Abstract
Table of contents
1. Quantum entanglement — the Queen of paradoxes 2. Bohr, Einstein and others 3. Experiments, interpretations, mysticism 4. Communication, teleportation and satellite 5. Computers and satellites References
Quantum entanglement — the Queen of paradoxes
Not so long ago, physicists showed the first results of the QUESS mission and the Mozi satellite launched into orbit within its framework, providing a record separation of quantum entangled photons with 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.
From the point of view of science
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[2]. At the same time, no interaction can propagate at a speed faster than light: this makes physical reality local. An apple can't slap Newton on the head without physically reaching her. A solar flare cannot instantly affect the operation of satellites: charged particles will have to travel the distance to 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 can be called the Heisenberg uncertainty principle, according to which it is impossible to accurately determine the magnitude of both "paired" characteristics of a quantum system. The position in space (coordinate) or the speed and direction of motion (momentum), current or voltage, the magnitude of the electric or magnetic components of the field are all "complementary" parameters, and the more accurately we measure one of them, the less certain the second will become[1].
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 uncertainty principle is a consequence of the fundamental indeterminacy of our world. And again we come to the incongruity of scales and levels of reality: where we exist, everything is quite definite: if you unclench your fingers and let go of the apple, it will fall, attracted by the gravity of the Earth. But at a deeper level, there are simply no causes and effects, and there is only a dance of probabilities.
The paradox of the quantum entangled state of particles is that a "blow to the head" can occur exactly simultaneously with the separation of an apple from a branch. Entanglement is non–local, and changing an object in one place instantly – and without any obvious interaction - changes another object completely in another. Theoretically, we can take one of the entangled particles at least to the other end of the universe, but all the same, if we "touch" its partner remaining on Earth, and the second particle will respond instantly. It was not easy for Einstein himself to believe this, and his argument with Niels Bohr and colleagues from the "camp" of quantum mechanics became one of the most fascinating plots in the modern history of science. "Reality is definite," 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 heart of our world is never fully defined," objected the adherents of quantum mechanics.
Speaking against its paradoxes, in 1935 Einstein, together with Boris Podolsky and Nathan Rosen, formulated his paradox[8]. "Well," they reasoned, "let's say it's 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 us knowledge of the coordinates of the first." Such particles were a purely speculative construction, a thought experiment – perhaps that's why Niels Bohr (or rather, his followers) managed to find a worthy answer only 30 years later.
Perhaps the first ghost of quantum mechanical paradoxes was observed by Heinrich Hertz, who noticed that if the electrodes of the spark gap are illuminated with ultraviolet light, the passage of the spark is noticeably facilitated. The experiments of Stoletov, Thomson and other great physicists made it possible to understand that this happens due to the fact that matter emits electrons under the influence of radiation. However, this does not happen at all as logic suggests; for example, the energy of the released electrons will not be higher if we increase the intensity of radiation, but it will increase if we reduce its frequency. By increasing this frequency, we will come to the border beyond which the substance does not show any photoelectric effect – this level is different for different substances.
Einstein managed to explain 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 "microportions", 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 free[6]. The photon energy is inversely proportional to the wavelength, and when the boundary of the photoelectric effect is reached, it is no longer enough even to communicate to the electron the minimum energy needed for the output. Today, this phenomenon occurs to us everywhere – in the form of solar panels, whose solar cells 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 managed to propose 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[3]. Already in the early 1970s, such experiments were conducted by Stuart Friedman and John Clauser, and then by Alain Aspen. 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 accurate and perfect – and the result is the same.
It is worth adding that the mechanism explaining quantum entanglement is still unclear, there is only a phenomenon – and various interpretations give their explanations. So, in the multi–world interpretation of quantum mechanics, entangled particles are only projections of the possible states of a single particle in other parallel universes. In a transactional interpretation, these particles connect the standing waves of time. For "quantum mystics", the phenomenon of entanglement is another 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 be understood: if you think about it, the consequences make your head spin.
The simple experience 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 Einstein's horror – vague and indefinite. This does not mean that interaction or information can be transmitted instantly, due to entanglement[5]. The separation of entangled particles in space proceeds at the usual speed, the measurement results are random, and until we measure one particle, the second one 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 are we interested in all this?
How to confuse particles: take a crystal with nonlinear optical properties – that is, one whose interaction with light 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 break up into pairs of entangled photons of lower energy (this phenomenon is called "spontaneous parametric scattering") and polarized in perpendicular planes[7]. It remains to keep the entangled particles intact and spread as far as possible from each other.
It seems that when we were talking about the uncertainty principle, we dropped an apple? Pick it up and throw it against the wall – of course, it will break, because another quantum mechanical paradox does not work in the macrocosm - tunneling. When tunneling, a particle is able to overcome an energy barrier higher than its own energy. The analogy with an apple and a wall, of course, is very approximate, but it is clear: the tunnel effect allows photons to penetrate into the reflective medium, and electrons to "not notice" 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, but we can benefit from it today. The tunneling effect underlies the work of some modern microchips – in the form of tunnel 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 of them.
Communication, teleportation and satellite
In fact, let's imagine that we have "quantum confused" two apples: if the first apple turns out to be red, then the second one is necessarily green, and vice versa. We can send one from St. Petersburg to Moscow, preserving their confused state, but that would seem to be all. Only when an apple is measured as red in St. Petersburg, the second one will turn green in Moscow. Until the moment of measurement, there is no way to predict the state of the apple, because (all the same paradoxes!) they do not have a very specific 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 "not completely" measure, and therefore fix the states of particles.
Using "weak quantum measurements", one can, as it were, look at an apple with half an eye, briefly, trying to guess its color. You can do this over and over again without actually looking at the apple properly, but you can quite confidently decide that it is, for example, red, which means that an apple confused with it in Moscow will be green. This allows you to use entangled particles again and again, and the methods proposed about 10 years ago allow you to store them by running around in circles indefinitely. It remains to take one of the particles away – and get an exceptionally useful system.
Frankly speaking, it seems that there is much more use in entangled particles than is commonly thought, it's just that 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. So, 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 "writing" it to another, as if the first one had simply been transported to the appropriate distance. The prospects of quantum cryptography are more realistic, the algorithms of which promise almost "unbreakable" communication channels: any interference with 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 – "Quantum experiments on a cosmic scale") comes on the scene[9].
Computers and satellites
The problem is that on Earth it is difficult to create a reliable connection for entangled particles spread over a long distance. 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 long – half the distance from the center of quantum science in Delinghe to the centers in Shenzhen and Lijiang – you can expect to catch a tangled pair in about 30 thousand years. Another thing is space, in the deep vacuum of which photons fly such a distance without encountering any obstacles. And then the experimental satellite Mozi ("Mo-Tzu") comes on the scene[10].
A source (a laser and a nonlinear crystal) was installed on the space orbiter, which produced several million pairs of entangled photons every second. From a distance of 500 to 1700 km, some of these photons were sent to the ground–based observatory in Delinghe in Tibet, and the second - in Shenzhen and Lijiang in southern China. As might be expected, the main particle losses 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 of 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 technique.
In the near future, the Chinese are planning to launch more advanced satellites to organize 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.
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