. Both the Sender and the Recipient must belong to the form homo sapiens. Automation is not suitable here, because any measurements made by automation over a quantum particle still remain in the probability field until they are seen by the Observer, that is, a person .
Such garbage is happening in the world of quantum mechanics. However, everything is so confusing and interesting there that scientists will never leave this field and will still come up with how these and many other, in fact, magical quantum effects to use in practice.
So what we have. It is impossible to transmit information with a speed exceeding the speed of light through any classical communication channels. On Earth’s scale, this may be enough, but alas, no for long-distance space communications. 299792 km / s - of course, the value is huge, but not infinite. So the signal from the Moon to the Earth will go one and a half seconds (this circumstance, incidentally, spoiled a lot of blood for the operators of the Soviet moon rovers), from Mars - more than six minutes, and from Voyager, which was launched back in 1977 and is now 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, due to which weak emitters can be "lost" in the background cosmic noise very quickly.
But it is too early to lower paws due to the injustice of the laws of the Universe. There is such a magical 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 some measurable quantum state (for example, polarization). Before the measurement, the quantum state (polarization of the photon) is in the probability field, that is, it is not clearly defined. But after the measurement, 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 you measure the parameters of one particle, the second appears (it appears, and not become known) the corresponding characteristics. In this case, the distance to which the particles are spaced does not play any role. Moreover, time does not play a role, which creates additional difficulties both in the implementation of the experiment and in the likely practical use of quantum teleportation.
The trick 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. At the same time, everyone is silent as partisans. After some time, the parameters of their particle are also measured in Yaroslavl. The key point is that in Yaroslavl they don’t know whether measurements were taken in Moscow or not. Later, both groups of experimenters meet and share their impressions. Of course, the results for both groups were the same. BUT! The trick is that from the point of view of quantum mechanics, it’s impossibleanswer the question whose measurements influenced the result. Yaroslavl or Moscow. Because time does not play a role in this case. For definiteness, experimenters should exchange data via the classical communication channel, and they should signal simply about the fact of the experiment, but not about the results. In this case, the Sender and the Recipient of information are precisely determined.
Let us repeat the same experiment with Moscow and Yaroslavl. This time the measurements are carried out in Yaroslavl, after which exactly one bit of information is sent to the Muscovites. "1". What symbolizes that the 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 troubles of quantum teleportation.
First trouble:as far as I know (though I'm not sure about this), it is impossible to predict the measurement results in advance. That is, if we take the same polarization of the photon, then after the measurement we get just a random variable, and we can not influence it in any way. Thus, using quantum teleportation, we transmit not meaningful data, but a set of random variables. For example, the polarization of a photon can be "+1" or "-1". Let's say "+1" we will have a logical 1 and "-1" - a logical zero. Like bits. And we want to transfer logical zero from Moscow to Yaroslavl using quantum teleportation. Measuring - bam! And when measuring, we got a logical unit. It’s just that the probability coincided. And in Yaroslavl, the data received is taken at face value and they think, “Ba! logical unit came. Damn me
The second misfortune: as I wrote above, time does not play a role. Thus, in order to get the correct results, it is necessary to notify the Recipient that the Sender has already taken measurements. In this case, what is the point of muddying this entire quantum teleportation if a classical communication channel is still required for data transfer? (True, it’s worth clarifying that an infinitesimally small amount of information is transmitted through a classical channel, and theoretically, arbitrarily large, through a quantum channel, so within computer networks, for example, there’s also some sense, you want the speed of information exchange, say, in 100 Gbit / s? And 100 Tbit / s? =) The
trouble is the third:follows from the misfortune of the second. Theoretically (again, here I am not sure of my conclusions) if the Sender and the Recipient have synchronized clocks and have agreed in advance that the Sender will take measurements at 2 pm, the Recipient can take measurements at 3 pm and get the correct result. But, with regard to space communications, there is also a problem. Both the Sender and the Recipient must belong to the form homo sapiens. Automation is not suitable here, because any measurements made by automation over a quantum particle still remain in the probability field until they are seen by the Observer, that is, a person .
Such garbage is happening in the world of quantum mechanics. However, everything is so confusing and interesting there that scientists will never leave this field and will still come up with how these and many other, in fact, magical quantum effects to use in practice.