Not so long ago, physicists showed the first results of the QUESS mission and the Mozi satellite launched in its framework into orbit, providing a record separation of quantum entangled photons at a distance of more than 1200 km. In the future, this may lead to the creation of a quantum communication line between Beijing and Europe.
The world around is large and diverse - so diverse that laws appear on some scales that are completely unthinkable for others. The laws of politics and Beatlemania do not follow from the structure of the atom - their description requires their own "formulas" and their own principles. It is difficult to imagine that an apple - a macroscopic object whose behavior usually follows the laws of Newtonian mechanics - took and disappeared, merged with another apple, turning into a pineapple. And yet it is precisely such paradoxical phenomena that manifest themselves at the level of elementary particles. Having learned that this apple is red, it is unlikely that we will turn green another, located somewhere in orbit. Meanwhile, this is exactly how the phenomenon of quantum entanglement works, and this is exactly what the Chinese physicists, with whose work we began our conversation, have demonstrated. Let's try to figure it outwhat is it and how can it help humanity.
Bohr, Einstein and others
The world around is local - in other words, in order for some distant object to change, it must interact with another object. Moreover, no interaction can propagate faster than light: this makes physical reality local. An apple cannot slap Newton on the head without physically reaching it. A solar flare cannot instantly affect the operation of satellites: charged particles will have to cover the distance to the Earth and interact with electronics and atmospheric particles. But in the quantum world, locality is violated.
The most famous of the paradoxes of the world of elementary particles is Heisenberg's uncertainty principle, according to which it is impossible to accurately determine the value of both "pair" characteristics of a quantum system. Position in space (coordinate) or speed and direction of movement (impulse), current or voltage, the magnitude of the electric or magnetic component of the field - all these are "complementary" parameters, and the more accurately we measure one of them, the less certain the second will become.
Once upon a time, it was the uncertainty principle that caused Einstein's misunderstanding and his famous skeptical objection, "God does not play dice." However, it seems to be playing: all known experiments, indirect and direct observations and calculations indicate that the principle of uncertainty is a consequence of the fundamental indeterminacy of our world. And again we come to a discrepancy between the scales and levels of reality: where we exist, everything is quite certain: if you unclench your fingers and release the apple, it will fall, attracted by the gravity of the Earth. But at a deeper level, there are simply no causes and effects, but there is only a dance of probabilities.
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The paradox of the quantum entangled state of particles lies in the fact that the "blow to the head" can occur exactly simultaneously with the separation of the apple from the branch. Entanglement is not local, and changing an object in one place instantly - and without any obvious interaction - changes another object entirely in another. Theoretically, we can carry one of the entangled particles to at least the other end of the Universe, but anyway, as soon as we “touch” its partner, who remained on Earth, and the second particle will respond instantly. It was not easy for Einstein to believe this, and his argument with Niels Bohr and colleagues from the "camp" of quantum mechanics became one of the most fascinating subjects in the modern history of science. "Reality is certain," as Einstein and his supporters would say, "only our models, equations and tools are imperfect." “Models can be anything,but the reality itself at the base of our world has never been completely determined,”the adherents of quantum mechanics objected.
Opposing its paradoxes, in 1935 Einstein, together with Boris Podolsky and Nathan Rosen, formulated his own paradox. “Okay,” they reasoned, “let's say it is impossible to find out the coordinate and momentum of a particle at the same time. But what if we have two particles of common origin, whose states are identical? Then we can measure the momentum of one, which will give us indirectly information about the momentum of the other, and the coordinate of the other, which will give knowledge of the coordinate of the first. Such particles were a purely speculative construction, a thought experiment - perhaps that is why Niels Bohr (or rather, his followers) managed to find a decent answer only 30 years later.
Perhaps the first specter of quantum mechanical paradoxes was observed by Heinrich Hertz, who noticed that if the spark gap electrodes were illuminated with ultraviolet light, the passage of the spark was noticeably easier. The experiments of Stoletov, Thomson and other great physicists made it possible to understand that this happens due to the fact that, under the influence of radiation, matter emits electrons. However, this is completely different from what logic suggests; for example, the energy of the released electrons will not be higher if we increase the radiation intensity, but it will increase if we decrease its frequency. Increasing this frequency, we come to the border, beyond which the substance does not exhibit any photoeffect - this level is different for different substances.
Einstein succeeded in explaining these phenomena, for which he was awarded the Nobel Prize. They are connected with the quantization of energy - with the fact that it can be transmitted only by certain "micro-portions", quanta. Each photon of radiation carries a certain energy, and if it is enough, then the electron of the atom that absorbed it will fly out to freedom. The energy of the photons is inversely proportional to the wavelength, and when the boundary of the photoelectric effect is reached, it is no longer enough even to impart to the electron the minimum energy required for the release. Today this phenomenon is encountered everywhere - in the form of solar panels, the photocells of which work precisely on the basis of this effect.
Experiments, interpretations, mysticism
In the mid-1960s, John Bell became interested in the problem of nonlocality in quantum mechanics. He was able to offer a mathematical basis for a completely feasible experiment, which should end with one of the alternative results. The first result "worked" if the principle of locality is really violated, the second - if, after all, it always works and we have to look for some other theory to describe the world of particles. Already in the early 1970s, such experiments were performed by Stuart Friedman and John Clauser, and then by Alain Aspan. To put it simply, the task was to create pairs of entangled photons and measure their spins, one by one. Statistical observations have shown that the spins are not free, but correlated with each other. Such experiments have been carried out almost continuously since then,more and more precise and perfect - and the result is the same.
It should be added that the mechanism explaining quantum entanglement is still unclear, there is only a phenomenon - and different interpretations give their explanations. Thus, in the many-worlds interpretation of quantum mechanics, entangled particles are only projections of the possible states of a single particle in other parallel universes. In transactional interpretation, these particles are linked by standing waves of time. For "quantum mystics" the phenomenon of entanglement is one more reason to consider the paradoxical basis of the world as a way of explaining everything incomprehensible, from the elementary particles themselves to human consciousness. Mystics can understand: if you think about it, then the consequences are dizzy.
The simple experiment of Clauser-Friedman indicates that the locality of the physical world on the scale of elementary particles can be violated, and the very basis of reality turns out - to the horror of Einstein - vague and indefinite. This does not mean that interaction or information can be transmitted instantly, at the expense of entanglement. The separation of entangled particles in space proceeds at normal speed, the measurement results are random, and until we measure one particle, the second will not contain any information about the future result. From the point of view of the recipient of the second particle, the result is completely random. Why does all this interest us?
How to entangle particles: Take a crystal with nonlinear optical properties - that is, one whose interaction of light with which depends on the intensity of this light. For example, lithium triborate, barium beta borate, potassium niobate. Irradiate it with a laser of a suitable wavelength and high-energy photons of laser radiation will sometimes decay into pairs of entangled photons of lower energy (this phenomenon is called "spontaneous parametric scattering") and polarized in perpendicular planes. All that remains is to keep the entangled particles intact and spread them as far apart as possible.
It seems that we dropped the apple while talking about the principle of uncertainty? Lift it up and throw it against the wall - of course, it will break, because in the macrocosm another quantum mechanical paradox - tunneling - does not work. During tunneling, a particle is able to overcome an energy barrier higher than its own energy. The analogy with an apple and a wall is, of course, very approximate, but clear: the tunneling effect allows photons to penetrate into the reflecting medium, and electrons to "ignore" the thin film of aluminum oxide that covers the wires and is actually a dielectric.
Our everyday logic and the laws of classical physics are not very applicable to quantum paradoxes, but they still work and are widely used in technology. Physicists seem to have (temporarily) decided: even if we do not yet fully know how it works, the benefits can be derived from this already today. The tunneling effect underlies the operation of some modern microchips - in the form of tunneling diodes and transistors, tunnel junctions, etc. And, of course, we must not forget about scanning tunneling microscopes, in which particle tunneling provides observation of individual molecules and atoms - and even manipulation by them.
Communication, teleportation and satellite
Indeed, let's imagine that we have “quantum entangled” two apples: if the first apple turns out to be red, then the second is necessarily green, and vice versa. We can send one from Petersburg to Moscow, keeping their confused state, but that would seem to be all. Only when in St. Petersburg an apple is measured as red, the second will turn green in Moscow. Until the moment of measurement, there is no possibility to predict the state of the apple, because (all the same paradoxes!) They do not have the most definite state. What is the use of this entanglement?.. And the sense was found already in the 2000s, when Andrew Jordan and Alexander Korotkov, relying on the ideas of Soviet physicists, found a way to measure, as it were, "not to the end", and therefore to fix the states of particles.
Using "weak quantum measurements", you can, as it were, look at an apple with half an eye, catching a glimpse, trying to guess its color. You can do this over and over again, in fact, without looking at the apple properly, but quite confidently determine that it is, for example, red, which means that an apple in Moscow that is confused with it will be green. This allows entangled particles to be used over and over again, and the methods proposed about 10 years ago allow them to be stored by running in a circle for an indefinitely long time. It remains to carry one of the particles away - and get an extremely useful system.
Frankly speaking, it seems that the benefits of entangled particles are much more than is commonly thought, just our meager imagination, constrained by the same macroscopic scale of reality, does not allow us to come up with real applications for them. However, the already existing proposals are quite fantastic. Thus, on the basis of entangled particles, it is possible to organize a channel for quantum teleportation, a complete “reading” of the quantum state of one object and “recording” it into another, as if the first were simply transferred to the appropriate distance. The prospects of quantum cryptography are more realistic, the algorithms of which promise almost "unbreakable" communication channels: any interference in their work will affect the state of entangled particles and will be immediately noticed by the owner. This is where the Chinese experiment QESS (Quantum Experiments at Space Scale) comes into play.
Computers and satellites
The problem is that, on Earth, it is difficult to create a reliable connection for entangled particles that are far apart. Even in the most advanced optical fiber, through which photons are transmitted, the signal gradually fades, and the requirements for it are especially high here. Chinese scientists have even calculated that if you create entangled photons and send them in two directions with shoulders about 600 km thousand years. Space is another matter, in the deep vacuum of which photons fly such a distance without encountering any obstacles. And then the experimental satellite Mozi ("Mo-Tzu") enters the scene.
A source (laser and nonlinear crystal) was installed on the spacecraft, which every second produced several million pairs of entangled photons. From a distance of 500 to 1700 km, some of these photons were sent to the ground observatory in Deling in Tibet, and the second - in Shenzhen and Lijiang in southern China. As might be expected, the main loss of particles occurred in the lower layers of the atmosphere, but this is only about 10 km of the path of each photon beam. As a result, the channel of entangled particles covered the distance from Tibet to the south of the country - about 1200 km, and in November this year a new line was opened that connects Anhui province in the east with the central province of Hubei. So far, the channel lacks reliability, but this is a matter of technology.
In the near future, the Chinese are planning to launch more advanced satellites for organizing such channels and promise that soon we will see a functioning quantum connection between Beijing and Brussels, in fact, from one end of the continent to the other. Another "impossible" paradox of quantum mechanics promises another leap in technology.
Sergey Vasiliev